Choosing an ND Filter: f-stops, T-stops and Optical Density

Imagine this scenario. I’m lensing a daylight exterior and my light meter gives me a reading of f/11, but I want to shoot with an aperture of T4, because that’s the depth of field I like. I know that I need to use a .9 ND (neutral density) filter. But how did I work that out? How on earth does anyone arrive at the number 0.9 from the numbers 11 and 4?

Let me explain from the beginning. First of all, let’s remind ourselves what f-stops are. You have probably seen those familiar numbers printed on the sides of lenses many times…

1      1.4      2      2.8      4      5.6      8      11      16      22

They are ratios: ratios of the lens’ focal length to its iris diameter. So a 50mm lens with a 25mm diameter iris is at f/2. If you close up the iris to just under 9mm in diameter, you’ll be at f/5.6 (50 divided by 5.6 is 8.93).

A stills lens with its aperture ring marked in f-stops
A stills lens with its aperture ring (top) marked in f-stops

But why not label a lens 1, 2, 3, 4? Why 1, 1.2, 2, 2.8…? These magic numbers are f-stops. A lens set to f/1 will let in twice as much light as (or ‘one stop more than’) one set to f/1.4, which in turn will let in twice as much as one set to f/2, and so on. Conversely, a lens set to f/2 will let in half as much light as (or ‘one stop less than’) one set to f/1.4, and so on.

 

If you think back to high school maths and the Pi r squared formula for calculating the area of a circle from its radius, the reason for the seemingly random series of numbers will start to become clear. Letting in twice as much light requires twice as much area for those light rays to fall on, and remember that the f-number is the ratio of the focal length to the iris diameter, so you can see how square roots are going to get involved and why f-stops aren’t just plain old round numbers.

A Zeiss Compact Prime lens with its aperture ring marked in T-stops
A Zeiss Compact Prime lens with its aperture ring marked in T-stops

Now, earlier I mentioned T4. How did I get from f-stops to T-stops? Well, T-stops are f-stops adjusted to compensate for the light transmission efficiency. Two different f/2 lenses will not necessarily produce equally bright images, because some percentage of light travelling through the elements will always be lost, and that percentage will vary depending on the quality of the glass and the number of elements. A lens with 100% light transmission would have the same f-number and T-number, but in practice the T-number will always be a little higher than the f-number. For example, Cooke’s 15-40mm zoom is rated at a maximum aperture of T2 or f/1.84.

So, let’s go back to my original scenario and see where we are. My light meter reads f/11. However,  I expressed my target stop as a T-number though, T4, because I’m using cinema lenses and they’re marked up in T-stops rather than f-stops. (I can still use the f-number my meter gives me though; in fact if my lens were marked in f-stops then my exposure would be slightly off because the meter does not know the transmission efficiency of my lens.)

By looking at the series of f-numbers permanently displayed on my light meter (the same series listed near the top of this post, or on any lens barrel) I can see that f/11 (or T11) is 3 stops above f/4 (or T4) – because 11 is three numbers to the right of 4 in the series. I can often be seen on set counting the stops like this on my light meter or on my fingers. It is of course possible to work it out mathematically, but screw that!

CameraZOOM-20140309092150072_zps94e90ea4
A set of Tiffen 4×4″ ND filters

So I need an ND filter that cuts 3 stops of light. But we’re not out of the mathematical woods yet.

The most popular ND filters amongst professional cinematographers are those made by Tiffen, and a typical set might be labelled as follows:

.3      .6      .9      1.2

Argh! What do those numbers mean? That’s the optical density, a property defined as the natural logarithm of the ratio of the quantity of light entering the filter to the quantity of light exiting it on the other side. A .3 ND reduces the light by half because 10 raised to the power of -0.3 is 0.5, or near as damn it. And reducing light by half, as we established earlier, means dropping one stop.

If that fries your brain, don’t worry; it does mine too. All you really need to do is multiply the number of stops you want to drop by 0.3 to find the filter you need. So to drop three stops you pick the .9 ND.

