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Focal reducers: a viable option?

V

Vivek

Guest
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.
 

apicius9

New member
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
 
V

Vivek

Guest
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.:)
 
M

milapse

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

Vivek

Guest
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.
 
P

pix2pixels

Guest
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.
 
R

Ranger 9

Guest
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?
 

apicius9

New member
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...|66:4|65:12|39:1|240:1318|301:1|293:1|294:200

It screws into the front of the lens.

Stefan
 
P

pix2pixels

Guest
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.
 
V

Vivek

Guest
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).

 
P

pix2pixels

Guest
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.
 

apicius9

New member
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
 
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pix2pixels

Guest
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.
 
R

Ranger 9

Guest
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!
 
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P

pix2pixels

Guest
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."
 
V

Vivek

Guest
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.
 
P

pix2pixels

Guest
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 $@280 for top of the range.
 
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