Using Focal Reducers On Full-Frame Bodies

Cullings wrote:

ProfHankD wrote:

_sem_ wrote:

ProfHankD wrote:

I'm particularly curious about some of the Samyang 8mm lenses being used this way... because they might actually give a usable circle covering a bit more than 180 degrees.

Samyang 8mm are made for APS-C, and I guess you can find relevant info on spherical pano sites. I know shaved Nikkors DX 10.5mm are used on FF bodies (even Canons) for such purposes without focal reducers, indeed covering more than 180deg.

The expectation is a roughly 21.6mm diameter usable image circle with a view angle of approximately 191 degrees... but we'll soon see. I have the Opteka version of the Samyang 8mm (with removable hood) coming to me as I write this....

Hi, did you get your 191 degree image circle using the Opteka?

Well, I hadn't measured... until now.

My best measurement (and this is a pretty touchy measurement) is that I'm getting an uncropped circle with between 184 and 188 degrees of usable view on the A7 using an original Lens Turbo. The last few degrees are pretty dicey, and are somewhat dependent on the aperture and focus distance used, but it is definitely a touch more than 180 -- so two-shot 360x180 is possible and three-shot should give quite good IQ. Then again, with the camera in portrait orientation, you could get an even better three-shot stitch using an ordinary adapter.

The usable image circle measures approximately 20.75mm diameter -- smaller than the 21.6mm I expected (which probably means the LT magnification isn't exactly as advertised, but it could be a more complex interaction between lens projection and LT). At f/22, that is with a very clean, sharp, edge that is surrounded by just a hint of blue fringe (pretty common for fisheyes). Wide open at f/3.5, the usable diameter is about the same, but the last 0.5mm of edge is quite smoothly fading to black without blue fringing, so how you call the view angle depends on how much vignetting correction you're willing to apply to that last 0.5mm. Here are two crude hand-held examples:

Opteka 6.5mm f/3.5 @ f/3.5 + Lens Turbo on A7

Opteka 6.5mm f/3.5 @ f/22 + Lens Turbo on A7

In sum, it works, but I'm not really sure it's all that useful a hack. The projection of this lens also gives less apparent distortion than typical fisheyes, so the circular image isn't as dramatic... although there's nothing preventing you from changing images into a more traditional fisheye projection in postprocessing -- the image data is all there. You are gaining a stop of light using the LT this way, so the f/3.5 setting is more like f/2.5 and f/22 is more like f/16, but meh.

That would be 12 megapixels within the circle assuming you have a 24 megapixel sensor??

If the circle had the expected 21.6mm diameter, you'd have about 10.2MP. As measured, it is actually about 9.4MP within the usable image circle using a 24MP sensor. Keep in mind that the magnification of the Speed Booster is somewhat less than the Lens Turbo, so you'd get a somewhat smaller circle. In fact, I wouldn't be surprised to see a few percent variation in image circle size just as sample variation on various versions of the Lens Turbo... and the later version that has the rectangular masking in it probably would clip the circle.

Overall, this hack does give pretty good IQ as circular fisheyes go. Actually, the Samyang (even my $150-new Opteka-branded copy) is awesomely good all by itself stopped down a little. The LT doesn't seem to hurt IQ much, but it's so sharp to begin with that the LT doesn't really improve upon the lens by itself... you actually loose just a touch of contrast.

I am considering using a Mamiya 24 mm medium format fisheye lens with a Mamiya-to-Canon adapter and a 0.7 Canon-to-Sony focal reducer. Do you think that would also achieve a true circular image? That could yield a 12 or 18 megapixel image circle depending upon A7 or A7R. - John S.

I would not expect that to work. The Mamiya 24mm is for their 645, which means the native image circle is possibly as big as 40mm diameter. 0.7*40mm=28mm, which means it wouldn't fit on the 24mm height of the A7/A7R sensor. Beyond that, focal reducers made for 135-format SLR lenses don't expect such a large image area as input, so you'd probably see some additional vignetting from the focal reducer itself.