Unusual Lens (not necessarily a macro lens)

[PaulFurman]

As a gear addict, I picked up this oddity on ebay; an old-ish digital projector lens:

It's a wide angle, about 20mm* but preposterously large and complex. Does anyone have any ideas why it's such a mammoth beast or what use I can apply it to?

[*20mm corrected from 26mm as originally posted]

It does make a nice focus-tilt lens for crop frame DLRs but is (as I said already) preposterously large. I guess the design is guided by a need for extra back-focus distance given the projection technology but I'm still kind of befuddled. It appears to be only about f/2.8 but the front element is about 142mm diameter.

I hoped maybe I could disassemble it and have fun as it has a bunch of hex-wrench slots up and down the body but those didn't seem to loosen up much that I could determine.

I thought maybe it's telecentric on the back side but no, the image circle tapers down from almost 2 inches (45mm) to less than a full frame 35mm sensor, more like a crop frame DSLR frame, which is 28mm diameter.

There is no aperture, as it's designed for a single use; projecting to a flat screen. Presumably it's got excellent flat field properties. Perhaps that's a reason for the design.

Ideas... maybe figuring out the pivot point would help with panoramas or determining the exact focal length... or figuring out where to put an aperture stop...

Sorry if this is not a macro lens but this is the best place I know to get lens knowledge. As best I can tell from comparable models, this thing is good for projecting a 1-inch subject (1-inch diameter (0.91 or 0.95-inch) video tube?) with room for perspective shift?) rated for a projection size between about 5 feet wide and about 40 feet wide... they typically have another lens design suitable for closer focus, for rear projection at smaller sizes than that. I haven't tried it much at closer distances but it should be good for focus stacked landscapes up to reasonably close focus.

Basically any thoughts about it (or similar) would be appreciated.

My best guess at a exciting use is to remove some of the camera-side elements and maybe have a less corrected but super-fast wide angle lens with some interesting properties. So any ideas about disassembly would be appreciated as well.

[Bob^3]

Hi Paul,

As a fellow "gear addict", I wouldn't automatically give up on this thing as a possible macro lens. I have similar fast WA lenses that make interesting macro lenses when used reversed. It may look down right silly to mount such a lens with its large front element on a bellows or other extension. But with the long rear focus you describe for this lens, you might just get a flat field, long working distance macro with good resolution not so limited by diffraction at f/2.8---or you could get complete mush! . Of course if it works, the hard part will be finding a way to mount it reversed on your bellows, using some kind of support bracket so it doesn't bend your mount!

[g4lab]

What a cool lens. Dead digital projectors are goldmines of interesting optical electronic and mechanical parts. Especially first surface mirrors.

What is it about the above image that made me wonder whether it was a real scene or a model layout? Maybe the limited DOF?

It does have vivid colors. I am reminded of the idea that cinema taking lenses have to have good color correction which I think is one reason why they have been pressed into service for macro use. It think the same
applies to the projection lenses.

I have a box full of parts from a dead projector too. It had what looked like a miniature funace plenum system to divide the light from a Metal Halide arc lamp into three medium format sized frames. These then illuminated three LCD panels each with a red green or blue interference filter applied. Then recombined in t big cube and on to the lens. Which was a zoom. I felt terrible taking it apart but its resolution was way obsolete and it didn't work and noone was going to fix it.

[PaulFurman]

Bob,
Well, it does sort of work reversed, I just reversed the two-tripod arrangement shown:



That has about three inches between the lens and subject with the camera almost touching the front element and some black material draped over for a bellows. The subject is blue microfiber cloth backlit in a window by sunlight softened by a curtain. Magnification is about 5x. It wasn't a very serious attempt. The thing is, this would work best with at least 5-foot wide film The lens does appear to be quite sharp, right up to the edge of the image circle even though it's only designed for 1400 pixels wide projection.


g4lab,
Yes, people use tilt lenses for a faux miniature effect, it's kind of a goofy trend on flickr, etc. There is considerable axial chromatic aberration for out of focus areas behind and in front but no apparent lateral red/green CA.

[Bob^3]

Humm, not bad Paul---beautiful bokeh!

Does it appear to be symetrical, PMF=1? If so, it should be running at about f/16.8 at 5x. And 3 inches of working distance is amazing for a 26mm focal length lens. Could be useful if you can deal with the obvious mechanical issues.

[PaulFurman]

Humm, not bad Paul---beautiful bokeh!

Does it appear to be symetrical, PMF=1?

Far from symmetrical; from the rear, the pupil appears to be 1-3/4 inches wide, looking into the front it looks about 1/4 inch diameter, so that's a pmf of 0.14? I always mess up the math though! Does that mean at 5x, it would be effective f/2.4? Probably not because that's a half-second exposure for a bright subject. The lighting has changed since then but I get 1/60 sec with a regular f/2.8 lens focused to infinity. Figuring pmf the other way as 7, it works out to f/118, which is about right for the difference in exposure time, about 5 or 6 stops difference.

Anyways, I tried again setting everything on a table both forward and reversed and it's got pretty bad CA in either direction. It's really not designed to be used closer than 5 feet away, they have a different lens for that.



Here's a shot in it's usable focus range:
hmm, it does have some red CA in the upper right corner...

