Alexa ProRes ISO Tests

My Cousin Rachel

I’ve shot three features on Arri Alexas, but I’ve never moved the ISO away from its native setting of 800 for fear of noise and general image degradation. Recently I read an article about the cinematography of My Cousin Rachel, in which DP Mike Eley mentioned shooting the night scenes at ISO 1600. I deliberately set off for the cinema in order to analyse the image quality of this ISO on the big screen. Undoubtedly I’ve unwittingly seen many things that were shot on an Alexa at ISO 1600 over the past few years, but this was the first time I’d given it any real thought.

To my eye, My Cousin Rachel looked great. So when I was at Arri Rental the other week testing some lenses, I decided to shoot a quick ISO test to see exactly what would happen when I moved away from the native 800.

But before we get to the test footage, for those of you unsure exactly what ISO is, here’s an introduction. The more experienced amongst you may wish to skip down to the video and analysis.

 

What is ISO?

ISO is a measure of a camera’s light sensitivity; the higher the ISO, the less light it requires to expose an image.

The acronym actually stands for International Organization for Standardization [sic], the body which in 1974 combined the old ASA (American Standards Association) units of film speed with the German DIN standard. That’s why you’ll often hear the terms ISO and ASA used interchangeably. On some cameras, like the Alexa, you’ll see it called EI (Exposure Index) in the menus.

A common ISO to shoot at today is 800. One way of defining ISO 800 is that it’s the sensitivity required to correctly expose a key-light of 3 foot-candles with a lens of T-stop 1.4 and a 180° shutter at 24fps, as we saw in my Barry Lyndon blog.

If we double the ISO we double the effective sensitivity of the camera, or halve the amount of light it requires. So at ISO 1600 we would only need 1.5 foot-candles of light (all the other settings being the same), and at ISO 3200 we would need just 0.75 foot-candles. Conversely, at ISO 400 we would need 6 foot-candles, or 12 at ISO 200. Check out this exposure chart if it’s still unclear.

ISO is one of the three corners of the Exposure Triangle, well-known to stills photographers the world over. You can read my posts on the other two corners: Understanding Shutter Angles and F-stops, T-stops and Optical Density.

Just as altering the shutter angle (exposure time) has the side effect of changing the amount of motion blur, and altering the aperture affects the depth of field, so ISO has its own side effect: noise. Increase the ISO and you increase the electronic noise in the picture.

This is because turning the ISO up causes the camera to electronically boost the signals it’s receiving from the sensor. It’s exactly the same as turning up the volume on an amplifier; you hear more hiss because the noise floor is being boosted along with the signal itself.

I remember the days of Mini-DV cameras, which instead of ISO had gain; my Canon XL1 had gain settings of -3dB, +6dB and +12dB. It was the exact same thing, just with a different name. What the XL1 called 0dB of gain was what today we call the native ISO. It’s the ISO at which the camera is designed to give the best images.

 

ISO and Dynamic Range

The Alexa has a dynamic range of 14 stops. That means it can simultaneously record detail in an area of brightness x and an area of brightness times 2 to the power 14. At its native ISO of 800, those 14 stops of dynamic range are equally distributed above and below “correct” exposure (known as middle grey), so you can overexpose by up to 7 stops, and underexpose by up to 7 stops, without losing detail.

If you increase the ISO, those limits of under- and overexposure still apply, but they’re effectively shifted around middle grey, as the graphic to the left illustrates. (The Pro Video Coalition post this graphic comes from is a great read if you want more detail.) You will see the effects of this shifting of dynamic range very clearly in the test video and images below.

In principle, shooting at ISO 1600 is the same as shooting at ISO 800, underexposing by a stop (giving you more highlight detail) and then bringing it back up a stop in post. The boosting of the signal in that case would come right at the end of the image path instead of near the beginning, so the results would never be identical, but they’d be close. If you were on a bigger project with a DIT, you could create a LUT to bring the exposure up a stop which again would achieve much the same thing.

All of the above assumes you’re shooting log ProRes. If you’re shooting Raw then everything is simply recorded at the native ISO and any other ISO you select is merely metadata. But again, assuming you exposed for that other ISO (in terms of iris, shutter and ND filters), you will effectively get that same dynamic range shift, just further along the pipeline.

