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Relay Lens System to adapt short back focal lenses
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PostPosted: Mon Jun 29, 2009 6:31 am    Post subject: Relay Lens System to adapt short back focal lenses Reply with quote

There has been a question if and how lenses with short or very short back focal length may be adapted to a camera. For instance c-mount or cine lenses to a DSLR.

Of course there is a way to get c-mount lenses to work on a camera with a larger back focal register and a larger sensor than the c-mount lens will fill - just not a very obvious and not that easy solution: a relay lens system. This is a well known method from microscopy and other science areas, whenever a small image needs to be enlarged to fit the sensor of a camera etc.

I made a few experiments myself, since I wanted to adapt a wide angle CCTV lens (140 degrees, 4.8mm focal length in my case) onto my camera. I'm still experimenting, but one solution was to use an older microscope objective as relay lens which takes the aerial image of the CCTV lens, magnifies it up by about a factor of three (3x) and projects that image to the camera sensor. [that has to be modified according to the image circle of that lens which ones wants to adapt and the sensor size of the camera to be filled]

One side effect is that the image is upside down of course. Using that principle, quite some strange lenses can be coupled to a camera or sensor system, nothing really new. The problem ist to find a suitable good enough front lens, since sharpness, contrast and CA are always a problem and a suitable high quality relay lens (for that a good special macro lens is always a very good solution like a low mag. microscope lens or the Zeiss Luminar, leitz Photar, Nikon Macro Nikkor series of special macro lenses).

Only the front lens should be stepped down btw., the relay lens has to be wide open to avoid severe vignetting (you will anyway run into that problem soon, since it is very common with most lens combinations - don't be frustrated about this). One method to overcome that is to enlarge only the center of the image of the front lens (by using a higher magnification of the relay lens), so as to achieve a better image quality, but at the cost of losing some FOV (view angle).

Here one iterim result which is far away from being accetable and it is uncropped to show the effect of vignetting, CA etc.:


and here a later stage using the same front CCTV lens, but a somewhat better relay lens:


Since that is quite some mechanical effort, a simpler method (yet with not so good image quality usually) is to use a reversed wide angle lens as relay lenses, a tube of appropriate length to attach the CCTV lens in front, like what has been done here (both images (c) Jukka Lehtonen, Finland)


to achieve a result like that:



Bjorn Rorslett has done that too here, to press a special fisheye lens into action:
http://www.naturfotograf.com/D1_fisheye_1,html.htm

This relay lens procedure is a well know method in filming when a large FOV and deep DOF is required (spare me to elaborate on the optics details here). One of the most discussed ones is the Panasonic FRAZIER lens having quite unique properties (google it if you like).


PostPosted: Mon Jun 29, 2009 8:52 am    Post subject: Reply with quote

I had earlier read Bjrn's article where he mentioned relay lenses, but without understanding what they are.

So, they take a part of the image circle and magnify it. In what way is a relay lens different to a teleconverter, which also does this?


PostPosted: Mon Jun 29, 2009 1:36 pm    Post subject: Reply with quote

No, that is sth completely different, since no auxiliary (aerial) image is being formed, thus no reversal of the image.

http://books.google.com/books?id=CU7-2ZLGFpYC&pg=PA434&lpg=PA434&dq=relay+lens+system+teleconverter&source=bl&ots=d8xPzKVucu&sig=yu8Kp6V1UKUzShY__UdGalG6zHk&hl=de&ei=t8JISoz_LtWOsAbX3rzXCQ&sa=X&oi=book_result&ct=result&resnum=3


PostPosted: Tue Jun 30, 2009 1:48 pm    Post subject: Reply with quote

I've tried something similar with a Zunow 6.5mm f/1.4 D-mount lens and a range of auxilliary lenses. Fun to do, but very unwieldy - long lens/extension ring/bellows combinations, reversed image, extremely short working distance. Try getting light onto your subject when it's touching the recessed front lens..

Still, for macro photos with the subject and a large recognisable landscape (almost) in focus it works. Maybe a small sensor compact digital camera would get you the same/similar results with much less hassle. Is there a particular reason why you want to use a DSLR Klaus? I know it's much more fun of course to do it the hard way..

Maybe a 4.5mm Sigma fisheye defished would get similar results.


