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[OM] Re: focusing question

Subject: [OM] Re: focusing question
From: Moose <olymoose@xxxxxxxxx>
Date: Wed, 28 Mar 2007 14:04:42 -0700
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.

In practice, I can see variation in focus sharpness in the 5D viewfinder 
using the 600/8 at 50 ft. that are due to distance variations of a foot 
or less. Maybe shooting a distant landscape the DOF is 18 feet, but you 
won't be able to tell because atmospheric effects will be a greater 
limitation on sharpness than DOF.

In the uses to which I put lenses like this, the practical DOF is very 
shallow. Seems to me that putting out numbers like 18' is just going to 
lead to disappointment for the inexperienced who try it.

Moose

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