And that’s why you need a .9 ND to shoot at T4 when your light meter says f/11. Clear as mud, right? Once you get your head around it, and memorise the f-stops, this all becomes a lot easier than it seems at first glance.

Here are a couple more examples:

  • Light meter reads f/8 and you want to shoot at T5.6. That’s a one stop difference. (5.6 and 8 are right next to each other in the stop series, as you’ll see if you scroll back to the top.) 1 x 0.3 = 0.3 so you should use the .3 ND.
  • Light meter reads f/22 and you want to shoot at T2.8. That’s a six stop difference (scroll back up and count them), and 6 x 0.3 = 1.8, so you need a 1.8 ND filter. If you don’t have one, you need to stack two NDs in your matte box that add up to 1.8, e.g. a 1.2 and a .6.

 

Choosing an ND Filter: f-stops, T-stops and Optical Density

Lensing Ren – episode 2

Here’s my video breaking down the cinematography of episode two of Ren: The Girl with the Mark. This week I discuss lighting Ren’s house, tweaking wide-shot lighting for close-ups, and depth of field.

Here is the lighting plan for Ren’s house:

Rens-house-1080

And here is a video blog from the set of Ren’s house:

Check out the article I wrote during the shoot about lighting Ren and Dagron’s house if you’re still hungry for details.

If you want to know more about using kinoflos as indirect window light, have a look at Lighting Technique #3: The Window Wrap.

Want to know more about Depth of Field? This post will give you the basics.

Check back next Saturday for another instalment of Lensing Ren, and meanwhile watch the next episode of Ren itself from Tuesday at 8pm GMT at rentheseries.com

Lensing Ren – episode 2

Black-screen & White-screen: The Best Kept Secrets in Compositing

Accessing the compositing modes in Final Cut Pro 7
Accessing the compositing modes in Final Cut Pro 7

When it comes to shooting elements for VFX, green-screen gets all the press. But certain kinds of elements can be tricky to key well, and sometimes it’s not the right look. In the last few days Kate Madison and I have needed to shoot last-minute elements for some shots in Ren: The Girl with the Mark, and we turned to monochromatic backgrounds.

Why? How does it work? Well certainly you can key out black or white just like you’d key out green, but the most powerful way to use these backgrounds is not with keying at all, but by a bit of basic maths. And don’t worry, the computer does the maths for you.

If you’ve ever used Photoshop, you’ll have noticed some layer modes called Screen and Multiply. Final Cut Pro has the same modes (it also has Add, which to most intents and purposes is the same as Screen) and so do all the major editing and FX packages.

Screen adds the brightness of each pixel of the layer to the layer underneath. Since black has a brightness of zero, your black screen disappears, and the element in front of it is blended seamlessly into the background image, with its apparent solidity determined by its brightness.

Multiply, as the name suggests, multiplies the brightness of each pixel with the layer underneath. Since white has a brightness of one, and any number multiplied by one is that same number, your white screen vanishes. Whatever element is in front of your screen is blended into the background image, with darker parts of the element showing up more than lighter parts.

One of the elements Kate and I needed to shoot was a flame, to be comped onto a torch. We lit a torch and clamped it to a stand, shooting at night with the pitch black garden in the background. It was the work of moments to comp this element into the shot using Screen mode.

The flame element, shot at night in the garden to ensure a seamless black background
The flame element, shot at night in the garden to ensure a seamless black background
I adjusted the flame's size and used Screen mode to composite it over the background.
I adjusted the flame’s size and used Screen mode to composite it over the background.

Fire is the perfect partner for black-screen shooting, because it generates its own light and it’s not solid. Solid objects composited using Screen/Add or Multiply take on a ghostly appearance – perfect for, er, ghost effects – but not ideal in other situations; because of the way Screen mode works, anything that’s not peak white will be transparent to some degree.

We shot some fast-moving leaves and debris against black, but only the high level of motion blur allowed us to get away with it. In fact, if you know you’re going to have a lot of motion blur, black-screen might be the ideal method, because it will be tricky to get a clean key off a green-screen.

