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[OM] Re: How AF is better

Subject: [OM] Re: How AF is better
From: Chuck Norcutt <chucknorcutt@xxxxxxxxxxx>
Date: Sun, 09 Apr 2006 07:49:56 -0400
In my opinion, the article is geometrically correct but uses exaggerated 
claims to make his point.  Chris has already addressed the girl in the 
example so I decided to tackle the B&W image of the man.  The claim is 
that this image was taken with a 100mm lens at f/2 and "Depth of field 
is about 2 inches, maybe less..."

I note first that the image has been cropped since it's not the 2:3 
aspect ratio that would be gotten from the author's Canon 1Ds. 
Considering the crop, I estimate that the vertical field of view was 
about 3 feet and must have been shot from about 9 feet using a 100mm 
lens.  Using conventional DOF calculations I get a DOF of close to 4 
inches.  Not the 2 or less he claims.  Tight to be sure but not super 
critical.

Furthermore, assuming a 3 foot vertical field at 9 feet, the distance 
spanned by the center and rightmost focus sensor on a 1Ds would be about 
9".  A little math tells me that the distance to the plane of focus 9" 
off the center line at 9 feet is only 0.37" further.  So, by my 
estimates, the focus error he speaks of is less than 10% of the 
avialable DOF... even at f/2.  Hardly the "2-3 inches of focus error 
[that] can make the difference between just right and just crap".

I'd give his article a whole lot more credence if he illustrated his 
points with precise, comparative pictures.  I think I know why he hasn't 
done so.

Chuck Norcutt


Chris Barker wrote:

> I suppose that it is an interesting idea, Winsor, but I don't think  
> in terms of a plane of focus; so tilting the plane of focus  
> (especially the miniscule degree that this chap is describing) is of  
> academic interest.
> 
> And in the example, the girl's eyes are going to be at the centre of  
> focus, with the depth of field chosen to include the other parts of  
> the composition that the photographer wishes to be in focus.  Again,  
> the plane of focus is academic for most people, I suggest.
> 
> Chris
> ~~ >-)-
> C M I Barker
> Cambridgeshire, Great Britain.
> +44 (0)7092 251126
> www.threeshoes.co.uk
> homepage.mac.com/zuiko
> 
> 
> On 9 Apr 2006, at 01:27, Winsor Crosby wrote:
> 
> 
>>That is not exactly what it says. It does say that using center focus
>>(split image or center sensor) and recomposing is not good. I
>>mentioned it before but this explains it much better:
>>
>>http://visual-vacations.com/Photography/focus-recompose_sucks.htm
>>
>>
> 
> 
> 
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