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Re: [OM] Depth of field preview

Subject: Re: [OM] Depth of field preview
From: Nicolas Mitchell <oldspeckledhen@xxxxxxx>
Date: Wed, 24 Jun 2009 20:54:40 +0100
And jolly good mud it is.
N

On 24 Jun 2009, at 19:28, Jan Steinman wrote:

> Well, since we're being pedantic, there are only *two* things that
> affect DOF:
>
> 1) focal ratio
>
> 2) reproduction ratio
>
> #1 is what everyone calls "aperture." Not saying anyone is wrong for
> using the latter term, just that "focal ratio" is more precise and
> descriptive in this context.
>
> #2 is the ratio between the real-life size of something and its
> eventual rendering. This is often expressed as the ratio to film-size
> for macro work, but as Chuck points out, is generally expressed as the
> ratio to an 8"x12" print size in DOF tables and markings on lenses.
>
> It's a technical fallacy that focal length has anything to do with it:
> crop and magnify a portion of a wide-angle shot so that it is
> identical to a telephoto shot *taken from the same location*, and they
> will have the same DOF, if they were shot at the same aperture.
>
> But *practically*, wide-angle has a well-deserved (if false)
> reputation for "having more DOF" simply because it tends to increase
> the reproduction ratio, just as telephoto has a well-deserved (if
> false) reputation for "having less DOF" simply because it tends to
> reduce the reproduction ratio.
>
> Another reason focal length has a reputation for affecting DOF is that
> one often changes perspective when changing focal length. Move back
> and forth until the subject is the same size, and take pictures with
> both a wide angle and a telephoto, and the DOF of a shallow subject
> will be about the same.
>
> But by moving back and forth and changing perspective, YOU have
> altered the relative reproduction ratios of the foreground-background
> objects in your scene, and so the wide angle lens WILL "have greater
> depth of field," but because you have moved, NOT because it's a wide
> angle lens. You could construct a mosaic wide-angle with the telephoto
> from that perspective and have identical DOF.
>
> Clear as mud yet? :-)
>
>> From: Chuck Norcutt <puhpxabephgg@xxxxxxxxxxxxxxxx>
>>
>> And in the real world of prints (as opposed to the viewfinder) the
>> size
>> of the print.  Standard DOF tables assume about an 8X print
>> magnification.  Larger prints result in less DOF and smaller prints
>> result in greater DOF.
>>
>> NSURIT@xxxxxxx wrote:
>>> Three things will  affect
>>> your depth of field.
>>>
>>> Focal length of the lens.
>>>
>>> Aperture you set on the lens.
>>>
>>> The distance between you and your subject.
>>>
>>> Bill Barber
>>
>
>
> :::: They that can give up essential liberty to obtain a little
> temporary safety deserve neither liberty nor safety. -- Benjamin
> Franklin ::::
> :::: Jan Steinman <http://www.Bytesmiths.com> ::::
>
>
>
> -- 
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