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Re: [OM] Comparing 50 mm lenses

Subject: Re: [OM] Comparing 50 mm lenses
From: Moose <olymoose@xxxxxxxxx>
Date: Mon, 21 Nov 2011 14:12:08 -0800
On 11/21/2011 6:41 AM, Chuck Norcutt wrote:
> Still much difference in brightness and very difficult to compare.

What are you trying to compare? Although not what I was looking for when I took 
these test shots, a clear result IS the 
differences in 'exposure' or 'brightness'

I was looking first for differences in bokeh. I am not happy with the bokeh of 
many modern lenses, and was curious about 
an older, non-symmetrical design with fewer elements. Second, I was also hoping 
to find differences in that elusive 
quality sometimes discussed here and elsewhere, 'drawing', 'rendering' and 
similar fuzzy terms. Third, I was interested 
in any obvious difference in sharpness.

Those weren't what I found. Bokeh, at least at these apertures and 
subject/background distances, isn't much different. 
"Rendering"? I can certainly see the difference between the soft, but somehow 
sharp rendition of good LF portrait lenses 
and the edgy sharpness of small format lenses. But I saw no real difference in 
that area here.

Sharpness? Although I mis-focused a bit, so that the Tessar didn't have the 
same focal plane as the others, with the 
limited parameters of this test, I don't see any significant differences. With 
the 60D and FF coverage lenses, I'm also 
not looking at the whole output of the lens. It would take quite a different 
test target and set-up to really test that. 
It's also useful to remember that 'sharpness' isn't a measurable thing. It's a 
poorly defined combination of resolution 
and edge contrast. With contemporary PP tools, modest edge definition and edge 
contrast failings in a lens are 
relatively easy to correct.

What I did find was interesting and complex differences in luminance 
distribution and in color. In my initial 
processing, I was careful to distort the lens response as little as possible 
while compensating for differences in 
mechanical aperture accuracy, which is secondary to optical qualities.

So I used Exposure in ACR to put the top of the recorded image data just at the 
top of the histogram, 'kissing the 
post', and set Blacks to zero, with no other adjustments like Recovery, Fill 
Light, etc. to distort the histogram.

That approach "failed", in that the central tones were in different positions 
relative to the ends of the histograms. 
But it was a success in that it showed that these very different lenses 
actually distribute the light that travels 
through them differently. Exposure to place midtones in the center would work 
well with one and result in blown 
highlights with another.

Then I went a step further. I used the Filter=>Blur=>Average tool to measure 
the average brightness of each image and 
adjusted the midpoints of the histograms in Levels to make them all the same 
average brightness. I can't think of a more 
accurate measure of equal brightness than that.

And still you see "much difference in brightness". And yes, they do appear 
different, but I submit that the difference 
is in the distribution of luminance and color, not overall brightness. Move 
between them looking at different details. 
What I see is differences in the relationships of nearby brightness and color 
in areas much smaller than the whole image.

My conclusion so far is that there is more difference in these details between 
lenses than I would ever have imagined. 
Perhaps I shouldn't be surprised. They are very, very different lenses, a four 
element design from early in the history 
of lens design with single coating, a multi-coated, six element, relatively 
modern version of the old double-Gauss 
formula and a recent 11x zoom with 15 elements, including two kinds of exotic 
glass, aspheric surfaces and much more 
highly developed multi-coatings.

> Maybe it's a hopeless task.  Pick the one you like.

Not hopeless; the differences you see ARE the differences in the lenses, not 
some failure of exposure. And indeed, it's 
"Pick the one you like." That's the point of such a test, isn't it?

I like the Tessar best, at least for this kind of subject. But I am happy that 
the Tamron is very close, perhaps even 
more accurate, and that's what I use day to day.

Testy Moose
-- 
What if the Hokey Pokey *IS* what it's all about?
-- 
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