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Thread: Focal reducers: a viable option?

  1. #1
    pix2pixels
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    Focal reducers: a viable option?

    Did anybody had a look into the possibility of using astro focal reducers with this camera?

    Really worth exploring!

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    Re: Focal reducers: a viable option?

    I use a few (not the ones from the link).

    Question for you- what is the difference between an astro focal reducer and a photo/video wide angle converter?

    Also, as a further hint, check out the Cine Ektar 25mm lens. A dedicated reducer was made for it by Kodak.

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    Re: Focal reducers: a viable option?

    Quote Originally Posted by Vivek View Post
    Also, as a further hint, check out the Cine Ektar 25mm lens. A dedicated reducer was made for it by Kodak.

    Mmmhh, with the 15mm reducer for the 25mm Ektar I just get a nice round circle image - is that what I would have to expect from a focal reducer?

    Stefan

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    Re: Focal reducers: a viable option?

    Typically, yes.

    But, it depends entirely on the lens used and the converter used.

    A 0.5X converter turns my 25/1.9 Xenoplan into a 12.5mm equivalent lens with heavy dark corners whereas a 0.65X New Century converter works very well on one of my 25mm lenses with minimal vignetting.

    The possibilities are there and it is yours to explore and use it.

  5. #5
    milapse
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    Re: Focal reducers: a viable option?

    is there such a thing as a focal reducer for nikon 35mm lenses?

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    Re: Focal reducers: a viable option?

    Quote Originally Posted by milapse View Post
    is there such a thing as a focal reducer for nikon 35mm lenses?
    Yes. Nikkorex camera had a wide and tele converter.

    Coolpix cams have an array of converters.

    The one that would mount on the back of a Nikon F mount lens are also there but they cost upwards of $8,000/each.

  7. #7
    pix2pixels
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    Re: Focal reducers: a viable option?

    From my understanding these astro focal reducers also increase the amount of light of the projected image (like a magnifier). So a 24mm f2 will become a 24mm T1.4 (100% more light for 0.5x magnification), a.s.o...

    They are mounted between the lens and the camera.

    I looked at one in telescope shop yesterday, and hope to talk to someone who actually builds some of them.

  8. #8
    Ranger 9
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    Re: Focal reducers: a viable option?

    Quote Originally Posted by Vivek View Post
    Question for you- what is the difference between an astro focal reducer and a photo/video wide angle converter?

    Also, as a further hint, check out the Cine Ektar 25mm lens. A dedicated reducer was made for it by Kodak.
    Okay, here's my attempt at an essay answer to Professor Vivek's pop quiz:

    The common photo/video wide-angle converters ("tele" converters also are available) mount on the front of the lens. They are afocal -- the converter itself isn't capable of forming a focused image. It needs to be attached to a lens that is capable of forming a focused image. (Incidentally, this is the original and correct usage of the widely-abused term "prime lens" -- a prime lens is the image-forming component of a system that also includes an afocal converter.) You can see the effect of the converter by holding it up to your eye and looking through it; your eye's lens becomes the "prime lens" in this optical system.

    It sounds to me (based on what I saw on the linked sites) as if the astro focal reducers instead mount behind the prime optical system. This means that optically, they would be just the opposite of the tele-converters that many of us have used on SLR lenses, or the "Barlow" lenses that the astros use to get more magnification out of a telescope.

    These tele converters use a negative (diverging) lens to enlarge the image, and a spacer to move the system farther from the image plane to correct the focus.

    So the focal reducer would have to do just the opposite: use a positive (converging) lens to reduce the image onto a smaller area, and some kind of mount that can bring the optical system closer to the image plane.

    It's this latter requirement that seems as if it might be tricky for camera use. Moving the optics closer usually isn't a problem in a reflecting telescope, which has a long focal length to play with and lots of empty space inside. But it could be tricky with still-camera optics, where the lens has less free space behind it and the camera has various mechanical bits that would get in the way.

    So while there might be a way to use one of these attachments to make a long, slow lens faster and brighter on Micro Four Thirds, I'm guessing that most of the time it would make more sense to use a readily available shorter, faster lens in the first place!


    So that's my essay answer. I'm not worried about my grade from Professor Vivek, because I'm just auditing this course.

