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View Full Version : Lens Charts - Can anyone explain?


CaptainD
7th March 2008, 06:28 PM
I have to be honest, even though I keep thinking I understand lens charts the reality is that I simply do not. Has anyone seen any good idiots guide about them?

If anyone is keen enough to try any explain them to me, I am all ears. It has to be simple but .........................

Thanks

Chris :o

Nova Invicta
7th March 2008, 10:34 PM
Depends on the chart to be honest but lens test charts are generally to test a. sharpness (full field, centre & edge sharpness)& the fall off b. distortion c. colour fringing d. colour balance (by comparing lenses) e. resolution you can also check whether the focus scales are correct by measuring from the film plane marking to the chart the distance you wish to check
Black & White charts are best and professional ones will have specific markings for specific tasks, searches on the internet will throw up professional advice on how to do certain tests.
We would project lenses onto charts to do the above tests and the projector can be moved to check distances. For us this is only one group of tests as we would also check collumation, MTF, and the F-stop / T-stop
With mass produced lenses the lens scales are worked from certain distances and the others extrapolated from these so they are worth checking if you use them manually.

CaptainD
10th March 2008, 11:51 AM
Sorry for the late response, thanks so much for taking the time to reply. It is helping me along the way to get more out of them.

Cheers

Chris

CaptainD
10th March 2008, 12:11 PM
Now this is a new one for me, I am going reply to my own question!

I have been searching on the web and found some interesting web pages on the subject.

http://www.luminous-landscape.com/tutorials/understanding-series/lens-contrast.shtml

http://www.luminous-landscape.com/tutorials/understanding-series/understanding-mtf.shtml

http://www.punitsinha.com/resource/understanding_mtf_chart.htm

http://artlessbeauty.blogspot.com/2007/05/understanding-mft-charts.html

If you are brave this one is a very techie one, but I fear this has confused me again!!!

http://www.normankoren.com/Tutorials/MTF1A.html

I thought I would post these web links to aid any others that are trying to understand lens charts better.

Please add any other comments or thoughts, they would be much appreciated.

Cheers

Chris

Nova Invicta
10th March 2008, 10:05 PM
CaptainD
What exactly do you want to test? Are you wanting to test lenses you own or understand how lenses are tested or maybe both?
MTF on a chart is misleading to a degree, we use charts to test lenses on a projector to see how well they resolve centrally and over the whole field as stated previously, you would also be checking colour fringing, distortion and you could also see if the lens flares, we walk up to the screen to view the results which obviously yoy cannot do. However if you simply purchase charts and shoot a stills test its only on high magnification that you could tell how well your lens was performing because aside from distortion you cannot tell much by simply looking at a photo.
MTF for lens manufacturers is carried out on at MTF machine which looks a a slit graticule and the results are shown on a computer screen and normally at certain frequency responses. Interestingly they are higher generally for digital lenses than for film (frequency) because they require slightly different contrast ratios.
My best advice on lenses is buy the most expensive you can afford as in optics you only get what you pay for and the common mistake everyone makes is worrying about the camera more than the lenses, it should be the other way around.

CaptainD
10th March 2008, 10:28 PM
My real interest in lens charts is really to get a feel for a good or a bad lens by looking at the chart. Lens manufacturers seem to always include them in their press releases and I thought I had better try and understand this "eye candy". But I guess that there will never be a substitute for actually trying a lens to see just how good it is.

Mind you, I could not agree more about the more you spend the better the glass is. I really do think that the quality of the lens really does make a big difference. Having said that, some of the results I have seen from even the budget digital Zuiko is amazing even when blown up to large sizes. I will never forget a picture that I saw on a visit to the Digital Depot that was taken with a standard lens, good technique clearly adds a lot the final result. There is always something to improve when taking photos which is what makes photography so interesting for me.

Cheers

Chris

Nova Invicta
11th March 2008, 08:41 PM
CaptainD
If you want to do a "real life" test choose a building you know well with detailed brickwork, always shoot from the same postion in roughly the same lighting conditions and using a magnifying glass on your finished images (or use Photoshop in the "Actual Pixels" mode) to see how well the brick resolves, this then becomes your "Constant" something you are always measuring against.