If this all got a bit too technical for you, don’t worry. Just remember:

Doubling the ISO

  • increases overall exposure by one stop,
  • gives you one more stop of detail in the highlights,
  • gives you one less stop of detail in the shadows, and
  • increases picture noise.

Halving the ISO

  • decreases overall exposure by one stop,
  • gives you one less stop of detail in the highlights,
  • gives you one more stop of detail in the shadows, and
  • decreases picture noise.

 

The Test

I lit the subject, Rupert “Are You Ready?” Peddle, with a 650W tungsten fresnel bounced off poly, and placed a 40W candle globe and some LED fairy lights in the background to show highlight clipping. We shot the tests in ProRes 4444 XQ on an Alexa XT Plus with a 32mm Cooke S4, altering the shutter angle to compensate for the changing ISOs. At ISO 400 the shutter angle was maxed out, so we opened the lens a stop for ISO 200.

We tested five settings, the ones corresponding to a series of stops (i.e. doublings or halvings of sensitivity): 200, 400, 800, 1600 and 3200. I have presented the tests in the video both as recorded in the original log C (mislabelled as S-log, sorry!), and with a standard Rec.709 LUT.

You’ll need to watch the video at full-screen at 1080P to have any chance of seeing the differences, and even then you might see the compression artefacts caused by the noise more than the noise itself. Check out the stills below for a clearer picture of what’s going on. (Click on them for full resolution.)

 

Analysis

To me, the most important thing with every test is how skin tones are rendered. Looking at the original ProRes of these comparisons I think I see a little more life and vibrance in the skin tones at lower ISOs, but it’s extremely subtle. More noticeable is a magenta shift at the lower ISOs versus a green shift at higher ones. The contrast also increases with the ISO, as you can see most clearly in the log images.

At the lower ISOs you are not really aware of any noise in the picture. It’s only at ISO 1600 that it becomes noticeable, but I have to say that I really liked this level of noise; it gives the image a texture reminiscent of film grain. At ISO 3200 the noise is quite significant, and would probably be unacceptable to many people.

The really interesting thing for me was the shifting of the dynamic range. In the above comparison image, look at the globe in log – see how it starts off as one big white blob at ISO 200 and becomes more detailed as the ISO rises? Now look at the dark wall around the globe, both here and in the previous image – see how it subtly and smoothly graduates into darkness at the lower ISOs, but becomes a grainy mess at the higher ones?

I can see an immediate benefit to shooting at ISO 1600 in scenes lit predominantly with practicals. Such scenes tend to have a low overall level of illumination, while the practicals themselves often blow out on camera. Going to ISO 1600 would give me extra exposure and extra detail in the practicals. I would be sacrificing shadow detail, so I would have to be a little more careful not to underexpose any faces or other important elements of the frame, but I can deal with that. In fact, I often find myself determining my exposure in these types of scenes by how blown out the practicals are, wishing I could open up a little more to see the faces better but not wanting to turn the lamps into big white blobs. Increasing the ISO would be the perfect solution, so I’m very glad I did this test to alleviate my ungrounded fears.

What about scenarios in which a lower-than-native ISO would be useful? Perhaps a scene outside a building with an open door, where the dark interior is visible in the background and more detail is required in it. Or maybe one of those night scenes which in reality would be pitch black but for movie purposes have a low level of ambient light with no highlights.

I hope you’ve found this test as useful and interesting as I have. Watch this space or subscribe to my YouTube Channel for the lens test.

Thanks to Rupert Peddle, awesome steadicam op and focus puller – check out his site at pedhead.net – for appearing in front of the lens. Thanks also to Bex Clives, who was busy wrangling data from the lens tests while we were shooting these ISO tests, and of course Arri Rental UK.

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Alexa ProRes ISO Tests

The 2:1 Aspect Ratio

Last autumn I wrote a post about aspect ratio, covering the three main ratios in use today: 16:9, 1.85:1 and 2.39:1. The post briefly mentioned a few non-standard ratios, including 2:1. Since then, I’ve noticed this ratio popping up all over the place. Could it be on its way to becoming a standard?

Today I’ll give you a little background on this ratio, followed by a gallery of frame grabs from 2:1 productions. The aim is simply to raise awareness of this new(ish) tool in the aspect ratio toolkit. As ever, it’s up to the director and DP to decide whether their particular project is right for this, or any other, ratio. However, I would caution low-budget filmmakers against picking what is still not a common ratio without considering that smaller distribution companies may crop your work to a more standard ratio either because of convenience or negligence.

