2 stops wider is exactly correct for a 2X crop factor as long as you
also apply a 2X factor to the focal length and the CoC. 20mm at f/1.7
on 4/3 gives exactly the same DOF as 40mm at f/3.4.
Dr. Focus
usher99@xxxxxxx wrote:
> Yes, the 20mm f1.7 is a nice one indeed but the selective focus crowd
> would like a stop or more faster for more flexible dof control.
> DOF equivalence is about F 3.4 on FF. Not as impressive when viewed
> from that perspective. Dr. Focus and AG have calculated the exact
> dof difference, but 2 stops is close enough for gov't work.
> Mike
>
>
>
>
>
>
>
>> 0.95 is pretty spectacular - the fastest available by anyone.
>> I'm more concerned by the rather lousy looking bokeh. I think this is
>> one for movie use where the criteria for sharpness and out-of-focus
>> rendition are less important than some other characteristics, such as
>> DOF control.
>> Andrew Fildes
>> afildes@xxxxxxxxxxxxx
>>
>>
>>
>> On 03/03/2010, at 5:10 PM, Nathan Wajsman wrote:
>>
>>> But there is already a fast prime, the excellent 1.7/20mm. Not
>>> Noctilux fast, granted, but 1.7 is still pretty fast.
>
>
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