rear filter for RB fisheye lens

Discussion in 'Medium Format' started by neil_grant, Jun 7, 2010.

  1. I would like to fit a Hoya R72 whose smallest available size of 46mm exceeds the 40.5mm thread of the rear element. Looks also that the dedicated rear filters for this lens are slim-frame types to avoid collision with the reflex mirror. I'm wondering if it's possible either to:
    1. remove and cut down a 46mm glass to 40.5mm and mount into a 40.5mm frame?
    2. make some sort of collar to fit neatly between the rear group and the protecting collar with a 46mm thread.
    Anyone been there?
  2. I don't use infrared but my 37mm. came without any filters - annoying to think there must be dozens of ex-owners of these lenses with filter sets lying forgotten in drawers, isn't it?
    Mine is the RB version and I've just taken some measurements with a digital caliper gauge.
    Acceptable projection of a filter behind the lens varies with diameter of course but I wouldn't risk more than 4mm. at the 12 o'clock position.
    So here's what I'm going to do. Because step-up rings are much slimmer than filters (often around 2.5mm. + male thread) and because 46mm. filters are available from decent manufacturers, I'm going to take the glass from a 46mm. filter and bond it into a 40.5 to 46mm. step-up ring. First, if necessary, I'll file and dress the male thread of the step-up ring until it screws into the lens thread all the way to the shoulder of the step-up ring. This to ensure minimum rearward projection of course.
    If I'm not too lazy I may even bevel the filter & peen the edge of the step-up ring instead of bonding them together.
    Hope this helps, good luck!
    1. Grind glass wet to reduce localised heat input.
    2. Any insecurity of attachment could result in broken mirror/jammed-on lens/scratched rear element.
  3. It's going to be difficult to focus with that R72 in place !
  4. Thanks David. I started to think along the lines of stepping rings too. How would you remove the glass filter from it's rim?? I also have several non-Mamiya 40.5mm filters whose thick rims prevent complete mirror movement - one of my friends is trying to machine them down a bit.
    Wayne, I realise that the R 72 is visually opaque. I currently use a 77mm one on the rest of my RB lenses and Rollei IR 400 film. I just want to keep things consistent.
  5. Hi Neil, some filters have a notched locking ring screwed in from the front which is the same thread as the filter, most have a circular wire spring - used as a circlip - which is removed from the rear, as is the glass. A few have a pressed-in ring with an interference fit, ie. tight and, effectively, permanent.
    The Hoyas I have all use the spring circlip, can't remember if they always did though. Use a magnifier and you'll see the tiny gap in the spring. Its free diameter is greater than that of the filter frame and it's compressed into the groove.
    I have spring-hooks which I would use but two small-to-medium-sized sewing needles could easily be used instead. Wear eye protection & remember needles are brittle.
    Work on one end of the spring only. Pry an end slightly with one needle, push the other needle behind the spring and lever the end right out. Usually comes out easily once you can grab it, if not pull towards the centre of the filter, not upwards. The glass will fall out if you invert it so work over something soft.
    If it's a locking screw use a lens key/pair of dividers/pair of scissors/crossed screwdrivers.
    For a pressed-in ring I would sacrifice the filter mount by cutting right through it at opposite sides to remove the glass safely. I haven't seen this type for many years.
    Usually I would use UV-setting Loctite Glass Bond but not sure about an R72... keep everything scrupulously clean, give it a couple of days of daylight to cure and it should be OK.

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