[rollei_list] Re: xenar test roll is back
- From: CarlosMFreaza <cmfreaza@xxxxxxxxx>
- To: rollei_list@xxxxxxxxxxxxx
- Date: Thu, 4 Feb 2010 05:13:08 -0300
My Rolleicord IV Xenar is very sharp at f4, the portrait I sent for a
postcard exchange to some members on this list too was taken with this
camera (camera hand-held) and lens at f4 and they can judge it, I
obtained bigger optical prints from that neg keeping the sharpness;
anyway I agree f8-f11 are excellent for this lens.
I never used this lens at f3.5.
Carlos
2010/2/4 Stephen Attaway <stephen@xxxxxxxxxxxxxxxxxx>:
> Hi Richard:
> f8 - f11 for the Xenar sounds right. Thanks!
>
> --------------------------------------------------
> From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
> Sent: Wednesday, February 03, 2010 10:53 PM
> To: <rollei_list@xxxxxxxxxxxxx>
> Subject: [rollei_list] Re: xenar test roll is back
>
>> Stephen
>>
>> FWIW, the main residual aberrations in a well-designed Tessar type lens
>> are all reduced by stopping down. For many lenses of "normal" coverage the
>> residual aberrations are essentially gone over the field when stopped down
>> about two stops. For an f/3.5 lens this is around f/7 so stops of about f/8
>> or smaller should be quite good. The "optimum stop" may be somewhat smaller,
>> probably about f/11. Beyond this the loss of resolution due to diffraction
>> begins to become significant. This is the reason that lenses on small camras
>> rarely stop down beyond f/22 while those on large cameras often go to f/32
>> or even f/45 to f/64. At f/64 a Rollei lens would be very blurred.
>> Beside the improvement in sharpness there may also be some improvement
>> in contrast as the optimum stop is approached. This is because spherical
>> aberration and its relatives like oblique spherical and coma also tend to
>> scatter some light and reduce contrast. For the most part well-designed
>> Tessars do not have much residual spherical, mostly they have some oblique
>> spherical. This is similar to coma and, like coma, increases with image
>> angle and decreses as the lens is stopped down.
>> Adding more elements is a way for the designer to control higher order
>> aberrations. For fast lenses, or wide angle lenses additional elements over
>> a Tessar are necessary if the lens is to have good performance and decent
>> speed. Note that some quite simple lenses (wide angle Protar) will cover
>> surprizingly wide angles (around 100 degrees) but onlyl at very small stops
>> (c f/64).
>>
>> --
>> Richard Knoppow
>> Los Angeles, CA, USA
>> dickburk@xxxxxxxxxxxxx
>>
>>
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