Using focal reducers with GH1/2 and old lenses?

oscillian

Am I correct in thinking that coma is due to the focal reducer lens diameter is equal or smaller than the lens aperture size? When stopping down the 50 1.4 in the example above coma disappears, so maybe a focal reducer with a bigger size lens diameter would do the trick?

Psyco

@balazer Take a 50mm lens for a big view camera and put it on a m4/3 sensor - you will get the exact same picture when using a m4/3 50mm lens. Image circle has nothing to do with beeing a 50mm lens or not.

The image circle is only limited by the finite glas diameter. The term 50mm focal length referes to the distance of the focal point and THAT determines the angle of light rays getting to a certain point on the focal plane. So, focal length and projection size is all you need to calculate the FoV (iris diameter has nothing to do with that).

So, using a 50mm full frame lens and getting a lens (focal reducer) that squeezes the FF picture down to m4/3 size will give you the exact same picture as using the 50mm lens on a FF camera. And the image will be even the same brightness while using the same f-stop.

(A 25mm m4/3 lens has to be at f1.4 if you want it to be the same brightness as a 50mm f2.8 lens on a FF camera.)

Psyco

@oscillian No, I was using a focal reducer with a larger diameter than the rear lens element.

balazer

Actually, the image will be brighter for the same aperture setting on the lens. Reducing the focal length with a focal reducer reduces the f-number and brightens the image. You'll need to decrease the exposure by decreasing the exposure time, or lower the ISO setting.

Psyco

Yes, the squeezed image will be brighter, but on a smaller area - so the same amount of photons on the sensor.

Thats why f1.4 on m4/3 is about f2.8 on FF, as FF is 4x the area of m4/3 (without any focal reducer).

Brian202020

Here is another update: Not all my parts came in yet, but enough to do some more rough tests. To my surprise it looks like the coma issue is not present, or at least I can't see it in my GH2's screen. I don't have my SD cards home to analyze a still or video, so the GH2 screen is all I have right now. Also for some reason the flange distance between my Nikon lens and GH2 body seems to decrease slightly when putting the focal reducer in. My other parts should arrive soon. More updates to come.

Here are some iPhone pictures of the screen. Lens used was the Nikon 20mm f3.5 wide open (it's the only lens I had home).

aki_hartikainen

Looks rather promising, and from your histogram one could see increase in brightness (assuming the same lens aperture).

@Psyco "Thats why f1.4 on m4/3 is about f2.8 on FF, as FF is 4x the area of m4/3 (without any focal reducer)."

This is quite true. To enlarge the m4/3 lens image circle to cover full frame would mean taking the lens twice as far from the film compared to m4/3 sized circle. Now the lens would be equivalent to a 100 mm f2.8. Only it would focus very very close, so doing this will in fact create a macrotele adapter.

I've learned that this could be a rather tricky concept and could get one easily into heated debates that might last all summer. But it is the basis of the "system" in a system camera. All aspects of the system require consideration, not just the lens.

peterosinski

@brian2020 with most focal reducers you will have a decrease in flange depth. Avoiding this is the real trick and it is why Kodak owns the patent specifically on focal reducers that do not change back focus distance.

I think that the real value of focal reducers will be on a camera like the BMC where you could replace the entire front of the camera and build the focal reducer into the new camera/ mount. Right now I am trying to talk to some machine vision camera manufacturers about building a full frame uncompressed camera because everyone I talked to said that focal reducers were not worth the effort. If you are able to make this work I think that it would be huge for a camera like the mft bmc.

Going from full frame to mft should give you a 2x crop which I would assume would be a 2x increase in brightness (1 stop). That would mean that if you reduced a f/2.8 FF lens it would show up as a f/2 on sensor. That being said, your original post showed that coma disappeared when stopping down. How many stops does it take? If you could do it with 1 stop wouldn't that mean that you could get full frame fov on a mft sensor with no coma if a 2.8 lens was stopped to f/4 but it would still give you the same amount of light as an f/2.8 lens?

aki_hartikainen

"Going from full frame to mft should give you a 2x crop which I would assume would be a 2x increase in brightness (1 stop)."

It is actually 2 f-stops all else being equal. Two times the field of view in two directions quadruples area.

Adding optical element at the back of the lens reduces flange depth because of change of light speed in another medium. The thinner the optical element, the less the flange depth is affected. I've put color compensating filters in the adapter and gelatin filter changes the focus only very little, whereas a glass filter changes focus scale quite a bit, sometimes a centimeter or two in the turn of the focus ring. Obviously a big difference in the distance and the scale. Compensating this would mean taking the lens farther from the film, but then the magnification increases eating some of the field of view.

The compensation would be best done on the lens front element and focus ring and there is often room for some adjustment.

goanna

@Brian202020

Keep up the good work!

Psyco

@Brian202020

Which focal reducer are you using? Do you have a link?

I'm not sure if f3.5 is already so stopped down that coma will be small. Can you repeat your test with a lower f-stop, f2.0 or f1.4 would be very interesting.

Brian202020

I won't be heading to my office to get another lens until after the weekend, and hopefully by then I'll have the rest of my parts to make the adapter. The link below is the Focal Reducer I'm using. Also note that in order to make the flange distance small enough I had to actually remove the glass itself from the housing.

http://www.ebay.com/itm/221125028103?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1438.l2649

JohnBarlow

Have you tried a positive acromat? SurplusShed has some cheap ones for testing

peterosinski

@brian2020 is it possible that the reason you had no coma in the pictures where you shot your face from a low angle is because the 3.5 f stop is already relatively small? Did you have a chance to try it out with a faster lens?

edit: sorry I saw psyco already asked the same question. That being said, the 3.5 should still show up as a 1.8 when used with a reducer right?

edit 2: in the before and after pictures you posted using the 20mm f3.5 how can it be that the exposure remains the same when using a focal reducer if you said you had the lens wide open and we can see that your shutter speed and iso remained the same

peterosinski

updates?

Brian202020

@peterosinski

My parts haven't all arrived yet. I purchased them on ebay and their shipping and handling seem to be slow. I'm hoping to get everything soon.

peterosinski

any updates yet?

Brian202020

Got my last part today actually. I will assemble everything next week and do tests.

GH2_fan

Please do let us know, I think the whole Focal Reducer thing is absolutely great, in my opinion, it actually restores the original focal length of any lens that is used on the GH2 due to the crop factor/sensor size.

I really do wonder if there is one that will make us able to use lenses at wide apertures like f1.8 or lower witho

GH2_fan

Please do let us know, I think the whole Focal Reducer thing is absolutely great, in my opinion, it actually restores the original focal length of any lens that is used on the GH2 due to the crop factor/sensor size.

I really do wonder if there is one that will make us able to use lenses at wide apertures like f1.8 or lower without any image degradation.

kris

I apologize for my bad english!!! Welcome the idea of Focal Reducer, but there is no danger of a little softness in the corners of the frame- reduced visual acuity in the periphery? Thank s!

peterosinski

http://www.abelcine.com/store/P-S-Technik-PRO35-Digital-Image-Converter-for-2-3-Cameras/ looks like there is an existing product that acts similar to a focal reducer, but at the cost of a mid size sedan

Brian202020

@peterosinski

The P+S Tenhnik Pro35 adapter is the first 35mm lens adapter made. The M2, M3, and Letus adapter were based of that. It uses a oscillating ground glass as a mini screen inside it to record the image off of.

Also my tests will have to wait another week maybe 2. I'll keep you all updated.

GH2_fan

So...any updates? :D

mnryan

@GH2_fan I'm pretty sure Brian just said he'd be another week or two yet with updates.

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