A smoke element shot against a black drape and backlit so that the smoke is visible but the drape is not
A smoke element shot against a black drape and backlit so that the smoke is visible but the drape is not
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Shooting dirt in a vase of water against white

Other things that work well against black-screen are sparks, smoke and water/rain, again because they’re not solid. If you want to add rain or snow to a shot, black-screen is the way to go – check out my post about that here.

Yesterday Kate and I needed to shoot a whirlwind element. One of the VFX team suggested swirling sand in a vase of water. After a few experiments in the kitchen, we ended up using dirt from the garden. We used fluorescent softboxes for the background, ensuring we got a bright white background, and made weird arrangements of white paper to eliminate as many of the dark reflections in the vase as we could.

One of the tornado elements shot with the set-up pictured above. We let the dirt settle in the bottom of the water, then swirled the water with a spoon (which had to kept out of frame).
One of the tornado elements shot with the set-up pictured above. We let the dirt settle in the bottom of the water, then swirled the water with a spoon (which had to be kept out of frame).

A few weeks back we shot hosepipe water against black, inverted it and used Multiply to superimpose it as blowing dirt.

With a little thinking outside the box, you can shoot all kinds of elements against white or black to meet your VFX needs. I’ll leave you with this featurette I made in 2006, breaking down the various low-tech FX – many of them black-screen – that I employed on my feature film Soul Searcher.

Black-screen & White-screen: The Best Kept Secrets in Compositing

How to Cut a Behind-the-Scenes Featurette

Cast&Crew_RAU_4273_20Sept_Unger copy copy
Cameras roll on the set of Kate Madison’s web series, Ren. Photo: Richard Unger

Despite my big plan to quit editing last year, I somehow ended up cutting nearly all the behind-the-scenes material for Ren, including a dozen YouTube videos and 30-odd exclusive set diaries which have just been released for sale. Guess I just have a fondness for BTS stuff.

Brett Chapman shoots B roll on Stop/Eject as Hadrian Cawthorne looks on. Photo: Paul Bednall
Brett Chapman shoots B-roll on Stop/Eject as Hadrian Cawthorne looks on. Photo: Paul Bednall

So here are some tips for editing BTS videos for the web. Many of these apply equally to any talking-head-based documentary.

  • Plan for it before the shoot by lining up a competent BTS camera crew and being clear about the kind of material you need. Here are some tips for shooting B-roll.
  • Start the edit by creating a new timeline and putting in some text generators with category headings you think you’ll want to cover, e.g. “plot”, “characters”, “casting”, “action scenes”, “concluding remarks”.
  • Watch through all the interview material. Every time you hear something you think you can use, dump it on the timeline after the relevant text.
  • Play back your timeline. You’ll immediately see that some of the material you’ve included is dull or repetitious. Whittle down the material until your timeline is only a little longer than you intend the finished piece to be. (I suggest 2-3 minutes should be your target length for a web piece.)
  • Pay attention to your in and out points. Don’t cut while someone is drawing breath – cut before or after. Beware of breathing time if you’re hacking someone’s sentences around. If your editing makes a couple of words sound unnaturally close together, interpose a few frames of atmos or silence. If you cut someone off in the middle of a sentence, firstly be sure the intonation doesn’t make it sound cut off, then add in some silence or atmos before the next clip, and paper over the edit with B-roll as the interviewee’s face will often give away that they’re not finished speaking.
  • Speaking of papering over the talking heads with B-roll, it’s time to do that now. I often start with the obvious stuff. Clearly shots of the fight scenes being rehearsed need to go over the actors talking about fight scenes. Then I’ll move onto the less obvious stuff – an actor talking about their character might go with almost any shot of that actor on set, so I’ll see what’s left at the end.
  • Avoid cutting in the middle of quick movements – an arm going up, a head turning- unless that action will be continued in the next shot. This goes for the talking heads too – don’t cut on or close to a blink. Also avoid cutting on an emphasised or particularly loud syllable, because this too will jar.
  • Take out the text generators and replace them with a few seconds of B-roll that doesn’t have any interview sound under it. This gives you dividers between topics without blatantly signposting them, and allows the audience a breather. You could bring up the audio on the B-roll, or put in a bit of music. Usually it’s best for this B-roll to serve as an introduction to the topic that’s up next. For example, if the next topic is “what it was like working with the director”, kick it off with B-roll of the director explaining the next scene to the actors. After hearing him or her talk for a sentence or so, fade down the audio and bring in the interview sound.
  • Get some music from somewhere like incompetech.com, if your composer hasn’t started work yet, and cut opening and closing montages of B-roll to it.
  • Put in your lower thirds and opening and closing titles. If the video’s going on You Tube, it’s a good idea to allow for annotations linking viewers to other videos on your channel. Do not put in credits – sorry, but no-one cares who made this.
  • Watch the whole thing through and try to take out another 10-30 seconds. Remember, pace is everything. Do not give people the slightest excuse to stop watching.
  • Do a colour correction pass so everything matches.
  • Go through again balancing the audio. People start their sentences loudly and get quieter as their lungs deflate, so counter this by ramping the audio up over the course of the sentence. Use EQ filters if necessary to counter tinny or boomy sound, or reduce hiss or wind noise. See this Nofilmschool article for some handy audio tips. If any of the audio cuts are popping or clicking, put on a 1 frame cross fade. If you don’t have decent speakers, do this on every cut because you won’t know which ones are dodgy.
  • If any of the speech is still hard to make out – and remember that your viewers haven’t heard it a million times like you have – then subtitle it.
  • Watch it one last time to check everything’s smooth, then compress and upload it. You’re done!

