# 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).

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.

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!

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.

# 4 Reasons to Use a Light Meter

In the celluloid era, light meters were essential to ensure proper exposure of the film negative. In today’s digital world, where you can immediately see your images on a monitor, it may seem like light meters are obsolete.

But these devices still have their place in modern cinematography. On a bigger production, when you may not be at the camera the whole time, they can be very useful. Interrupting your ACs (as they set up the sticks, swing the lens, put on the eyepiece extension, balance the fluid head, run cables to the monitors, etc.) in order to see if your image is correctly exposed on camera can be inefficient.

And having the reliable, hard number a light meter gives you can be more reassuring than judging false colours or histograms.

Here are four ways in which I used my brand new light meter on my last production, The Little Mermaid:

1. To call ND filters. When shooting outdoors, I would take light readings in the sun and in the shade, and when then the sun was in, to understand the range of light levels I was dealing with. I could then pick an ND filter that would put me at a stop at where I would still have the room to adjust the iris a little either way if the light got brighter or darker. This was particularly important when we were shooting on water in a splash bag, when changing NDs or even just adjusting the iris was a longer process. (In my next post I’ll look deeper into stop maths and ND filters to demonstrate exactly how to select an ND filter based on a light reading.)
2. To measure contrast ratios. The Alexa can handle up to seven stops of over-exposure and eight stops of under-exposure. Knowing this, I could use my meter to see if certain areas were going to blow out or crush, before the camera was even set up. I could also measure how many stops the key side of an actor’s face was above the fill side, and thus work out the key-to-fill ratio. At present this is still something I judge by eye on the monitor, but the more I get to know the numbers, the more I suspect I will start determining it in advance.
3. To check green-screens are properly lit. The visual effects supervisor, Jafar, told me that green-screens should be exposed at key, or up to half a stop over key. So if I was shooting at T4, I would walk along the green-screen and take readings at various points to make sure the meter was generally giving me between f4 and f4½.
4. To schedule a dusk shot. For a twilight scene on a beach, I needed to know in advance exactly what our window of opportunity was. Looking up the sunset time is all well and good, but it doesn’t help you figure out how long afterwards there will be enough ambient light left to shoot with. So while at location the day before, I went out onto the beach and took light readings every few minutes after sundown. These told me I had 20 minutes from sunset until the ambient light dropped below what the lenses could expose.

Do you use a light meter? And if so, how?

# 10 Ways Low Budget Shoots Differ from Micro Budget Ones

I’ve been working in the film business for 16 years now, but until very recently I hadn’t really worked on a ‘proper’ production, one that had a budget above five figures. Here are some differences I noticed stepping up from micro-budget to low budget…

1. Formal crew structure. There is a proper separation between departments, even between camera and lighting (which is quite strange for the DP, in charge of both). Woe betide anyone who moves set dressing without asking the art department, or who plugs something in without checking with the sparks, or who stores equipment in a room without asking locations.
2. Proper production and locations departments. The feature I worked on last year had two producers, a line producer, a production manager and a production co-ordinator, plus a locations department. I’m used to productions where one person does all those jobs, and often directs as well. Figuring out which person to approach about any given issue was fun! (Creative Skillset’s website is a good place to check if you’re not sure who does what.)
3. Advance prep. With a large crew, time cannot be wasted waiting for things that could have been pre-rigged. Heads of department are expected to think ahead and splinter their crew if necessary to be ready for things coming later in the day or week. For a DP this most commonly means pre-rigging distro and/or lighting.
4. Delegation. Aside from operating the camera, I did little hands-on work on the recent feature shoot. Lens changes, grip rigging and lighting set-ups are all handled by other people on the instructions of me, and of the gaffer and the 1st AC. Sometimes this means the DP can go and have a cup of tea, but often it provides important thinking and planning time – an opportunity to reccie the next set and design the lighting, or to review footage in the edit room, or reccie a possible location with other HoDs, or discuss the afternoon’s shots with the director. It’s impossible to do this sort of forward planning if you’re changing your own lenses and setting your own lamps up.
5. Hard wrap times. On micro-budget shoots the wrap time is a theoretical concept, with no more relevance to reality than an episode of Sponge Bob Square Pants. On a bigger production, you wrap at wrap time, because if you don’t then the gaffer might pull the plug. Occasionally the crew will be asked if they are willing to go over by half an hour, say, in order to complete a scene. But everyone must agree, and that half hour must be deducted from the next day.
6. Lunch break, not just lunch. In micro-budget land, getting lunch at all is not a given. But when you do get it, you’re often expected to eat as quickly as possible and get straight back to work. On a bigger production you get your hour lunch break come hell or high water. And there’s proper catering. With desserts!
7. Reliance on the crew. If you’re working with a small camera and mains power, you can stay late with the director and steal a few extra shots, if necessary. But when everything’s run off a generator, which only the gaffer is qualified to operate, and your camera package is almost too heavy to lift onto your own shoulder, and you have no idea how half the bits and bobs connected to it work because your ACs always deal with it, you really can’t do anything on your own.
8. Permissions and qualifications. For insurance reasons you must have qualified people overseeing the electrics and the rigging. You must also check with the locations department before using any space or equipment or filming in any area that was not discussed and signed off in preproduction.
9. Paperwork. Most HoDs seem to have some kind of daily paperwork to do on a larger production.The DP happily escapes this (the ACs handle the camera reports), though they do have to complete a risk assessment before shooting commences.
10. People management. Because of the size of the team under you, people management becomes a major part of an HoD’s job. I’ll go into more detail on this in a future post.

