23 February 2009

Depth of field in movies

One of the best things about the EOS 5D Mark II movie function is that it shoots on the full-frame sensor and therefore you get really shallow depth of field. Great.....except when you want lots in focus.
As far as I can tell, photography is all about problem solving and so it was the other day. I was trying to take a short movie clip of myself (yes, I am that vain) and needed to make sure I was in focus. I had a couple of attempts and it wasn't working out as the depth of field was just too shallow - any time I moved, I moved out of the plane of focus and it was ruined. So how to get around it?
With no control over aperture, there is not much you can do, but one thing will help - your knowledge of depth of field in relation to focus distance.
Initially I was using a EF35mm f/1.4L USM lens and DofF was very shallow. I switched lenses to the EF24-105mm f/4L IS USM and moved the camera and tripod further back. This had two effects:
1. The lens only has an f/4 maximum aperture so instantly more depth of field as the lens couldn't open as wide
2. By increasing my camera to subject distance, I increased the depth of field

Bingo! Enough depth of field for me to move around while staying in the plane of focus.

It's that second one that people forget. While everyone is busy using aperture to control depth of field, they forget that actually the most over-riding function of depth of field is the focus distance, or camera to subject distance.
Get further away from your subject and wham! more depth of field. Conversely, if you want less depth of field, get closer in to your subject and suddenly you're in blur city.


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