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[OM] Re: Apertures, where is f22 ?

Subject: [OM] Re: Apertures, where is f22 ?
From: Moose <olymoose@xxxxxxxxxxxxxxx>
Date: Tue, 07 Jun 2005 13:50:26 -0700
Der Eiserne Reiter wrote:

>List,
>
>let me offer a new technical subject:
>
>I have often wondered why some lenses in the 28-50mm focal range do
>not offer an additional f stop. I understand that is is technically
>difficult to provide both wide and small apertures, and good bokeh,
>but for landscape photography, f22 is very helpful sometimes.
>
>Why do some lenses stop at f16 and others continue to f22 ? Do other
>people care about f22 ? For instance, the 50/3.5 features f22, while
>the 1.2, 1.4, 1.8 do not.  Technically it is feasible to get f22 with
>a wider lens, as was demonstrated by C*non in the 50s with their
>50/1.2 lens. Why do the 35/2.8, 28/3.5 and 28/2 not have f22 ?
>
The problem is diffraction. Theoretically, any edge diffracts some light 
that passes very close to it. The classic photographic example is the 
pinhole lens, where a very small hole act as a lens with no glass at 
all. As an aperture gets smaller, it starts acting more like a pinhole 
lens element, throwing off the optical formula and starting to decrease 
resolution and contrast.

Since this is a phenomenon of the physics of light, it is independent of 
focal length and depends only on the wavelength of the light and the 
size of the hole. The effect is not dependent at all on the focal length 
of the lens, but the size of opening for a given f-stop is dependent on 
the focal length. So, for example, a physical aperture size that gives 
f22 on a 200mm lens gives f5.6 on a 50mm lens. (With complex lens 
designs, this isn't exactly true, but the generalization is accurate 
enough.)

If you read Gary's lens tests, you will see that he comments in many 
cases that resolution at the smallest apertures of lenses is 
"diffraction limited". What that means is that sharpness starts to get 
worse at these apertures than wider open. The choice by any lens maker 
of the minimum aperture to put on a lens will depend on a number of 
factors, including the trade off of benefits from increased DOF vs. the 
drop in sharpness from diffraction and features/marketing 
considerations. Just because someone makes a 50m lens that stops down to 
f45, doesn't mean it will take sharp pics at f45.

The 50/3.5 has its best performance at f8-11 and declines below that. 
F22 is available, but shouldn't be used for critical work. If you look 
at all the 50mm tests, you will notice a distinct tendency for f16 
ratings to be lower than wider apertures. The 135/4.5 macro offers 
apertures down to f45, but the performance slide starts at f22 and by 
f45 is really iffy, worse than the 50/3.5 at f22.

Take a look at small sensor digicams. Many have no apertures below f8 
for the same reasons.

Moose


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