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10-12-2014, 04:22 AM   #1
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Measuring lens sharpness - some advice?

Hyy guys, since I'd right now have to learn for some exams and am therefore periodically frustrated and/or bored while sitting around at home quite a lot, I've started taking pics of a printed out (simple laserprint, A4-sized) sharpness table. I've already taken some pics with most of my K-mount (and two M42 adapted) lenses, and I'm glad to share the results, but at the same time I'd like to ask for some advice - first of all, while center sharpness is what I adjust the lens for, how can you make sure that the lens has a good chance of even picking up the peripheral sharpness at the same time? With the 28mm lenses (which don't exactly count as ultra-wides on an APS-C camera in my book), I'm already only about 50cm away from the sharpness table, and since A4 paper is about 36cm in diagonal, I arrive at a distance of 50cm to the center and squareroot((50cm)^2+(18cm)^2)=53cm distance for the edges. That is obviously a significant difference when it comes to focus at F/2.8, so I'm not too surprised to see that the periphery always looks blurry until I step down to f/5.6 or so. Is there a better way to measure peripheral sharpness than this?

Actually, is there some sort of accepted and simple-to-replicate testing method for this kind of thing?

In total I have tested the following lenses:
SMC-M 50/1.7
SMC-M 28/2.8 Version I
SMC-A 28/2.8
Sigma Mini-Wide I 28/2.8 (M42 adapted)
Porst Weltblick 35/2.8 (M42 adapted)
Rikenon 50/2
Cosina Cosinon-S 50/1.4
Pentax 18-55 WR kit lens (just for reference at wide-open apertures, since I just wanted to see how the tele side competes wide-open against some old primes)

Testing conditions were: Cheap tripod, artificial light only (did this last night at 1:00AM, after finally coming to the conclusion that I won't be able to shove much more into my head anymore), manual exposure set to same shutter speeds for each aperture (might have screwed that up every once in a while), manual focus through viewfinder+magnification function, tried several times with each lens until I found the best (or in some cases, the least blurry) focus spot.

Well, first batch of results for the Asahi SMC-M 50/1.7 coming up... First set of pics are center and periphery at F/1.7, second set is shot at F/4.

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Last edited by dec.net; 10-12-2014 at 04:51 AM.
10-12-2014, 04:25 AM   #2
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And here we have the Rikenon 50/2, at F/2 and at F/4:
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10-12-2014, 04:30 AM   #3
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And then the Cosina 50/1.4 at 1.4 and F/4... If I may say so myself, it looks like either I screwed up the focus or the lens is indeed quite soft, compared to it's more standard-apertured 50mm colleagues. Which I suppose is quite alright for a portrait lens.
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10-12-2014, 04:32 AM   #4
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And finally in the 50's game, the Pentax kit lens 18-55 WR, @55mm, F/5.6 - it seems a bit disappointing really, and yes I did manually focus the lens using the viewfinder+magnification method several times - maybe I got a bad example?

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10-12-2014, 04:38 AM   #5
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And now the 28mm battles. First up, the SMC-M 28/2.8, Version 1 (with the chrome ring). Sets shot at F/2.8 and F/5.6. As you can see, the peripheral focus is quite noticeably off on the 28mm lenses (same principle applies to the 50s, but there the difference in distance is smaller).
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10-12-2014, 04:42 AM   #6
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And here the newer-and-slightly-improved version of that same lens (it's certainly improved in the usability part) the SMC-A 28/2.8. I'll just go ahead and just post center sharpness pics for the 28mm for now. F/2.8 and F/5.6:
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10-12-2014, 04:46 AM   #7
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And last, the adapted-from M42 candidates. First the Sigma Mini-Wide I 28/2.8 at F/2.8 and F/5.6, then the Porst (Cosina-made?) Weltblick 35/2.8 at, again, F/2.8 and F/5.6:

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10-12-2014, 04:59 AM   #8
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QuoteOriginally posted by dec.net Quote
Is there a better way to measure peripheral sharpness than this?
Yes, give up on test charts, find a suitable office block etc. far away and on a clear day test your lenses at infinity at the centre and at the corners. From some simple trig you can work out resolutions etc. Or invest in an optical bench and some proper test gear.
10-12-2014, 05:22 AM   #9
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Alright, that's a good idea - unfortunately, the view outside my flat is pretty limited (I can see a dirt wall with unappealing shrubbery on top of it and that's basically it) - I've had that problem several times, no good way to check infinity focus on lenses when I'm at home - would have saved me some trouble with my first M42 adapter if I could have just stepped outside and focussed a building 1000m away instead of what I see . Obviously, I don't have too much ambition (or funds) to turn this into a real science, so the investment in an optical bench (I have no idea how that really looks like and what that costs, but since it's got the words "optical" in it and presumably isn't mass produced, I suppose it's expensive) would be a bit over the top. I don't really need an absolute measurement of lines-per-area, just a relative information such as "lens A is sharper wide open at the center but loses sharpness on the edges, while lens B starts with less center sharpness but has better overall sharpness throughout the picture" etc.. Well, at least I can get a general feeling of center sharpness from what I already got, I suppose. I guess I'll have to find a good "infinity" motive, maybe on the other side of the house .

