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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