Using Focal Reducers On Full-Frame Bodies

I've posted a variety of things about this before (mostly in the "forum formerly known as NEX" because the Sony E-mount A7/A7R/A7S are best bodies for this trick), but I think it's time to get the general principle out there: focal reducers are not designed to work with full-frame sensors, but they sort-of do.

Note that for everything in this posting, my A7 was set to use the full sensor area -- not to crop to APS-C nor to automatically select between the two modes.

One would expect a focal reducer, such as the Speed Booster or Lens Turbo, to vignette when used on a full-frame camera, and they generally do. Here's a 35mm f/2 SMC Takumar on my A7:

35mm f/2 full-frame lens wide open on A7

And here's what one gets using a Lens Turbo instead of a glassless adapter:

35mm f/2 full-frame lens wide open on A7 using a Lens Turbo

Notice that the image doesn't cover the full frame; it vignettes pretty much like this no matter what f/number you use. However, the usable view angle is wider, and cropping would allow that wider view to be sampled various ways -- including different aspect ratios, such as square. You can easily extract a square with the same angle of view as the usual 3:2 full frame, or even a bit wider! Here's the proof:

A7 Lens Turbo view; green duplicates native FF view, red is an otherwise impossible square crop

Do all full-frame lenses vignette the same? It seems the answer is yes. Obviously, some lenses designed for smaller formats (e.g., APS-C) might vignette more when used this way, and zooms will often vignette a little more at certain focal lengths. Some lens hoods, or even internal baffles in some lenses, can also cause increased vignetting. However, the vignetting with various full-frame lenses generally appears to be from the Lens Turbo itself... which means if you have a different focal reducer, your mileage may vary, but I'm never going to get an non-vignetted full-frame view with mine. You probably will not get one with yours either.

Then again, for some lenses we don't care. Fisheye (and bird's eye) lenses normally create circular images. Placing a fisheye lens on a focal reducer might allow your cropped fisheye to produce a full circular image, and can widen the view in some cases. For example, my strange old Accura 12mm fisheye on full frame natively creates a circular image clipped on the top and bottom, but on my Lens Turbo:

12mm fisheye on LT, A7 full frame -- poof! A circular fisheye!

If you've got a focal reducer and a compatible full-frame body, give some lenses a try.  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. Be sure to post any surprises....  

Interesting. I wonder what the 12mm 2.8 seaming full frame fisheye would produce with a focal reducer?

You can take the hood off it, and it reduced enough to give a circular image then you would have two fisheyes in one; a full frame one and a circular.

Pity my 12mm is in FE mount already, can't use a reducer...

_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....

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

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.

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.

ProfHankD wrote:

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.

Hi, I hope to make it work! Look at the Mamiya 24mm 360 degree panorama at http://08.ae/panos/Mamiya24FEtest/virtualtour.html. I'm impressed by the fine resolution when zooming way in. I found this url at:

http://www.getdpi.com/forum/medium-format-systems-and-digital-backs/49332-advice-mamiya-24mm-f4-0-fisheye-lens.html

I've been unable to get that resolution with my fisheye panoramas. I currently use a Madoka 180 circular fisheye (Sony e-mount for aps-c) with my a6000 24 megapixel camera. I think my lens quality may be the major problem (??) I usually stitch together 4 circular images using PTGui.

So I'm in process of upgrading my equipment. Mamiya 24 lens due tomorrow. Zhongyi Lens Turbo II due next week. I already have the Mirex Mamiya/Canon shift adapter. All that remains is choosing the best A7 camera - trying to avoid the very expensive A7RII.  I think 36 megapixel sensors would be best for the smaller-sized circular images.

I'll report later on my success or failure -  but it will be a while until I get the camera.  In the meantime I will try the Mamiya 24 with Mirex adapter on my a6000.  I also have a Rhinocam (Fotodiox) with Mamiya adapter which might work well on the a6000 (??)                --  John S

ZeilSam wrote:

-note, I'm realizing that the ef to E is the exact opposite of what I want which may not be possible

Actually, you're just not being clear about what you want.

There are focal reducers to take medium format lenses to FF sensor, but not many. There are also lots of focal reducers to take FF lenses to APS-C or MFT sensor. There also are lots of 1.4X or 1.5X teleconverters that will take an APS-C lens to a FF sensor, but none that take E mount lenses.

What are you looking for?

A very interesting thread Hank

My only focal reducer is a EF-MFT model  which I have tried adapted onto a NEX. It seemed to work suprisingly well, I guess I'll have to give it a go on the A7ii now.

It might just be the lens that was miss-sold as a medium format model (it turned out to have a M42 mount, although the lenses was also sold in several medium format mounts) might finally prove to have some worth, but I won't hold my breath!

Experiments with UWA/fisheye lenses on formats they're not designed for have generally shown me the inside of the hood - I've seen this using an APSC fisheye reduced on MFT (which theoretically would be close to the intended FOV), and on a 10mm 2.8 lens on the A7ii (it turned out to be an APSC lens giving almost identical visible  FOV to my FE 14/2.8). There;'s clearly not going to be any benefit try to reducers on these to FF.

This focal reducer for Sony full-frame cameras is quite interesting, the Kipon Baveyes MF Focal Reducer:
https://www.youtube.com/watch?v=URZBQvzBy98

dan pv wrote:

I have tried some MF and LF lenses with my Leica R-Sony Nex Metabones SB. It doesn't matter that the lenses have a huge image circle - the SB will only cover APS-C.

False. Most designed-for-APS-C focal reducers will cover a 24mm square, and perhaps a bit more... which is useful.

However, coverage is very lens dependent. MF lenses tend to assume a wider obstruction-free path, so it wouldn't surprise me if they vignette MORE than most FF lenses when used on a focal reducer.

I am also interested in a 0.5 FF SB. I've seen some attempts to make a scan-back for LF cameras in order to revive the unic rendering of some LF lenses, but all were low resolution captures of the ground glass. A 0.5 FF SB would offer the full resolution of the 6x9 lenses with a gain of 2 F-stops while preserving their unique "character". I did some experiments with achromats placed at the back of the lens and it seems to work, but not without some flaws. I've used a Leica Elpro II achomat and, while preserving (or even increasing a bit) the central resolution of the lens it slightly softness the edges when the lens is used fully opened. Closing the lens down improves corners but you lose the "character". However, I think that this setup is usable for portraits, some closeups and soft focus images but not for landscapes or images where one needs very good corner sharpness at large apertures.

It's very hard to make a large-reduction focal reducer... until somebody figures-out a way that it isn't.

dan pv wrote:

ProfHankD wrote:

dan pv wrote:

I have tried some MF and LF lenses with my Leica R-Sony Nex Metabones SB. It doesn't matter that the lenses have a huge image circle - the SB will only cover APS-C.

False. Most designed-for-APS-C focal reducers will cover a 24mm square, and perhaps a bit more... which is useful.

Precise measurements are difficult to make but the circle they iluminate is slightly larger than their covering circle without being really useful. However, what's important in this discussion is that they don't cover FF.

I know you're disappointed, but just because it doesn't fulfill your dreams doesn't mean it isn't useful. The ability to take a FF lens and get a square aspect image with the same view angle as full frame is really useful -- if you like square images.

I am aware of how difficult should be to make a 0.5 FF SB. My intention is only to make an useful tool for me in order to enjoy the "character" of some of my older LF lenses on a digital camera in situations when the corner sharpness is not so important - that's all .

Well, that will take a larger-format focal reducer. However, being able to get a square image out of your FF lenses with the same field of view sure makes them feel a lot more 6x6-ish to me.  YMMV.