Manuel Viet wrote:
> Le lundi 06 Novembre 2006 14:02, Chuck Norcutt a écrit :
>> here's Khem when we need him? Why did Olympus decide to produce a 40mm
>> f/2? Optimum focal length for producing a low cost moderately fast
>> normal lens? If so, why was that Zuiko 50/1.8 the "normal" lens.
>>
> I seem to remember reading an interview where Maitani stated that he wanted a
> full line of lenses at f/2, and was using the 40mm himself. Ah, there it is :
>
> http://www.geocities.com/maitani_fan/om_interview_2.html
In a broader and more technical sense, a 40/2 'pancake' lens makes
eminent sense, and they have been made by others than Oly. The distance
from film plane to lens mount flange for most 35 mm MF SLRs is between
42 and 46.5 mm, with OM at 46 mm.
As lens focal length gets shorter, there comes a point where a
conventional, semi-symmetrical lens design like that used for almost all
'normal' lenses in the 50 mm range, can no longer be used, as the rear
element would get hit by the mirror. This point is at around 40 mm. So
down to there, the shorter the focal length, the smaller the lens can
be. (F2 @ 40 mm requires a 20 mm front element; a 50 mm lens requires 25
mm, 85 mm=>42.5 mm.)
Beyond that point, wide angles require retrofocus optical designs, which
locate the rear node behind the actual last element. These get bigger
and more complex, so around 40 mm is the smallest lens that can be made
for SLRs. Interchangeable lens RF cameras don't have mirrors, so the
lens can project back almost to the film. As a result, their lenses
shorter than 40 mm are smaller and lighter and optically simpler than
comparable SLR lenses.
As to why f2, why not? Faster retrofocus designs are complex, large and
expensive compared to slower ones. The original OM WAs are relatively
slow for those reasons. Most lenses using the same basic optical formula
used in 40/2s are actually faster than that. Take a look at the
similarity in optical layout of the 40/2 with the 50/1.8 and 1.4. So f2
at 40 mm isn't any big deal, nor anything like the achievement it is at
35 mm, let alone 21 mm.
Moose
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