I have a point and shoot camera that I use half the time, and I have a DSLR that I use half the time. I usually leave the DSLR in full auto mode, but I occasionally switch to program mode to turn off the auto-flash or pin the ISO to a fixed setting; I've never used it in full manual mode. In an effort to develop some technical proficiency with the DSLR, I took the plunge and switched to full manual control. I thought repeating the depth of field assignment I did in high school would be a good way to exercise my brain cells and practice manual control of the camera. It was a good exercise but not as easy as I thought it would be.
There's a beautiful building on 7th Ave. that has a row of pillars in front of it; each one has a sphere or globe on the top. This would be the perfect subject for my exercise. I hand-held the camera and focused on the pillar in the center. I took severa pictures from the west and the east. I waited several times while people walked past. For each shot, I pushed the f-stop to one end or the other and adjust the shutter speed accordingly.
I looked at the results on the computer when I got home and was absolutely dumbfounded! Every picture was completely in focus; none of them showed depth of field. I didn't understand why. I went back and repeated the exercise paying extra attention to what I was doing with each shot. I got the same results! Clearly there was something I didn't know. Perplexed and frustrated—I went to lunch.
Over lunch, I pondered the difference between how I did this exercise in high school and what I was doning now. In high school I had used a manually operated camera with a fixed focal length lens. That shouldn't be a problem. I'm operating the camera in manual mode, controlling both the f-stop and shutter speed, and I have the zoom lens set on 50mm. The formats are different. Back then I used ASA-400 black & wight 120 film; today I'm using a DSLR, but the format differences shouldn't be a factor.
I used an old wagon wheel as my subject when I did this assignment in high school. I set the ASA to 400, took a light reading, set the f-stop to f-5.6, the largest apature available, and adjusted the shutter to get the correct exposer. I moved in close and focused on the center of the wagen wheel and snapped a picture, then I changed the apature to f-32, the smallest apature available, adjusted the shutter speed, and snapped again. I got great results! How is that any different than what I'm doing today? Then it dawned on me, I was much closer to my subject than I am today. My focal point is 30 feet away; for a 50mm lens that just as well be infinity.
I took my camera and tripod and went back outside. This time I used a fence close to the apartment building. I cheated a little this time and set the camera to apature priority. I controlled the apature and ISO, and let the camera do everything else. I focused on the fence just a couple of feet away. With the ISO fixed at 400 and the apature at f-32 the shutter dropped to 1/20 sec, hence the need for a tripod. This produced clear focus across the whole picture. With the apature set to f-5.6, distant objects were out of focus and near objects were sharp. I finally got the results I was looking for.
I learned that apature isn't the only thing that controls depth of field. The proximity to the subject also affects dept of field. The closer you are to the subject the shallower the depth of field. Depth of field becomes deeper the farther you get from the subject. The focal length of the lens has a huge affect on depth of field, as does the magnifying power of the lens. A wide angle lens will provide a deep depth of field while a telephoto will provide a much shallower depth of field. A x300 magnifying glass has almost no depth of field while a x20 pair of binoculars has good depth of field.
I'm really glad I did this exercise. I not only became much more proficient with the camera, but I also gained a much better understanding of "depth of field".