    But here's a follow-up question about that 25mm Cine-Ektar: is its focal reducer a front-of-lens or behind-lens variety? Anyone got a picture of it?

  9. #9
    pix2pixels
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    Re: Focal reducers: a viable option?

    There is also a thread at DVXuser that could also shed some light.

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    Senior Member apicius9's Avatar
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    Re: Focal reducers: a viable option?

    Quote Originally Posted by Ranger 9 View Post
    But here's a follow-up question about that 25mm Cine-Ektar: is its focal reducer a front-of-lens or behind-lens variety? Anyone got a picture of it?
    Thanks for the explanation. My personal conclusion is I'd rather wait for someone else to try that out

    Reg. the Kodak: I don't have a picture pf my own, but here is a borrowed one from ebay (no relation to the vendor):

    http://cgi.ebay.com/Kodak-Ektar-f-1-...%3A1|294%3A200

    It screws into the front of the lens.

    Stefan

  11. #11
    pix2pixels
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    Re: Focal reducers: a viable option?

    I am talking about this adaptor.

  12. #12
    pix2pixels
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    Re: Focal reducers: a viable option?

    After further reading, it looks like a 0.5x focal reducer will increase the light 4 times.

    So, a 50mm at f1.4 will project an image 2 stops brighter: ~ f 0.75 and keeping the equivalent FOV of a 50mm on FF camera.

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    Re: Focal reducers: a viable option?

    Xenoplan 25/1.9 with a 0.5X converter (screws on the front), whole frame, near infinity (which is just past 1 meter mark) f/2.8:




    The same setup, closer distance (~45cm)



    No detectable exposure differences with the converter, though images do seem bright.

    If you want to reduce the image circle and increase the brightness, try this-

    stack a (say) 50mm f/2 AI nikkor on another 50/2 AI Nikkor and mount it on the G1. Do not expect an f/1 brightness!

    Does this work for you Ranger?

    Here is an example through my favorite 25mm lens plus a 0.65X New Century Converter (40.5mm to 37mm ring needed, this is the cheapest 0.65X converter from New Century/Schneider. Check eBay (HK)). The lens is a Computar-TV 25/1.3 (already in Monza's list).


  14. #14
    pix2pixels
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    Re: Focal reducers: a viable option?

    This is based on the telecompressor principle.
    Stacking two 50mm lenses front/front is one of the best macro solutions: much cheaper than a dedicated macro lens.

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    Senior Member apicius9's Avatar
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    Re: Focal reducers: a viable option?

    Quote Originally Posted by pix2pixels View Post
    This is based on the telecompressor principle.
    Stacking two 50mm lenses front/front is one of the best macro solutions: much cheaper than a dedicated macro lens.
    I heard about that but never really thought about it much. So, just an example to make sure I get that right: I have a Canon FD 28/2.0 and a 85/1.8, both with 55mm threads - if I get one of these reverse coupling adapters in 55mm I can just stack them together and get a macro lens? Can I combine any lenses? How can I calculate what length and aperture I will be ending up with?

    Stefan

  16. #16
    pix2pixels
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    Re: Focal reducers: a viable option?

    I really don't kow.
    Get a male to male adapter, or for testing, just gaffer tape the lenses.
    Some tips:
    - Use the 85mm on the camera.
    - Do not stop down the front lens (leave it wide open, otherwise you'll get serious vignetting)
    - Ideally, two identical lenses will work great.

  17. #17
    Ranger 9
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    Re: Focal reducers: a viable option?

    Prime (image-forming) photo lenses always have a net positive power; if they didn't, they wouldn't be able to form an image.

    Screw-on close-up lenses (aka "diopter lenses") also have a net positive power.

    So when you stack two lenses as described above, what you're really doing is using one of them as a multi-element, highly-corrected close-up lens. It can work really well as long as the design of the lenses avoids vignetting.

    Close-up lenses have no influence on the effective aperture of the prime lens, so the combination should require little or no exposure compensation (you might lose a small amount to absorption.) As pix2pixels says, leave the front lens wide-open; it's only being used as a close-up lens, so closing down the diaphragm wouldn't reduce aberrations or extend depth-of-field.

    All this is completely different, though, from the astro 'focal length reducers' that mount behind the prime lens. I'm still curious to see whether these significantly increase brightness by reducing the size of the image. I still think fitting one into a Micro Four Thirds or DSLR camera would be a challenge... but a view camera with a digital back might be a good test case. Anyone own both one of those and a telescope?


