Depth of field is a print or output characteristic. The greater the
enlargement the less the depth of field. The depth of field is reduced
in a cropped image by virtue of needing greater enlargement. It is
also, of course, influenced by viewing distance. The shorter the
viewing distance the less the depth of field. It all boils down to
whether the enlargement is big enough or the viewing distance close
enough to be able to see something as small as the CoC. And the desired
CoC is itself a variable based upon viewing distance, enlargement and
the assumed resolution of the human eye (at a given light frequency of
course).
And we haven't even begun to discuss focal length and aperture yet. No
wonder folks gets confused. Then you've got folks who bop in and say,
wait a minute! That's all based on thin lens theory which is just a
simplificaction. The *real* story is... too complicated to tell. :-)
Chuck Norcutt
On 11/19/2010 3:43 PM, Andrew Fildes wrote:
> Yes I was bothered by this. I mean, how does mere cropping or masking
> change the DOF? Doesn't make sense to me. Andrew Fildes
> afildes@xxxxxxxxxxxxx
>
>
>
> On 20/11/2010, at 7:26 AM, usher99@xxxxxxx wrote:
>
>> BP writes:
>>
>> "Let's remember, before this gets too clouded. A 50 mm lens has
>> the same dof as a 100mm lens, if the 50 is cropped to the same
>> coverage area as the 100."
>>
>> Well almost but not quite. Even with the thin lens assumption the
>> FL term for near and far limits of dof do not fall out. The
>> distribution of the dof changes with a higher percentage of the dof
>> distal to the plane of focus with shorter FL's. Not really much
>> shift at the 50 and 100mm choices but pick something wider and
>> there will be a larger difference. Dr. Focus concurred with this
>> from a post a year or two ago.
>>
>> Would be thrilled to see a full wave dof calculator for even one
>> complex lens--perhaps ping Zeiss.
>>
>> Skeptical about thin assumptions, Mike
>>
>>
>>
>> http://www.cambridgeincolour.com/tutorials/depth-of-field.htm --
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>
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