gasrocks wrote:
I think Paul is just setting us up to buy his new lens testing rig.
Why let the cat out of the bag? I could have let you in on the deal. I have 200 pinholes on order from a Chinese manufacturer....!!! Already have a box of paperclips here, so I'm saving a bundle there. Got a source for cheap white rulers?
Learn something new every day, and of course this makes perfect sense. However, it's demonstrably moot for testing wide-to-normal camera lenses. This is a uninterpolated 400% crop:
This test was produced with an OM Zuiko 28mm f/2 at f/2 on a 4/3" camera. The crops are from the extreme bottom right corner of the frame. The "real star" was Vega, and the "artificial star" was an incandescent Maglight with a pinhole taped to the front bezel. The pinhole was constructed of a piece of .002" brass shim stock with a roughly 0.5mm hole poked through it with a pin. Vega was very distant, and the Maglight was roughly 25m from the camera.
Only the brightest few stars were visible to naked eye, under typical Vancouver conditions. No tracking mount was used, which necessitated higher ISO, hence the grainy shot.
I couldn't find my LED Maglight, so the red-biased incandescent lamp's spectrum has made the cyan side of the CA hard to discern in that shot.
The test demonstrates that this lens shows red/cyan lateral CA, and, at least wide open, quite a bit of Coma. It also shows that no specialized equipment is required to get useful data out of this kind of testing.
Do you really need to be that far away from the pinhole? A really high resolution camera has got ~5µm pixel c-c distance, on a 50mm lens that gives that a 0.5mm hole (normal pinhead needle) only needs to be 0.5*50/0.005mm = 5m away before the pinhole is smaller than a single pixel in the camera...
A 0.5mm hole is easy to calculate minimum distance to, 28mm gives 2.8m, 35mm gives 3.5m, 85mm gives 8.5m and so on.
The comatization in the above example actually makes the Lateral CA much harder to judge, as Coma is often corrected very differently for the three different primary wavelengths. For lateral CA, the USAF target, or even any other high-contrast pure black/pure white edge will give more pictorially relevant information - without making things to hard on the more non-scientific target users who only want an overview of classic lenses' CA behaviour.
LoCA smudges the information available in a lateral CA pinhole target so much that you almost have to deconvolve the channels separately to get a good overview of what's going on.
theSuede wrote:
Do you really need to be that far away from the pinhole? A really high resolution camera has got ~5µm pixel c-c distance, on a 50mm lens that gives that a 0.5mm hole (normal pinhead needle) only needs to be 0.5*50/0.005mm = 5m away before the pinhole is smaller than a single pixel in the camera...
No, but the pinhole was bright enough, and I wanted to test at infinity, for best comparison with an actual star.
theSuede wrote:
The comatization in the above example actually makes the Lateral CA much harder to judge, as Coma is often corrected very differently for the three different primary wavelengths. For lateral CA, the USAF target, or even any other high-contrast pure black/pure white edge will give more pictorially relevant information - without making things to hard on the more non-scientific target users who only want an overview of classic lenses' CA behaviour.
Maybe, but I think most photographers can produce an image like that, and see how far the various aberrations extend, and how intense they are, and then make a good assessment on how they will affect their images. For example, I knew that this lens delivered good detail but low contrast wide open, and figured this was probably due to Coma. The star test confirms this speculation, and shows its extent.
theSuede wrote:
LoCA smudges the information available in a lateral CA pinhole target so much that you almost have to deconvolve the channels separately to get a good overview of what's going on.
The images shown were chosen to compare a pinhole star with a real star, rather than look at any particular aberration, so you're right to call them a smudge. They're greatly overexposed, for one thing, to bring up the Coma. I did take additional images at f/16 in order to look at Lateral CA more or less by itself, and out of focus shots to look at SA. On-axis shots could be used to inspect Longitudinal CA in better isolation.
Is not a goal ability to compare images from different people? At least same object at same magnification, so easy to compare dimensions of each CA, see which is less.
CA target and test #2: knife edge against sunny 16 sky as measured with lightmeter.
siriusdogstar;
not in Sweden we don't: the sun is not up long enough for me to draw the knife, stop down the lens and shoot. We'll have to substitute the sun for artificial lighting.
olyacme: it is always rewarding to read your posts on the technical side of things.
Though I don't quite understand all of it (my head starts to hurt after a while) I do look forward to reading them. Thanks!
I don't want to hold up this extremely learned conversation, but it seems to me, and I am a technical ignoramus, that one of the main attractions of a forum is that even idiots can learn from their betters. So, when I see an opinion posted by a member I trust, why would I bother to replicate his/her test? As a sadist trying to catch him/her out? As a masochist knowing that I am going to all this trouble even though the outcome is already a foregone conclusion?