    Incidentally, getting back to those screw-in close-up lenses again: the diopter of a lens is equal to 1/focal length in meters. For example, a "+1" close-up lens has a focal length of 1000mm. A +2 lens would have a focal length of 1/2 meters, or 500mm, and so forth.

    Going the other way, a lens with a focal length of 100mm (0.1m) would have a diopter of 1/0.1 or +10. A garden-variety 50mm lens would have a diopter of +20, which would make very close shots possible... but you also might not be able to get the lens "stack" close enough to the subject to bring it into focus, let alone get any light on it!
    Last edited by Ranger 9; 7th May 2009 at 19:54.

  18. #18
    pix2pixels
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    Re: Focal reducers: a viable option?

    The image circle from the focal reducer according to Optec is 17.5 -18mm:
    Quoting from their brochure:

    " NextGEN Ultra WideField 0.7XL (NGUW) NEW DESIGN
    ...The speed of a f/8 telescope would be increased to f/5.6 with this reducer and the optics would still be nearly diffraction limited. This four-element system also maintains the parfocality of the telescope since these types of telescope have more limited focus compared to SCTs. The image field is highly corrected to 18mm diameter so that large format CCD cameras can be used."

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    Re: Focal reducers: a viable option?

    Quote Originally Posted by Ranger 9 View Post

    All this is completely different, though, from the astro 'focal length reducers' that mount behind the prime lens. I'm still curious to see whether these significantly increase brightness by reducing the size of the image. I still think fitting one into a Micro Four Thirds or DSLR camera would be a challenge... but a view camera with a digital back might be a good test case.


    The astro stuff would be useful only with a telescope of long- very long focal length- lenses.

    For use with a G1, the stacking I mentioned would be more useful. BTW, do not reverse any lenses while stacking!

    Can you brighten up the image circle? Yes. Are there limits to it? Yes.
    The dreams of a 0.7 lens doing this or using an astro focal reducer have limitations.

    I have the ingredients to make the focal reducers that Nikon sell (special order) for a lot of cash (Arri, etc mount). It involves a Repro Nikkor 85/1 (acting as a relay lens). I never made it as even a semi-permanent set-up because the Repro Nikkor weighs nearly 1 kg and is ~15 cm long.

  20. #20
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    Re: Focal reducers: a viable option?

    Vivek,
    Have alook at this test.

    There's no Nikon made telecompressor that I know of. Please give us a link.

    The astro ones, sell between $<90 and [email protected] for top of the range.

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    Re: Focal reducers: a viable option?

    pix2pixels, Search for FC-ENG TMW-B1 (B4 mount, I have seen those converted for Arri mount).

    While the thread you link is fascinating and informative, actual use on G1 and the image quality would entirely depend on the lens used and the reducer used (the combination). Good luck with your experiments and I very much look forward to what you come up with.

    Also, those who are interested in history would find what Nikon did interesting.
    See: http://www.mir.com.my/rb/photography...2E2s/index.htm

  22. #22
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    Re: Focal reducers: a viable option?

    I'd be the last one to discourage experimentation, but other than for Feynman's "pleasure of finding things out" (and I certainly don't want to underrate this)...

    ...To get enough space behind the lens to mount the reducer on your G1, you'd probably have to start with a 35mm-format or medium-format lens. The reducer would convert this into a lens with a shorter effective focal length and a faster effective aperture.

    But since there already are so many short-focal-length, fast-aperture cine lenses available, and it's so easy to use them on the G1 with readily-available adapters, it's hard for me to see the point of this exercise.

    Don't let that stop anyone from trying it, though, and like Vivek, I'd love to see some pics!

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    Re: Focal reducers: a viable option?

    Quote Originally Posted by Vivek View Post
    pix2pixels, Search for FC-ENG TMW-B1 (B4 mount, I have seen those converted for Arri mount).

    Make that FW-ENG TMW-B1.

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    Re: Focal reducers: a viable option?

    I am also curious about that because I also find something with a similar design of Century optics does.

