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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