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Thread: 25mm lens for Contax 645 and some other MF...

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    25mm lens for Contax 645 and some other MF...

    Had a research on a solution for C645 (me among them) users to go wider... The Pentax 25mm f4 lens with a 6.87mm spacer will work on the Contax exactly the same as it would on a Pentax 645 camera, the lens will also work on M645 mount with 7.57mm spacer and will also be compatible with the Leica S cameras with additional spacing added.

    Of course, this means that all Pentax 645 lenses can be fitted on the same MF cameras, but with the new 28-45mm f4.5 and the 25mm f4 attracting special attention to C645 users as they can't go wider than 35mm with what was available up to now.

    The aperture control of the lens is fairly easy to adjust as Pentax D & DA lenses use mechanical coupling via a lever on the lens mount flange which works much the same as it happens with Nikon G lenses which also don't have an aperture ring (but aperture coupling is still mechanical).

    I'm thinking to build the adapter by saving off the back of a Pentax 645 old extension ring and add a C645 lens mount to the back of it out of a Contax extension ring. To control the aperture, I'll use a thin (1mm) ring machined so that its inner diameter "just" fits on the outer diameter of the (what used to be) the Pentax extension ring and connect it through a slot with another thin ring to the inner diameter of the same spacer, that will have a small extension at the same place as the Pentax 645 camera mount has the lever to automatically adjust the aperture of the lens. (Any other suggestions on the matter are welcomed)

    As of AF... who needs it on a 25mm f4 lens?

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    Re: 25mm lens for Contax 645 and some other MF...

    OK... Having checked what is available, one can preferably shave off the back of a Pentax 645 to Canon or Nikon adapter and keep the Canon (or Nikon) mount too which may prove handy for any other application. He will then have to add the Contax lens mount on the shaved side of the adapter.... For the "aperture ring" on the adapter, I am thinking of the classic "ball" & spring solution, but one must use a lens first as to accurately calibrate the "clicks"...

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    Re: 25mm lens for Contax 645 and some other MF...

    Ooops... just found another two lenses that will work on MF with some serious modicafication... The Nikkor 24mm f3.5 & the respective Samyang T-S... the sift mechanism at the back of them has to be removed completely (it's not connected with the lens system) and it has to be replaced with a shorter construction that will have the lens mount for the camera... There is no aperture needed as both lenses have mechanical aperture ring...

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    Re: 25mm lens for Contax 645 and some other MF...

    Quote Originally Posted by T.Dascalos View Post
    Ooops... just found another two lenses that will work on MF with some serious modicafication... The Nikkor 24mm f3.5 & the respective Samyang T-S... the sift mechanism at the back of them has to be removed completely (it's not connected with the lens system) and it has to be replaced with a shorter construction that will have the lens mount for the camera... There is no aperture needed as both lenses have mechanical aperture ring...
    Possible but maybe the back lens touches the mirror of the Contax 645.
    I did the such mod with the Leica 28f2.8 PC lens (machined away the shift mechanism and added C645 mount. Works but needs mirror pre-release.

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    Re: 25mm lens for Contax 645 and some other MF...

    Quote Originally Posted by chrismuc View Post
    Possible but maybe the back lens touches the mirror of the Contax 645.
    I did the such mod with the Leica 28f2.8 PC lens (machined away the shift mechanism and added C645 mount. Works but needs mirror pre-release.
    No, there is no such danger with either the Nikkor or the Samyang.... there is need for both lenses to be 19mm reduced in rear depth, but the last element is about an inch from the mount...

    By the way, the Nikkor's aperture is electronically coupled to the existing aperture ring... So the aperture won't stop down mechanically... No big deal as the Samyang has even wider image circle, is sharper in other than the center and has good control of chromatic aberrations on its outer circle ...and costs 1/3rd of the price.....

    The good thing with both lenses, is that there is no machining needed as to remove the rear shift mechanism... it is bolted, so one only needs to unscrew it... I think I'll use a Hasselblad to Contax adapter to do the conversion... I'll remove the V mount and use it on the shift mechanism as to create a Hassy to Nikon shift adapter (it should need very little of metal to be saved off) and then save off the remains of the adapter to the right thickness as to bolt it back on the Samyang lens!