    Opteka 0.3x wide angle

    This lens is supposed to use on Video. The comment is not bad in some forum especially the skate broad player they need wide angle to capture the fast movie shot in limited space. Have someone used it? The only question is that would this lens degrade the image quality since it is resolution may not be superior enough for still photo.

  25. #25
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    Re: Focal reducers: a viable option?

    Quote Originally Posted by Vivek View Post
    Make that FW-ENG TMW-B1.
    I had a go with Google and couldn't find anything relevant except a patent application for a 'telepresence' system that suggested this as a way to mount an 8mm Nikkor fisheye on a video camera. It didn't give any information about the converter.

    Searching Nikon's global website didn't turn up anything either.

    Any further leads? I don't need to know, but I'm curious...

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    Re: Focal reducers: a viable option?

    The one from Century (no idea why I kept typing "New Century" )
    is this 0.65X adapter: http://www.schneideroptics.com/ecomm...=1069&IID=1757

    I bought it (new) for $50 (shipped)from Hong Kong. It works just fine with the 25/1.3 Computar-TV lens (see sample shot). There is a slight hint of a CA that can be taken care of in the post. Sharpness wise, the lens becomes sharper!

    I chose that particular one for these reasons:

    1. Size/weight.
    2. Price.

    The 0.3x is a fisheye, likely to be a very large one and heavy.

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    Re: Focal reducers: a viable option?

    Ranger, I have saved images of the FW (for wide lenses) and the other converter (boxed with Nikon's box) from eBay vendors. I do not want to post them here.

    See this current listing (another system) which is similar in concept:

    http://cgi.ebay.com/ws/eBayISAPI.dll...m=250404593920

  28. #28
    pix2pixels
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    Re: Focal reducers: a viable option?

    Vivek,
    JVC, and many other video cameras have use a trichroic beamspliter prism to divert the image formed by the lens to the 3 RGB sensors as described in this article.

    The lenses (stills, movie) designed for flat field projection will cause severe chromatic aberrations if one attempts to use them in a 3 chip camera.

    There are two different types of adapters:
    - For using Cine, stills lenses on 3 chip cameras
    - For using ENG lenses on cameras with single chip Bayer type sensor.

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    Re: Focal reducers: a viable option?

    Indeed. The link I gave is to show how the Nikon adapter might look like (particulalry the FW or the wide angle use one).

    OTOH, the Computar-TV 25/1.3 I use is most likely made for B/W security cameras. I doubt if Panasonic's proposed 20/1.7 can better its performance and ease of handling.

  30. #30
    borissimo86
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    Re: Focal reducers: a viable option?

    Don't know if I should resurrect an old thread. I've been a long time visitor of these forums, but this is my first post. Specifically, this is one of the threads that inspired me, so I'm playing it forward:

    YES, it is possible to use an astro focal reducer with a micro 4/3 camera. I've done it. If anyone's interested, I'll post some pictures of both the adapter and sample images, but here's the gist of it:

    PROS:
    + Wider field of view, more lens coverage
    + Faster lens speed. Great for F4 lenses or slower

    CONS:
    - *Slightly* reduced sharpness and contrast, but not much
    - Doesn't make fast lenses any faster. In fact, it makes them look terrible

  31. #31
    jwestra
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    Re: Focal reducers: a viable option?

    I am interested.

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    Re: Focal reducers: a viable option?

    Quote Originally Posted by Vivek View Post
    The astro stuff would be useful only with a telescope of long- very long focal length- lenses.
    Well, I've seen them used on scopes from f5.9 to f10 focal ratios. The converters actually improve the image by flattening the field, producing better edge definition. Camera lenses (most of the teles) already provide flat fields. The reducers come in ranges from f3.3 to f6.3 (so your 2000mm f10 telescope becomes an f6.3 1260mm, as an example or even a f3.3 660mm!). However, except for the f6.3 from Celestron, Meade and generic sources which was made to support 35mm film formats, the others are made for smaller astronomical CCDs and might not support the whole 4/3rds field. Unlike the relatively poor quality camera converters which screw on the front of lenses, the astro units go behind the lens so they would need to be adapted from their threaded mounts. However, it might be worth experimenting with.

  33. #33
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    Re: Focal reducers: a viable option?

