[rollei_list] Re: xenar test roll is back

  • From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
  • To: <rollei_list@xxxxxxxxxxxxx>
  • Date: Wed, 3 Feb 2010 22:53:04 -0800


----- Original Message ----- From: "Stephen Attaway" <stephen@xxxxxxxxxxxxxxxxxx>
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Wednesday, February 03, 2010 10:10 PM
Subject: [rollei_list] xenar test roll is back


Hi all ...

Got the test roll I shot last weekend back. This was with my 'cord IV with Xenar.

I did three wide open pix - good contrast, color and focus was spot on - but frankly a bit soft.

Especially compared with the results stopped well down (f11-22). Sharpness was really excellent and held well into the picture edges. Contrast was moderate, giving a pleasing, classic drawing.

In fairness, I think there was just a touch of camera shake in the wide open shots, because I was at 1/50 and am not quite used to the 'cord shutter release. Later, in soft sunlight, with Provia 400x, I was up at 1/100 and 1/250 - and my handheld shots all looked like the 'cord had been bolted to a granite plinth.

I even tried the Xenar with a Mutar 1.5x and bay I rings. The sharpness of this combination was also very impressive, but contrast had dipped, perhaps partly due to a touch of underexposure. I had opened up about 1/3 to 1/2 stop with the Mutar (I get good results that way with my Planar), but should have opened up 2/3 of a stop to a full stop on the Xenar.

I'll post a few 100% crops from my scans this weekend, in case any of you are also pixel peepers like me.

My conclusion: the Xenar is a fine performer stopped down but I'd be reluctant to open it up past f5.6. Its really splendid at f11-22. The 'cord IV is a very good match for Provia 400x, which is good because I bought way too much of that stuff when it first came out.

And the 1.5 mutar is quite usable with the old 'cord, if you can overlook the really nasty vignetting in the ground glass. Just a touch of vignetting in the actual picture, though.

But I think I'll be using the 'cord IV more as a lightweight, less imposing option to the 'big ride' for daytrips, with a minimum of accessories. Its obviously capable of really first class results, if you are aware of its limitations and respect them.

And obviously there is nothing wrong with my Xenar ... no need to trade in or trade up.

Thanks for everyone's help, advice and wisdom. I've learned a lot the last few days.

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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