Rokinon/Samyang 7.5 Lens Profiles

Started Feb 22, 2012 | Discussions thread
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kenw
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Rokinon/Samyang 7.5 Lens Profiles
Feb 22, 2012

I recently acquired the excellent Rokinon/Samyang 7.5 Fisheye for m43.

The original review for it on Photozone claimed it was a stereographic projection fisheye. This was incorrect, it is in fact an equal area projection just like nearly all fisheye lenses made (the Samyang 8mm for APS-C is the only exception I know of). I contacted Photozone and they've corrected the review. If you aren't familiar with what projections (or mappings) are, here is something to read:

http://en.wikipedia.org/wiki/Fisheye_lens#Mapping_function

Now, the stereographic projection is nice in that it is a bit less "distorting" than equal area. In the same way you can "de-fish" a fisheye lens to make it look like a rectilinear lens (with a significant loss in angle of view) you can also change the projection to be stereographic (with a small loss in angle of view). Many panorama programs will let you do this.

However, it would be nice to do it directly in LR or ACR. LR and ACR offer "Lens Profiles" but unfortunately the default implementation is to "de-fish" rather than just correct to the intended projection so they aren't all that useful.

I've done a standard calibration of the 7.5 using the Adobe Lens Profile Creator software and then derived modified profiles that will produce any of the typical projections. So now with these profiles you can do any sort of radial projection mapping you want right in LR or ACR. Note that if you want to "fine tune" a profile to get a bit less or more angle of view for a given composition you can use the "distort" slider in ACR/LR to adjust a bit.

Please note, I've removed the CA and vignette corrections from these projections. I wasn't exactly sure how best to transform them so I just left them out. The rectilinear profile does have CA and vignetting correction available (it is the one directly created by the Adobe tool).

The LCP files I generated have XML comments in them describing how to derive the profiles should someone mathematically inclined want to play with them more.

Without further ado, here is the link to a zip with the profiles:

http://dl.dropbox.com/u/46420541/Rok7_5_lcp.zip

And here are some examples of the different projections (camera oriented at an angle so you can see the effect on the corners clearly, really an architectural subject with lots of horizontal and vertical lines would be a better example but I don't have anything like that yet with this lens).

Default (no profile applied):

Equal Area:

Equidistant:

Stereographic:

Rectilinear:

Finally, there are various non-radially symmetric projections that are popular, in particular the Pannini projection that is used by the Hemi de-fishing tool. Unfortunately the lens profile correction models used by Adobe will not support any projections at all like Pannini. So if you want something like that you'll have to export out of LR/ACR to another tool.
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Ken W
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