measuring lines / mm at film

[Thomas78]

Hello,

just out of curiosity I tried to do some resulution tests of some 35 mm film and MF lenses.

I took photos of the USAF resolution test chart at a distance of 5 m, deveoloped the film and looked at it with a microsope with a 63x magnification (illuminated by a light table).

My question regarding the interpretation of the results:

Where do I determine the resolution of the lens?
At the smalles pair of lines which is still clearly visible as a pair of black and white lines with a sharp border
or
the last pair just before the lines merge together to a grey block ?

Regards,
Thomas

 

[Vince Lupo]

From my (small) experience of having done them back in undergrad, you need to use what they call a 'traveling microscope' (one that can accurately measure the number of lines in mm).

As well, did you only use one chart, or several? Best way to do it is to use 5 charts set up evenly spaced apart, then cant your camera on an angle on your tripod so that the first and fifth chart end up in opposite corners of the frame, and the third chart is in the middle of the frame.

 

[Thomas78]

From my (small) experience of having done them back in undergrad, you need to use what they call a 'traveling microscope' (one that can accurately measure the number of lines in mm).

At the chart (similar to this one
http://en.wikipedia.org/wiki/File:1951usaf_test_target.jpg
) the lines are in grops of known size so I don't have to measure them with the micoscope. I just look at the designation of the line pair and I can calculate the size of it.)


At the center I got values of around 50-60 l/mm for a Canon 50 mm f/1.2 which needs a CLA, about 40 - 50 l/mm for a Canon 50 mm f/1.5 and 30 - 35 lines for some FSU lenses.
(Using the concept "smalles pair of lines which is still clearly visible as a pair of black and white lines with a sharp border ")

As well, did you only use one chart, or several? Best way to do it is to use 5 charts set up evenly spaced apart, then cant your camera on an angle on your tripod so that the first and fifth chart end up in opposite corners of the frame, and the third chart is in the middle of the frame.

For my first test, I used one chart in the center and one near one ot the corners.

 

[Vince Lupo]

Just pulled out my old test from undergrad, and you're right -- I didn't use a traveling microscope for the procedure (I was confusing it with something else we did in tech class -- sorry for the confusion).

According to the RIT (Rochester Institute of Technology) instructions that I have in my project, they advise that you "read the chart by determining the smallest group in which you can clearly read all three letters in the line". Then, you check the 'lines per millimetre' chart that they provide with the tests. Do your charts have numbers and letters that correspond to each line?

As far as film goes, we used Tech Pan 2415 (exposed at ISO 25), so I'm assuming that you used a really slow, fine-grained film to provide the most accurate results?

Here again, it's been almost 20 years since I did this project (comparing a 50/2 Summar, a 50/1.5 Summarit and a 50/2 collapsible Summicron), so I'm likely a bit fuzzy on the details -- but fortunately I held onto all the info and negatives!

 

[Vince Lupo]

Other thing I just read in the RIT instructions is how to set the 'camera-to-target' distance:

If you're using 35mm film (24x36), you should multiply the focal length of the lens by 49.7. So if you're testing a 50mm lens, multiply 50x49.7, which would give you 2485mm, or 248.5cm, or about 8.15 feet.