'Depth' of depth of field

Hi All,

Bit of a techy one... I'm still toying with the idea of getting a 50mm f1.4 for my 300D (Bid for a few on eBay but 2nd had are so close to the price of new - what's the point!).

However - does anyone know how to work out the 'depth' of the area in focus for such a lens? I've noticed with my other lenses, even at only f4.0, that taking a close up portrait of a face at 45 degrees can often end up with one eye in focus and the other out.

I seem to remember, a while back when I was learning about DOF, that the area of focus is related to the magnification of the lens, and that it's different for say a 50mm at f4 and a 120mm at f4.0, but I don't know which way round it works.

I'm worried that I buy the f1.4 and find I can only use it from f4.0, and would have been better to stick with what I've got...

Colin

 

http://www.dofmaster.com/
If you go there you can download a program which will help you work it all out. Then you can print it off and put it in your bag if you want.
There's also a tool which does the same for hyperfocal distance as well.

http://en.wikipedia.org/wiki/Depth_of_field
There's loads on DOF there, including the maths.

 

Excellent, thanks ferny. Very useful little tool that.

Colin