Woody Allen and Vittorio Storaro shooting Café Society

Vittorio Storaro, ASC, AIC – the highly-regarded cinematographer of Last Tango in Paris and Apocalypse Now amongst many others – began championing the 2:1 ratio around the turn of the millennium. It was one of the most complicated times in the history of aspect ratios. The horror of pan-and-scan (butchering a movie to fit its 1.85:1 or 2.39:1 ratio into 4:3 without bars) was starting to recede with the introduction of DVD, which was in fact still 4:3 but could contain squeezed 16:9 content. Widescreen television sets were starting to build in popularity, but some programmes and films were being broadcast in the middle-ground ratio of 14:9 so as not to offend the majority of viewers who still had 4:3 sets. And Storaro recognised that HD would soon supplant celluloid as the primary capture and exhibition method for cinema, likely bringing with it fresh aspect ratio nightmares.

Storaro proposed “Univisium”, a 2:1 aspect ratio that fell between the two cinema standards of 1.85:1 and 2.39:1. It was a compromise, designed to make everyone’s life easier, to produce images that would need only minor letterboxing no matter where or how they were screened. However, the industry did not share his vision, and until recently 2:1 productions were relatively rare, most of them lensed by Storaro himself, such as Frank Herbert’s Dune, Exorcist: The Beginning and Storaro’s first digital picture, Café Society.

John Schwartzman shooting Jurassic World

Perhaps the biggest 2:1 movie to date is Jurassic World. DP John Schwartzman, ASC wanted to shoot anamorphic 2.39:1, while Steven Spielberg, exec producing, advocated 1.85:1 (like his original Jurassic Park) to provide more height for the dinosaurs. 2:1 was arrived at, again, as a compromise.

And compromise is likely what has driven the recent explosion in 2:1 material – not in the cinema, but online. Recent shows produced in this ratio include The Crown, A Series of Unfortunate Events, Stranger Things and House of Cards on Netflix, and Transparent on Amazon. I expect the producers of these series were looking to give their audience a more cinematic experience without putting off those who dislike big black bars on their screen, not unlike the reasoning behind the 14:9 broadcasts in the noughties.

2:1 may be a ratio born out of compromise, but then so was 16:9 (invented by SMPTE in the early eighties as a halfway house between 2.35:1 and  4:3). It certainly doesn’t mean that shooting in 2:1 isn’t a valid creative choice. Perhaps its most interesting attribute is its lack of baggage; 16:9 is sometimes seen as “the TV ratio” and 2.39:1 as “the big movie ratio”, but 2:1 has no such associations. One day perhaps it may be thought of as “the streaming ratio”, but for now it is simply something other.

Anyway, enough of the history and theory. Here are some examples of the cinematography that can be achieved in 2:1.

 

Cafe Society

DP: Vittorio Storaro, ASC, AIC

 

Jurassic World

DP: John Schwartzman, ASC

 

House of Cards

Season 5 DP: David M. Dunlap

 

Stranger Things

Season 1 DP: Tim Ives

 

The Crown

Season 1 DPs: Adriano Goldman, ASC, ABC & Ole Bratt Birkeland

 

Broadchurch

Season 3 DP: Carlos Catalan

 

A Series of Unfortunate Events

Season 1 DP: Bernard Couture

The 2:1 Aspect Ratio

24fps or 25fps?

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It’s a common dilemma in the UK for filmmakers: do you shoot at 24 or 25 frames per second? Until a couple of years ago, I would have said 25 every time, but with DCPs and Blu-rays now about, and most TVs capable of handling a range of frame rates, the answer is not so clear-cut. Unlike aspect ratio or shooting format, the decision has no discernible creative impact on your project, merely a technical one. And it’s so easy to convert between the two that it often feels like it makes no odds. Nonetheless, to help anyone on the horns on this dilemma, here’s my round-up of the respective advantages of each frame rate.

The Case for 25fps

  • If you need to record to any kind of tape format at any point in your process, 25fps is what you need.
  • The same goes for PAL DVDs.
  • If your film is going to be broadcast on UK TV, it will be transmitted at 25fps.
  • Since your camera’s running in sync with the UK mains supply’s alternating current, you don’t need flicker-free ballasts for your HMIs. Having said that, pretty much every time I’ve hired an HMI, it’s come with a flicker-free ballast as standard anyway.
  • If you’ve made a 25fps feature film that isn’t quite long enough for distributors to classify it as a feature, the extra running time you squeeze from exhibiting it at 24fps might make the difference.

