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Favor to Ask of Samyang/Rokinon/Bower 7.5mm Fisheye Owners

Discussion in 'Native Lenses' started by ijm5012, Oct 24, 2014.

  1. ijm5012

    ijm5012 Mu-43 Hall of Famer

    Oct 2, 2013
    Pittsburgh, PA
    Ian
    Could somebody please let me know what the diameter of the front element is? I want to know because if it is small enough, I plan on buying a MagFilter (link), adhere it on the plastic portion that surrounds the front element, and either use 58mm filters, or buy a step-up ring and use 67mm filters, so that I can use my CPL and 10-stop ND filters to do some long exposure shots with a fisheye lens.

    If someone could let me know what diameter the front element is, I would greatly appreciate it!
     
  2. kwalsh

    kwalsh Mu-43 Top Veteran

    775
    Mar 3, 2012
    Baltimore, MD
    Glass element is just about 32mm. Around that is a raised plastic ridge surrounding the glass, OD of that is about 39mm. The ID of the permanently attached hood petals is about 56mm.
     
  3. ijm5012

    ijm5012 Mu-43 Hall of Famer

    Oct 2, 2013
    Pittsburgh, PA
    Ian
    Thank you very much, extremely useful information!
     
  4. wjiang

    wjiang Mu-43 Hall of Famer

    Are you expecting full frame coverage? I just tried a 58mm filter on the hood, the entire edge of the filter is within the frame. I also tried a 55mm filter inside the frame, same problem. Given that it's a 180° diagonal fish eye, anything vaguely in front of that bulbous front element that's inside the hood boundary (e.g step up rings) is likely to show up in the frame.
     
  5. ijm5012

    ijm5012 Mu-43 Hall of Famer

    Oct 2, 2013
    Pittsburgh, PA
    Ian
    When converting to a rectilinear image, I'd likely crop the corners anyways to help eliminate some of the smudging. The other option I'm looking at is doing the same but with the Olympus 9mm fisheye.
     
  6. Edmunds

    Edmunds Mu-43 Regular

    158
    Oct 16, 2012
    Yeah, I just tried placing an ND filter inside the lens hood, with the filter glass hitting the lens glass. You can still see the whole filter. I have a Cokin filter holder, if I place that in front, its way worse, because you can see the whole filter holder.

    Of course, you can still get a usable image, but you're gonna have to crop away quite a lot.
     
  7. ahinesdesign

    ahinesdesign Mu-43 Veteran

    432
    Dec 6, 2011
    NC, USA
    Aaron
    One solution I explored for filters on the 7.5mm was to add gel filters to the back of the lens. This - in theory - eliminates any vignetting or visible filter mount in the image. In reality, it is a real challenge to fix a filter on the back of the lens where it won't interfere with the lens focusing mechanism or a camera's lens mount, lens communication pins, or shutter.

    I was able to successfully use 4-stop and 10-stop ND gel filters, but was never satisfied with the resulting image quality. I attached the filters with small pieces of gaff tape. Because the rear lens element moves when focusing, the filters only remained flat in a limited range of focus (near infinity, if I remember correctly). Both gel filters seemed to cause bright areas of an image to "glow" due to what I suspect is not being coated for reflection reduction (or perhaps that is just the nature of such dark ND gel filters behind the glass elements). The 4-stop filter seemed to cause an overall haziness to images, while the 10-stop did not. I never noticed the same effect with glass ND filters on the front of a lens (although I have seen increased flare and light reflection).

    I also purchased a gel filter holder for the Panasonic 8mm fisheye to attach to the 7.5mm, but never found a non-permanent way to affix it. If one were willing to use a good adhesive, or dissemble their lens and drill/tap for mechanical fasteners, it would work great.
     
  8. eteless

    eteless Mu-43 All-Pro

    Jun 20, 2014
    While all of these measurements were taken using a mitutoyo caliper the difference in angle from square would induce far greater error, if there was any doubt multiple measurements were taken at different points to confirm a reading - If you plan on machining one yourself, measure your copy and confirm any number you plan on using. Any measurement of the glass to do with protrusion is generally an estimation (I wasn't prepared to touch the glass). The serial number of the lens I measured was S114C0081 just in case there is batch variance.


    The lens hood petals are 56.5mm ID, 60mm OD, and 1.75mm thickness and protrude 6.5mm and 14mm from the flat part with text on the front of the lens, this flat part is 33.5mm from the flange.

    The glass part of the front element is 33.5mm OD, the metal surround is 39.5mm OD, Thickness is therefor 3mm. The hole in the surround it moves inside is 40mm ID (0.25mm clearance either side of moving element). The glass protrudes 5mm at infinity and 8mm at the closest focusing distance. The flat surround is raised 0.5mm and 3.5mm above the surround at infinity and closest focusing distance respectively (3mm movement).

    The camera end of the lens has a 36mm ID on the inside of the mounting bayonet (inside of the silver part), the glass element is 12.5mm OD and protrudes around 1mm from the flat metal moving surround. The moving surround is 18mm OD however it has a beveled edge (the top of the bevel is 17.2mm diameter however it's hard to take an exact measurement, on my copy it wasn't 17.0mm despite me feeling that this would be the number they aimed for), the lower edge of this bevel is level with the surrounding face at infinity so it's harder to measure, the top edge protrudes 0.5mm at infinity and is recessed 1.2mm at closest focusing distance (At closest focusing distance I put a flat across it and it had roughly 0.2mm clearance from the surface of the surround, however I didn't do a contact measurement just an estimation). The hole which the rear element moves has an ID of 19mm. The rear edge of the bayonet is 4.2mm from the flange which roughly the same as Olympus lenses (4.25mm).



    Given the size of the rear element and the distance elements protrude in many Olympus designs you would have enough room to mount one filter on the rear as long as it was below 28mm in OD (over 28mm and it will interfere with the electronic contacts). 28mm would be ideal as it is a standard size however you may need to machine the outside edge to reduce its diameter, smaller sizes are standard however there isn't as much choice in the type of filter (24mm might be the next best option). It wouldn't be hard to machine the rear baffle and thread it to allow the fitting of one rear filter. If you were to machine the two longer petals of the hood lower so they were equal to the other two petals you could easily thread it to allow a filter to be mounted (I would suggest mounting a ring on the outside of the petals in a larger size so that you can use a larger filter size, if you mount on the inside of the hood stronger vignetting will result).

    One thing I didn't think about that I discovered on closer examination of the lens is that it is not a unit focusing design, it is actually a floating element design to provide better correction at closer focusing distances. No idea if it was in the marketing or not, I don't think I ever read any of it it.
     
  9. eteless

    eteless Mu-43 All-Pro

    Jun 20, 2014
    It's sort of amusing to me that what's happening in the gel is identical to in a film without an antihalation layer, bright areas flare out due to internal reflection inside the film base (or gel in this case). Most gel filters are designed for lighting rather than optical use, so this isn't normally an issue.

    The optical degradation is caused by the gel not being completely uniform in thickness, the filter needs to bend the light from the edge of the picture the same amount as the middle. Using thicker foam based double sided tape (to provide stand off) and glass filters might be a better option if modification wasn't an option.