How Does One Test Their Lenses ala Perez/Thalman??

[Scott Rosenberg]

good day...

i thought i remembered reading on this site a thread or article detailing the procedure one should follow to test the resolution of their lenses. however, search as i might, i am coming up empty. does anyone have a link to this information, or if i'm not remembering correctly, how would one go about testing their lenses? i'd like to run tests similar to the ones kerry thalmann and chris perez did for my lenses.

thanks,
scott

[Ernest Purdum]

The usual method is to obtain a lens "chart". There are several designs, but they all have series of patterns iin sizes which can be related to lines per inch or millimeter. To use it, you fasten several (at least five) copies of the chart onto a flat board, place it at a specified distance from the camera. You then take photographs and carefully examine the result.



There are many potential problems with this approach. Your board has to be very flat and placed accurately perpendicular to the camera. Your film must be perfectly aligned with the groundglass and the groundglass must be perfectly perpendicular to the lens. Over or under exposure will prevent accuracy. The contrast of the film and development has an affect on the results. Partly as a result of the above, and partly because of recognition that resolution is not the only factor affecting perceived "sharpness", other methods of testing have been developed.

One of the popular charrts is the United States Air Force (USAF) pattern. Thbe Air Force once did some uncommon testing by painting a version of their test chart onto the fuselage of a U-2 airplane, usually used for taking photographs of ground targets from extremely high altitiude. In this instance the process was inverted, the patterns on the aircraft being photographed from the ground.

[Scott Rosenberg]

ernest...

thanks for the information. it sounds like the first method of testing would bias the results in favor of lenses intended for close-up and 2-D copy work, as the test procedure closely resembles what they are optimized for. not to disparage those optics, but i wonder if this gives lenses like the fuji a series and the schneider g-claron series an advantage on these types of tests. please don't flame me for that observation... i have and love two fuji A lenses (the 300 and 240).

is there standard way to test lenses using 3d objects at infinity? it sounds like simply taking the same picture with a variety of lenses and then comparing the fine details, such as letters on a distant sign, shingles on a roof, or some other small detail, might give results more in-line with how i actually use my lenses.

[Dan Fromm]

Shooting distant subjects, which you suggested as an alternative to shooting test charts closer than lightyears away, is one of the things I do to help me decide which of my lenses not to use. But it doesn't give resolution numbers that can be compared with other peoples' tests and it is vulnerable to inaccurate shutters, focusing errors, use of different emulsions for different shots, ...

The standard approach with test charts, probably USAF 1951, at 50 focal lengths away is not biased towards performance closeup. 50 focal lengths away isn't that far from infinity. And it has two great advantages. It gives a better sense of just what each lens tested can and can't resolve than shooting at subjects that don't have many, many scales of relatively fine detail. And it allows relatively easy comparisons with other peoples' results.

I started testing macro lenses formally by photographing a microscope stage micrometer that had 100 marks/mm. Your idea. This was somewhat informative, but I got a better sense of what my lenses couldn't do by using a USAF 1951 on glass target. Similarly, if you look at Klaus Schmidt's pictures of microchips -- see www.macrolenses.de -- you'll see that they really aren't as informative as one could want.

As is often the case, quick and dirty lens tests are easy but not always the best. If all you want to know is which of your lenses not to use, quick and dirty will serve. But if you want to know more than that, you should ask the question more formally.

Cheers,

Dan

[Emmanuel BIGLER]

Scott. May I foresee a (small) problem using real objects at infinity in a landscape, i.e. potential blur due to haze or other atmospheric effects. I like to test film & lenses with my favourite urban landscape, I just have to open my windows at home and take a picture of the old roofs of my old city, plus the cathedral's clock ;-) this is more fun than even the French CSP target (the French equivalent of the 1951 USAF target).
BTW when the roofs have been renewed recently it is an excellent test for moiré effects in digital inkjet prints ;-);-) older roofs being less regular are less prone to moiré ;-) More seriously, I have noticed that old rows of tiles when they look super sharp on a print hardly ever exceed the value of 40-50 lp/mm on film, as far as the periodicity of the tile pattern is concerned. Going back home, the advantage of doing tests indoors is that you get rid of potential atmospheric blurring defects ; however indoors you have to test your lenses at a finite distance.
50 times the focal length is considered reasonably far away for general-purpose lenses optimized for infinity.
However I remember reading some posts on photo.net by Kornelius Fleischer who insisted on taking at least 100 times the focal length for proper testing of high performance long tele lenses. But he insists on finding 200 line pairs per millimetre, so ;-);-)
May be a sports hall would be the best place.. except that you have to properly lit the targets ! so testing outdoors with a generous sun solves the question of an homogeneous arrangement of artificial lights, something tricky to properly adjust if you want all your targets to be evenly illuminated.

[Bob Salomon]

Not to mention the variables that lead to less then perfect comparisons:
Lighting, are the emulsions the same, developing identical, atmospheric conditions identical (dust or smoke in the room), loupes for evaluating results the same, eyestrain or lack of it when examining, shot at optimal aperture on each lens, developing time identical, chemistry dilution identical, etc.

There are so many variables that it is almost impossible to repeat the test with the same lens and chart a few months later and obtain the same result.

[Ralph Barker]

As has been suggested by the other responses so far, there are both a variety of methods and a number of variables to consider. I would suggest, however, that perhaps the first thing you need to determine is what you want the tests to accomplish, Scott. If you want to compare your results with those of Kerry Thalmann or Chris Perez, so you can see if your lenses are in line with those they tested, then I think you need to duplicate their testing methods in order for direct comparisons to be valid. In contrast, if you simply want to make comparisons between your lenses under typical shooting conditions, a different methodology would be appropriate.

For the latter style of testing, you might, like Emmanuel, select a convenient and consistent target, and do test images where certain factors (e.g. magnification and film/developer) are kept consistent. The "best" approach depends on what you want the tests to accomplish.

[sanking]

Another consideration in testing of this type is that there may be slight differences in the T-dimension of your sheet film holders , or in how close to the film plane your roll film camera is able to hold the film. Or your groundglass may be off with respect to the film plane. These differences may seem inconseqeuntial but in practice they can easily skew the result of your tests and suggest incorrect differences in the performance of your lenses.

[Scott Rosenberg]

thanks for all the information...

the piece about testing at 50 focal lengths is the one is was missing. i guess i should have been more clear in my original post - i don't want to compare my results to those of kerry or chris, as whatever results i conclude from that comparison would not be statistically valid. there are too many sources of variation that can not be accounted for. furthermore, there is no information as to the gauge reliability / repeatability of the study chris and kerry conducted. comparing my findings to their's would not reveal any useful information... it would be like two people using non-matched scales to measure the same distance - it's a fools errand to be sure.

i was looking for a way to test my lenses, and possibly a friend's lenses as well, controlling as many of the variables as possible ... using the same film holder, the same camera, the same lighting, the same film, equivalent magnifications (now i know it's 50 focal lengths), etc. i would only vary the lenses in question and do my best to keep all other variables constant. this would yield data for my particular set of lenses only, drawing comparisons to anyone else's tests would be invalid, as many of you have noted, these results are driven as much by outside variables as the lens on the camera, variables that chris and kerry controlled using methods of which i have no knowledge.

if i wanted to venture a little experiment, where does on obtain the test charts?

thanks,
scott

[Glenn Kroeger]

Scott:

Edmund Scientific used to (I haven't checked lately) sell a 24x36" chart with AF test targets arranged in all possible orientations and also in each primary color. This is the one I use, dry mounted to a gator board.