I would much prefer to see this thread going not towards a standard test for absolutely everybody to replicate, but a test the validity of which is recognised by every one but conducted by only one member, whose findings can be adopted by all once one person conducts it.
What say you?
philber wrote:
So, when I see an opinion posted by a member I trust, why would I bother to replicate his/her test? As a sadist trying to catch him/her out?
I can't see the harm in more people being able to perform consistently reproducible and easily interpreted tests. Being able to unambiguously compare results between multiple copies and similar lenses is a powerful insurance against expensive duds, though I suppose a "tests before words" ideal could decimate the posting rates on photo forums...
I agree with you, Acme. But statistics show that just a few tests will be required to describe the whole lens population. So I will be content to read about the findings of experts like you, Paul, Henrik and some others whom I happen to trust. And, beyond that, I will test with real-life shots and come to my own determination....
philber wrote:
I don't want to hold up this extremely learned conversation, but it seems to me, and I am a technical ignoramus, that one of the main attractions of a forum is that even idiots can learn from their betters. So, when I see an opinion posted by a member I trust, why would I bother to replicate his/her test? As a sadist trying to catch him/her out? As a masochist knowing that I am going to all this trouble even though the outcome is already a foregone conclusion?
I would much prefer to see this thread going not towards a standard test for absolutely everybody to replicate, but a test the validity of which is recognised by every one but conducted by only one member, whose findings can be adopted by all once one person conducts it.
What say you?...Show more →
My main drive is to find something simple that is available to everyone so we can compare our results. I'd love to know how the CA of much more expensive lenses compares to some of my bargains or vise versa (etc., etc.).
I'm not willing to invest all sorts of money to find out for myself and I can't expect others to buy samples of the lenses I own and shoot CA targets for me.
I think others would find such comparisons, at the very least, interesting. I think the issue is not enough comparable data, rather than too much.
philber wrote:
I agree with you, Acme. But statistics show that just a few tests will be required to describe the whole lens population. So I will be content to read about the findings of experts like you, Paul, Henrik and some others whom I happen to trust. And, beyond that, I will test with real-life shots and come to my own determination....
That's a thing, though - I don't claim to be an expert. I do claim to be able to test stuff, though, and figure most people can as well. The more people who replicate a given test and share their results, the more reliable the conclusions that can be drawn. Star testing is the standard method for assessing telescopes, though the longer the focal length the more care is required for good results. With typical camera lenses it's actually dead easy to perform, and its austerity makes it simple to interpret. It's really a shame it's not more commonly applied.
OK, I stand happily corrected. Design the exact test, and I will put my two left hands to work on replicating it. I have finally been able to file down my Contax 24mm enough to avoid mirror problems on my 5D MkII, so I am feeling brave right now...:-)
note: forget the knife target; the edge produces CA the lens can see, adds to lens-produced CA.
Next test setup only requires two identical objects, one red and one blue. Place objects together at same focusing distance, focus on either red or blue object, but don't try to get both in focus at once. How much the other color is out of focus on the image is the measurement of CA.
Can use ruler with Red marks and one with Blue marks. Focus on red or blue measurement mark; see how far out of focus is the other color mark, what other color different mark is in focus. Difference is CA for that lens focused at that distance with that f/# setting.
I haven't tried that yet; mm markings may be too big to show difference, might need finer rulers. I may try this this weekend by printing colored rulers on piece of paper for angled target.
Can try three objects, Red, Yellow, Blue -- some lens' image may show Red and Yellow in focus, but Blue out-of-focus; some lens image may show Red and Blue in focus but yellow out-of-focus, etc., depends on lens CA correction formula(e) used. Maybe even try side-by-side rulers each color from color chart!
How about a large Dalmation jumping up to catch a black Frisbee on a bright sunny day with the owner in the background dressed in a black pinstriped suit shouting "At A Boy!" That way, if you like the picture, who cares about CA, LoCA, Coca Cola, or whatever?
Excellent pdf resource for Lens Aberrations including CA, the complete Chapter 29 "Aberrations" from the 6-volume multi-K$US bible of lens design, "Handbook of Optical Systems":
DocsPics wrote:
How about a large Dalmation jumping up to catch a black Frisbee on a bright sunny day with the owner in the background dressed in a black pinstriped suit shouting "At A Boy!" That way, if you like the picture, who cares about CA, LoCA, Coca Cola, or whatever?
