DIY Lens Turbo mk I

[iangreenhalgh1]

Was playing around with a Schneider WA-Cine-Xenon 2/45 projector lens that I had modified by fitting an iris and helicoid and discovered that the short back focus made it impossible to hit infinity on a M4/3 camera. Then I had a brainwave - put some optics behind the lens to increase it's back focus. All you need is a converging group, basically the reverse of a teleconverter. I was going to try usig the optical block from a Kiron 7 element TC but couldn't get the damn thing out, the retaining rings are glued. So then I had a think and a cup of tea, and thought some more and realised that the rear optical block of a double gauss lens would also work as a converging lens. Dug into my box of bits and found the rear block out of a Petri 1.8/55. Mounted this block inside the m4/3 mount so it is as close to the sensor as possible, fixed in position. The objective goes in front and moves back and forth to achieve focus.

The result is interesting. No added CA and the sharpness isn't ruined, the corners look okay too. What it has done is introduced some spherical aberration which means there is a diffuse glow. This is very noticeable with the objective's aperture wide open but is greatly reduced by closing the aperture 1 stop.

The intention for this lens is to use it for video so character is everything and sharpness is not too important as 1080p is still less than 2mp resolution.

First wide open, showing glow, then closed 1 stop to show how the glow is reduced:




Abysmal light today so just made a few close ups to test it out:





No idea how much faster or wider this thing is compared to the 45mm objective alone, but it works and should do what I need it to.

[Attila]

Interesting , idea, you can compare easily with a normal 50mm f2 lens, what you get .

[iangreenhalgh1]

Good idea. I've made another one using an ISCO projector lens and a block from a Mamiya 645 lens, I'll try that one out tomorrow, I think it has much less SA.

[Attila]

Look forward it!

[iangreenhalgh1]

Cheers, will need some luck I expect. One good thing - it doesn't cost me anything because I am just re-purposing bits from my junk box.

[DConvert]

Very interesting TFS.
The close up test shots certainly show it can work well!

BTW I believe the remainder of your double gauss lens can be used (on bellows or similar) as a soft focus lens. I converted a Pentax M50/1.7 for this after reading about it on the web. It leaves it with a focal length of ~100mm so added extension is needed to reach as close as infinity. Similar extreme glow wide open reducing as the aperture is closed.

I'm sure I must have a lens or two somewhere that could benefit from this sort of focal reducer

[iangreenhalgh1]

Hello.

Yes, you can indeed do that with the remainder of the lens. As you say, the focal length doubles. Also, the back focus distance increases, roughly double so you need to put it on bellows or extension tubes. You get a sharp enough image but with lots of uncorrected aberrations, especially spherical, makes for a dreamy look with glow, as you describe. I did it with a Helios-44 once.

[iangreenhalgh1]

Here is the much modified lens - the Cine-Xenon was hacked up and an iris and helicoid fitted. Inside the helicoid, placed as close to the mount as possible is the 'speed booster' part.



Here's the optical schema of the mutant thing: