The more I practice with depth of field, the more I struggle with it. I understand depth of field well, and how aperture and focal length play a role. I'm just not consistently getting the results I want. I don't own it yet. But I'm getting closer.
The other day I was taking photographs of two women on their horses. Although they were jumping the horses, I didn't want a "sports" shot. I was looking for a portrait of them while jumping. Not much distinction maybe, but for me that means I needed a very shallow depth of field. Specifically, I wanted the rider and the horse to be in sharp focus, and the background to be as diffuse as possible. No prob, I brought out my sweet 50mm f/1.4 lens. It was a sunny day, so I was at ISO 100. I like to shoot in aperture priority mode, so I set for f1.4 and checked my exposure. Not good. Too sunny, the shutter speed couldn't go high enough to compensate. I ended up at f/3.5, 1/2500 at ISO 100. The histogram was good, and the display looked ok. I checked my DOF calculator app on my iTouch and determined that my depth of field was about 10 feet, from 20 feet to 30 feet away. Perfect.
So I shot the session with those general settings, making minor adjustments as the sun went in and out. When I got home and reviewed the photos, the exposures were all great. But I was unhappy with depth of field. With the sun, I had no choice but to move up to f/3.5. But even at that aperture I expected some diffusion in the background. Nope. The background was sharp as a tack. This is not the way I like my portraits.
So what went wrong? I checked in with my local Penn Camera guy and, after spending some money, asked him what he thought. His answer elevated my understanding of depth of field quite a bit. The problem was distance to the subject.
Using my 50mm, I had to be about 25 feet away from the jump to get all of the rider and horse in the frame. That's in the range of infinity for the 50mm lens. That is, the autofocus set the lens to infinity focus. As my Penn guy reminded me, by definition when the focus is set to infinity everything in the background is in focus. My calculator didn't remind me of that fundamental principle. In order to get some diffusion in the background, I had to be closer to the subject. But then I couldn't fit the subject in the frame.
The ultimate answer: bad choice of lens. What I should have used was my 70-200mm f/4L. Now when I take my test shots, I'll look to see where I am on focus and adjust so I'm not at infinity. Another valuable lesson learned. So will I relegate my 50mm to the bottom of my camera bag. No way hoser. I'm shooting an indoor wedding on Saturday. I'll be in dim light, where every stop counts so my aperture will be f/1.4 for sure. I'll be about 25 feet away from the couple, which puts me at infinity for focus, which makes everything sharp on the alter, which is exactly what I want.
Now I just need to get out to the horse farm again and re-shoot that session with the right lens.