If you’ve found this post useful, please consider supporting Ren by purchasing or sharing the trailer for the Daily Diary videos. Buyers get the first 7 videos now and the remaining 29 when the series is released this summer. They’re all different, some following the above pattern and others being much more candid, fly-on-the-wall affairs. There are plenty of bloopers, interviews and filmmaking tips to be enjoyed throughout. Or check out our free behind-the-scenes videos on YouTube.

How to Cut a Behind-the-Scenes Featurette

Converting Blackmagic Raw Footage to ProRes with After Effects

My 4K Blackmagic Production Camera
Blackmagic Production Camera

One of the big benefits of the Blackmagic cameras is their ability to shoot raw – lossless Cinema DNG files that capture an incredible range of detail. But encoding those files into a useable format for editing can be tricky, especially if your computer won’t run the processor-intensive DaVinci Resolve which ships with the camera.

You can usually turn to the Adobe Creative Suite when faced with intractable transcoding problems, and sure enough After Effects provides one solution for raw to ProRes conversion.

I’ll take you through it, step by step.  Let’s assume you’ve been shooting on a Blackmagic Cinema Camera and you have some 2.5K raw shots which you want to drop into your edit timeline alongside 1080P ProRes 422HQ material.

1. In After Effects’ launch window, select New Composition. A dialogue box will appear in which you can spec up your project. For this example, we’re going to choose the standard HDTV resolution of 1920×1080. It’s critical that you get your frame rate right, or your audio won’t sync. Click OK once you’ve set everything to your liking.

step1

2. Now go to the File menu and select Import > File. Navigate to the raw material on your hard drive. The BMCC creates a folder for each raw clip, containing the individual Cinema DNG frames and a WAV audio file. Select the first DNG file in the folder and ensure that Camera Raw Sequence is ticked, then click OK.

step2

3. You’ll then have the chance to do a basic grade on the shot – though with only the first frame to judge it by.

step3

4. Use Import > File again to import the WAV audio file.

step4

5. Your project bin should now contain the DNG sequence – shown as a single item – along with the WAV audio and the composition. Drag the DNG sequence into the main viewer window. Because the BMCC’s raw mode records at a resolution of 2.5K and you set your composition to 1080P, the image will appear cropped.

step5

6. If necessary, zoom out (using the drop-down menu in the bottom left of the Composition window) so you can see the wireframe of the 2.5K image. Then click and drag the bottom right corner of that wireframe to shrink the image until it fits into the 1080P frame. Hold down shift while dragging to maintain the aspect ratio.

step6

7. Drag the WAV audio onto the timeline, taking care to align it precisely with the video.

step7

8. Go to Composition Settings in the Composition menu and alter the duration of the composition to match the duration of the clip (which you can see by clicking the DNG sequence in the project bin).

step8

9. Go to the Composition menu again and select Add to Render Queue. The composition timeline will give way to the Render Queue tab.

step9

10. Next to the words Output Module in the Render Queue, you’ll see a clickable Lossless setting (yellow and underlined). Click this to open the Output Module Settings.

step10

11. In the Video Output section, click on Format Options… We’re going to pick ProRes 422 HQ, to match with the non-raw shots we hypothetically filmed. Click OK to close the Format Options.

step11

12. You should now be back in Output Module Settings. Before clicking OK to close this, be sure to tick the Audio Output box to make sure you don’t end up with a mute clip. You should not need to change the default output settings of 48kHz 16-bit stereo PCM.

step12

13. In the Render Queue tab, next to the words Output to you’ll see a clickable filename – the default is Comp1.mov. Click on this to bring up a file selector and choose where to save your ProRes file.

step13

14. Click Render (top far right of the Render Queue tab). Now just sit back and wait for your computer to crunch the numbers.

step14

I’ve never used After Effects before, so there are probably ways to streamline this process which I’m unaware of. Can anyone out there suggest any improvements to this workflow? Is it possible to automate a batch?

Converting Blackmagic Raw Footage to ProRes with After Effects

How to Soften Harsh Sunlight with Tinfoil and a Bedsheet

On a pick-up shoot a couple of months back I found myself in the position of needing to mould the natural light without any equipment whatsoever. The sun was shining brightly with no clouds in sight, and we needed to shoot a close-up that would match an existing overcast wide shot. Fortunately there was a B&M just around the corner. Unfortunately no-one took any pictures of the ridiculous set-up that ensued, hence the comedy illustration.

Here’s how to do it:

  1. Get yourself round to B&M. (Other depressing discount stores are available.)
  2. Purchase a white bedsheet (£3.99) and a roll of foil (79p).
  3. The sheet is going to be your “silk”. First of all, cut the elastic corners to make it easier to wrangle.
  4. Assign Unfortunate Crewmember #1 to hold up the sheet so that it casts a shadow on the talent. It will soften the direct sunlight falling on the them, but there will probably still be darker shadows on their face than you want, so…
  5. Unroll a couple of feet of the foil. This is going to be your bounce card.
  6. Assign Unfortunate Crewmember #2 to stand on the shade side of the talent and use the foil to reflect some direct sunlight into that side their face.
  7. Hope that this is not the day the photographer from the local paper visits the set.

Even if you’re not matching to overcast shots, this softening of harsh sunlight is usually desirable to some degree when shooting CUs. Click here for more on moulding natural light.

How to Soften Harsh Sunlight with Tinfoil and a Bedsheet

Slating 101

Slating Brendan O'Neill's Fled. See www.sticklebackproductions.blogspot.co.uk
Slating Brendan O’Neill’s Fled. See www.sticklebackproductions.blogspot.co.uk

With dual system sound now the norm for even micro-budget shoots, a clapperboard (or slate as they call them in the US) is an indispensable bit of kit. It’s always best to keep this under the purview of the clapperloader or 2nd AC, rather than giving it to whichever crew member is free at the time. Otherwise you often end up with the camera operator calling “mark it” followed by an awkward pause because that crew member has left the set to perform some other duty, or has been too busy with other duties and is now scrambling to update the numbers on the slate. Incorrect slates can give the editor headaches down the line, so it’s important to get it right.

Slating Harriet Sams' The First Musketeer. See www.firstmusketeer.com
Slating Harriet Sams’ The First Musketeer. See www.firstmusketeer.com

With that in mind, here are the basic rules of slating.

Labelling the Board

The production name, scene number, DP’s and director’s names and the date are self-explanatory. DAY/NIGHT and INT/EXT (interior/exterior) are intended to ensure the labs process the film footage correctly, but should still be circled appropriately on a digital shoot. Shutter and frame rate information can be obtained from the camera operator or DP. Some slates will have a space for a roll number, and since “rolls” (memory cards) are recycled on a modern shoot, it is best to ask the DIT (Digital Imaging Technician, or data wrangler) how they would like these numbered.

