Thomas Bryhn wrote:
>
> You should be warned that this DOF calculator uses inaccurate values for
> the circle of confusion, so I'll try to show what's going on:
>
> 1) Using the same DOF calculator,
> http://members.home.net/gillettm/DOF.html, enter realistic values for a
> normal lens on 4x5 film: 150mm, f/6, and a distance of 2 meters/feet. Note
> the DOF.
> 2) A normal lens for 35mm is 3 times shorter (50mm), so the circle of
> confusion should also be 3 times smaller if the final print on the wall is
> to be the same size (less magnification required).
4x5 is 101.6mmx125mm, the diameter is 161mm, while 35mm's diameter is
43mm. It is roughly four times. But the COC he used was 0.03 and 0.15
seems still some errors. I haven't notice about that since I only use
6x6 for compare, the COC he used was 0.6mm, which is roughly right as
the diameter of 6x6 is 84mm. At the same aperture, a 85mm lens for
35mm and a 150mm lens for 6x6, the 35mm one got larger DOF value.
> The numbers in this
> program are a little off, so you'll have to enter 645 as the film format to
> have the correct relative value for COC. Use 645 film, 50mm, and f/2 to
> keep the absolute lens opening constant (150/6=50/2, very important!), and
> finally keep distance constant at 2 meters/feet. Note DOF, it's practically
> unchanged.
I'm sure you will get different out-focus effect even you stop down
the 150mm lens of 6x6 to get the same DOF value of 35mm. The 150mm
lens still get much softer look background.
C.H.Ling
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