A little while back there was a post on a photography forum that I frequent alot. The post was about a photography "tip" that was given by a well known photographer on a site from a major photography retail store. The photog was to give 100 tips for 100 days, a tough calling if you ask me. This particular tip was to underexpose a shot by 2 stops in 100 iso and then pull the exposure back up in LightRoom or Photoshop. The point was to attain a faster shutter speed while maintaining the good quality of 100 iso. This of course would have to be while shooting in RAW.
My thoughts regarding this technique as the author described using it in this circumstance, I disagree with. His point was to keep the quality of a 100 iso image while getting a faster shutter speed by underexposing and pulling the exposure back up in post processing, using a RAW file. (one should always be shooting in RAW if your concerned with getting the most out of your images). Whenever you under expose (especially by as much as 2 stops) and then pull your exposure back in post processing, you introduce noise and image degradation, period. So, your 100 iso quality image is no longer the quality you expect from a 100 iso image. A correctly exposed 400 iso image will give you better quality than a 2 stop under exposed 100 iso image that has been pulled back in PP. Especially with the quality of todays cameras and sensors, a 400 iso image will yield nice results.Here are a few close-up examples of Benny the Polar bear (good ole Benny) taken in 100 iso correctly exposed, 400 iso correctly exposed, and 100 iso under exposed 2 stops and pulled back in LightRoom 2.3
Pulling up exposure to that degree in PP introduces alot of undesired effects. The highlights pull up more so than the shadows and so you have a more contrasty image, less shadow detail, more noise, and also some unusual color shifts begin to occur.
I am not saying that using the under exposing technique is bad, as a matter of fact I use it at times for various things. I might use it if I were already at a higher ISO and I had no other way of getting a faster shutter speed. . .then I would underexpose as much as I needed to tweak that extra speed out. I only do not agree with the particular application the author suggested as an example. In the situation he described, the shooter would have been better off simply switching to 400 iso. the point of the authors claim was to achieve the high quality of the 100 iso image while having a faster shutter speed. Can't get something for nothin, and in this case the correctly exposed 400 iso would yield a better quality image.
These test examples were done with a Canon XTi. I chose that over my 5D because most shooters out there most likely have a camera compatible with the XTi quality.Click on the images shown below to see them large for a better view at the detail. Here is the original thread started in PhotographyReview.com My screen name on this forum is gahspidy.