what is the downside?

[jim monroe]

Following the advice of the experienced members of this forum I am slowly moving toward more and more shooting using the raw format. Also following the advice of many on this forum I just purchased me first L lens a 400mm f5
.6.
Amongst other things I'm interested in nature photography, especially birds. This time of year in PA things are pretty bad, especially this year so visited the local aviary for some practice.
Inside the aviary the lighting is not great and with the lens full open at f=5.6 and shooting with a 10D in Av mode, at 400 ISO, the shutter speed under autofocus is sometimes 1/15-th to 1/30-th of a second. During that time the bird often moves, even if only slightly so I get some blur. So I now have the idea: why not shot in manual mode setting the shutter speed say two stops faster than what autofocus suggests thereby decreasing the possibility of blur because of movement of the bird. Then I come home and using the exposure +/- 2 stops exposure adjustment I can get in raw then I correct for my underexposing.
Seems like I gain a lot but since I don't believe in free lunches what am I missing? I'm guessing I pick up noise by doing this but since I don't understand what the software is really doing when you make the exposure correction it is not totally clear to me.
So what is the downside with this approach?

[dtrayers]

Alain Briot, who is arguably one of the best landscape phtographers today, has some thoughts on the exposure latitude gained in shooting in RAW mode:

http://www.outbackphoto.com/reviews/equipment/canon_300D_alain/Canon_300D_diary_alain.html

[Scottes]

The sensor captures less detail on the darker end.

In a nutshell, the sensor captures 4096 levels of light. Looking at an exposure on the histogram the far right 1/5th of the histogram captures 2048 levels of light. The next 1/5th captures 1024, then 512, then 256, and the leftmost 1/5th captures only 128 levels. (Or something like that.)

So you're going to lose a lot of detail if you expose like you wish. To capture the most detail the general rule of thumb is to try to overexpose without blowing highlights, then use the RAW converter to "underexpose" that picture.

http://luminous-landscape.com/tutorials/understanding-series/understanding-histograms.shtml
http://www.luminous-landscape.com/tutorials/expose-right.shtml

[jim monroe]

Dave,
Excellent link, I really appreciate it and furthermore it looks like I can get away with doing this. I especially appreciated the 100% crop presented because first seeing the two comparisons of the full scene I was still skeptical.
I read only the first section on the exposure question but will read the rest later today because it looks like a very interesting site.
One question you can perhaps answer. On my monitor the original shot which was underexposed and the correction of it had strange horizontal lines crossing the image while the other exposure, correct from the start, did not. I don't know if its my monitor or what. Any explanation?
Again thanks, Jim Monroe

[jim monroe]

Scottes,
Now I have an interesting dilemna. I understand your histogram arguement, at least the theory or in general. However Dave's link and the presentation there seems pretty convincing that I don't lose much.
Perhaps I'm missing something special about the presentation in Dave's link and if so would appreciate your reading of that presentation. In other words do you see anything special there that might not be true in general?
I understand histograms in theory but suppose I have absolutely nothing in the right 1/5 of the historgram it is hard for me to "see" what that means in practical terms, that is how much of an effect do I see in a particular photo. I need to do some experimenting.

[Scottes]

Well that's a hell of an argument, and if Alain Briot says it I'm going to listen.

However a couple things come to mind - statements to the effect "in my eye" and "from what I could see" are certainly valid in the real world, but possibly inaccurate. If you show me a picture with 40,000 subtly different colors and another with 10,000 colors I'm probably not going to notice on my screen. But print that at 16"x24" and I may very well notice. Or if I bought a better monitor I might notice.

Yes, might. I dunno.

Also be aware that underexposing by 2 stops means you might completely lose details because you've "blown out" the blacks. Same issue as when overexposing by 2 stops and blowing out the white. Try taking a picture of a crow at -2 and another at +2 I'm guessing that you're going to see a difference in details in the feathers.


But I'm intrigued, and plan on doing some tests. I'm in the same boat that you are in - wildlife in the early hours. If I can pull details back when underexposing by 2 stops then you'd better belive that I'll do that from now on.

[dtrayers]

I can share my experience... it's similar to Alain Briot's.

I shoot all the time in RAW and typically convert with PS-CS. I've noticed that if the dynamic range of the image is less than the full 5 stops the camera is capable of, the histogram is shifted to the left.

There is usually very little latitude for moving the shadow control without clipping, but I can typically move the exposure control to expand the histogram to the right 1-1.5 (sometimes 2) stops.

It's like the camera is "exposing to the left". I think it's very conservative in preventing a blown out highlight.