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Thread: Alpa Back Adaptors & Shims

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    Alpa Back Adaptors & Shims

    I'm starting out in the Alpa corner of Dante's Inferno...I'm interested to know a little bit about experience with shimming the back adapter.

    I've seen the video that Paul Slotboom did and I have a fair understanding of the process. What I don't understand is which bit of the image chain you are correcting? Are you correcting for the lens infinity focus, for imprecision in the camera body, for imprecision in the back adapter itself or for imprecision in the back? Or are you correcting for the whole chain - all of those things?

    Depending on the answer to that question...is a shimmed adapter ideally specific then to a combination of camera, back and lens or is it specific to that back on whatever camera/lens combo you choose to use it on?

    Sorry if the answers to these questions are obvious...for some reason they aren't obvious to me.

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    Subscriber & Workshop Member GrahamWelland's Avatar
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    Re: Alpa Back Adaptors & Shims

    Basically assuming that the lens helicoid is correctly set up for infinity you are adjusting for the distance between the sensor plane and the body. With this you will be able to accurately focus any other lens with that back / mount correctly at infinity and hence also all of the other calibrated distance points on the helicoid - ie. if it says 1m then you should be focused at exactly 1m (well it does assume no focus shift with the lens I suppose).

    The presumption is that every lens is calibrated at the factory to infinity and the tolerances on the machined lens mount are extremely tight so that any lens when mounted on the body will be identical in terms of accuracy. Ditto the body is machined in the same manner to be accurate. The unknown as far as Alpa is concerned is the distance from the sensor plane to the mount of the digital back and so you use the shims to bring it into the correct distance from the body.

    You want to use a reasonably long lens such as 70/90mm to set up the shims. There's actually a better video out there by Mark Dubovoy that was on LuLa (it was part of the IQ180 review but the link seems to be dead now https://luminous-landscape.com/Alpa-Phase-Shim-1.3.mov - maybe worth pinging Michael Reichmann / Kevin Raber to get an updated link on their new site).

    Of course a mechanical engineer would argue that it's probably impossible for everything to match precision but in theory so long as the lenses are correctly calibrated to infinity everything should align to be accurate (enough).

    Now if you have live view this all doesn't really matter quite so much because you'll be focusing everything off the sensor anyway (so long obviously that you can focus to infinity or beyond).

    Is the shimming body/lens specific? In theory no. Once the back/adapter are shimmed together they should be usable on any other Alpa camera and any other correctly setup lens on an Alpa mount. (The reality is though that I know that I'm not alone in having to have a lens adjusted/recalibrated and that's not just on Alpa but also Leica).
    Last edited by GrahamWelland; 8th September 2015 at 17:53.
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    Re: Alpa Back Adaptors & Shims

    Graham,

    Thanks for a great explanation of the process. I'll try and track down that other video. What are the implications of using a shorter lens for the shimming process? I understand that the longer lens will both be more sensitive to focus changes and be able to show more distant detail. Anything else? The only lens I currently have access to is an SK Apo-Helvetar 48.

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    Subscriber & Workshop Member GrahamWelland's Avatar
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    Re: Alpa Back Adaptors & Shims

    Pretty much as you mentioned. A mid to long lens makes setting infinity a little easier but I see no reason why using your SK 48 wouldn't work well either, in fact if that is your only lens it'll be perfect I would wager.

    i used to shoot the 47 SK APO Digitar myself. A very versatile lens and under-rated in this world of $6-9k Rodenstocks.
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    Senior Member danlindberg's Avatar
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    Re: Alpa Back Adaptors & Shims

    Allow me to shim in....

    Graham has covered the topic very well, I just wanted to add that I (too) find it extremely important to take the time and effort to shim your back to your reference lens. In my case I use the SK 72 L as my reference which is ideal length, but ofcourse if I had a one lens setup I would most definitely use that one to go through the process as well.
    The whole point is to get that distant mountainrange silhuette ridge as crisp and sharp as possible and ofcourse piece of mind that all the rest of your distances on the scale then are spot on.

    Now, I have an Aptus II 5 and a Credo 60 with their own adaptors. These are shimmed differently to achieve perfect infinity!!

    One of my 5 tech lenses has a slight override at infinity stop, I have made a 'cheap' correction by mounting the HPF ring at the exact and true infinity position rather than at the stop.

    It really is a revelation to see the difference adding or taking away thin shims. I use a telegraphpost around 3 km away from my terrace. I cannot recomend it enough to be patiant and do this carefully. Oh, and shoot wide open ofcourse....
    Alpa FPS MAX TC | Alpagon 32Hr | Helvetar 75 | Schneider 120N | Leaf Aptus II 5 Leaf Credo 60 | www.danlindberg.com
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    Re: Alpa Back Adaptors & Shims

    Graham & Dan,

    Thanks so much for your input. This adapter will be with an Aptus II-7 back and I will use a very distant object.

    Thanks for the support and incredibly helpful input.

    Regards
    Mike

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    Re: Alpa Back Adaptors & Shims

    For those that might discover this thread in the future...

    "Shimming" is one form of accounting for the cumulative variation in the exact thickness of each component (back + adapter + body + lens mount + lens). If focusing primarily by number (e.g. measuring a distance and then focusing on that distance) accounting for this distance is critical. If focusing primarily using live view or guess-and-check it is less important, but still helpful (in that you'll more often start off closer to the right answer).

    Each tech camera company provides some manner of doing this. In each case it's a process you do once (e.g. when you first buy the system or add/change a component) or can be done for you by a knowledgable dealer.
    - Cambo: adjust the helical of each lens
    - Alpa: add/remove thin metal spacers (shims)
    - Arca: determine an offset number

    Each process has advantages and disadvantages, but they all get the job done.
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    Re: Alpa Back Adaptors & Shims

    For what's its really worth I've done this so much that I'm very tired of it. Almost all of the lenses have an infinity stop that can be adjusted...... yes Alpa/Schneider/Rody are supposed to assure all of us that this is done correctly but it isn't..... they all vary. Most importantly is if you have one of the older (now only one - Schneider 150mm) non adjustable lenses then that should be your standard lens for setting infinity. Shim for that and then adjust all other lenses to match that lens. This is all, of course, moot with CMOS live view. At that point I would shim the back to slightly beyond infinity and focus accurately with live view and a loupe.

    Victor

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    Re: Alpa Back Adaptors & Shims

    An update: When I first got this lens/back setup it "looked OK" at infinity so I didn't bother going through the whole rigmarole of shimming.

    Wind forward a few hundred shots and I wasn't happy with the sharpness of distant objects. Interestingly near objects - close to minimum focus - looked pretty good.

    I set up yesterday afternoon and went through the whole process of shimming the back. I wouldn't like to have to do it too often. However the end result is stunning. The distant images are much sharper, with more apparent contrast. Everything looks much better. The end result - after lots of different combos - is a single 0.20mm shim.

    It's worth saying that, as others pointed out at the beginning, this process isn't so easy with a 48mm lens. It would be much easier to see really distant detail with a longer lens. The other interesting thing is that something around 1km away is _not_ far enough away. More distant objects yield the ultimate result.

    Thanks again for all the assistance.
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