Slates and Takes

The slate number should start at 1 for the first shot of the first day, and increment every time the camera position and/or lens is changed. Sometimes a director will ask instead for the slate number to match the numbers on their shotlist or storyboards, but this is a bad idea because inevitably shots will be dropped or added and it becomes very confusing. Besides, if the slate number simply starts at 1 and goes up, the DIT can easily tell if a shot is missing from their hard drive due to a card being overlooked or some technical fault.

The take number should reset to 1 each time the slate number changes, and increment every time the camera stops rolling, with certain exceptions and variations outlined below.

(The American system differs in that it omits slate numbers. Instead a letter is appended to the scene number, so the first shot filmed of scene 7 would be 7, then 7A, 7B, 7C, etc.)

Procedure

The clapperloader should always have the board up to date and ready to go. He or she should have checked the length of the lens being used and found a position for the slate in which it’s fully in frame and legibile. A torch may be required if the set is moodily lit.

The sound mixer will roll their device and announce “sound speed”.  The camera operator will then roll and ask the clapperloader to mark it. By this point the slate should already be in frame so that the first frame recorded, when the DIT looks at it as a thumbnail on their hard drive, has the slate on it.

Only the slate and take number need be announced, e.g. “30 take 3”. The board should then be clapped nice and cleanly to produce a sharp click on the soundtrack that is easy for the DIT or assistant editor to sync. If it’s necessary to clap a second time, the clapperloader should announce “second clap” or “second sticks” immediately before.

Pick-up
Slating a pick-up for The Deaths of John Smith. See www.thedeathsofjohnsmith.com

PU and AFS

If the director decides to do another take but to begin the action part way through rather than from the top, the take number should still increase but pick-up (PU for short) should be appended to the number. For example: take one, take two, take three pick-up, take four pick-up.

If camera and/or sound roll but cut before the board is clapped, the take number remains the same for the next attempt.

If camera and sound roll, the board is read and clapped, but the crew cuts before action is called, the take number remains the same but AFS (After a False Start) is appended.

If action is called, even if it’s immediately followed by cut, the take number always increases for the next attempt.

A mute slate for The First Musketeer
A mute slate for The First Musketeer

MOS

Sometimes the camera rolls without sound, if the mixer feels he or she cannot get any useful sound. In these cases the clapperloader should circle MOS (Mute Of Sound) on the slate. They don’t need to clap the board or announce the slate and take number; they simply need to hold the board up long enough for it to be read by the editor. As an additional indicator that there is no accompanying sound file, the clapperloader should hold the board with their fingers between the sticks.

An end board on Fled
An end board on Fled

End Board

Sometimes it’s impractical or inconvenient to shoot the slate at the start of a take, so instead it’s shot at the end. At the start of the take the camera operator announces “end board” instead of “mark it”. When the action is finished, the director typically forgets that it’s an end board (American term: tail slate) and calls “cut”. Hopefully the sound mixer and camera operator remember not to obey this command, and the latter calls “mark it”. The clapperloader should then mark the take in the usual manner, except that the board should be held upside-down. They should conclude their verbal announcement with “end board” or “on the end”, e.g. “27 take 2 on the end”. Only then can camera and sound cut.

Slating 101

Where to Put Your Key Light

What angle the key light hits a character at is a KEY (groan) decision for a director of photography. The lighting featurette in my last post looked at some of the options, but today I’d like to expand on those with some more up-to-date examples.

Imagine a clock face. You’re looking down on the scene and your talent is at the centre with their eyeline to twelve o’clock.

The key light clock
The key light clock

With that in mind, consider the following shots. The camera is at between twelve and one o’clock in each case.

Rewind
Key light at almost twelve o’clock
Half past eleven
Half past eleven
Eleven o'clock
Eleven o’clock
Quarter to eleven
Quarter to eleven
Half past nine (Aimee Denaro in See Saw)
Half past nine (view the See Saw trailer)

Clearly the level of fill makes a big difference, but you can already see that a key light at noon gives a flawless, evenly lit look which is great for a leading lady, while a half-past-nine casts half of the face into darkness for a threatening or mysterious feel. There is a sweet spot I love at about quarter to eleven where, on one side of the face, only the eye and a triangle on the cheek are lit. But generally between half past ten and half past eleven models the face nicely.

