With its latest firmware, the Canon EOS R5 gains the Canon Log 3 profile for video. In this article, we’re taking a closer look at C-Log3 and how it differs from the existing C-Log profile. In doing so, we hope to make clear why cameras sometimes offer multiple Log profiles, to help you decide which one to use.

Why Log?

To understand why cameras have different Log profiles, it’s worth going back to basics to explain why videographers shoot Log at all.

In stills photography we generally choose between JPEG files (a small and efficient compressed format but one that has limited editing flexibility), and Raw files (larger but retaining the greatest flexibility). Log is an attempt at a middle ground between these two: it’s subject to a lot of the processing and compression that a JPEG file is, but has been expressly designed to retain enough information about the original capture to remain flexible. Crucially, it tends to be relatively small, which is valuable in video: when you’re measuring file sizes in data per second, then the clock is ticking from the moment you hit the [REC] button.

The simplest way to think of Log files is like JPEGs but with tone curves designed to share their data values out more equally between the number of stops you captured. Colors are also desaturated, to reduce the risk of saturated parts of the scene clipping and hence losing flexibility when color grading.

Why are there different versions of Log?

In the previous section I wrote that Log curves are designed to retain similar amounts of information about every stop of light you captured. This isn’t quite correct. More precisely, Log curves are designed to encode a pre-determined number of stops of light. And there are a number of reasons why more is not always better.

Firstly, there’s simply no point using a Log curve that encodes 15 stops of light if you sensor can only capture 12 before the signal becomes indistinguishable from noise. For a start, it would mean that the bottom three stops of your file would be a noisy mess, but it would also encourage you to use an inappropriate exposure.

C-Log is designed to accommodate around 11 stops of DR. In order to capture around 2EV more highlights than a standard JPEG, it’s supposed to be exposed as ISO 400 (rather than the ISO 100 that’s the base setting for the Standard color mode). Canon Log 3’s base ISO setting is rated as 800, to capture an additional stop.

If we assume the Log curve is designed to incorporate another two stops of highlights over what a standard sRGB JPEG would include, you’d need to reduce exposure by two stops to capture this additional highlight information. A two-stop reduction in exposure means a two-stop increase in noise in all those tones that overlap with the standard color mode.

The final consideration is data availability. Your camera can only record a certain number of brightness values. If you try to share those values roughly equally between 11 stops of light, you retain more information about each one than if you try to squeeze 15 stops into the file. This is less of a problem on higher-end cameras that shoot 10-bit footage (and so have 1024 values to share), but is worth considering if your camera only captures 8-bit Log, and hence has only 256 values available.

This should start to explain why there are multiple Log options: you want to find a Log curve that’s well suited both to the capabilities of your camera and to the scene you’re shooting.

C-Log vs C-Log3

We took a closer look at the Canon Log and Canon Log 3 curves and what difference they make. The first thing you may notice upon engaging C-Log3 is that the base ISO rating jumps to 800: one stop higher than the existing C-Log mode. We’ve checked, and there’s no change in amplification: that one stop change in ISO rating is solely an instruction to give the camera one stop less exposure, to capture more highlights.

Here are the waveforms of C-Log and C-Log3, exposed with this change in ISO rating taken into account. As theory leads us to expect, this results in an extra stop of highlight capture but with a noise increase (the waveforms of each step becoming taller/fuzzier), for the rest of the tones.

C-Log	C-Log3

Of course you don’t have to expose this way, if you’d prefer not to. The inherent flexibility of Log means you could expose C-Log3 as if it were C-Log, clip highlights at the same point and gain your extra stop in the shadows, instead.

C-Log	C-Log3 (matched exposure)

But the difference between the two profiles isn’t just a question of C-Log3 being slightly flatter, to incorporate an extra stop of DR. Look closely at the noise floor, represented by the thick white band at the bottom of the waveform. At first glance, this appears taller (noisier) for C-Log3. But this shouldn’t be the case, since we gave identical exposure to both modes.

Look closer and you’ll see that the noise in C-Log3 mode extends the same distance above and below the 128 marker line, which is what you’d expect: noise is variation above and below the ‘true’ value. But in C-Log mode, it extends upwards by the same amount as C-Log3, but cuts off cleanly just below a value of 128. Canon is using a very high black clipping level to make the darkest tones appear less noisy.

The impact of this can sometimes reveal itself when you try to grade the footage: C-Log will sometimes plug-up blacks, which look unnatural if they’re brightened during color grading. C-Log3 doesn’t clip its blacks so aggressively, so there’s less risk of the darker tones looking strange when lifted.

It’s also worth noting that C-Log3 gives you a choice of Cine or Rec 709 color response (rather than the EOS or Neutral options for Canon Log), making it compatible with the LUTs Canon has created for its Cinema EOS cameras, and giving a more interesting starting point.

So C-Log3’s greater processing flexibility comes from more than just the additional stop its tone curve is designed to provide. However, given how noisy the shadows get, it should be pretty clear that it’s already pushing at the limits of what the EOS R5 is able to capture.

This makes sense: the R5 creates its video from a 12-bit sensor readout. And, although the process of downscaling from 8K to 4K will boost the number a bit, it’s never going to significantly exceed 12 stops of DR capture. As such, the C-Log3 curve seems a pretty good match for the camera, and it’s unlikely that Canon will add the even wider DR C-Log2 profile.

When should I use it?

Canon Log 3 offers a little more dynamic range than Canon Log, but it’s also less prone to clogged-up shadows and is more readily compatible with the LUTs designed for Canon’s Cinema EOS cameras. Both of these factors make it more useable than the existing C-Log profile.

If you’re worried about the additional noise and don’t need the additional stop of dynamic range, there’s no reason you can’t overexpose C-Log3 by a stop and pretend you’re still shooting with a slightly more sophisticated version of C-Log. The wider DR captured does seem to slightly increase the risk of posterization, though.

Of course this doesn’t mean it suddenly makes sense to shoot Log all the time: the reduced, highlight capturing exposures mean it may not be ideal in low light, and it may be unnecessary or ill-suited to low contrast or controlled lighting shoots. But when it makes sense to shoot log on the R5, we think it makes sense to choose C-Log3.

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