# 10 Tips for Meetings

Late last year I secured a great feature film job as DP, on the basis of a personal recommendation followed by a meeting with the director which went really well. Making a good impression at a meeting like this is clearly crucial. But although such meetings are essentially job interviews, they are much less formal and rely much more heavily on the director and DP having similar tastes. Here are a few tips to help you give your next one your best shot.

1. Be prepared. This means reading the script and any other documents provided, ideally more than once if you have the time and you’re serious about wanting the job. Look up the director’s previous work to get a sense of their tastes.
2. Dress to impress. What you wear to a meeting can influence its outcome, just as wearing a smart suit to a traditional job interview can. During the shooting of the feature, the director commented that the Highlander t-shirt I wore to the meeting reassured him that my cinematic tastes were broadly in line with his own.
3. Be willing to travel. If you don’t live in London, you’re going to have to travel there for most meetings. Don’t complain about it, don’t even mention it if you can avoid it. But also don’t do it if you have doubts about the quality of the production and what it’s going to do for your career.
4. Bring showreel footage. The director will likely have seen your showreel before you meet, but it doesn’t hurt to bring additional clips or stills that are particularly relevant to this project. In my feature meeting, frame grabs from Ren: The Girl with the Mark helped demonstrate what I could do with a period setting.
5. Bring some creativity to the table. Put some reference images together to show the visual ideas that came to your mind when you read the script, and how you think the cinematography of the project could be approached. I found an image of some monks with a shaft of light coming in the window that perfectly summed up how I saw the feature, and the director really responded to it.
6. Be flexible. Be prepared to listen to the director’s vision and bounce off their ideas.
7. Bring people and/or kit to the table. What do you have access to that puts you ahead of other applicants? Often in the micro-budget world this will be your camera, or maybe a drone or a jib, but once you get into the realm of more reasonable budgets, directors and producers appreciate skilled crew more. The feature director really wanted to use a lot of steadicam in the film, so before being offered the job I contacted a talented steadicam op I knew and got an expression of interest from him which I was then able to go back to the director with. I think this was a big part of the reason I got the job.
8. Be OK with the budget. If it’s late enough in preproduction that the crew fees and the kit hire budget are fixed, don’t grumble about them. All you will achieve is to make the director think you’re going to be difficult to work with. Instead cite examples of how you achieved great results with similarly limited resources in the past.
9. Don’t be cheap. Offer to pay for the drinks. I’d probably take it as a bad sign if the director allowed me to, but offer nonetheless!
10. Follow up. We all think of great things we should have said when we’re halfway home. Send an email with those extra thoughts, any links you may have discussed in the meeting, and a thank you for their time taken in meeting you.

# 9 Tips for Easier Sound Syncing

While syncing sound in an edit recently I came across a number of little mistakes that cost me time, so I decided to put together some on-set and off-set tips for smooth sound syncing.

