I promised to write about histograms, but I’ve been pretty busy. However, here’s something quick I wanted to share. I went whale watching today with a couple of friends, and so of course I took the camera along. The thought process that I used here is pretty much my standard way of thinking about what I want to shoot.
Packing the camera bag was easy– the Canon 70-200 f/4L IS was the perfect lens choice, since it was long enough to bring the whales in close, and as a bonus image stabilization would help a little bit with the motion of the boat. Well, maybe, but it couldn’t hurt.
What about settings? I didn’t really care about depth-of-field other than wanting to have enough to get my subject in focus, so I set the camera to aperture priority mode and f/8. I realized that I was going to be shooting moving animals from a moving boat on choppy waters, and so shutter speed would be critical. I set the ISO to 100 and pointed the camera at the water, looked through the viewfinder, and pressed the button halfway to see what the camera would want to do. It picked 1/320 for a shutter speed. Under normal circumstances that would be fast enough, since it was faster than one over the focal length of the lens (1/200 in this case), but with all of that motion I wanted something faster. I took a wild guess and thought that 1/800 was a good minimum.
If I got 1/320 at ISO 100, then at ISO 200 I should be able to shoot 1/640. That still wouldn’t be enough, but at ISO 400 I should be able to shoot 1/1250. (Note: shutter speeds don’t always change in precise multiples of two. It’s confusing until you get used to it.) Since 1/1250 was well above my arbitrarily-chosen 1/800 threshold, ISO 400 should be perfect.
The sky was gray when we left the harbor, but I expected some of the fog to burn off over the course of the trip. I knew that I would have to doublecheck the settings if it started getting sunny– the fastest shutter speed my camera could do was 1/4000, which meant that I only had around two stops of room before my camera would be forced to overexpose the image. Sure enough, the sun came out a few hours later, but I was still able to shoot at around 1/2000. Otherwise, I would have either dropped down to ISO 200 or changed the aperture to f/11.
The thought process for this is one that you should use whenever you pick up a camera. What do I want this image to look like? What range of apertures and shutter speeds will get me the results I want? What camera settings should I use to get the right apertures and shutter speeds? As a novice you won’t always know all of the answers to those questions, but you can start guessing. “Do I want the background blurry or sharp? Do I want to capture or freeze motion?” Just answering those two questions before you pick up the camera will give you a huge leg up in getting the image you want.
Oh, yeah, a quick note about the gull. When I first looked at this picture on the camera’s LCD, I was rather shocked– the image looked blurry! It was taken at a shutter speed of 1/2500, so that hardly seemed possible. Either I’d seriously misjudged what shutter speeds would work, and I didn’t think I had, or something else was amiss. I zoomed in on the image and realized that the fuzziness I thought I was seeing was really the transition in feather colors at the rear edge of the wing. The image was actually so sharp that I could pick out individual feathers quite clearly.
Stop Shooting Auto! 
So why f/8 and ISO 400 instead of f/5.6 and ISO 200? So far I’m seeing more useful differences in noise performance than DOF differences in the middle apertures, so I’d tend to choose the latter. Is this a prime/zoom difference?
It has nothing to do with prime vs. zoom– noise is completely a function of the camera’s sensor.
Do you remember the f/8 and be there entry? f/8 is kind of the default middle-of-the-road aperture. It’s where many lenses perform best. And it’s kind of just the right aperture where if the autofocus guesses wrong you’ll probably still be OK.
I’m usually shooting with the Canon 5D, and on that camera there’s very little noise difference between ISO 100 and ISO 400. On the Rebel XT, the camera I took on the boat, the difference is more pronounced. Still, noise can be fixed fairly well in post-processing, while missed focus generally can’t.
Patti, I didn’t understand this part:
Under normal circumstances that would be fast enough, since it was faster than one over the focal length of the lens (1/200 in this case), but with all of that motion I wanted something faster.
What did you mean when you said it was “faster than one…?”
The rule of thumb for hand-holding a shot is that the minimum shutter speed is one divided by the focal length of the lens. If you’re using a 50mm lens, shoot at 1/50 or faster. If you’re using a 200mm lens, shoot at 1/200 or faster.
What that not-entirely-clear sentence meant was “faster than (one over the focal length of the lens)”.