Push Processing

[Chinese YoYo rehearsal in Fidelity Chinese School]
[I on purposely underexposed the picture by 1 stop and then adjust it back using digital darkroom]

Pushing processing (also called uprating) is a developing skill used in film era. Can it be used in digital world?

I covered some indoors events around the year, mainly for our Chinese school. The problems for shooting these events are: 1. the ambient light is really weak and 2. the flash is prohibited. On top of that is very often I encountered some fast moving dances. Even with the aperture set to f/2 to f/2.8, I still can't freeze the motion. To produce sharp images, the rule of thumb is to maintain "safe shutter speed". The safe shutter speed says if you want a sharp photo (no motion blur), you should use the shutter speed equal to or higher than the focal length. For example, if you are using a 100mm lens, your shutter speed should be 1/100 sec. But this is a rule for full frame cameras, when using crop body, you have to multiply that number with crop factor, so for Canon, the safe shutter speed for 100mm lens will be 1/160 sec.

The rule says nothing about the ISO or aperture. It's up to the photographers to manipulate these two factors to achieve the safe shutter speed. For my situations, I have to achieve at least safe shutter speed so I can catch up with those fast moving dancers. Extreme high ISO is the last thing I will do since it generates too much noise. So the only way is to use largest aperture say f/2.8 on my 70-200mm lens. But what if I still can't achieve 1/320 sec shutter speed (200mm is 320mm in full frame)? The only way to raise the shutter speed in this situation is use exposure compensation, you intentionally underexposure the photos (i.e. do negative compensation) to achieve the faster shutter speed.

When you view your photo on the computer, it will be dark since we under-exposed it on purpose. Now it's time to bring it back to its correct exposure. You use software to do it. I believe Canon ships Digital Photo Professional (DPP) software along with the cameras. DPP allows you to adjust the exposure of the photo. Adobe's Photoshop or Lightroom can also help to bring the photo back to live. One thing needs to be remembered is on the software, the correct term of doing this is to adjust the "level". But nowadays, the software are made easy to do that. When you see the slider for adjusting exposure or brightness, it actually adjusts the level for you.

This is like doing the pushing processing in the old film days. The difference is that in film days, we adjust the film ISO wheel to higher value (e.g. if the film is rated 200, we uprate it to a higher value, say 400). Cameras will think this is a 400 film so allow higher shutter speed. But the film is actually a ISO 200 film, so when developing the film, it needs to be exposed for a longer time. Now every one of us has a digital darkroom at home so the pushing processing becomes available for many of us.

The last thing about this digital pushing processing is that it also brings up the noise, especially in dark area. So don't push (your luck) too much!