Filters to control CA?

[Ags]

I was looking at some test shots taken with a Canon 135 SF lens and it appears the lens is sharp enough but it falls down in the CA department - color fringes centrally, shifting to lateral purple flares to the edge of the sensor. The central CA is greatly reduced at F3.5, by the way.

I have a theory that this would be dramatically reduced when using a UHC or UHC-E Astronomik filter, as these cut out the extreme blue wavelengths. I was just wondering if anybody had any experiences in this regard?

The Astronomik CLS filter looks like it lets through too much blue, by the way:

http://www.karmalimbo.com/aro/pics/filters/extrawide.jpg

http://www.karmalimbo.com/aro/pics/filters/medium.jpg

[davew]

Hi Agnes,

You know I'm a sucker for a good graph !

There are a few ways to mitigate. One is crop the worst out. Bit severe.

I assume you're going to use the lens with a DSLR ? If so, and the camera can read the lens type, then you can take the CR2 file into the provided software and get rid that way.

PS has a blue fringe tool but I've not had great success with it myself. It also has a pincushion / barrel distortion tool that can align channels a bit better. Again I've used it with mixed success.

Then come filters. Fringe killers and apo filters attempt to cut the very short blue so could work ok. The LP filters not only cut some blue but also great chunks of green. If you intend to shoot in light pollution anyway then that's ok. If you aren't then they cut too much. The Baader fringe killer also cuts NIR.

If you look at fringe killer graphs you'll notice they cut back on blue but don't reduce it all ( For a purpose not by accident ). Could be something to try if you can borrow one ?

As you already know, some CA can be reduced by stopping down. Depending on the CA this doesn't always work in the corners.

Bottom line - can you find all the filter types to fit your camera ?

Dave.

BaaderFringeKillerSpectrum.bmp

[Ags]

I had to update my version of Canon DPP to work with my collaborator's raw files from a 1Dx, and it turns out that DPP does not have a lens profile for the 135 SF! But it does have a color blur setting that seems to work well enough. It would mean fixing all the CR2s in DPP before passing them on to DSS.

There is also no lens profile for the 35 F2.0, the other lens I've got my eye on... :-(

[Ags]

I decided not to go for the 135 SF because it would be daft to spend hundreds on something with the same CA as an ST80.

I think my next purchase will be a filter - if i get a 2" CLS filter it should screw on the front of my nifty fifty (it takes 52mm filters). Then a 58-52mm step-down ring can adapt the filter to go on the front of my EF-S 55-250 zoom, which I think gives quite acceptable images (sharp and no CA, but only working at F5 or slower). Hopefully the filter will let me double the exposure time before the images fog out.