The Case for 24fps

  • For maximum compatibility, Digital Cinema Packages should be authored at 24fps.
  • The same goes for Blu-rays. (Blu-rays do not technically support 25P, but they support 50i, which can contain progressive 25fps content. However, discs authored to the 50i spec apparently will not play on most US machines.)
  • If you shoot at 24fps and need to convert to 25fps for any reason, your film will become 4% shorter, making it that extra bit pacier and able to squeeze into a shorter slot at a film festival.
  • If shooting on film, your postproduction facilities will be much more comfortable with 24fps material. We really freaked out our lab on The Dark Side of the Earth by shooting 25.
  • Many traditional film projectors will only run at 24fps.

Can you think of any other factors that I’ve missed?

I’d say the balance has tipped in favour of 24fps. However, I think you’ll find that many people in the UK (outside of the celluloid world) are still more comfortable with 25…. for now.

24fps or 25fps?

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

Understanding Colour Temperature

As I was writing my last entry, in which I mentioned the range of colour temperatures in a shot, it occurred to me that some readers might find an explanation of this concept useful. What is colour temperature and why are different light sources different colours?

The answer is more literal than you may expect. It’s based on the simple principal that the hotter something burns, the bluer the light it emits. (Remember from chemistry lessons how the tip of the blue flame was always the sweet spot of the Bunsen Burner?)

Tungsten bulbs emit an orange light - dim them down and it gets even more orangey.
Tungsten bulbs emit an orange light – dim them down and it gets even more orangey.

Colour temperature is measured in kelvins, a scale of temperature that begins at absolute zero (-273°C), the coldest temperature physically possible in the universe. To convert centigrade to kelvin, simply add 273. So the temperature here in Hereford right now is 296 kelvin (23°C).

The filament of a tungsten light bulb reaches a temperature of roughly 3,200K (2,927°C). This means that the light it emits is orange in colour. The surface of the sun is about 5,778K (5,505°C), so it gives us much bluer light.

Colour temperature isn’t necessarily the same as actual temperature. The atmosphere isn’t 7,100K hot, but the light from the sky (as opposed to the sun) is as blue as something burning at that temperature would be.

Digital cameras have a setting called “white balance” which compensates for these differing colour temperatures and makes them appear white. Typical settings include tungsten, daylight, shade and manual, which allows you to callibrate the white balance by holding a white piece of paper in front of the lens as a reference.

Colour temperature chart
Colour temperature chart

Today there are many types of artificial light around other than tungsten – fluorescent and LED being the main two. In the film industry, both of these can be obtained in flavours that match daylight or tungsten, though outside of the industry (if you’re working with existing practical sources) the temperatures can range dramatically.

There is also the issue of how green/magenta the light is, the classic example being that fluorescent tubes – particularly older ones – can make people look green and unhealthy. If you’re buying fluorescent lamps to light a scene with, check the CRI (colour rendering index) on the packaging and get the one with the highest number you can find for the fullest spectrum of light output.

The Magic Lantern hacks for Canon DSLRs allow you not only to dial in the exact colour temperature you want, but also to adjust the green/magenta balance to compensate for fluorescent lighting. But if two light sources are giving out different temperatures and/or CRIs, no amount of white balancing can make them the same.

Left: daylight white balance preset (5,600K). Right: tungsten white balance preset (3,200K)
Left: daylight white balance preset (5,600K). Right: tungsten white balance preset (3,200K)

The classic practical example of all this is a person standing in a room with a window on one side of them and a table lamp on the other. Set your camera’s white balance to daylight and the window side of their face looks correct, but the other side looks a nasty orange (above left), or maybe yellowy-green if the lamp has an energy-saving bulb in it. Change the white balance to tungsten or fluorescent and you will correct that side of the subject’s face, but the daylight side will now look blue (above right) or magenta.

This is where gels come in, but that’s a topic for another day.

The beauty of modern digital cinematography is that you can see how it looks in the viewfinder and adjust as necessary. But the more you understand the kind of theory I’ve outlined above, the more you can get it right straight away and save time on set.

Understanding Colour Temperature