PostPosted: Tue Jun 30, 2009 3:29 pm    Post subject: Reply with quote

Well yes Dick, you "caught me" - I like challenges a lot!!


PostPosted: Tue Jun 30, 2009 4:06 pm    Post subject: Reply with quote

Very interesting -

Please correct me if I'm wrong -

So Mr. Lehtonens rig is, from the camera -

reverse adapter-
wide-angle lens reversed-
C-mount adpater for Nikon-
C-female-to C female tube -
C-mount lens

Correct ?

If this works with good quality there are many very wide-angle C-mount lenses to try, not to speak of other odd optics like telecentric lenses.

I think C-to-C tubes are in stock from Edmund Optics.


PostPosted: Tue Jun 30, 2009 4:42 pm    Post subject: Reply with quote

luisalegria wrote:
Very interesting -

Please correct me if I'm wrong -

So Mr. Lehtonens rig is, from the camera -

reverse adapter-
wide-angle lens reversed-
C-mount adpater for Nikon-
C-female-to C female tube -
C-mount lens

Correct ?

If this works with good quality there are many very wide-angle C-mount lenses to try, not to speak of other odd optics like telecentric lenses.

I think C-to-C tubes are in stock from Edmund Optics.


Yes, fully correct. You're also correct about the many other lenses available. The quality is certainly an issue, so don't expect miracles there....


PostPosted: Tue Jun 30, 2009 5:40 pm    Post subject: Reply with quote

Hi Klaus,

A few examples of my experiments two years ago:




The block of wood in the first picture is a clothes peg, about 1 cm wide.

Both are the 6.5mm f/1.4 Zunow. I'm not 100% sure which relay lens I used for these - a Luminar 16mm, a 24mm Minolta reversed or my 45mm f/1.0 S-Planar.

Stopping the relay lens down makes a big difference in DoF and image circle size.


PostPosted: Wed Jul 01, 2009 10:36 am    Post subject: Reply with quote

Thanks Dick, this is quite similar to teh results I get (got). The Zunow is a 8mm cine lens?


PostPosted: Wed Jul 01, 2009 10:52 am    Post subject: Reply with quote

You're welcome. It's a D-mount cine lens from a Yashica 8mm camera with a turret for three lenses. It has a recessed front lens, unpractical for lighting extreme close ups. I also have a Yashinon 6.5/1.4, similar, but with a less recessed front lens.

From memory, the best setup was the Yashinon with a 19/2.8 Macro-Nikkor as relay lens.


PostPosted: Wed Jul 01, 2009 1:35 pm    Post subject: Reply with quote

Thanks Dick! I just got a Biotar 2/12.5mm from a nice guy here, image circle is very small, need to try a 19mm Macro Nikkor or 16mm Luminar - we'll see.


PostPosted: Wed Jul 01, 2009 9:15 pm    Post subject: Reply with quote

This thread got me to try this again, with the Yashinon 6.5mm 1.4 this time. The weak link in my setup is my homemade d-mount/m42 adapter - easy to decenter the cine lens and too many reflections. For this picture I used a reversed 18-55. The reason for using this rather than a macro-nikkor 19/2.8 is ease of use. Unlike Klaus's assertion in the first post I find that closing the aperture of the cine lens beyond f/2 decreases the image circle, whereas stopping down the relay lens has no detrimental effect on the image circle. Using a reversed 18-55 gives me an automatic aperture and therefore a much brighter viewfinder. Plus, zooming in or out you can switch between a full circle image or a crop.



Chasing bugs with this combination is a bit of a challenge, as the subject just about touches the front lens. Cleaning the front lens is a good idea as well, since any speck will be in focus as well.

BTW Klaus, is a 12.5mm lens wide enough for your needs? I thought you wanted a shorter focal length for a steeper perspective.


PostPosted: Thu Jul 02, 2009 11:51 am    Post subject: Reply with quote

Wow this is amazing! thanks for sharing


PostPosted: Fri Jul 03, 2009 11:57 am    Post subject: Reply with quote

Just two more examples:



Yashinon 6.5mm f/1.4, reversed 18-55IS



Yashinon 6.5mm f/1.4, macro-nikkor 19/2.8

What helps for these photos are insects that aren't at all camera shy and a lot of light. A considerable amount of patience is also useful since focussing is tricky, with the dark viewfinder. Liveview works to a degree, but is not that easy to use upside down with moving bees.