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Re: [OM] 100 shootout

Subject: Re: [OM] 100 shootout
From: Dawid Loubser <dawid@xxxxxxxxxxxxxxx>
Date: Fri, 30 Sep 2011 10:43:11 +0200
I wonder if my interpretation is too simplistic, but to me it seems:

- The entry pupil of two 100mm lenses at f/2.8 should be two holes of  
exactly the same size, right?
- Light travels in a straight line, right? (in the absence of a black  
hole...)

This means that a "cone" of identical volume is formed between a point  
in the image field, and the entry pupil of the lens, for *any* lens of  
any design whatsoever, of equivalent focal length and aperture.

The diameter of this cone - at any point in front of or behind the  
image field - dictates the *magnitude* of defocus (not the "quality"  
or "smoothness", etc), but simply "how much it is out of focus" (size  
of airy disc).

Surely the magnitude of defocus in front of, and behind, the image  
field must be the same, because light travels in a straight line. So  
too, assuming the position and size of the entry pupils between any  
two 100mm lenses are the same, the magnitude of defocus must be the  
same between any two 100mm lenses at f/2.8?

The *rendering* of the defocused area (smoothness) has a great effect  
on the perceived magnitude of the defocusing, though. For example, a  
Zeiss C-Biogon 35mm f/2.8 is renowned for looking like a 35mm f/2.0  
lens, because of the supremely smooth and even OOF rendering.  
Similarly, the Zuiko 50mm f/1.4 actually looks more like an f/2.0 lens  
because of the less smooth OOF rendering, etc.

Am I wrong?

Dawid


On 30 Sep 2011, at 4:16 AM, Wayne Harridge wrote:

<snip>

>
> So perhaps the floating rear elements have some effect on the DOF ?
>
> ...Wayne
>
>>
>> Do either the 100/2.8 or 100/2 use a component of internal  
>> focussing ?
>>
>> ...Wayne

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