In all of the above examples, with the camera at one-ish, the key was between nine and twelve, i.e. the key was on the opposite side of the eyeline to the camera. This is known as lighting the “downside” – the side away from camera. Most cinematographers, myself included, consider this the most pleasing side to light from, as it shows the shape of the face and gives a nice shadow area on the camera side into which you can dial your preferred amount of fill.

Here is an example of lighting the upside:

Key light at eleven o'clock, camera at eleven thirty (click here to link through to this interview with renowned designer Dick Powell, by Astute Graphics)
Key light at eleven o’clock, camera at eleven thirty (Click here to link through to this interview with renowned designer Dick Powell, by Astute Graphics)

In many scenes, the position of the keylight will be dictated by the layout of the set or location, so a DP should consider this in pre-production discussions with the production designer, on location scouts or when the director is blocking the actors.

Where to Put Your Key Light

Understanding Shutter Angles

How many of us see that 1/50 or 1/48 in the bottom of our viewfinders and aren’t really sure what it means? Shutter angles or shutter intervals are part of the cinematographer’s toolkit, but to use them most effectively an understanding of what’s going on under the hood is useful. And that begins with celluloid.

This animation from Wikipedia shows how the shutter's rotation works in tandem with the claw moving the film through the gate.
This animation from Wikipedia shows how the shutter’s rotation works in tandem with the claw moving the film through the gate. The shutter angle here is 180 degrees.

Let’s imagine we’re shooting on film at 24fps, the most common frame rate. Clearly the film can’t move continuously through the gate (the opening behind the lens where the focused light strikes the film) or we would end up with just a long vertical streak. The film must remain stationary long enough to expose an image, before being moved on four perforations (the standard height of a 35mm film frame) so the next frame can be exposed. And crucially light must not hit the film while it is being moved or vertical streaking will occur.

This is where the shutter comes in. The shutter is a portion of a disc that spins in front of the gate. The standard shutter angle is 180°, meaning that the shutter is a semi-circle. A 270° shutter would be a quarter of a circle; we always talk about shutter angles in terms of the portion of the disc which is absent.

The shutter spins continuously at the same speed as the frame rate – so at 24fps the shutter makes 24 revolutions per second. So with a 180° shutter, each 24th of a second is divided into two halves, or 48ths of a second:

  • During one 48th of a second, the missing part of the shutter is over the gate, allowing the stationary film to be exposed.
  • During the other 48th of a second, the shutter blocks the gate to prevent light hitting the film as it is advanced. The shutter has a mirrored surface so that light from the lens is reflected up the viewfinder, allowing the camera operator to see what they’re shooting.

Frame rate * (360/shutter angle) = shutter interval denominator

24 * (360/180) = 48

So we can see that a film running at 24fps, shot with a 180° shutter, shows us only a 48th of a second’s worth of light on each frame. And this has been the standard frame rate and shutter angle in cinema since the introduction of sound in the late 1920s. The amount of motion blur captured in a 48th of a second is the amount that we as an audience have been trained to expect from motion pictures all our lives.

Saving Private Ryan's Normandy beach sequence uses a decreased shutter interval
Saving Private Ryan’s Normandy beach sequence uses a decreased shutter interval

A greater (larger shutter angle, longer shutter interval) or lesser (smaller shutter angle, shorter shutter interval) amount of motion blur looks unusual to us and thus can be used to creative effect. Saving Private Ryan features perhaps the best-known example of a small shutter angle in its D-day landing sequence, where the lack of motion blur creates a crisp, hyper-real effect that draws you into the horror of the battle. Many action movies since have copied the effect in their fight scenes.

Large shutter angles are less common, but the extra motion blur can imply a drugged, fatigued or dream-like state.

