@Brian202020

Keep up the good work!

@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?

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).

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.

@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.)

@RRRR "Personally, I don´t think optical adjustment is the way to go if you want a wider FOV. And the assumption that the image gets brighter by a wider focal length is just plain wrong.."

Focal length itself does not change brightness, but changing the magnification or size of the image circle does. If you took a lens farther away from the film thus creating a macro adapter, the focal length remains unchanged while magnification and image circle increases. The image is enlarged and becomes dimmer. This same would work the other way around and that is what the focal reducer would achieve.

Lots of interesting and good thinking in this thread. The back of the lens is more critical in the optical path than the front of the lens. It is even difficult to make a good quality tele converter and this problem is magnified when trying to create a wide converter at that end. If it is difficult to produce a good quality wide converter for the front of the lens, it would be more so at the back of the lens.

This must be the reason good quality optical adapters for adapting lenses of different flange distances are rare. Canon produced one years ago to fit FD lenses to EOS flange distance, but even they chose to make it a tele converting adapter and not the other way around.

Here is a quick update on my findings. I had to order more parts, so most of my testing has gotten delayed until they arrive. Here is what I believe I figured out from what I've seen with my limited testing so far. I believe I found out why @Psyco wasn't getting a full 2x image size, and if has to do with the distance between the back of the lens and the focal reducer. When I get my new parts I can verify this with images. As for the coma issue, it still looks present. once I get the new parts and can verify the 2x image increase, I might either buy a separate coma corrector and put it inline with the focal reducer or just buy a more expensive focal reducer with built in coma correction. These are expensive and I'm not willing to pull the plug on that unless I have a pretty good idea everything else works correctly.

Anyway my goal is to come up with a list of things to buy and where to buy them to make a M4/3 to Nikon F or Canon EF adapter with a built in focal reducer that works. I'll keep you all posted.

I was talking about the lens's angle of view. A lens's angle of view is a property of the lens, independent of what camera it is attached to. A focal reducer generally won't change a lens's angle of view.

Please don't talk about angles of view in millimeters. That's the original source of this confusion. Angles are expressed in degrees.

There seems to be some confusion...

With the focal reducer added, the new lens system that that you just created absolutely does have a shorter focal length, and also a smaller image circle. The angle of view does not change. Focal length alone does not determine a lens's angle of view.

@Brian202020 I'm curious to here your ideas :-)

@RRRR Its not like a shorter focal length, its more like compressing the FF image onto the smaler sensor - and that would make the image brighter (I got that as one of the expected results of my test).

@Brian202020 Nice! This is what the GH3 will need if the crap factor is greater than the GH2.