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[OM] Re: Great E-1 150/2.0 pix on dpreview.com

Subject: [OM] Re: Great E-1 150/2.0 pix on dpreview.com
From: Skip Williams <om2@xxxxxxxxxxxxxxxx>
Date: Thu, 2 Dec 2004 11:15:07 -0500
> On a film camera, for a given focal length, as the format gets smaller, so 
> the DoF gets narrower.

(I'm no optical engineer, just a hack.)

Calculation of DOF is independent of the film/sensor size.  This assumes that 
DOF is really a measure of apparant sharpness based on some minimum 
circle-of-confusion.

A 300mm lens that has an image circle large enough to cover a 5x7 piece of film 
produces the exact same image size as a 300mm lens on a 4/3 camera.  The 4/3 
camera only uses a small crop of the 5x7 lens' image.  The DOF characteristics 
are the same.  That's why you typically see LF photographers using f/45-128 to 
get enough depth of field to cover reasonable subjects.  You could stop a 4/3 
300mm lens down to f/22 or so, but the long exposure times wouldn't make is 
usable, whereas the LF lens is used for fixed subjects.

Your statement above should really say:  As image magnification 
on-the-film/sensor stays constant, the DOF increases as the film/sensor format 
gets smaller.

Skip


----- Original Message ---------------

Subject: [OM] Re: Great E-1 150/2.0 pix on dpreview.com
   From: "Piers Hemy" <piers@xxxxxxxx>
   Date: Thu, 2 Dec 2004 15:08:10 -0000
     To: <olympus@xxxxxxxxxx>

>
>I'm not sure if you are actually suggesting what I think you might be
>suggesting Skip - but this is my understanding of what follows from what you
>say.
>
>.... the DoF characteristics of a 150mm lens on an image format which is
>smaller than that of a 35mm camera.  On a film camera, for a given focal
>length, as the format gets smaller, so the DoF gets narrower.  I suspect
>something similar applies to digital, though what size the relevant circle
>of confusion would be - no idea! 
>
>Another way of visualizing this would be to ignore focal length, and say
>that an extreme telephoto has very little DoF, and a wide angle has plenty -
>and that applies regardless of what format you are using.
>
>I may have got the theory a*se-upwards - but never mind, what does the
>practice show in your experience?
>
>--
>Piers 
> 
>-----Original Message-----
>From: olympus-owner@xxxxxxxxxx [mailto:olympus-owner@xxxxxxxxxx] On Behalf
>Of Skip Williams
>Sent: 02 December 2004 14:49
>To: olympus@xxxxxxxxxx
>Subject: [OM] Re: Great E-1 150/2.0 pix on dpreview.com
>
>
>Yes, it produces the same image magnification on a 4/3 camera as a 300mm
>lens would on a 35mm camera.  But it does have the DOF characteristics of a
>150mm lens, not a 300mm.
>
>--snip
>
>
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