[Bob^3]

Far from symmetrical; from the rear, the pupil appears to be 1-3/4 inches wide, looking into the front it looks about 1/4 inch diameter, so that's a pmf of 0.14? I always mess up the math though! Does that mean at 5x, it would be effective f/2.4? Probably not because that's a half-second exposure for a bright subject. The lighting has changed since then but I get 1/60 sec with a regular f/2.8 lens focused to infinity. Figuring pmf the other way as 7, it works out to f/118, which is about right for the difference in exposure time, about 5 or 6 stops difference.

Yeah, when I rely on my aging memory I always screw that one up too. So invoking Lefkowitz (if I'm interpreting the equations correctly), PMF = RearPupil/FrontPupil. So your second value (PMF = 7) should be about right. But the effective f# also depends on whether or not the lens is used reversed.

non-reversed
f_eff = fr*(m/P+1) = 2.8*(5/7+1) = f/4.8

reversed
f_eff = fr((1/P)*(1+Pm) = 2.8*(1/7)*1+7*5) = f/14.4

Or invoking some guy called rjlittlefield (whoever he is ):

http://www.photomacrography.net/forum/v ... f&start=15

I’ll offer my apologies before hand Rik, if I got this wrong!

[Bob^3]

Anyways, I tried again setting everything on a table both forward and reversed and it's got pretty bad CA in either direction. It's really not designed to be used closer than 5 feet away, they have a different lens for that.

Looks like it does have significant CA and reduced sharpness when used at higher magnifcations. But you never know until you test.

[Harold Gough]

As a gear addict, I picked up this oddity on ebay; an old-ish digital projector lens:

Paul,

You are not the only one to be unable to resist such items!

In recent years, I picked up a film projector lens of similar proportions for a few pounds, the going price then being into four figures.

Mine is a Tamron Pro Zoom f 3.5 5-10 inch/ 127-254mm. It would fit my Kodak Carousel projector, taking over where the Tamron Zoom I bought with it leaves off. The Tamron optics have LD glass and curved field at the slide end, to allow for "popping" from the heat. There is also something to do with "anti-keystoning" but I forget the details.

Mine is 100mm wide at the front, of which 80mm is glass. It is about 275mm long and weighs about 1550g.

Mine remains in storage. I shall be interested in any application you find for the whole lens.

Harold

[PaulFurman]

Harold, what is the back focus for that lens? If it's for 35mm slides, it will cover a 35mm sensor but might not focus to infinity. I got started on this looking at movie theater projection lenses, which are supposed to work on DSLRs, old ones with about 100mm f/1.8 specs seem to go for $50 +-

The keystoning on your lens feature would allow shift for perspective correction, meaning it has a larger image circle. If there's enough back focus, it would allow tilting the focus also, like I did in the night scene.

PS, I did some more checking and my lens is actually very close to 20mm, not the 26mm I guessed before. I'll edit that in my original post. The aperture does look right at f/2.8 comparing the same exposure with a 20mm f/2.8 Nikon lens.

I also checked the location of the entrance and exit pupils; the entrance pupil is located way up front, on the first step down after the tapered segment, the exit pupil is a little bit in front of the slender unpainted metal mounting bracket. I'm not sure what that says about the lens... again, I'm hoping that if disassembled, it might have some other interesting properties, removing the rear part, like becoming much faster, or a larger image circle. I suspect it is glued together though. I seem to recall that carefully heating a lens in the oven can loosen some of those glues.

[Craig Gerard]

Paul,

I like the "faux miniature effect" of the image in your initial post

What happens when this lense is used in combo with another?

http://www.flickr.com/photos/47889673@N07/4732535550/

Above link from thread below:
http://www.photomacrography.net/forum/v ... hp?t=10084

Craig

[PaulFurman]

Craig,

I can use a macro lens to focus on the virtual image and get it to fill the FX frame that way, or I can put a 1.4x teleconverter on but that effectively stops it down, and appears to increase DOF, which sort of loses interestingness. I did try various other lenses as shown in your link without luck.

I also played with just holding various lenses behind it and examining with my eyeballs, the only interesting result was an oddball Rodenstock XR-Heligon 75mm f/1.1 which held non-reversed behind the projector lens, gives a nice big bright peep-hole image if I put my eye right up to the back of it. Without that I have to put my eye about a foot away and it's not as big. Either way the image is upside down. If I mount the Rodenstock on the camera and use it like a macro lens as described above, then it doesn't have nearly enough magnification. Conceivably with more extension that might work but I couldn't easily get it to form an image hand held.

It seems like the rear half of this thing is already a relay lens, which is why I wonder what would happen if that portion was removed. If I google images for dmd projection lens optics there are few diagrams that look like this:



I suppose I'd lose back focus distance and it would no longer work on my camera but maybe on micro 4/3 as a 40mm f/1.2 or something?

Here's another odd relay lens setup: http://www.naturfotograf.com/D1_fisheye_1,html.htm

And a tutorial on relay systems which I do not understand (yet): http://www.edmundoptics.com/technical-s ... /?&viewall

[Harold Gough]

Harold, what is the back focus for that lens? If it's for 35mm slides, it will cover a 35mm sensor but might not focus to infinity.

It is for 35mm slides. It doesn't focus at infinity but there is about an inch of recess behind the rear element which might cause this. The only focusing is via the long-pitch, coarse helix which meshes with guides in the projector.

It may be a very long time before I do anything with it, except, if the need arises, what the manufacturer intended.

Harold