In today’s digital environment, only the top-end cameras like the Arri Alexa have a physical shutter. In other models the effect is replicated electronically (with some nasty side effects like the rolling shutter “jello” effect) but the same principles apply. The camera will allow you to select a shutter interval of your choice, and on some models like the Canon C300 you can adjust the preferences so that it’s displayed in your viewfinder as a shutter angle rather than interval.

I advise always keeping your shutter angle at 180° unless you have a solid creative reason to do otherwise. Don’t shorten your shutter interval to prevent over-exposure on a sunny day; instead use the iris, ISO/gain or better still ND filters to cut out some light. And if you shoot slow motion, maintain that 180° angle for the best-looking motion blur – e.g. at 96fps set your shutter interval to 1/192.

Understanding Shutter Angles

How to Light a Church

Roger Harding (left) and Jeremy Heynes in The Deaths of John Smith.
Roger Harding (left) and Jeremy Heynes in The Deaths of John Smith. A 1.2K HMI punches through the window on the right, while a fluorescent softbox illuiminates the arches on the left. Background light comes from two 500W halogen work-lights rigged to a dimmer, while fill (given that it was getting dark outside at this point) comes from a blue-gelled 1K Arrilite behind and to the left of camera.

This weekend shooting began on Roger Harding and Darren Scott’s feature-length comedy The Deaths of John Smith. As director of photography I was called on to light a beautiful rural church on a limited budget. Here are some tips for ecclesial cinematography:

  • Hire HMIs – powerful, daylight-balanced lamps. Without at least one you will never have enough light to illuminate anything but the tiniest of churches. As a backlight on a mezzanine level, a 2.5K HMI will illuminate most churches. Better still, put them outside the windows and create artificial sunbeams. (A blue-gelled blonde or redhead outside a stained glass window is pretty much useless; those windows cut out so much light.)
  • Use smoke. A £50 disco smoke machine is perfectly sufficient – use it to volumize the light and emphasise the depth and scale of the building. If you’re struggling to expose a bright enough image, smoke helps there too – because it catches the backlight and lightens up the shadows.
  • Candlelight is a good way to introduce colour contrast into your scene. Dedos are the best lamps to fake candelight with, as they can produce a small circular pool of light. Failing that, any tungsten source will do, ideally rigged to a dimmer board for a bit of flickering.
  • Assuming you’ve got your HMIs punching directly in through all the windows on one side of your church (that’s the side the “sun” is on), you now need soft light coming in through the opposite windows. Ideally these would be larger HMIs playing off bounce boards, but you might get away with soft boxes or bounced tungsten sources (gelled blue, of course) hidden behind pillars inside the building.
  • Sellotape together some old bits of coloured gel and rig them in front of a fresnel to simulate daylight through a stained glass window. Note that this doesn’t really work with unfocused lamps like redheads.
Left to right: David Draper, Bryan Ferriman and Adrian Moore.
Left to right: David Draper, Bryan Ferriman and Adrian Moore. Our single HMI shines through the lefthand window, suitably volumized with smoke, leaving natural light to deal with the other two. A blue-gelled 1K Arrilite off to the right of frame creates the edge-light on the righthand side of each character. An existing halogen spotlight over the organ was gelled with half CTB to cool it down a little. I chose to leave the nearside of the characters dark to contrast the foreground with the brighter background.

On The Deaths of John Smith I only had access to one HMI, so for every shot I needed to carefully choose which window to put it outside of for the maximum impact. I relied on natural light as well as blue-gelled redheads and fluorescent softboxes just out of frame for fill light. Nonetheless, I’m very pleased with the results. Next weekend we have to repeat the performance with a large congregation….

All images copyright 2013 Two Hats Films. Visit the Facebook page or the official website for more info on The Deaths of John Smith.

Here the "sun" (HMI) is outside of the lefthand background window.
Here the “sun” (HMI) is outside of the lefthand background window, but I couldn’t resist cheating a little and pushing a 1K Arrilite through a nice yellow stained glass window in the top centre background. Additional backlight comes from a blue-gelled Arrilite off frame right, while a softbox behind and to the left of camera illuminates the actor’s face.

How to Light a Church