How is Dynamic Range Measured?

The high dynamic range of the ARRI Alexa Mini allowed me to retain all the sky detail in this shot from “Above the Clouds”.

Recently I’ve been pondering which camera to shoot an upcoming project on, so I consulted the ASC’s comparison chart. Amongst the many specs compared is dynamic range, and I noticed that the ARRI Alexa’s was given as 14+ stops, while the Blackmagic URSA’s is 15. Having used both cameras a fair bit, I can tell you that there’s no way in Hell that the Ursa has a higher dynamic range than the Alexa. So what’s going on here?

 

What is dynamic range?

To put it simply, dynamic range is the level of contrast that an imaging system can handle. To quote Alan Roberts, who we’ll come back to later:

This is normally calculated as the ratio of the exposure which just causes white clipping to the exposure level below which no details can be seen.

A photosite on a digital camera’s sensor outputs a voltage proportional to the amount of light hitting it, but at some point the voltage reaches a maximum, and no matter how much more light you add, it won’t change. At the other end of the scale, a photosite may receive so little light that it outputs no voltage, or at least nothing that’s discernible from the inherent electronic noise in the system. These upper and lower limits of brightness may be narrowed by image processing within the camera, with RAW recording usually retaining the full dynamic range, while linear Rec. 709 severely curtails it.

In photography and cinematography, we measure dynamic range in stops – doublings and halvings of light which I explain fully in this article. One stop is a ratio of 2:1, five stops are 32:1, thirteen stops are almost 10,000:1

It’s worth pausing here to point out the difference between dynamic range and latitude, a term which is sometimes regarded as synonymous, but it’s not. The latitude is a measure of how much the camera can be over- or under-exposed without losing any detail, and is dependent on both the dynamic range of the camera and the dynamic range of the scene. (A low-contrast scene will allow more latitude for incorrect exposure than a high-contrast scene.)

 

Problems of Measurement

Before digital cinema cameras were developed, video had a dynamic range of about seven stops. You could measure this relatively easily by shooting a greyscale chart and observing the waveform of the recorded image to see where the highlighs levelled off and the shadows disappeared into the noise floor. With today’s dynamic ranges into double digits, simple charts are no longer practical, because you can’t manufacture white enough paper or black enough ink.

For his excellent video on dynamic range, Filmmaker IQ’s John Hess built a device fitted with a row of 1W LEDs, using layers of neutral density gel to make each one a stop darker than its neighbour. For the purposes of his demonstration, this works fine, but as Phil Rhodes points out on RedShark News, you start running into the issue of the dynamic range of the lens.

It may seem strange to think that a lens has dynamic range, and in the past when I’ve heard other DPs talk about certain glass being more or less contrasty, I admit that I haven’t thought much about what that means. What it means is flare, and not the good anamorphic streak kind, but the general veiling whereby a strong light shining into the lens will raise the overall brightness of the image as it bounces around the different elements. This lifts the shadows, producing a certain amount of milkiness. Even with high contrast lenses, ones which are less prone to veiling, the brightest light on your test device will cause some glare over the darkest one, when measuring the kind of dynamic range today’s cameras enjoy.

 

Manufacturer Measurements

Going back to my original query about the Alexa versus the URSA, let’s see exactly what the manufacturers say. ARRI specifically states that its sensor’s dynamic range is over 14 stops “as measured with the ARRI Dynamic Range Test Chart”. So what is this chart and how does it work? The official sales blurb runs thusly:

The ARRI DRTC-1 is a special test chart and analysis software for measurement of dynamic range and sensitivity of digital cameras. Through a unique stray light reduction concept this system is able to accurately measure up to 15.5 stops of dynamic range.

The “stray light reduction” is presumably to reduce the veiling mentioned earlier and provide more accurate results. This could be as simple as covering or turning off the brighter lights when measuring the dimmer ones.

