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02-27-2014, 02:31 AM   #1
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Significance of innaccuracy of focus in lens testing

So I recently set about testing a bunch of 135mm lenses: a M 135, K 135 3.5, Tamron 135 2.5, and a Kiron zoom 70-150mm at 135. The results surprisingly showed the M 135 coming in with the highest score using the QuickMTF software in trial mode. The set up was a home made slant edge test, using black paper on white paper at about 5 degrees from vertical.

Its quite a clever testing procedure as it gets around a problem with simple tri-bar target tests in that if the multiple of focal lengths is not the same on the bar tests, you are looking at different pairs of bars for the same resolution which is not fair. Its also a very quick thing to do with a digital camera. I really need to go back to it and improve the size and stability of the target, and get more consistent lighting. The solution is to mount the target on a wall and then illuminate it with a pair of flashes on flash stands in manual mode.

For anyone interested, there is a chapter towards the back of the Manual of Photography (9th ed) which covers image formation, and explains point spread functions, line spread functions and leads to understanding of how a single slant edge can enable the calculation of a lens' MTF curve. Its really very elegant.

But the initial result which strikes me having done this test is that focus accuracy dwarfs the difference between the primes. The Kiron zoom was substantially beneath the primes at f4, by a large margin, which means its not a contender, even when everything else is controlled. At f4/f3.5, the primes are all pretty similar, so that the inaccuracy in focus in my opinion is more of a consideration than the difference between lenses...this is not to say that the results I have previously obtained for tri-bar tests are wrong in terms of overestimating lens performance, but rather that its possible to get more out of a lens than I think I am getting. I begin to understand why Zeiss insist on continuing to produce manual focus lenses.

So for the next time I end up comparing lenses, the really important bit of the testing will involve bracketing the focus, and taking the best shot, in order to account for inaccurate focussing.

There are a couple of programs which are free alternatives to Quick MTF, called MTF calculator and MTF Mapper. In my opinion, for relative comparison of MTF's, MTF calculator is easiest to use.


Last edited by whojammyflip; 02-27-2014 at 02:36 AM.
02-27-2014, 03:54 AM   #2
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QuoteOriginally posted by whojammyflip Quote
The results surprisingly showed the M 135 coming in with the highest score using the QuickMTF software in trial mode.


Keep it quiet, once the Chinese lens hoarders discover that the M135/3.5 is actually very good eBay prices of this little gem will rocket from the current rock bottom of 10-20.

Is there a link to your test method anywhere ? Sounds interesting.
02-27-2014, 11:48 AM   #3
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QuoteOriginally posted by whojammyflip Quote
So for the next time I end up comparing lenses, the really important bit of the testing will involve bracketing the focus, and taking the best shot, in order to account for inaccurate focussing.
I would think focussing manually with magnified live view would be your best chance to ensure consistent accurate focus.
02-27-2014, 01:19 PM   #4
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QuoteOriginally posted by johnyates Quote
I would think focussing manually with magnified live view would be your best chance to ensure consistent accurate focus.
I have a split screen from focusingscreen, on a KM. The KM is not the best viewfinder, but a split screen is pin point accurate. You have to verify its shimmed correctly by shooting a ruler at a 45 degree angle, running away from you, and confirm that the visual focus corresponds to the "chemical focus"...this is an issue from the darkroom too, where light our eyes are sensitive to is not the same as light the camera is looking at. I dont think the KM does live view. I dont use it for anything other than digitizing negatives and lens testing

We have vision which peaks in the yellow/green area. If you want to do this all really accurately, you would use a beautiful old MX with a great big viewfinder, a yellow green Hoya X0 filter on the lens, and ADOX film with resolution which is higher than the lens. A yellow green filter ensures the film is being exposed to what your eye is sensitive to. You could do the filter on digital too, but it would muck around with the demosaicing, I think.

