Saturday, June 23, 2007

Depth of Field

I have a new camera and I have had the same problem over and over - insufficient depth of field (DOF) So I decided to try to figure it out. Here is the result.

Depth of Field is the distance from the focal plane of the camera over which a picture is in focus. With insufficient DOF, sometimes my subject is either out of focus, or partly out of focus. So I did a quick study for myself to see how DOF is controlled with my new camera. If anyone technical is reading this, here are the important specs on my camera. It is a Nikon D200, 10.2 Mpix, with a 17-35 mm f2.8 lense. The lense is designed for a film camera and due to some other techincal stuff, this lense performs like a 24 - 52 mm lense on my digital camera. All the other stuff will be cited below. Remember, I need a deeper depth of field in order to overcome my inexperience as a photographer. That's the goal.

For Scenario 1 I set a pencil on a ruler approximately 30 inches from the camera's focal plane. With my lense, that means the pencil is about 24 inches from the front edge of the lense. I took pics on Apperture Priority with onboard flash. Here are the results

At f2.8 (apperture wide open - pictured on the right here) the pencil is in focus and you can see from the ruler that the pic starts coming into focus at about 26 inches and stays in focus until about 33 inches. That means two things. First, the camera did not actually focus on the pencil at 30 inches. The area around 28 - 29 inches is actually the sharpest. Second, at 30 inches with a wide open apperture, I get a 6 or 7 inch depth of field. That should normally be sufficient but if I am taking a picture of Brenda, her face might be in focus, but some of her hair might be fuzzy. Not good. Of course it depends on the subject being photographed.

The other end of the DOF spectrum is with the lense stopped all the way down, f22 in this case. At this point the apperture is letting in as little light as possible, so shutter speeds have to slow down, but look what happens to DOF. This pic is in focus basically for the entire photo. Nothing is fuzzy.

The other two pics in this series are at F10 and F5 (left to right below). F10 seems to be a good compromise to get sufficient DOF and still let enough light in to get a fast shutter speed.

Scenario 2 is with the pencil at 12 inches, about 6 inches from the end of my lense. This lense is pretty big and you can see that the lense obscures the flash in the forground of the pic (that dark shadow on the ruler.) Big problem for tight shots with a flash, but there are ways to fix this little issue.

First pic is F2.8 (on right) and look how tight the DOF is. The pencil point is in focus at 12 inches, but the 11 on the ruler is blurred. Similarly, the 13 is in focus, but not the 14. If you had a high resolution pic like I have on my computer here, you would see that the DOF goes from about 11 3/4 inches to 13 inches. So I got the pencil into the sharp area in this pic by only 1/4 inch and total DOF was only 1 1/4 inch overall. Not much tolerance in this set-up.

Now look at F22 (stopped down) Again, everything is in focus, even the cup holder in the forground and the banannas in the background (might be too dark on the web to see, but it is clear on my computer.) Nice. The F10 and F5 settings gave similar results as in the 30 inch scenario.

So my conclusion is that with this new, very nice camera, a wide open apperture (f2.8) gives me the fastest shutter speed possible but gives me a DOF of about 10%-20% of the distance from the camera's focal plane to the subject. At 12 inches, I get a 1.2 inch DOF, at 30 inches I get a 6 inch DOF. For pics beyond 30 inches, DOF becomes less important with this lense. So to get my subjects in focus, I need to use Apperture priority more often and try to shoot most of my subjects in the f10- f5 range. If it is a very brightly lit area, I can go to F22 and get everything in the field of view in focus. As I get better, maybe I can start opening the aperture more and reducing the DOF.

fun stuff


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