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03-23-2008, 11:19 AM   #1
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Questions for the Lens Experts...

I was wondering if the lens experts here can offer some suggestions on how to test the performance of a "New" used lens?

The database and the discussion boards are great for searching out which glass is of Awesome, Average, or Mediocre performance, but once we get the new lens into our hands what type of shots can be taken to evaluate the performance of this specific lens.

I can feel LBA starting to kick in,and it would help me to analyze my new aquisitions.

Also, is there a list of terms with their explanations available ? ie. CA, PF, IQ...

I've searched and could not find one.
One problem is that the search engine requires a three letter word minimum to search on, and it seems that most lens terms are two.

Thanks in advance for the help.
Steve

03-23-2008, 12:41 PM   #2
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Well, I might be able to help with those abbreviations, if not with their definitions:

CA: "chromatic aberration", a defect of the eye that causes you to misfocus and then blame the lens

PF: "purple fringing", a purplish rash you get when you use a lens formerly used by someone with cooties

IQ: "image quality", how good an image the lens can create when it used at the extreme outer limit of its ability, or better yet, past its limits

QC: "quality control", an officer who comes to your door to check on a lens supposedly with CA

BS: well, you get the point....
03-23-2008, 01:18 PM   #3
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More Please!

Great definitions! I hope your cheek heals soon (from your tongue being planted so firmly there).

How about focus abbreviations like BF and FF and maybe even FPS?
Or the other FF regarding frame size?
And defocus stuff: OOF and a translation of the term "bokeh"

I'm certain that your take on these will be of interest to a few of us...
03-23-2008, 02:01 PM   #4
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This list could be used as a starting point for definitions and here is an extensive list of Photographic Acronyms.

Thanks

03-23-2008, 02:59 PM   #5
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Mikhail's links are good, but the abbreviation list didn't have PF, I think.

So, back to the difficult task of identifying and defining abbreviations (you asked....):

BF: "back focus" -- the lens inexplicably focuses on people's backs

FF: "front focus" -- the lens inexplicably focuses on people's fronts

FPS: "frames per second" -- the number of paintings a good criminal gang is able to move out of the museum quickly before getting caught

OOF: "out of focus" -- your eyesight is out of focus and so is every shot you take

Bokeh (NOTE: this is technically not an abbreviation): pronounced "Bow-kuh" -- as in "This lens has great bokeh." -- The ability of the lens to produce pleasing out of focus areas when the lens is focused carefully on sushi.
03-23-2008, 05:00 PM   #6
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the only test I do on a new lens (new to me, old K mount) is a check of exposure accuracy on each of my cameras. (*istD and K10D)

There are 2 sets of tests,

one where I start at wide open, focused on a uniformly exposed surface (block wall etc) and move the lens through each click on the apature ring, setting exposure each time with the green button. I then plot the results of grey scale (measured in photo editor) vs stop, to check how the camera metering works with each lens.

The second is starting with the lens stopped all the way down, (i.e. minimum apature) I take a shot deliberately -2stops under exposed. I then click through the apature settings leaving shutter alone (increasing exposure at each shot) until wide open. I do another run through apatures metering 2 stops over exoised wide open, and stop the lens down.

What you get from the first test is a line (hopefully streight) at aboout 110-120 on grey scale, showing the camera meters correctly at all apatures.

Note on the K10D this does not happen, lenses typically under expose by 1 stop at F1.4, move to correct exposure at F4, to 1.5 stop over by f8 2 stops by F11 and then taper back to +1.5 stops by F32.

The second test gives you a pair of lines, the first one should start at about 25 or so grey scale, and be linear with F stop until about a value of 230. it will then bend as the top of the grey scale is non linear. the second trace will start at about 200 and reduce (as you stop down) and should be linear down to about 25 where it will again start to bend.

Note in a photo editor, the middle range of grey scale from 25 to 230 is essentually linear, and divided into between 4 and 6 stops (depending on the camera's contrtast setting for jpeg and 5 stops in RAW)

With these 2 tests you can check that your apature is correct and the camera meters as you expect it to.

I do not other testing, but then go out and take pictures, usually simple things at first to try the lens out.

If you want to really check for purple fringing, or CA, take a shot through branches at different distances, it is probably the worst thing to show serious CA
03-23-2008, 09:19 PM   #7
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QuoteOriginally posted by Lowell Goudge Quote
If you want to really check for purple fringing, or CA, take a shot through branches at different distances, it is probably the worst thing to show serious CA
These should be preferably backlit. Branches against a bright Southern sky (Northern for you Anzacs) will show each and every flaw in the lens, believe me. My test ground is a pine forest to the South of my home, over a hill crest. I have some dandy shots (check out the "pretty girl" mule deer doe shot on my Flickr site - CAs galore with my 400 M) She is backlit against bright snow. You can look at the other 400 shots and see that it isn't quite as bad as she made it look.
03-24-2008, 03:57 AM   #8
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QuoteOriginally posted by Canada_Rockies Quote
These should be preferably backlit. Branches against a bright Southern sky (Northern for you Anzacs) will show each and every flaw in the lens, believe me. My test ground is a pine forest to the South of my home, over a hill crest. I have some dandy shots (check out the "pretty girl" mule deer doe shot on my Flickr site - CAs galore with my 400 M) She is backlit against bright snow. You can look at the other 400 shots and see that it isn't quite as bad as she made it look.
true, but if you do this, no one will ever purchase a lens again they will all be disillusioned by the CA's it produces

As a note the the OP, the only reason I do tests is to understand how the lens performs with a camera as a system, specifically with metering and consistency of exposure, too many people are into trying to prove a lens is "good" or "bad" with lens testing. Personallly, I don't find the focus test pattern, and blocks of newsprint as artistically interesting subjects.

03-24-2008, 12:50 PM   #9
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QuoteOriginally posted by Lowell Goudge Quote
true, but if you do this, no one will ever purchase a lens again they will all be disillusioned by the CA's it produces
I did the test to find out just how bad it was. I did some fiddling with the image after I shot it, and there is an 8.5x11 inch print on my wall. She's just soooo pretty!
03-25-2008, 09:18 AM   #10
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Original Poster
Thanks to all, for both the "Tongue in Cheek" humour, and actual useful information.
The links to the offsite acronym list was quite helpful, and I see now that my original question pegged me as a noob.

Thanks again,
Steve
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