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[OM] Re: focusing question

Subject: [OM] Re: focusing question
From: Chuck Norcutt <chucknorcutt@xxxxxxxxxxxxxxxx>
Date: Wed, 28 Mar 2007 17:26:28 -0400
For your use you are quite correct.  However, I did state my assumptions 
(30 lines/mm for 35mm and 60 lines/mm for 4/3) and, as you guessed, 
those are the more or less standard assumptions for 8X enlargement.  For 
a sharp 24x36 or equivalent crop you're talking much more stringent 
numbers.  For 24x36 at 200 feet the DOF for 600mm f/8 is 6 feet for 35mm 
images (24X enlargement) and 3 feet for 4/3 (48X enlargement).  Not much 
to play with.  These numbers also equate to 90 lines/mm and 180 lines/mm 
resolution at the sensor.  Getting into tough territory there.  It 
wouldn't be so bad if you made a real 24x36 where one would likely stand 
back to view the print but, if you cut out an 8x12 area then you're 
likely to view that at reading distance.

Chuck Norcutt

Moose wrote:
> Chuck Norcutt wrote:
>> Moose is correct but still underplaying the distance you need for 
>> infinity focus with a 600mm lens at f/8.  
> Lazy in the middle of the night Moose was sort of hoping you would do 
> the calculations. :-)
>> ......
>>
>> Long lenses can be hard to use.  At 200 feet this lens only has about 18 
>> feet DOF for 35mm and only about 9 feet on an E-thing.  
> I just can't agree with these numbers based on practical experience. I'm 
> sure the calculations are correct, but the assumptions that go into them 
> must not be.
> 
> The problem is that, at least for me, most super tele shots are of 
> things that are still relatively small in the frame. So I tend to crop, 
> often a lot. This means the default assumption in most calculators of an 
> 8x12 print from the full frame is wrong. It's more like a 24x36 or 
> larger print size.

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