Lens Correction on Canon FD 17mm

It’s nice to use modern cameras with old lenses, even though some lens correction may be needed. I bought

a Canon nFD 17mm f/4 (I still have a Canon F1 from the film days, wanted to start film, but never did), so anyway, this lens work on the Sony α7, also confirmed by phillipreeve.net. It covers full frame well too. The lens came out in 1979 (for nFD, first FD came out in 1971) and I bought it a few years ago on auctions.yahoo.jp

The Canon FD 17mm lens has very low distortion (compare to the Laowa 15mm f/4 that REALLY distorts a lot!) so I didn’t bother to correct that. But it has a little bit of chromatic aberration (CA), but much, but noticeable if you zoom in a lot.

This lens has no preset in Darktable, it’s not in Lensfun, and even not the Adobe database. So here is what I did:

The lens had no setting in Darktable, so when you go to the Lens Correction module it’s blank and you can edit anything. So I did chose my Laowa 15mm which I made last week. The Canon is now distorted, but I didn’t do any CA correction with the Laowa, so that part was untouched, CA was same for the Canon as before. I saw something red and green, but no blue. So I played a bit with the TCA red slider and came up with a value of 1.00073 to look good. Next I created the lens DB entry below for Darktable, and placed the code inside the misc.xml file:

<model>FD 17mm f/4</model>
<mount>Sony Alpha</mount>
<mount>Sony E</mount>
<tca focal="17" model="poly3" vb="1.0000" vr="1.00073"/>

Full file path in Windows 10:

C:\Program Files\darktable\share\lensfun\version_1\misc.xml

And this is the before/after change:

Canon FD 17mm
Canon FD 17mm

It worked!

Disclaimer: To make it really good you need to adjust the setting to the aperture. That is no big deal if you have a modern lens where the aperture gets recorded into the EXIF data Darktable can pick that up automatically. But otherwise you need to note down each aperture, a 1.00030 – 1.00090 should be the range. To add more trouble, the CA error is not distributed evenly and might differ from center to edges.

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