I found a bit more information about the test chart in a 2011 camera shoot-out video, from that momentous time when digital was supplanting film as the cinematic acquisition format of choice. Rather than John Hess’s ND gel technique, the DRTC-1 opts for something else to regulate its light output, as ARRI’s Michael Bravin explains in the video:

There’s a piece of motion picture film behind it that’s checked with a densitometer, and what you do is you set the exposure for your camera, and where you lose detail in the vertical and horizontal lines is your clipping point, and where you lose detail because of noise in the shadow areas is your lowest exposure… and in between you end up finding the number of stops of dynamic range.

Blackmagic Design do not state how they measure the dynamic range of their cameras, but it may be a DSC Labs Xlya. This illuminated chart boasts a shutter system which “allows users to isolate and evaluate individual steps”, plus a “stepped xylophone shape” to minimise flare problems.

Art Adams, a cinema lens specialist at ARRI, and someone who’s frequently quoted in Blain Brown’s Cinematography: Theory & Practice, told Y.M. Cinema Magazine:

I used to do a lot of consulting with DSC Labs, who make camera test charts, so I own a 20-stop dynamic range chart (DSC Labs Xyla). This is what most manufacturers use to test dynamic range (although not ARRI, because our engineers don’t feel it’s precise enough) and I see what companies claim as usable stops. You can see that they are just barely above the noise floor.

 

Conclusions

Obviously these ARRI folks I keep quoting may be biased. I wanted to find an independent test that measures both Blackmagics and Alexas with the same conditions and methodology, but I couldn’t find one. There is plenty of anecdotal evidence that Alexas have a bigger dynamic range, in fact that’s widely accepted as fact, but quantifying the difference is harder. The most solid thing I could find is this, from a 2017 article about the Blackmagic Ursa Mini 4.6K (first generation):

The camera was measured at just over 14 stops of dynamic range in RAW 4:1 [and 13 stops in ProRes]. This is a good result, especially considering the price of the camera. To put this into perspective Alan measured the Canon C300 mkII at 15 stops of dynamic range. Both the URSA Mini 4.6 and C300 mkII are bettered by the ARRI Alexa and Amira, but then that comes as no surprise given their reputation and price.

The Alan mentioned is Alan Roberts, something of a legend when it comes to testing cameras. It is interesting to note that he is one of the key players behind the TLCI (Television Lighting Consistency Index), a mooted replacement for CRI (Colour Rendering Index). It’s interesting because this whole dynamic range business is starting to remind me of my investigation into CRI, and is leading me to a similar conclusion, that the numbers which the manufacturers give you are all but useless in real-world cinematography.

Whereas CRI at least has a standardised test, there’s no such thing for dynamic range. Therefore, until there is more transparency from manufacturers about how they measure it, I’d recommend ignoring their published values. As always when choosing a camera, shoot your own tests if at all possible. Even the most reliable numbers can’t tell you whether you’re going to like a camera’s look or not, or whether it’s right for the story you want to tell.

When tests aren’t possible, and I know that’s often the case in low-budget land, at least try to find an independent comparison. I’ll leave you with this video from the Slanted Lens, which compares the URSA Mini Pro G2 with the ARRI Amira (which uses the same Alev III sensor as the Alexa). They don’t measure the dynamic range, but you can at least see the images side by side, and in the end it’s the images that matter, not the numbers.

How is Dynamic Range Measured?

8 Things to Look For When Buying a Cinema Camera

A couple of weeks ago I shared my thoughts about whether a director of photography should own equipment. My conclusion was that it can be useful early in your career, when you’re shooting corporates or tiny films with no hire budget. So what is the best camera for indie cinematography?

I’m not going to answer that, but I will tell you what to look for when investing in a camera. Hopefully these tips will help you choose the one that’s right for you from the huge and ever-changing array of professional cameras on the market, from the humble DSLR to the ubiquitous Reds and everything in between.

 

1. Image quality

Shooting on a Sony FS7 for “Finding Hope”

The quality of the image is of course the most imporant attribute of any camera. Rather than any technical specifications, I’m talking about the aesthetic quality here: how does it feel? Does it have that elusive “cinematic” quality? Is it “filmic”? Does it remind you of certain kinds of movies?

