Saturday, 2 March 2013

Depth of Field ~ understanding the focal plane


Depth of field (DOF) is the range of distance in an image over which the image is said to be "sharp". In actuality, only one plane of the image can actually be in focus, but all points lying within the DOF are considered to be "acceptably" sharp.

Think of it this way, you set your aperture, you choose your focal length and point your camera at a subject... the vertical plane of what is in focus will be different horizontal lengths from back to front of the plane of focus...

I was recently shooting owls and I was shooting at f11 to f16... when I got home some friends were making comments that I should have shot that owl at an f2.8 to f5.6 instead of bumping my ISO up to get the exposure I wanted.

While they had a point, I wanted sharpness in my birds from back to front... an f5.6 would have had focus just on the eyes, while the wing tips would have been blurred... I would rather deal with a little digital noise in post editing and have the bird in focus...

Here is a real world example to show you what the focal plane of my lens was shooting these owls...

From tip of wing to tip of wing these owls are over 6 feet long. If I had set lens to 200mm and the aperture at f5.6 and the bird was 25 feet away from me, the horizontal distance that would have been tack sharp would be from 24' 5" to 25' 7" from my camera ... 14 inches of tack sharp horizontal plane in focus. When I set my focal length to 200mm and my aperture at f11, the horizontal distance that would have been tack sharp at 25 feet would be from 23' 7" to 26' 2" from my camera ... 31 inches of tack sharp horizontal plane of focus.

I would rather have half the bird in focus with some minor softness at the tips....

The end result, my Sigma 50-500 f4-6.3 lens handled itself fantastically and it handled the aperture I used, the speed of the owl and I got the shots I wanted...

If you want to learn more about focal distance, depth of field or hyper focal distance; there has lots been written on the web... do some research and take practice shots with the camera to get a feel for your camera and lens combinations.
 
Thanks for reading,
 
Kev