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Photo Forum / Digital Photography / DSLR Cameras / May 2006Reducing teleconvertor
Today, while driving around with my camera, a thought struck me. I've got some lenses which I can use on my Nikon D70, but because of the crop factor, I lose some of the special features. The 20mm f2.8 is no real problem because the 18-70mm kit lens covers that nicely -- other than the slight loss of sensitivity. However -- the 16mm Fisheye, when cropped to the 1.5 crop factor loses much of its character. So -- has anyone attempted design and sales of a reverse teleconvertor? One with a 0.66 magnification, so a lens which would put a particular image on a full 35mm frame would put that same image on the 1.5 crop factor sensor of the Nikon digital cameras. Note that this should also result in a bit of gain of light sensitivity, unless it includes a ND filter as part of its design (and I see no reason to throw away the free sensitivity increase). And -- unlike the normal telephotos, which are designed to work mostly with telephoto lenses, this one should be optimized for wide angle lenses, as that is what you would be wishing to use them with. (Or, at least, that is what *I* would be wishing to use them with. :-) Any comments? Enjoy, DoN.  SignatureEmail: <dnichols@d-and-d.com> | Voice (all times): (703) 938-4564 (too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html --- Black Holes are where God is dividing by zero --- > So -- has anyone attempted design and sales of a reverse > teleconvertor? One with a 0.66 magnification, so a lens which would > put a particular image on a full 35mm frame would put that same image on > the 1.5 crop factor sensor of the Nikon digital cameras. My understanding is that such a thing would need to be too close to the image plane to function in an SLR with the mirror in the way, unless you're willing to make severe compromises in functionality (see below). > Note that this should also result in a bit of gain of light > sensitivity, unless it includes a ND filter as part of its design (and I > see no reason to throw away the free sensitivity increase). No, the exit pupil size would need to be too large. The old Nikon/Fuji SLRs that used optical reduction to avoid any crop factor with 35mm lenses had a *maximum* aperture of f/6.3 no matter how fast a lens you put on them. To get any more, you would have to allow it to get closer to the image plane, which isn't practical. Wide-angle adapters that go on the *front* of the lens would work. But they would need to be very large and expensive to do the kind of job you want; they would almost certainly cost more than simply buying a wide-angle lens and maybe a 10.5mm fisheye, which is what I'd suggest you do. :) SignatureJeremy | jeremy@exit109.com According to Jeremy Nixon <jeremy@exit109.com>: > > So -- has anyone attempted design and sales of a reverse > > teleconvertor? One with a 0.66 magnification, so a lens which would [quoted text clipped - 4 lines] > image plane to function in an SLR with the mirror in the way, unless > you're willing to make severe compromises in functionality (see below). O.K. Well ... it was a thought. > > Note that this should also result in a bit of gain of light > > sensitivity, unless it includes a ND filter as part of its design (and I [quoted text clipped - 5 lines] > To get any more, you would have to allow it to get closer to the image plane, > which isn't practical. Oops! > Wide-angle adapters that go on the *front* of the lens would work. But they > would need to be very large and expensive to do the kind of job you want; > they would almost certainly cost more than simply buying a wide-angle lens > and maybe a 10.5mm fisheye, which is what I'd suggest you do. :) Hmm ... not as bad a price for the 10.5mm as I feared, but the 12-24mm adds signifiantly to that. And, since I'm currently trying to save up enough to get a D200, and I would rather have that than the fisheye capability. Thanks, DoN. SignatureEmail: <dnichols@d-and-d.com> | Voice (all times): (703) 938-4564 (too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html --- Black Holes are where God is dividing by zero --- > Today, while driving around with my camera, a thought struck me. > I've got some lenses which I can use on my Nikon D70, but > because of the crop factor, I lose some of the special features. > The 20mm f2.8 is no real problem because the 18-70mm kit lens > covers that nicely -- other than the slight loss of sensitivity. > However -- the 16mm Fisheye, when cropped to the 1.5 crop factor > loses much of its character. > So -- has anyone attempted design and sales of a reverse > teleconvertor? One with a 0.66 magnification, so a lens which would > put a particular image on a full 35mm frame would put that same image on > the 1.5 crop factor sensor of the Nikon digital cameras. Yes, more