A good place to start is to look up sample footage on YouTube, or better still Vimeo for less compression muddying the issue. If you can borrow the camera and try it out before you buy, even better. Take away some test footage and try grading it too.

 

2. Resolution

Resolution, the sheer number of pixels a camera can record, is part of image quality, but I include it as a separate point because I see it as more of a technical consideration than an aesthetic one. You should ask yourself what longevity you require from your films – will people still be watching them, say two or three years from now, and if so what sort of resolution might be the norm by then?

Also consider your delivery platform. If everything you shoot is going on YouTube, perhaps you don’t need more than 1080P (standard HD).

 

3. Dynamic Range

Dynamic range is a measure of how much contrast a camera can handle. Too small a dynamic range and you will frequently struggle with bright areas “clipping” – i.e. losing details – or dark areas getting lost in the image noise. Also, the wider the dynamic range, the more flexibility you will have in grading.

For a cinematic image, 12 stops of dynamic range is the absolute minimum, with 14 or more being ideal.

 

4. Maximum ISO

Some ISO tests I conducted on an Arri Alexa Classic in 2017

The ISO (International Standards Organisation) scale rates the light sensitivity of a camera. The most important thing is the native ISO, the one at which the camera is optimised to give the cleanest image with the most detail. On some cameras, setting an ISO other than the native one reduces the image quality considerably.

The higher the ISO, the less light will be required to expose an image correctly. 800 is typical these days, but many cameras go much higher than that. It is worth thinking about spending more money to get a camera with a higher native ISO, because you may save a lot of money on lighting.

 

5. Lens Mount

This is crucial because you may already have a collection of lenses, or you may intend to hire certain lenses, and you need to be sure that they will fit your new camera’s mount.

The Canon EF mount is extremely common and will open up a huge range of options for stills glass as well as some low-end cinema glass. The smaller MFT (micro four-thirds) mount also has a wide range of lenses.

Top-end cameras have PL mounts which take all the beautiful cinema lenses used on big movies, but only choose this route if you are willing to part with a lot of cash!

 

6. Form Factor

A Blackmagic Micro Cinema Camera, not so micro once it’s rigged with rails, matte box, wireless follow focus, battery, monitor and video transmitter!

When I started in the industry, cameras were all ergonomically designed to sit on your shoulder, with a nice handgrip to the right of the lens and an EVF (electronic viewfinder) to provide a third point of stabilising contact. Nowadays cameras tend to be boxy, heavy and uncomfortable to hold without additional accessories (see below).

Again, try to gets your hands on the camera in a shop and see how it feels before you purchase. As well as handheld shooting, consider how easy it will be to rig onto dollies, sliders, gimbals, etc.

 

7. Required Accessories

Buying the camera body itself is unlikely to be the end of your expenditure. You will need lenses, batteries, a battery charger, cards, a card reader and almost certainly some kind of stabilising system, be it a simple shoulder rig or an electronic gimbal.

You may also want an EVF, a tripod, matte box, follow focus – the list can seem endless! Be careful to budget your essential accessories before buying the camera. Some cameras seem like bargains until you add up all the extras. Pay particular attention to the media, and to exactly what speed of media you need in order to shoot at the resolution and frame rate that you require, as this can get very expensive.

 

8. Codec

What file type and codec does the camera shoot? Does your editing system support that format? If not, how time-consuming will it be to convert everything?

What compression ratios does the camera support? How much hard drive space will you need to store an hour of footage at that ratio? What about ten hours, plus back-ups? Often there is a trade-off between a highly compressed format like H.264 which is light on disc space but may need converting before you can edit it, and a lightly compressed format like ProRes which burns through disc space but can be dropped straight into most editing software.

8 Things to Look For When Buying a Cinema Camera

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, 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.

Alexa ProRes ISO Tests

Black Magic Cinema Camera Review

Throughout September I got a crash-course introduction to the Blackmagic Cinema Camera as I used it to shoot Harriet Sams’ period action adventure web series The First Musketeer. The camera was kindly lent to us by our gaffer, Richard Roberts. Part-way through the shoot I recorded my initial thoughts on the camera in this video blog:

Here’s a summary of the key differences between the Blackmagic and a Canon DSLR.