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    Re: 25mm lens for Contax 645 and some other MF...

    Quote Originally Posted by T.Dascalos View Post
    No, there is no such danger with either the Nikkor or the Samyang.... there is need for both lenses to be 19mm reduced in rear depth, but the last element is about an inch from the mount...

    The good thing with both lenses, is that there is no machining needed as to remove the rear shift mechanism... it is bolted, so one only needs to unscrew it... I think I'll use a Hasselblad to Contax adapter to do the conversion... I'll remove the V mount and use it on the shift mechanism as to create a Hassy to Nikon shift adapter (it should need very little of metal to be saved off) and then save off the remains of the adapter to the right thickness as to bolt it back on the Samyang lens!


    Did you measure the Samyang back lens clearance? It would be great if you are right.

    From the Samyang section view, the space between rear lens element and mount flange looks less.
    Click image for larger version. 

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    But I guess their drawing is not in scale. (The lenses look larger than the housing.)

    Good idea. Good luck!

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    Re: 25mm lens for Contax 645 and some other MF...

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    From this pic the back lens looks quite deep recessed, still one has to check no interference with mirror of Contax 645 at infinity = least recessed.

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    Re: 25mm lens for Contax 645 and some other MF...

    Quote Originally Posted by chrismuc View Post
    Name:  shiftilt.jpg
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    From this pic the back lens looks quite deep recessed, still one has to check no interference with mirror of Contax 645 at infinity = least recessed.
    As is obvious from the picture you've posted, there is more than enough depth to the rear element. It looks like the distance to the rear element is more than an inch (may be more) deeper from the mount, while for the new mount for Contax, the lens needs to be "shortened" only by 19mm... The other thing that must be discussed, is whether the lens performance will be improved or not... It should improve (maybe less so at the center) since the new mount will be wider and nearer to the rear element and since the mirror box of the Contax is much bigger than on a DSLR as to "free" the image circle... shouldn't it?

    As far as I've seen from tests they claim that resolution at +6mm shift doesn't change much and that's exactly what I need as it is going to be used with a 37X49mm sensor with 6.8μm of pixel size... At least the numbers suggest that IQ should be nothing to complain with and it can even be very good indeed at f5.6 & f8.... don't know about distortion though towards the edges of the frame... but I guess some distortion is to be expected with a 25mm MF lens...

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    Re: 25mm lens for Contax 645 and some other MF...

    The P45+ sensor 37mmx49mm requires an image circle of 61,4mm
    This equals 10,25 shift movement along the long axis of the 24mmx36mm sensor, so not so little.
    Acc. to the lenstip test, the resolution towards the shifted corners diminishes quite a lot, so I am not sure if the lens would perform so well towards the edges/corners on a P45+ sensor.

    An increase of the lens barrel/mount diameter has no influence on the performance. Only if it would be too small it would lead to vignetting.

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    Re: 25mm lens for Contax 645 and some other MF...

    Quote Originally Posted by chrismuc View Post
    The P45+ sensor 37mmx49mm requires an image circle of 61,4mm
    This equals 10,25 shift movement along the long axis of the 24mmx36mm sensor, so not so little.
    Acc. to the lenstip test, the resolution towards the shifted corners diminishes quite a lot, so I am not sure if the lens would perform so well towards the edges/corners on a P45+ sensor.

    An increase of the lens barrel/mount diameter has no influence on the performance. Only if it would be too small it would lead to vignetting.
    If the rear element is recessed by a lot (like in this case) and the image circle projected is wider than the mount (measured at the mount plane), it does affect negatively the lens performance to some extend... The shift required for the Kodak 39mp sensor to be covered, is just a bit more than 6mm either side and both horizontally or vertically, so a measurement of resolution at 6mm shift applies for the edges of all Kodak sensor sensor area.... the extra 2.5mm (on the furthest corners only) that is not measured, can't be far off as the image circle projected by this lens is of 74mm diameter (!) and the shifts measured at 9mm shift on this lens, do show only some reasonable drop in resolution...

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