than one person has successfully done this. For example, see http://www.naturfotograf.com/D1_fisheye_1,html.htm. Andrew. According to Andrew Haley <andrew29@littlepinkcloud.invalid>: > > Today, while driving around with my camera, a thought struck me. > > > I've got some lenses which I can use on my Nikon D70, but > > because of the crop factor, I lose some of the special features. [ ... ] > > However -- the 16mm Fisheye, when cropped to the 1.5 crop factor > > loses much of its character. [quoted text clipped - 6 lines] > Yes, more than one person has successfully done this. For example, > see http://www.naturfotograf.com/D1_fisheye_1,html.htm. Thanks! This is what I hoped was possible. The implementation is rather klugy, but it demonstrates the principle, at least. Again, thanks, DoN. SignatureEmail: <dnichols@d-and-d.com> | Voice (all times): (703) 938-4564 (too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html --- Black Holes are where God is dividing by zero --- > According to Andrew Haley <andrew29@littlepinkcloud.invalid>: > > > So -- has anyone attempted design and sales of a reverse > > > teleconvertor? One with a 0.66 magnification, so a lens which would [quoted text clipped - 6 lines] > Thanks! This is what I hoped was possible. The implementation > is rather klugy, but it demonstrates the principle, at least. The optical system that Roslett's assembled is a full-blown relay, a reimaging camera with demagnification. That is, the fisheye produces an image, the next lens (a 58mm) is focused on that image plane and so the light exiting it is collimated, and the next lens (the 35mm) takes the collimated light beams and reimages them onto the detector. The intermediate image, between the fisheye and 58mm, is a real image (in the optical sense, that if you slid a ground glass in at that place you'd see an image). A teleconverter works somewhat differently. A TC is a negative (diverging) lens/group which intercepts the converging beam exiting the primary lens and makes the cone of rays converge less quickly, so that the combined focal length is longer. This is actually how simple telephoto lens designs work also - there's a positive lens/group followed by a negative lens/group, which allows the whole thing to be shorter. In the case of the TC, unlike the reimaging camera, there is not a collimated beam in the middle, nor is there a real image that you could access - the image formed by the primary is somewhere inside the TC body. To make a reverse teleconverter (a reducer) that wasn't a huge thing like Roslett's, you'd want a positive lens. For a variety of reasons - one is that the optical system would be very fast, which is hard to handle, and another is that it would probably have a lot of field curvature - I think it would be hard to build a practical high quality reducer. Another problem is physical. The reducer is to be short, positive focal length, so it probably wants to sit inside the mirror box. The way to get a short focal length lens further away from the focal plane is the reverse telephoto, or retrofocus wide angle, which is how basically all modern SLR wide angles are made. At this point the reducer might be at least as complex as the new wide angle we are trying to avoid buying ... Ben >> According to Andrew Haley <andrew29@littlepinkcloud.invalid>: > [quoted text clipped - 51 lines] > > Ben Nikon makes a collimated relay system to convert their 35mm wide lenses for pro video cameras with 2/3" chips, which are about 1/4" the area of a standard 35mm frame (normally you get about a 4x magnification as compared to 35 mm FF). There are a number of problems. First, of course, you can't use the diaphragm in the lens, so one is supplied with the adapter. They are long: about the length of a 200mm lens. They are expensive--about $4000--that's the model with manual diaphragm--the sevo model is about twice that. You probably lose a lot of optical quality, although, it's not evident on SD video. Finally, any speck of dust on the front surface of the collimator shows up as a hard-edged black dot--just like a piece of dust on your focusing screen--so you have to keep the thing extra clean. Great if you want fisheye video shots, but a huge pain in the a.s. Toby In amateur astronomy, they sell dedicated tele"compressors" for many telescopes. The telescopes are primarily refractive (same as a telephoto lens) or mirror-lens. The teleconverters are made by various companies like Meade, Celestron, TMB, TeleVue, Astro-Physics. They reduce the focal length of the "lenses" by anywhere from 40-75%. So, a 2000mm mirror lens at f10 becomes a 600mm mirror lens at f3. In addition, telecompressors flatten the photographic field. However, some are not meant to cover a whole 35mm frame though most are. Prices range from $150 to over $500. I have no idea how one would work on non-telephoto but people