Raw Dynamic Range

By Rishi Sanyal

The new 24MP CMOS sensors in the Canon Rebel T6s/T6i sparked some speculation as to whether or not the dynamic range of these new sensors showed any improvements over other offerings from Canon (that, compared to the competition, lag in this arena). Here, we put the dynamic range of the new sensors to the test, particularly by examining the ability to push shadow detail in Raw files, or simply 'rescue' underexposed shots.

Note: For simplicity, we'll limit our discussion to the T6s, but the T6i shows identical results. Note that most in-line hyperlinks below change widget states, so refer to the widget above the text as you click on them.

Exposure Latitude

In our first test, we look to see how tolerant of pushing exposure the T6s' Raw files are. We've done this by exposing our scene with increasingly lower exposures at the camera's base ISO of 100, then pushed them back to the correct brightness using Adobe Camera Raw. Examining what happens in the shadows allows you to assess the exposure latitude (essentially the dynamic range) of the Raw files.

There are two things at play here: less exposure means less light is captured, which itself leads to noisier images because of shot noise. Hence, larger pushes of progressively underexposed files will always be noisier than properly exposed shots (even for a perfect camera). However, a camera's sensor performance can also affect results by adding additional noise (or not), which leads to the difference in performance you'll see between cameras with the same sensor size in our widget above.

In normalized comparisons at common viewing size (8MP equivalent), the T6s is a tad bit more tolerant of shadow pushing than the previous T5i, showing slightly lower levels of noise after +3 EV or higher pushes (and even slightly less chroma noise after a +2 EV push). It's nice to see that the increased pixel count, which might have led to an increase in total read noise due to the extra pixels that have to be read, does not appear to have adversely affected dynamic range (quite the opposite, in fact).

That said, dynamic range improvements over the previous Rebels are rather incremental, and the new Rebels are not very competitive against the large dynamic range offered by Sony sensors in cameras like the Nikon D5500. This means that shadows and exposures in general are more tolerant of brightening in post-processing, which can both help you rescue mis-exposed shots, as well as deal with higher dynamic range scenes by exposing for the highlights, and brightening shadows in post.

ISO Invariance

Since the exposure latitude test above conflates both shot noise and read noise performance into an assessment of how recoverable a camera's shadows are at base ISO, we further investigate the camera's sensor and electronic performance by holding focal plane exposure the same, and assessing the benefit of in-camera ISO amplification to overcome camera read noise. A camera with very low read noise (a better performer) will show little to no benefit to performing this amplification, or image brightening, in-camera vs. in post-processing, and will therefore be 'ISO-invariant' (or 'ISO-less'). A camera with higher read noise (a worse performer) will show significant benefit to performing this amplification in-camera as opposed to in post-processing, and will therefore be 'ISO-variant'.

An 'ISO-invariant' camera with high base ISO dynamic range that comes from having a low noise floor can reduce the need to amplify the sensor's signal to keep it above that noise floor even under conditions traditionally demanding higher ISOs.

The benefit? Increased highlight retention ability if you shoot Raw and purposefully keep your ISO setting low so that you can brighten shadows and midtones in your favorite Raw converter, while holding back highlights and keeping them from clipping. Read about why increasing your ISO setting typically decreases highlight information, and dynamic range, here.

Below we've used the same aperture and shutter speed at different ISO settings to see how much difference there is between shooting at a particular ISO setting (and using hardware amplification) or digitally correcting the brightness, later. Using the same exposure means that all the images were created from the same amount of total light (so have the same shot noise). This means that any differences in noise must be the result of read noise added by the camera.

Above, you'll want to compare each pushed shot to the ISO 3200 shot at the bottom right, which is the reference, or 'best case'. An ISO-invariant camera will show little to no difference across all crops, while an ISO-variant camera will.

As we've come to expect from Canon cameras, the Rebel T6s and T6i cameras are, like its predecessors, ISO-variant. Pushed lower ISO files are noisier than their higher ISO counterparts, when the camera is given exactly the same exposure. This is because of higher levels of in-camera downstream read noise, which is overcome by a higher ISO setting (at the potential cost of highlights), but not at lower settings. The ISO 100 and 200 shots pushed by 5 and 4 EV in post, as opposed to in-camera by setting the ISO at 3200, show much higher noise levels across almost all tones. ISO 400 pushed by 3 EV still shows more noise than the same exposure pushed in-camera via ISO amplification. By ISO 800 (and a 2 EV push), differences become rather modest, and ISO 1600 with a 1 EV push looks virtually identical to the ISO 3200 'baseline' shot.

Essentially, there's some visible noise cost to almost every > 1EV push of lower ISO files, the extent of which will vary depending on what tone you're concerned with (darker tones suffer more). Contrast this with an 'ISO-invariant' camera like the Nikon D5500, which shows very little visible noise cost to pushing lower ISO files vs. brightening in-camera via ISO amplification. The 'textbook' example of an ISO-invariant camera being the Nikon D7200. These cameras allow you to save at least a portion of the brightening for post-processing of your Raw, which allows you preserve extra highlights by keeping ISO amplification down (increasing your ISO setting decreases dynamic range). Not so with the Rebels.

You'll note, though, that performance is slightly improved over the T5i. Not only are pushed lower ISO files cleaner than comparable shots from the T5i, the ISO 3200 shot itself is cleaner on the newer Rebels to begin with. This is particularly commendable given the resolution increase of the T6s/T6i, which requires smaller pixels. As we saw with the 5DS R, Canon is making small, incremental improvements to their sensors, which is encouraging. However, the new Rebels continue to show higher levels of read noise compared to their peers, likely due to continued use of analog-to-digital converters (ADC) on chips external to the image sensor itself. The extra read noise means that you won't be able to use less hardware amplification to retain extra highlight information as well as you could with some of the cameras' peers, and you'll have to be more careful about ensuring proper exposure and ISO at the time of Raw capture.

TL;DR

The Rebel T6s/T6i cameras show a slight improvement in Raw dynamic range compared to their predecessors, particularly commendable given the resolution increases they bring. However, the new cameras are not very competitive against the high base ISO dynamic range we see from on-chip ADC architectures from Sony sensors in cameras from competitors like Nikon, Pentax, and Sony itself. Whether or not this matters to the Rebel alliance user-base is of course debatable, but keep in mind that if you do intend to use these cameras to shoot high dynamic range scenes, you'll still have to rely on filters and HDR techniques more so than you might've had to if you were to shoot with some of the better performing peers. Also, in general, you will have to take more care to ensure proper exposure due to the more limited exposure latitude compared to some of the competition.