Generating Ptlens corrections from Imatest

Started Jul 16, 2005 | Discussions thread
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mikecol Senior Member • Posts: 1,132
Generating Ptlens corrections from Imatest

Ptlens is great for doing distortion corrections and has a good set of profiles for bodies and lenses. I found a way to generate new profiles for Ptlens using the parameters from Imatest. Here is the process:

1) take a shot of the grid pattern and run Imatest distortion. Note the k1 (which is the r^3) parameter.

2) divide my magic number = -3.397. This result is now used as the b (r^3) parameter in Ptlens.

3) Edit the profile file used in Ptlens to include this new lens. Use the bodies multiplier, lens name, focal length, a, b, c ( make a and c 0.000000; and b from calculation in 2) )

example:

begin lens
group: canonSLR
multiplier: 1.25
menu_lens: EF 14mm f/2.8L USM
cal_abc: 14 0.000000 -0.011009 0.000000
end

The results are excellent. Here is a list of 8 primes and a 70-200 with the original uncorrect distortion (Tv%) compared to the corrected distortion (Tv%).

You can see several of my 1-2% lenses can be corrected to

Now, can anyone explain the magic number of -3.397 ( for 1.25X body, -5.308 for 1X body) used in the conversion between Imatest and Ptlens?

Have fun,
Mike

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