---------- Post added 27th Feb 2014 at 20:31 ----------

QuoteOriginally posted by kh1234567890 Quote
Is there a link to your test method anywhere ? Sounds interesting.
I can post a link later when I have made a new and improved slant edge...rather than using black paper, I am going to use a light trap, so that there is no possibility of a shadow affecting the sharpness of the image edge. The idea is that a slant edge on a gradient gradually transitions across pixels on the sensor, as you move down the images beneath, the slight angle of the image means that the high contrast edge moves slightly relative to the grid of the pixels. The processing finds the gradient, rotates the image so that it is vertical, and then finds the average change in pixel value in the transition from left to right on the image. This is a digital equivalent to calculating the accutance. If the lens does not attenuate the sharpness of the original object, then the gradient would be completely flat until the transition where it would peak, and then it would be completely flat again. Practically, the gradient is kind of wave like. From the wave that is formed, a calculation is performed which works out what the lens can see in terms of sharpness...what frequencies have to be present in that wave, and how strong the frequencies are. The sharper the gradient, seen in the image, the greater the presence of the highest frequency waves...ie the higher the resolution capabilities of the lenses. Again, whats nice about this is that its not really distance related (a line looks like a line wherever you are standing, whereas a pair of lines doesn't), and thus does away with Edmund resolution targets. On the flip side, it involves a bunch of software and is less intuitive. I hope the explanation above is not too much like voodoo.

Its literally a simple question of generating images like these:

Pentax M 135 3.5 at 3.5



Pentax M 135 3.5 at 5.6


In the first image, its clear to see the colour fringing. The second image at f5.6 is sharper. Quick MTF said that for the 30% contrast level, the resolution was 1650LPH, in photozone.de speak...and this is with a dodgy test target....if I get a guillotine to ensure that the edge is dead straight, its possible this will be even higher. However, if you download the images and run QuickMTF now, you wont get the same results, as I have cropped the images. However, you will get .335 cycles per pixel...there are 172 pixels per mm, so this means 58lp/mm. This highlights the benefits of something like an Edmund test chart, in that you can see the bars with your eyes, and read off the resolution from a conversion chart. I guess this is work in progress. Aside from absolute lens resolution measurement, its useful just to go through the process comparing lenses to understand that the difference between good lenses is probably less than the variation in my ability to take a sharp shot, even when I am trying hard.

Software:
http://www.quickmtf.com/
http://mtfmapper.blogspot.co.uk/
http://sourceforge.net/projects/mtfcalculator/

MTF calculator is the quickest, but you can only use it for relative measurement. MTF mapper looks interesting but I have not had it working yet. Quick MTF is probably the most refined thing I have found, but its pretty expensive in comparison to other software...its quite tempting for me to actually write something myself. There is some shareware called imagej, and I might try taking the code into R.

PS, I am pretty sure my neighbours are wondering what the hell I am doing taking photos of black and white wedges!


Last edited by whojammyflip; 02-27-2014 at 02:46 PM.
02-27-2014, 09:05 PM   #5
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QuoteOriginally posted by whojammyflip Quote
But the initial result which strikes me having done this test is that focus accuracy dwarfs the difference between the primes.
I have found this too. Since setup takes most of the time, I repeat the whole test three times, then choose the best-focused shot for comparison. Sometimes all three shots are fine, mostly at least two. If I have a lot of trouble deciding exactly where the best focus is, that lens often has low contrast or coma or something.

I use a split prism, live view and if it's an AF lens, CDAF. They all work. CDAF is consistent but no good for Pentax-M lenses.
02-27-2014, 09:11 PM   #6
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My M135/3.5 is one of the sharpest lenses I own. It is a fairly easy lens to focus and the viewfinder seems brighter than other lenses that are also f/3.5 at their widest opening. I used it at my daughters college graduation over a couple of AF zooms that probably would have seemed to be a logical choice. I felt that confidant in my ability to get the focus right. Add to that the compact size, built in hood, and 49mm filter size and I think it's one of the best lenses you can buy for a cheap price. Because of it's size, it's always in my bag. I have always hoped the Pentax engineers would find a way to make an AF version and keep it the same size. A 135 Limited maybe? One can always dream!
02-28-2014, 07:57 AM   #7
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I'm glad I picked up a M 135 several years ago.
It's a very nice little lens indeed.
02-28-2014, 12:52 PM   #8
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Picked up an M135/3.5 30 years ago. Keeping it. Just too good to go.

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