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February 27, 2010
Depth of Field Calculations
I wanted to do some Depth of Field (DOF) calculations for my Canon EOS 50D with the EF 100-400mm f/4.5-5.6L IS USM lens with the 1.4X Extender. The extender/teleconverter costs about 1 f/stop of light. And I'm trying to understand exactly what the depth of field is for my photos. For instance, if I'm shooting an owl that's 50 feet away with the aperture set at f/8, then what, exactly, is my Depth of Field? Is it enough to make the entire owl appear sharp? This is really the goal, of course. That the owl appear in sharp focus, but that the tree limbs in front and behind him appear soft and out of focus. This is the ultimate goal. But the calculations are horribly complicated and vary with the distance to the subject, as well as the aperture of the lens.
So, I was going to use this online DOF calculator, but I wasn't sure whether I should use the 35mm equivalent focal length of the lens, or the actual focal length of the lens. For instance, if I have my 100-400mm lens zoomed in all the way to 400mm, the 35mm equivalent lens would be 640mm. So, I wasn't really sure what values to plug in. So, tonight I was browsing through the FAQ and found this little gem: "Always use the ACTUAL focal length when calculating depth of field."
OK. Fair enough. So, I'll use 400mm * 1.4X = 560mm. So, this is my best guess...that the true lens focal lenght is 560mm.
Camera: 50D
Focal Length: 600mm (there's not one for 560mm)
Selected f-stop: f/5.6
Subject distance: 50 ft
This gives me a DOF of .44 ft, or 5 1/4", or 2 3/4" before and behind my focus point.
I like their DOF Table even better. It allows me to put in 560mm, instead of having to select from a list. This table shows my DOF to be 3" at 50 ft with f-stop of f/5.6 for the 50D with a 560mm lens.
Honestly, though, I still don't trust these calculations. How can it possibly know that I'm using a lens that's not designed for my camera sensor size? I feel like that has to be taken into account in these calculations, and I'm reasonably sure that it's not taken into account.
http://forums.dpreview.com/forums/read.asp?forum=1019&message=34662249
Posted by Rob Kiser on February 27, 2010 at 11:15 PM
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