Chris
10-12-2014, 05:33 AM   #10
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I agree with previously given advice. Instead of wasting time for these ( been there done that !) go out and snap away some real life shots. Then learn to post process them and share some online. To have a chance to get some meaningful feedback on quality of lens - you would need to upload full size jpg images out of camera with no post processing. And multitude of colors, shades, highlights and darks in one picture will be far more accurately describing the quality of any lens , than even best shoot test charts. Photography is about freezing the time and events - for that you really don't need anything apart one simple light tight box and decent lens better part of my life I spent having only kiev 4 and later stepped up to slr zenit with one helios 58mm f2.0 lens. That combo documented the events from my life that I care for the most ( my wife, my newborn 1st son , several weddings of good friends, some trips and places that I've been to, etc.. ) Do not waste any more time on tests. Just shoot - shoot your family members, your cat, your neighbour , your ugly neighbourhood, your way to school , to work, whatever... you wouldn't believe how much you will appreciate them in the future
10-12-2014, 06:32 AM   #11
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I agree that shooting itself is the most important test for any lens! Please don't mistake this as a project that I want to continously invest lots of time in - it's pretty much what I did late last night in my room to get my mind off that tedious and looming exam, and (mostly) to see whether my newly arrived SMC-A 28/2.8 was indeed an optical improvement over the older SMC-M 28/2.8 (I'd say, not really, they are very close from what I see here, which makes the decision to sell the SMC-M that much harder unfortunately). I got a bit of knowledge in post-processing already, that's why this is not much of a bother to me (those were all shot as DNGs with no post-processing applied whatsoever to keep the results comparable, just batch-cropped and converted to JPG in Photoshop afterwards btw.).

I have to say it gets hard at time to tell how good a lens actually was when you play around with sharpening and clarity in PP. Also of course, any lens looks like it takes better pictures when you like the motive, and for me it's quite hard to go shooting and take exactly the same motive twice just to have a good comparison between lenses (instead of looking out for something else I like) . Just felt the need to get some as-objective-as-possible measurements and do a bit of straight-forward productive work last night, that's all.

Chris

Last edited by dec.net; 10-12-2014 at 07:17 AM.
10-12-2014, 07:26 AM   #12
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Try shooting something with a lot of fine detail to resolve. For example, take pictures of a pet, a couch or sofa with a rough texture, portraits, a carpet, a piece of rough wood. Do your best so that it fills up the entire frame. Process all the shots with the same color correction, sharpening, etc. That ought to keep you busy.

The bigger question that I think people are hinting at is, "Does it matter?" If your images look good on your 15" to 27" monitor at the right viewing distance then you're in good shape with your camera and lens. Personally, I only print between 4"x6" and 8"x12" and every single one of my lenses is good enough for that with my K-30.
10-12-2014, 07:42 AM   #13
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Most landscape images are taken at apertures of f/4 and smaller, wide open performance of a lens isn't all that critical for landscape photography.

QuoteOriginally posted by kh1234567890 Quote
Or invest in an optical bench and some proper test gear
...which will not be cheap at all.
10-12-2014, 08:29 AM   #14
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I used to do something similar - until I got bored with it.
10-12-2014, 09:49 AM   #15
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Using your printed test chart is fine, but you should work at a greater distance. You are not challenging the resolution of most of your lenses. If you have magnified live view or a well-calibrated split image focus screen, use that for focus. The camera focal plane should be parallel to the plane of focus. If not, your corner evaluations have little meaning.

If you can find instructions for your test chart, you should be able to calculate actual resolution values as well as evaluate for various aberrations.


Steve

---------- Post added 10-12-14 at 10:14 AM ----------

Here is a good reference for testing optical systems:

http://www.edmundoptics.com/capabilities/imaging-optics/imaging-resource-guide/


Steve

Last edited by stevebrot; 10-12-2014 at 09:58 AM.
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