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Using focal reducers with GH1/2 and old lenses?
  • 61 Replies sorted by
  • PsycoPsyco
    Yes, that exactly what I mean. Unfortunately we have only about 2-3mm to move the focal reducer back- or forward, so we will need a focal reducer or something similar with much stronger light bending.
  • patvigpatvig
    @Psycho
    I found here http://cgi.ebay.fr/Antares-1-25-0-5x-Focal-Reducer-for-Telescope-/110721122830?pt=LH_DefaultDomain_0&hash=item19c77e420e some interesting comments about reduction ratio variation:

    "The actual reduction will vary a little over or under 0.5x depending on exactly where the focal reducer is placed in the optical path."
  • PsycoPsyco
    Yes, thats exactly what I think it will be.
  • pingpongcomputerpingpongcomputer
    so if a focal reducer were found that put the entire ff image circle on the m4/3 sensor, and a 24mm f2.8 lens had a usable/crisp image at f5.6, then one would effectively have the equivalent of a 12mm 2.8 m4/3 lens?
  • PsycoPsyco
    @DanDOF
    Sad to here that you got stuck at the same point. But, as I know from my own experience, you learn so much more from trying than only thinking about it...;-)

    @andres
    Yes, 0.5 means half the focal lenght, but only when used in a telescope (these reducers are designed for telescopes). So I would guess the 0.5 only works for parallel light rays. Beeing so close to the main lens and sensor means everything but parallel rays.

    @pingpong
    If you get 20% more field of view, you also gain about 20% in brightness (in ISO or F-stop). Its a bit tricky to calculate as you have to take into account that you start with 20% width, a linear dimension, then convert it to area increase an then go from that to the ISO or F-Stop numbers. Sorry, I can't find the calculations I did when I tested this setup :-(
    But you really get the light of the bigger image circle on the m4/3 sensor - so if you get all the full frame image down to m4/3 size you would gain 4x the light per pixel = 2 F-Stops (minus the light the focal reducer eats up, but thats only a tiny fraction). With the 50mm F1.4 that would result in a crazy 50mm F0.7 lens!

    The image gets more and more of that coma glow when you open up the aperture - a smooth transition.
  • pingpongcomputerpingpongcomputer
    how much light do you gain from using the reducer?
    how does the image look at f2.8/4/5.6?
  • andresandres
    This is an interesting topic, I think many have the same ideas, glad to see you are putting your hands on it. Funny I thought that a 0.5 reducer would divide the focal lengh by 2 instead it seems like 0.8. In the other thread there is a lot of work, I wonder what lenses have a built in reducer for making that effect.
  • DanDOFDanDOF
    This really pains me to see. Why? Because I spent 8 months between 2009-2010 developing a focal reducer by doing all the math, prototyping, and considering legal cracks between the definitions claimed in different patents. I did all of this - just to find out the same issue as you've come to - COMA (i.e. field curvature).

    I was so looking forward to the day where I presented my product to you all. It sucks to try something and fail, but I learned a lot in the process.

    Congrats on what you've done here (as shown in the first post). I'm impressed. I wish I could help get to the next step of solving the physical/optical limitations presented by designing a custom focal reducer + field flattener small enough to fit into the cavity of a m43 body. My dream murmurs in the back of my mind. Oh sigh!
  • bubbabubba
    yep'
    ...was for an 8mm movie camera.
    I'll dig around for it, but was a "mirror box" type.
  • PsycoPsyco
    There are old c-mount anamorphic adapter O.O

    I didn't know that - can you give me some names to look for (e.g. on ebay)? That might be a nice way of getting the desired pieces of glas.
  • bubbabubba
    I like the way you're thinking.
    I bought an early 90's Nikon/Fujix E3 digital camera to chop
    up for the excellent focal reducer, inside. This lens reduced FF to 2/3 chip size. Damn thing is now worth too much to do it.
    Next idea was to put an ole delph c-mount anamorphic adapter
    between the lens and sensor. ....This may just work....
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