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March 10, 2007
Depth of Field Calculator
I found a nice website that allows you to calculate depth of field so that you can make sure your subjects will be in focus. I've been going around and shooting some of the early blooming plants Dogwood, Bradford Pears, etc. One of the most complex challenges when shooting flowers is getting the depth of field correct, so that all of the flower is in focus, and the background is out of focus. This, of course, is the goal.
Calculating the depth of field is not easy. Depth of field is affected by so many variables, that it's nearly impossible to figure out in your head. Depth of Field is affected by the len's aperture, focal length, and the distance to the subject. Additionally, you have to consider the "Circle of Confusion" , which is determined by the size of the camera's film or sensor.
I have discovered that I'm frequently unable to get the depth of field correct, particularly when shooting subjects like flowers, where the depth of field is extremely important. One simple trick to increase the depth of field is to put the camera on a tripod and crank down the aperture from 4.0 to 22 or something like that. This will increase the depth of field. But, again, normally, I'm just going through the motions. I don't really have any concrete way of knowing exactly what the depth of field will be in these cases. Just that it will be deeper.
So, today I plan to go out in the field with my laptop and see if I can't get better results using this handy Depth of Field calculator.
Update: You can download a verion of the DOF Calculator for your Windows laptop here (so you don't have ot be online). Or download a version of the DOF Calculator for your Palm Pilot here.
Update 2: That last DOF Calculator program made my head spin. Here's another DOF Calculator download I found that I'm going to try now. This is the one to go with. Bob Atkins has it figured out. I changed it to use feet instead of meters and set my Circle of Confusion to 23 microns since I'm shooting an EOS 20D with a 1.3 LMF. This little program is slick, and of course, the price is right.
Update 3: I have subsequently realized that the LMF for the Canon EOS 10D, 20D, 30D, and 40D is 1.6, not 1.3. So, I should be using a Circle of Confusion of 18.75 microns. And, what I'm seeing is that if I use my long lens (400mm), then I run into a problem when I close the aperture down to an f-stop greater than 14. It says "*** WARNING: Diffraction limited spot larger than chosen COF". So, this means that with the 400 mm lens, if I hold the subject at the nearest focal point the lens can focus on at 1.8m(5.9')... (if I get as close to the subject as possible to get the largest image possible), then my maximum depth of field is .33". The shallowest depth of field under the same conditions, with the aperture set at f/5.6, is .13".
Focal Length....Aperture.........Distance...... DOF
400mm..............f/5.6...............5.9'..........0.13"
400mm..............f/14................5.9'..........0.33"
So now, I'm wondering if I change the focal length of the lens...if that will allow me to get a greater depth of field. In theory, of course, it will. But now I'm not sure what all changes when I go to say....300mm instead of 400mm. OK...after some playing around, what I found is this...that I can change to 300mm, for instance, but I can't get any closer to the subject, and I can't get my aperture any better than f/14 without getting the error. So, the circle of confusion doesn't change, the aperture doesn't change, and the distance to the subject doesn't change. So, the effect is that the subject appears smaller on the image, which is not what I want, of course. At 300mm, the numbers crunch out like this:
Focal Length....Aperture.........Distance...... DOF
300mm..............f/5.6...............5.9'..........0.25"
300mm..............f/14................5.9'..........0.63"
So, it does increase my DOF to 0.63", but the image appears smaller. Arghh...
Posted by Peenie Wallie on March 10, 2007 at 7:35 AM
Comments
Your 20D has a crop factor of 1.6, so the CoC should be set to 19 microns. Only the 1D series has a 1.3 crop factor. And don't just crank the aperture to f/22. Beyond f/13, diffraction stats impacting image quality. Only go smaller than f/13 if you're willing to trade more DoF for less sharpness. The Atkins calculator will warn you when you've passed the diffraction limit.
Posted by: Rich on July 5, 2008 at 9:40 PM