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01-10-2014, 03:18 PM   #61
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QuoteOriginally posted by fyby Quote
Really? How different sensor can change plane of sharpness of lense? Easiest explanation is that there was need to microadjust af in case of K3.
the answer is: they are not measuring "sharpness" -- they are measuring acutance:

From Wikipedia: "In photography, acutance is the edge contrast of an image. Acutance is related to the amplitude of the derivative of brightness with respect to space."

Michael

01-10-2014, 03:21 PM   #62
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There's definitely something fishy going on when they test these lenses on Pentax cameras. The following screen grab where the 35/1.4 "A" was tested on K-5 and D7000 show completely different MTF curves, both S&M (pardon),

I don't buy that explanation about getting the most out of a sensor (These two cameras have the same sensor, right?).

These sensors don't have micro-lenses that act differently toward the edge of the sensor, and I am assuming the lens is of a telecentric design, so it's doubtful the sensors would effect the outcome as much as is shown (unless they had the previously said microlenses). Sigma also offers a lens mount conversion service, which doesn't change any optical elements to my knowledge, so I doubt any major optical tweaking takes place when designing for "normal" sensors. (that service may be a con as they may just swap out your lens instead of converting it)



Last edited by bossa; 01-10-2014 at 03:53 PM.
01-10-2014, 03:33 PM   #63
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Adding in a Canon 70D it's easy to see the MTF curves are similar to the Pentax apart from being a tad "sharper".

All lenses seem to have variations though.. I wonder if it is just Sample variation at work.



Last edited by bossa; 01-10-2014 at 06:57 PM.
01-10-2014, 03:43 PM   #64
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QuoteOriginally posted by bossa Quote
Adding in a Canon 70D it's easy to see the MTF curves are similar to the Pentax apart from being a tad "sharper". All lenses seem to have variations though.. I wonder if it is just Sample variation at work.
what's up with that Nikon aperture shown in the middle of the "compare 3" image? It is showing f/4??? Can't be right.

M

01-10-2014, 06:49 PM   #65
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QuoteOriginally posted by MJSfoto1956 Quote
what's up with that Nikon aperture shown in the middle of the "compare 3" image? It is showing f/4??? Can't be right.

M
an accident.

Sorry, it's corrected.

Last edited by bossa; 01-10-2014 at 06:58 PM.
01-10-2014, 07:33 PM   #66
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well, my experience is that lenses + sensors don't always perform the way you expect them to. Here's a test. The following image shows three "sharpness/CA" tests for the SAME LENS ON THE SAME BODY (Sigma 18-35mm on Nikon D7100). If I give you four different f/stops to choose from can you guess which of the three f-stops illustrated (A, B, C) is which? Your choices are:
  • f/2
  • f/4
  • f/8
  • f/11

Have fun choosing!!! (btw, there is lots to learn from this exercise. One of which there is no such thing as an "optimal" f-stop)
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01-10-2014, 08:52 PM - 1 Like   #67
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Before I go and have a look at the page I'll guesstimate:

A = f/2
B = f/8 or f/11
C = f/5.6

My choices are related to the sensor and diffraction limits more than the lens.

After looking on the site I see that I am wrong on all counts. (well, 1/2 right on one anyway)

Last edited by bossa; 01-10-2014 at 09:01 PM.
01-10-2014, 09:12 PM   #68
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QuoteOriginally posted by bossa Quote
After looking on the site I see that I am wrong on all counts. (well, 1/2 right on one anyway)
LOL! there is a lot to learn! I am fascinated by the "trends" the data show. One might quibble with a single result, but over all the trends are quite revealing. And not entirely what I expected. For example, for certain kinds of subjects, this lens might be better to shoot slightly below its so-called diffraction limit since at f/11 it delivers a remarkable even sharpness center to edge. Who would have guessed that?

Michael

01-10-2014, 09:32 PM   #69
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Diffraction, the new AA!

QuoteOriginally posted by MJSfoto1956 Quote
LOL! there is a lot to learn! I am fascinated by the "trends" the data show. One might quibble with a single result, but over all the trends are quite revealing. And not entirely what I expected. For example, for certain kinds of subjects, this lens might be better to shoot slightly below its so-called diffraction limit since at f/11 it delivers a remarkable even sharpness center to edge. Who would have guessed that?

Michael
Exactly!
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