have used them for this. > In amateur astronomy, they sell dedicated tele"compressors" for many telescopes. > The telescopes are primarily refractive (same as a telephoto lens) or mirror-lens. [quoted text clipped - 6 lines] > I have no idea how one would work on non-telephoto but people have used > them for this. That's a focal reducer. It compounds with the telescope to reduce its focal length (increase the lens's power). This page gives a little schematic of how a focal reducer is used (second figure): http://www.rc-astro.com/resources/reducer.html In practice a focal reducer might have a 300-700mm focal length and be used on telescopes that have a ~1000-2000mm focal length at f/5 to f/10 or so. Because the lenses and telescopes have long focal length compared to the field that needs to be covered (35mm or 120, say) and the primary telescopes are relatively slow, this is doable without major compromises. Once you get to focal lengths that are much shorter, close to the field diagonal, I think it would be hard to keep image quality up or keep the field flat. > Today, while driving around with my camera, a thought struck me. > [quoted text clipped - 20 lines] > angle lenses, as that is what you would be wishing to use them with. > (Or, at least, that is what *I* would be wishing to use them with. :-) Here's one for $150-ish that screws on the front for 0.7x reduction: <https://emporium.olympus.com/innards/empproddetails.asp?sku=200864-410> Designed for P&S digital. It appears to have a 55mm thread so I guess it's possible to adapt to a standard 52mm filter size lens such as a 50mm or even a 28mm f/2.8 but it looks like your 20mm f2.8 is 62mm although vignetting might not show on a small sensor? Of course there is going to be some image degradation but these are supposed to be pretty decent. According to Paul Furman <paul-@-edgehill.net>: > > Today, while driving around with my camera, a thought struck me. > > [quoted text clipped - 6 lines] > > However -- the 16mm Fisheye, when cropped to the 1.5 crop factor > > loses much of its character. [ ... ] > Here's one for $150-ish that screws on the front for 0.7x reduction: > <https://emporium.olympus.com/innards/empproddetails.asp?sku=200864-410> > Designed for P&S digital. It appears to have a 55mm thread so I guess > it's possible to adapt to a standard 52mm filter size lens such as a > 50mm or even a 28mm f/2.8 but it looks like your 20mm f2.8 is 62mm And -- as I mentioned above, the 20mm f2.8 is not really in consideration here, because the 18-70mm "kit" lens covers the 20mm in its range -- and appears to be sharper in the corners than the 20mm, based on some brick-wall tests. The real lens which I wanted this for is the 16mm Fisheye, which has *no* front filter threads (the front element bulges too much), so the lens actually has a filter turret inside it to allow red, orange, and yellow filters (IIRC). > although vignetting might not show on a small sensor? Of course there is > going to be some image degradation but these are supposed to be pretty > decent. But it is not what I wanted, and the discussion has gone on to explain how what I *really* wanted is not practical, however it *is* possible to make a relay lens from a collection of older lenses to accomplish the task -- if you don't mind the result looking more like a 600mm lens in overall length. :-) I wanted to preserve the curved image which the 16mm Fisheye offers on full sized film exposures. Thanks, DoN. SignatureEmail: <dnichols@d-and-d.com> | Voice (all times): (703) 938-4564 (too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html --- Black Holes are where God is dividing by zero --- > I wanted to preserve the curved image which the 16mm Fisheye > offers on full sized film exposures. I have the 10.5mm DX fisheye, and while it's a tad expensive, it's really nice. Personally, I find it more useful than a super-wide like the 12-24. The widest non-fisheye I have is 17mm, and I have yet to find myself wishing for any wider. Anyway, I can highly recommend the 10.5. SignatureJeremy | jeremy@exit109.com > > I wanted to preserve the curved image which the 16mm Fisheye > > offers on full sized film exposures. [quoted text clipped - 3 lines] > The widest non-fisheye I have is 17mm, and I have yet to find myself wishing > for any wider. Anyway, I can highly recommend the 10.5. Interesting. I had a 17mm for my film camera, and found it quite useful (and the 20, and the 24). So I'm glad I have a 12-24mm for my 1.5x crop DSLR now, and rather like the idea of taking that out to 10mm (say the Sigma 10-20mm). I've never owned a fisheye. Are you using the 10.5mm as a fisheye, or for electronic linearization? As a fisheye, I guess it's full-frame, not the circular image the 6mm gives? SignatureDavid Dyer-Bennet, <mailto:dd-b@dd-b.net>, <http://www.dd-b.net/dd-b/> RKBA: <http://www.dd-b.net/carry/> Pics: <http://dd-b.lighthunters.net/> <http://www.dd-b.net/dd-b/SnapshotAlbum/> Dragaera/Steven Brust: <http://dragaera.info/> > Interesting. I had a 17mm for my film camera, and found it quite > useful (and the 20, and the 24). So I'm glad I have a 12-24mm for my > 1.5x crop DSLR now, and rather like the idea of taking that out to > 10mm (say the Sigma 10-20mm). I'm not crazy about the perspective distortion you get with super-wide angle; they're very difficult to use well for that and other reasons, and while I'm sure I could find a use for it, I can't see myself using it very often just because it's not my thing. (I never had anything that wide back in the film days, either.) A fisheye is difficult as well, but it suits me better. If I had all the money in the world, I'd have the super-wide too, of course. :) > I've never owned a fisheye. Are you using the 10.5mm as a fisheye, or > for electronic linearization? Just as a fisheye; I've never tried "de-fishing" the pictures. > As a fisheye, I guess it's full-frame, not the circular image the > 6mm gives? Yes, it's a full-frame fisheye, 180 degree angle of view on the diagonal, on DX format. I've used it recently for street shooting: http://defocus.net/temp/jesus.jpg and nature: http://defocus.net/temp/prairieplant.jpg SignatureJeremy | jeremy@exit109.com >>Interesting. I had a 17mm for my film camera, and found it quite >>useful (and the 20, and the 24). So I'm glad I have a 12-24mm for my >>1.5x crop DSLR now, and rather like the idea of taking that out to >>10mm (say the Sigma 10-20mm). Somebody recently posted some great architectural shots with that, angles corrected in photoshop. From Sweden perhaps. > I'm not crazy about the perspective distortion you get with super-wide > angle; they're very difficult to use well for that and other reasons, > and while I'm sure I could find a use for it, I can't see myself using > it very often just because it's not my thing. (I never had anything > that wide back in the film days, either.) I really really like shooting with a 12-24. There are many situations where the distortion is not noticeable like in nature shots: <http://www.edgehill.net/1/?SC=go.php&DIR=California/Bay-Area/San-Francisco/Natur al-Areas/clipper/2006-04-22&PG=1&PIC=5> For architectural, I corrected the angles on some of these: <http://www.edgehill.net/1/?SC=go.php&DIR=California/Bay-Area/San-Francisco/gritt y/2006-05-05-mission-balmy&PG=1&PIC=5> Sometimes it's freaky: <http://www.edgehill.net/1/?SC=go.php&DIR=California/Bay-Area/San-Francisco/gritt y/2006-05-05-mission-balmy/full-set&PG=2&PIC=11> But sometimes the distortion is just fine: <http://www.edgehill.net/1/?SC=go.php&DIR=California/Bay-Area/San-Francisco/gritt y/2006-05-05-mission-balmy/full-set&PG=4&PIC=19> -that set has the 12-24 mixed up with 45mm. > A fisheye is difficult as well, but it suits me better. If I had all > the money in the world, I'd have the super-wide too, of course. :) [quoted text clipped - 17 lines] > > http://defocus.net/temp/prairieplant.jpg Nice! > >>Interesting. I had a 17mm for my film camera, and found it quite > >>useful (and the 20, and the 24). So I'm glad I have a 12-24mm for my [quoted text clipped - 20 lines] > <http://www.edgehill.net/1/?SC=go.php&DIR=California/Bay-Area/San-Francisco/gritt y/2006-05-05-mission-balmy/full-set&PG=4&PIC=19> > -that set has the 12-24 mixed up with 45mm. Are you talking about lens distortion, or just the perspective effects from not having the camera absolutely level? Sometimes it's hard to tell after the fact what you started from! And the uncorrected shots seem to me to be showing mostly convergence, not distortion. SignatureDavid Dyer-Bennet, <mailto:dd-b@dd-b.net>, <http://www.dd-b.net/dd-b/> RKBA: <http://www.dd-b.net/carry/> Pics: <http://dd-b.lighthunters.net/> <http://www.dd-b.net/dd-b/SnapshotAlbum/> Dragaera/Steven Brust: <http://dragaera.info/> >>>>Interesting. I had a 17mm for my film camera, and found it quite >>>>useful (and the 20, and the 24). So I'm glad I have a 12-24mm for my [quoted text clipped - 23 lines] > Are you talking about lens distortion, or just the perspective effects > from not having the camera absolutely level? Just perspective effects. The lines are not curved. Buildings get bizzare angles (sometimes) and sometimes it looks good to have dramatic angles. It's simple to change angles in photoshop though there is some loss of detail. Also if you get a person in the edge of the frame they look really fat and there's no way to really correct that other than making it into a fisheye. > Sometimes it's hard to > tell after the fact what you started from! And the uncorrected shots > seem to me to be showing mostly convergence, not distortion. > > Interesting. I had a 17mm for my film camera, and found it quite > > useful (and the 20, and the 24). So I'm glad I have a 12-24mm for my [quoted text clipped - 6 lines] > it very often just because it's not my thing. (I never had anything > that wide back in the film days, either.) I "was never" a wideangle person (this is now so long ago it's ancient history, but never mind that). But I decided to add a 24m for a trip to Australia and New Zealand in 1983 or some such (previous wide limit: 28mm), and found I liked it a LOT. So I added a 20mm rather later, and liked that enough I added the 17. > A fisheye is difficult as well, but it suits me better. If I had all > the money in the world, I'd have the super-wide too, of course. :) Well, naturally. > > I've never owned a fisheye. Are you using the 10.5mm as a fisheye, or > > for electronic linearization? [quoted text clipped - 14 lines] > > http://defocus.net/temp/prairieplant.jpg Oooh, now *that* one is really striking, and I like it a lot. SignatureDavid Dyer-Bennet, <mailto:dd-b@dd-b.net>, <http://www.dd-b.net/dd-b/> RKBA: <http://www.dd-b.net/carry/> Pics: <http://dd-b.lighthunters.net/> <http://www.dd-b.net/dd-b/SnapshotAlbum/> Dragaera/Steven Brust: <http://dragaera.info/> According to Jeremy Nixon <jeremy@exit109.com>: [ ... ] > > I've never owned a fisheye. Are you using the 10.5mm as a fisheye, or > > for electronic linearization? > > Just as a fisheye; I've never tried "de-fishing" the pictures. Nor have I. > > As a fisheye, I guess it's full-frame, not the circular image the > > 6mm gives? > > Yes, it's a full-frame fisheye, 180 degree angle of view on the diagonal, > on DX format. O.K. But not the full 180 degrees on both horizontal and vertical axis, thus not yielding the circular image of the 6mm. > I've used it recently for street shooting: > [quoted text clipped - 3 lines] > > http://defocus.net/temp/prairieplant.jpg O.K. Nice, but not quite what I want. FWIW, I've taken my 16mm and mounted it on a full frame camera to see whether it did what I thought that it did. On the Nikon F, it was cropping to a circle, but the behavior made me suspect the focusing screen, so I popped it on the NC2000d/e*, and discover that it is similar (with full frame) to the images which you posted here. So -- I *never* could get what I really want from it. :-( * Yes, I know that the NC2000d/e is a crop frame sensor based on the N90s film camera, and I have two of them. One has the outer part of the focusing screen blanked out totally (white with no image), but the other has a black line outlining the active area -- which I preferred while I was using those cameras, since I could see what was in the surrounding area to get an idea of what benefit I might get from re-composing or zooming. Thanks, DoN. SignatureEmail: <dnichols@d-and-d.com> | Voice (all times): (703) 938-4564 (too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html --- Black Holes are where God is dividing by zero --- > O.K. Nice, but not quite what I want. > [quoted text clipped - 4 lines] > (with full frame) to the images which you posted here. So -- I *never* > could get what I really want from it. :-( If you want a circular fisheye for DX format, well, you can't have one. On 35mm format you'd be looking for the 8mm f/2.8 fisheye, which can be found used for a few thousand dollars. There is no current model circular fisheye from Nikon itself, but you can get a third-party one from one or two of the Russian or Ukrainian lens places. There was also a 6mm fisheye -- 220 degree angle of view, I think -- but I've never seen one. The 8mm f/8 fisheye can't be used on current cameras (it requires the mirror be locked up to mount it). The 10mm orthographic fisheye was non-AI, but I suppose it could be modified. SignatureJeremy | jeremy@exit109.com > Today, while driving around with my camera, a thought struck me. > [quoted text clipped - 15 lines] > sensitivity, unless it includes a ND filter as part of its design (and I > see no reason to throw away the free sensitivity increase). Adding glass won't increase the lens speed, it will still reduce it. >> Today, while driving around with my camera, a thought struck me. >> [quoted text clipped - 17 lines] >> > Adding glass won't increase the lens speed, it will still reduce it. Not necessarily. He's talking about using a reverse converter to turn, say, a 12mm lens into something equivalent to a 6mm. There since the image circle is smaller for the same luminous flux (minus slight losses in the converter) the lens will effectively be faster. This is the opposite of what a teleconverter does. Such things have existed in the past for various purposes. For example the Minolta 3000RD had one built in that adjusted the focal length of Vectis lenses to the slightly smaller sensor in the 3000RD. Thinking about it, at the wide end it seems like it could be a useful device. Signature--John to email, dial "usenet" and validate (was jclarke at eye bee em dot net) |
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