Push Processing and Unity Gain

Explosions in the Sky @ the Pageant -- 2008.03.29

Explosions In The Sky. Nikon D3 and 14-24mm at 14mm. f/2.8, 1/60, and ISO 12800.

Answers to the Pepsi Challenge of high ISO and push processing, and an introduction to unity gain. Earlier in the week I posed a simple question: Can you tell the difference between an image shot natively at ISO 12800 and one underexposed at ISO 6400 and pushed +1EV in post?

Between responses in the original thread, comments on Flickr, and votes through Twitter, the opinions broke down as follows:

Sample A was pushed: 6

Sample B was pushed: 10

Undecided/too close to tell: 8

By a very slight margin, the majority opinion was that sample A was more appealing due to slight differences in noise, contrast, and perceived sharpness, though most everyone agreed that the samples were almost identical.

The Answer


Sample A was the ISO 6400 image with +1EV in post. Sample B was the “native” ISO 12800 file.

The Explanation

So, how can an underexposed RAW file possess image quality that is the same (or arguably better) than an image properly exposed at a higher ISO? Shouldn't software-enabled amplification of the signal result in inferior results?

At the heart of the issue is the way in which the camera at hand, the Nikon D3, achieves the sensitivity of ISO 12800, which is specified as an “extended” setting (“HI 1.0”) above the camera's standard range of 200-6400. Specifically, unlike lower settings, ISO 12800 is achieved in-camera by multiplication, rather than analog amplification via hardware.

Thus, the ISO 6400 file and the ISO 12800 file were both essentially “pushed” in parallel means, albeit different points of the data chain, which accounts for the similarity in the final images.

An important concept here is the “unity gain ISO,” which describes threshold at which the A/D converter digitizes 1 electron into 1 data number of a digital file. Since 1 electron the minimum amount of information necessary in this process, the unity gain ISO represents the point after which digital multiplication must take over, analog amplification.

Shooting above the unity gain ISO, the signal-to-noise ratio decreases as the limit of real information collected by the sensor and digitized by the A/D converter is reached. Or in other words, above the unity gain the camera is going to be working to multiply more noise and less real, image-forming information.

Furthermore, raising the sensitivity above the unity gain ISO results in decreased dynamic range due to highlight clipping as a result of the multiplication of data.

What does this mean for the D3 and these samples?

From these results, we can assess that ISO 12800 is above the unity gain ISO of the Nikon D3 (which, depending on who you ask, is ISO 3200 or 6400). As such, with regard to image quality, there are few to no practical advantages of shooting at ISO 12800 over ISO 6400, since the former is a multiplied product of the same threshold of information.

All digital cameras and DSLRs possess have a unity gain ISO, and it will vary from model to model depending on the pixel pitch, signal amplification, and other factors of sensor efficiency. If you want to curl up with a lot of hardcore graphs and talk about quantum efficiency, you can check out an in-depth comparison of many DSLRs and a better explanation of unity gain at clarkvision.com.

For reference, here are the unity gain ISO ratings for some current cameras:

  • Canon 5D Mark II: 1600
  • Nikon D50: 1500
  • Nikon D300: 1100
  • Canon 50D: 900

But I like to photograph rock bands

OK, you're wondering, “What does this mean in practical terms for real world shooting like concert photography?”

Knowing the unity gain ISO of your particular camera can translate into smarter choices of picking in-camera sensitivities, allowing you to maximize image quality and dynamic range. In terms of the latter, the drop going from ISO 6400 to ISO 12800 is roughly one-stop. Contrast, which many people picked up on in the test samples, may take a hit above the unity gain ISO, as can color saturation/fidelity.

While the goal of ultimate image quality and the pursuit of those iconic, Jimmy-Hendrix-burning-his-guitar-on-stage moments may not seem particularly aligned, knowing the limits and capabilities of your camera's files can only benefit your output.

Of course, shooting in this manner necessitates post processing and conversion from RAW, so there are no benefits for JPG shooters. In addition, the ends will not justify the means for most photographers out there, given the extra work involved and the extremely similar results, as the previous samples indicate.

End Notes

There you have it – the long story of why an underexposed shot at IOS 6400 basically looks the same as an image at ISO 12800 with the D3.

In summary, the samples from the Pepsi Challenge look almost identical because both the ISO 6400 RAW file and the ISO 12800 RAW file are working with essentially the same info to begin with.

At the very least, the next time you think you've underexposed a high ISO shot, you may just have done yourself a favor.