    I used a Vixen 0.6x C-mount reducer. It was made for 2/3" CCD cameras but it actually covers the whole 4/3 sensor! Funny enough, it never actually worked with my C-mount lenses. The rear fit into a C-mount just fine, but the front (female part) was made for 1.25" eyepieces. It might be good for astronomy and telescoping, but pretty useless for general photography out of the box. It was expensive too, something like $185 used if I remember correctly. I know there are cheaper reducers out there, but I bought this one thinking it will work with C-mount lenses, as the name implies. I was wrong. Because of micro 4/3's flange focal distance, this reducer will only work with Nikon lenses, and even longer flange focal mounts like T-mount, and PL-mount. By the way, this particular reducer has been discontinued now and is almost impossible to find online.

    I unscrewed the actual reducer from its original mount and put it through a regular C-mount to micro 4/3 adapter that I got off Ebay. I filed the adapter's opening to allow for the reducer's larger diameter. Then, I had to file the front and back "hoods" of the reducer to get the optic closer to the sensor in the back, and closer to the lens in the front. It is all held in place by superglue. When mounted, it's a very tight fit; I really can't get the reducer any closer to the sensor without hitting something or getting it jammed.

    Here is how it looked like modified to c-mount :


    And this is how the finished adapter looks, with the Nikon mount:



    It fits my E-P1 like a glove to a hand! I calibrated it for infinity focus with Nikon lenses (actually, it goes past infinity, but only slightly). There is no blur, no tilt/shift effects, no stray light pollution, or any other outside factors affecting the image quality. Lens speed is increased, and it's pretty sharp for what it is, although contrast is decreased. Here's a test... it was shot at ISO800 to avoid blur from my handshake, but I'll have some low-ISO shots soon for you guys:


    ROKINON FISHEYE 8/3.5 NO REDUCER:


    OLYMPUS E-P1, F3.5, P MODE, -1.7 EV, ISO 800, 1/13 sec



    ROKINON FISHEYE 8/3.5 WITH 0.6x REDUCER:


    OLYMPUS E-P1, F3.5, P MODE, -1.7 EV, ISO 800, 1/25 sec


    LINKS TO FULL SIZE IMAGES:
    NO REDUCER: http://lh5.ggpht.com/_vO7nk0ZUy_k/TN...no-reducer.jpg
    WITH REDUCER: http://lh3.ggpht.com/_vO7nk0ZUy_k/TN...p1-reducer.jpg

    Being a fisheye, the distortion is (obviously) caused by the lens and NOT the reducer. The vignetting seen above is also not from the reducer but from the lens. The fisheye lens is DX format, made for Nikon's APS-C, so it creates a smaller circle than a lens designed for a full-frame would. I actually shaved off the fisheye's hood to get more coverage for this test! I get no vignette with other, full-frame 35mm, lenses.

    Here is a vignette test for this fisheye with the reducer. This tests the lens vignette, NOT the reducer's: http://img822.imageshack.us/img822/5...heyevignet.jpg
    And here is a test with the same lens on full-frame (Canon 5D) and APS-H (Canon 1D) cameras: http://www.360pano.de/en/samyang_5D_1D.html

    As you can see, the lens coverage with the 0.6x reducer is somewhere between APS-C and Canon's APS-H. This means that some of Nikon's DX lenses will vignette with this reducer.

    I have a couple of more Nikon lenses to test the reducer with: Nikkor 105/2.5, Nikkor Sigma 30/1.4, Nikkor 43-86mm F3.5, and Nikkor 28/3.5. I will provide more sample images soon :happy:

  34. #34
    jwestra
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    Re: Focal reducers: a viable option?

    Thanks for the response. I already found this on a different forum before your update .
    Looks promising but difficult to judge with the fisheye. Do you also have some examples with a different lens (I think a full frame lens would be perfect)

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    Re: Focal reducers: a viable option?

    Hi Vivek. Do you have a link where you bought the Century 0.65x for 50/-

    I can only see the old Sony PD150 ones on ebay. And a New Century adapter for 300/-

    I assume you used a40.5mm - 37mm step down adapter for it..

    Quote Originally Posted by Vivek View Post
    Indeed. The link I gave is to show how the Nikon adapter might look like (particulalry the FW or the wide angle use one).

    OTOH, the Computar-TV 25/1.3 I use is most likely made for B/W security cameras. I doubt if Panasonic's proposed 20/1.7 can better its performance and ease of handling.

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