#### On set: tips for the 2nd AC

1. Get the slate and take number on the slate right. This means a dedicated 2nd AC (this American term seems to have supplanted the more traditional British clapper-loader), not just any old crew member grabbing the slate at the last minute.
2. Get the date on the slate right. This can be very helpful for starting to match up sound and picture in a large project if other methods fail.
3. Hold the slate so that your fingers are not covering any of the info on it.
4. Make MOS (mute) shots very clear by holding the sticks with your fingers through them.
5. Make sure the rest of the cast and crew appreciate the importance of being quiet while the slate and take number are read out. It’s a real pain for the editing department if the numbers can’t be heard over chit-chat and last-minute notes from the director.
6. Speak clearly and differentiate any numbers that could be misheard, e.g. “slate one three” and “slate three zero” instead of the similar-sounding “slate thirteen” and “slate thirty”.

For more on best slating practice, see my Slating 101 blog post.

#### Off set: tips for the DIT and assistant editor

1. I recommend renaming both sound and video files to contain the slate and take number, but be sure to do this immediately after ingesting the material and on all copies of it. There is nothing worse than having copies of the same file with different names floating around.
2. This should be obvious, but please, please, please sync your sound BEFORE starting to edit or I will hunt you down and kill you. No excuses.
3. An esoteric one for any dinosaurs like me still using Final Cut 7: make sure you’ve set your project’s frame rate correctly (in Easy Setup) before importing your audio rushes. Otherwise FCP will assign them timecodes based on the wrong rate, leading to errors and sound falling out of sync if you ever need to relink your project’s media.

Follow these guidelines and dual system sound will be painless – well, as painless as it can ever be!

# 5 Tips for Perfect Shafts of Light

There’s nothing like a good shaft of light to add production value to your cinematography. But you can’t just shine a lamp through a window and expect to get Hollywood shafts. Here are the essential conditions you need:

1. You need focused light, i.e. a lamp with a lens. Source 4s work extremely well. HMI or tungsten fresnels will also do the job, and sometimes Dedos.

2. You need a smoke machine or hazer to volumise the light. A cheap one from Maplin will work, but as a general rule the cheaper the machine, the more its output will be wreaths of smoke rather than just thickening up the atmosphere. However, given time to disperse and some vigorous wafting with a flag or the clapperboard, any smoke will work.

3. The smoke/haze needs to be backlit. The closer the light source is to being directly behind the smoke, the more the smoke will show up. So shoot towards windows.

www.rentheseries.com

4. A dark background will show up the smoke best. If you’re shooting in a house with white walls then you’re probably flogging a dead horse.

5. Keep other light sources away from the shaft. Competing lamps can muddy the shaft of light or maybe make it disappear altogether. Often I find that shafts of light work well as background interest, with the actors well in front of it, lit by other sources.

Follow all these guidelines and you’ll get lovely shafts of light every time!

# Lighting Techniques #7: Gobos and Shadows

Gobos are shapes that you fit onto a lamp in order to break up the light. If you’re using Source 4s you can get gobos especially for the purpose, which slot into the front of the lamp.

A cucoloris is a piece of wood or metal with vaguely leaf-life shapes cut into it. You would mount this on a C-stand or clamp of some kind. You can easily make your own cucoloris by punching holes in black-wrap or cardboard.

In fact you can create patterns of light and shadow by placing almost anything in front of a light, varying the distance from the source to make the pattern sharper or softer. Be careful to observe the minimum safe distances printed on the side of the lamphead though, or you might set fire to your shadow-maker.

Here are some examples of breaking up the light that I’ve tried over the years…

On more than one occasion I’ve taped up some of the PVC pipes which my dolly uses as tracks, to create the impression of vertical bars or pillars. In the below example the French windows (when closed) didn’t have enough bits of frame to break up the light sufficiently, so I had my spark tape a pipe to the window…

I don’t have a picture, but I remember once on a horror feature sticking lots of blobs of gaffer tape to a window.

In this shot from Stop/Eject I blacked out the room’s real window and rigged a fold-up director’s chair in front of a 1K Arrilite to cast a window frame-like shadow…

Look for things in the set that you can shine lights through, like this partition window….

or a fence…

or blinds…

If you want the venetian blinds effect and you don’t have any, stick strips of gaffer tape to the window.

