What you say is true if you're comparing equal fields of view which
means that you're comparing a 56mm lens on 35mm to a 28mm lens on 4/3.
But for 28mm to 28mm (and different fields of view) the result is quite
to the contrary. The depth of field on 4/3 for a given focal length and
aperture will be reduced over 35mm by the simple fact that the image is
normally magnified twice as much. On the other hand, depth of field
will be increased when using equivalent (not actual) focal lengths. A
14mm lens will give the same field of view as a 28mm lens on 4/3 and the
14mm lens will yield greater depth of field.
You can easily prove it to yourself by using any good depth of field
calculator. But the key in the computation is the size of the circle of
confusion. For 4/3 the CoC needs to be 1/2 the size of 35mm. When you
make that change the depth of field has to be smaller.
And, of course, an f/2 lens is still f/2 with respect to exposure which
is what I was actually talking about.
Chuck Norcutt
On 1/7/2013 10:07 AM, Sawyer, Edward wrote:
> As has been mentioned and proven innumerable times, the effective (as in,
> visually in terms of DOF, blur, et al) f stop in this case is f/4. It
> doubles as does everything else.
>
>
> On 1/5/13 6:16 AM, "olympus-request@xxxxxxxxxxxxxxxxx"
> <olympus-request@xxxxxxxxxxxxxxxxx> wrote:
>
>> The crop factor (angle of coverage) makes it a 56mm equivalent but the
>> real focal length is still 28mm and the real focal ratio is still f/2.
>> No f/4 about it.
>
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