The perfect CA target?

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Archive 2009 · The perfect CA target?

siriusdogstar
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p.3 #1 · The perfect CA target?

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.

Everybody already has equipment!

Sep 24, 2009 at 11:16 PM

kosmoskatten
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p.3 #2 · The perfect CA target?

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!

Sep 25, 2009 at 11:37 AM

philber
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p.3 #3 · The perfect CA target?

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?

Sep 25, 2009 at 11:59 AM

olyacme
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p.3 #4 · The perfect CA target?

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...

/Acme

Sep 25, 2009 at 12:19 PM

philber
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p.3 #5 · The perfect CA target?

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....

Sep 25, 2009 at 12:38 PM

cogitech
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p.3 #6 · The perfect CA target?

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?

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.

Sep 25, 2009 at 12:55 PM

olyacme
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p.3 #7 · The perfect CA target?

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.

/Acme

Sep 25, 2009 at 01:06 PM

philber
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p.3 #8 · The perfect CA target?

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...:-)

Sep 25, 2009 at 01:17 PM

siriusdogstar
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p.3 #9 · The perfect CA target?

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!

Sep 26, 2009 at 01:24 AM

DocsPics
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p.3 #10 · The perfect CA target?

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?

(Just trying to be helpful......)

Sep 27, 2009 at 12:50 PM

siriusdogstar
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p.3 #11 · The perfect CA target?

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":

http://media.wiley.com/product_data/excerpt/95/35274037/3527403795.pdf

lots of mathematics and color illustrations of lens aberrations!

Oct 31, 2009 at 04:38 PM

cogitech
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p.3 #12 · The perfect CA target?

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?

(Just trying to be helpful......)

I like this idea the best, so far!

Oct 31, 2009 at 07:50 PM

gasrocks
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p.3 #13 · The perfect CA target?

But not everyone has a large Dalmation that jumps.

Oct 31, 2009 at 10:43 PM

kosmoskatten
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p.3 #14 · The perfect CA target?

I wrapped the cat in tinfoil and tried to make her sit under the desk top light with a white back drop. She bit me.

Twice.

I'd give anything for a Dalmatian that jumps. And I would wrap the frisbee in tinfoil as well.

Nov 01, 2009 at 09:08 AM

trifox
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p.3 #15 · The perfect CA target?

We have to live with CA in photography -- the tests are for engineers only

Nov 01, 2009 at 01:58 PM

cogitech
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p.3 #16 · The perfect CA target?

kosmoskatten wrote:
I wrapped the cat in tinfoil and tried to make her sit under the desk top light with a white back drop. She bit me.

Twice.

I'd give anything for a Dalmatian that jumps. And I would wrap the frisbee in tinfoil as well.

Nov 01, 2009 at 03:11 PM

philber
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p.3 #17 · The perfect CA target?

ROTFL

Nov 01, 2009 at 03:22 PM

ISO1600
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p.3 #18 · The perfect CA target?

how about anything, when shooting the Nikon 85/1.4D?

haha

Nov 02, 2009 at 02:19 AM


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