Canon DSLR Blackmagic Cinema Camera
Rolling shutter (causes picture distortion during fast movement) Rolling shutter (though not as bad as DSLRs)
Pixels thrown away to achieve downscaling to 1080P video resolution, results in distracting moiré patterns on fabrics, bricks walls and other grid-like patterns Pixels smoothly downscaled from 2.5K to 1080P to eliminate moiré. Raw 2.5K recording also available
On-board screen shuts off when external monitor is connected On-board screen remains on when external monitor is connected
Some models have flip-out screens which can be adjusted to any viewing angle and easily converted into viewfinders with a cheap loupe attachment On-board screen is fixed and highly reflective so hard to see in all but the darkest of environments
Maximum frame rate: 60fps at 720P Maximum frame rate: 30fps at 1080P
50mm lens is equivalent to 50mm (5D) or 72mm (other models) full-frame lens 50mm lens is equivalent to 115mm full-frame lens
10-11 stops of dynamic range 13 stops of dynamic range
Recording format: highly compressed H.264, although Magic Lantern now allows for limited raw recording Recording format: uncompressed raw, ProRes or DNXHD
Battery life: about 2 hours from the 600D’s bundled battery in movie mode Battery life: about 1 hour from the non-removable internal battery
Weight: 570g (600D) Weight: 1,700g
Audio: stereo minijack input, no headphone socket Audio: dual quarter-inch jacks for input, headphone socket

Having now come to the end of the project, I stand by the key message of my video blog above: if you already own a DSLR, it’s not worth upgrading to a Blackmagic. You’d just be swapping one set of problems (rolling shutter, external monitoring difficulties, aliasing) for another (hard-to-see on-board screen, weight, large depth of field).

The BMCC rigged with a lock-it box for timecode sync with the audio recorder, on a Cinecity Pro-Aim shoulder mount
The BMCC rigged with a lock-it box for timecode sync with the audio recorder, on a Cinecity Pro-Aim shoulder mount

The depth of field was really the killer for me. Having shot on the 600D for three years I’m used to its lovely shallow depth of field. With the Blackmagic’s smaller 16mm sensor it was much harder to throw backgrounds of focus, particularly on wide shots. At times I felt like some of the material I was shooting looked a bit “TV” as a result.

The small sensor also creates new demands on your set of lenses; they all become more telephoto than they used to be. A 50mm lens used on a crop-chip DSLR like the 600D is equivalent to about an 72mm lens on a full-frame camera like the 5D Mark III or a traditional 35mm SLR. That same 50mm lens used on the Blackmagic is equivalent to 115mm! It was lucky that data wrangler Rob McKenzie was able to lend us his Tokina 11-16mm f2.8 otherwise we would not have been able to get useful wide shots in some of the more cramped locations.

As for the Blackmagic’s ability to shoot raw, it sounds great, but will you use it? I suggest the images you get in ProRes mode are good enough for anything bar a theatrical release, and are of a far more manageable data size. You still get the high dynamic range in ProRes mode (although it’s optional), and that takes a little getting used to for everyone. More than once the director asked me to make stuff moodier, more shadowy; the answer was it is shadowy, you just won’t be able to see it like that until it’s graded.

The colour saturation is also very low, again to give maximum flexibility in the grade, but it makes it very hard for the crew huddled around the monitor to get a sense of what the finished thing is going to look like. As a cinematographer I pride myself on delivering images that looked graded before they actually are, but I couldn’t do that with the Blackmagic. But maybe that’s just a different workflow I’d need to adapt to.

The biggest plus to the BMCC is the lovely organic images it produces, as a result of both the down-sampling from 2.5K and the high dynamic range. This was well suited to The First Musketeer’s period setting. However, I think next season I’ll be pushing for a Canon C300 to get back the depth of field.

I’ll leave you with a few frame grabs from The First Musketeer.

Note: I have amended this post as I originally stated, incorrectly, that the BMCC has a global shutter. The new 4K Blackmagic Production Camera does have a global shutter though.

Black Magic Cinema Camera Review