On Ren I built an openable and closable little door (complete with tiny barred window) for light to shine through, since the set didn’t have a door.

On the same show, the roof of Karn’s house became a giant gobo for the 2.5K HMI placed above and behind the set, creating these incredible God rays when smoke was added. The roof was made of interlocking branches and had been covered by sheets by the art department – presumably to block light – but I removed the sheets because I wanted this lighting effect…

Branches make great gobos. I often sneakily break one off a nearby tree and rig it to a C-stand to cast some summery shadows or break up a moonlight or streetlamp source that’s looking too bright and flat.

If you’ve missed the other posts in my Lighting Techniques series so far, here are the links:

#1: Three Point Lighting

#2: Cross-backlighting

#3: The Window Wrap

#4: Health Bounce

#5: Smoke

#6: Cross-light

# 5 Ways to Use LED Panels

LED technology is transforming the way cinematographers can light. Running off batteries and not getting hot are two of their biggest advantages over other sources, making them much more flexible. I tend to avoid keying with them, because even the most expensive brands don’t render skintones as accurately as incandescent sources, but there are many other uses they can be put to. Here are a few of my favourite.

### 1. Eye-light on overcast day exteriors

If it’s one of those dark days when reflectors just don’t seem to do anything, or you’re under the tree canopy of a forest, an LED panel can give you a bit of fill and eye-light.

Visit rentheseries.com to learn more about Ren, or read my blog post about lighting the above scene.

### 2. Background spots on night exteriors

So you’ve spent a while lighting the master shot of your big night exterior scene, and everyone’s ready to shoot. Then you notice that there’s an area in the background of frame which looks dark and empty, and you’d love a bit of extra light in there. Just slap a battery on your LED panel and run over there with it. No need to run power cables!

Read my blog post about lighting the above scene.

### 3. Off-screen TV set

An LED panel makes a good “TV” source because during the take your spark can mess with not only the brightness control but the colour balance as well, to suggest changing images on the screen.

Browse the blog posts about my cinematography on The Gong Fu Connection.

### 4. Mobile fill

If you’re shooting a long scene with your talent on the move and you need to maintain a little fill when they’re between lamps, an LED panel is easy for your spark to hand bash as they walk with the actors.

Read my blog post about the above scene from Synced.

### 5. Hidden sources

Because they don’t get hot, and you don’t need power cables to them, it’s easy to hide LED panels behind bits of furniture or set dressing, to give interesting pools of light or punch up practicals.

Read my blog post about lighting the above scene from Ren.

What interesting uses have you found for LED panels?

# How to Cut a Behind-the-Scenes Featurette

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.

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 Correct Cosmetic Issues with Lighting

Every cinematographer needs to make the cast look good. Here are some quick tips for minimising blemishes and undesirable physical attributes. To any readers who have been lit by me, please don’t get a complex! These techniques can also be used to make someone who’s already flawless look even more amazing. Conversely, if you have a bad guy, or a character who needs to look ill, or a prosthetic monster make-up, you might want to do the opposite of what I suggest below.

• Thinning hair – Avoid toplight and strong backlight, which will show up the scalp under the hair.
• Wrinkles, spots and scars – Avoid lighting that will throw shadows from these features, e.g. cross-light (meaning light from the side). Instead put the key light as close to the camera as possible. Ideally use a soft source. If you’re still seeing shadows, add more fill.
• Double chins, bags under the eyes, general appearance of tiredness – Use Health Bounce – a reflector placed under the talent’s face to eliminate shadows cast from above.
• Small or deep-set eyes – Again, use Health Bounce. It will help get light into the eye sockets and put a sparkle of reflection in the eyeballs.
• Weak jawline – Use three-quarter backlight (a.k.a. “kicker”) to create a rim along the jawline on one side.
• Shiny skin – This may be a make-up issue, but you can help by using bounced light. Kinoflos, though they are soft sources, are amongst the worst culprits for creating shine.
• Big nose – Keep the key light close to the camera to minimise the shadow the nose casts.

To learn more about lighting, check out my post on key light angles and my series of lighting techniques.