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Adjusting Alpa system to shoot both digital and film using HPF rings

epforever

Member
I could use a hand figuring this out. Right now, my Alpa is perfectly shimmed for my H5D-50 (CCD) back, so that using the HPF ring produces nice sharp images, wide open. However -- the ground glass doesn't perfectly match the HPF's focus. For example, if I set the HPF ring at 2.00 meters to focus on an object 2.00 meters away, that object will be sharp on my digital back but not on the ground glass. To get it optimally sharp on the ground glass, I need to focus slightly closer.

I'd like to shoot both digital and film with the system. (And I'm assuming the ground glass matches my film backs, which I haven't tested yet.)

How can I get it so that the HPF, digital back, ground glass and film backs all match each other in terms of focus? That way I could trust the HPF and Disto whether shooting digital or film.

I do understand how to adjust the helicoid's point of infinity focus. One thought I had is, perhaps I need to adjust the lens so that infinity on the lens shows sharp infinity on the ground glass, and then shim the back to match that same infinity point? The problem with that procedure is, it's quite difficult to discern exact infinity sharpness on the ground glass, even with the Silvestri 12x loupe. Objects are extremely small at that distance.

Hope that's all clear. Any ideas? Thanks in advance.

Ethan
 

jotloob

Subscriber Member
Don't know your Hassy, but why not shim it for film and then use LV for digital focus?
This seems to be a possible solution . But how ?
What do you mean by "shim it" ? ? ? As far as I know , you can not shim the groundglas nor the filmholder . All you can do is , check if the HPF ring is properly mounted .
Am I missing something here ? ? ?
 

Geoff

Well-known member
Another way is to keep it as you have it, figure out the offset required for the film, and just compensate when you use the film back. Although not pure, it’s pretty easy to do.
 

dchew

Well-known member
To get it optimally sharp on the ground glass, I need to focus slightly closer.
Well, that makes this a bit easier. First some reference: Focusing closer means turning the helical out, i.e. moving the lens further away from the film/GG/sensor plane. If you have to focus to a slightly closer distance using the GG, that means your GG is a little closer to the lens than your DB (you have to turn the helical out a bit when the GG is mounted to compensate).

Step 1: Remove some shims from the DB adapter until it matches the GG. Now your HPF rings are a little off; if you are 2.00 meters from subject to film plane, you have to set the HPF at 1.95 or whatever (something <2.00). Likewise, if you are at infinity, you have to set the HPF ring just short of INF. But that offset will now be the same for the GG and for the DB.

Step 2: Loosen the HPF ring and rotate it just enough so the the INF stop on the lens goes past the INF mark on the HPF ring. This is a bit tricky and will probably take a few tries to get it right. Once you have it right, the HPF ring will now read 2.00m when your Disto says 2.00m.

Everything is now registered, except you have to remember when focusing at infinity not to just turn the helical to its hard stop. You have to turn it to the infinity mark on the HPF ring, which is now just before the infinity stop on the lens.

Of course you have to do the same thing for all your HPF rings on all your lenses! The good news is your GG was off the right way. If it was off the other way, you would have to mess with the lens helical to keep infinity in focus since, as you point out, you can't shim the GG. Messing with lens helicals is not near as fun as simply rotating the HPF rings to compensate.

Dave

PS: Every time I think this through I get it backwards six times... Hopefully I didn't this time!
:loco:
 

dchew

Well-known member
Also note you can estimate the shims to remove from your DB adapter: Each small tick mark on the HPF ring is ~ 0.03mm of shim (little different for sk vs Rodi's, but close enough). So 1/3 of a tick mark equals ~0.01mm shim.

Dave
 

epforever

Member
Well, that makes this a bit easier. First some reference: Focusing closer means turning the helical out, i.e. moving the lens further away from the film/GG/sensor plane. If you have to focus to a slightly closer distance using the GG, that means your GG is a little closer to the lens than your DB (you have to turn the helical out a bit when the GG is mounted to compensate).

Step 1: Remove some shims from the DB adapter until it matches the GG. Now your HPF rings are a little off; if you are 2.00 meters from subject to film plane, you have to set the HPF at 1.95 or whatever (something <2.00). Likewise, if you are at infinity, you have to set the HPF ring just short of INF. But that offset will now be the same for the GG and for the DB.

Step 2: Loosen the HPF ring and rotate it just enough so the the INF stop on the lens goes past the INF mark on the HPF ring. This is a bit tricky and will probably take a few tries to get it right. Once you have it right, the HPF ring will now read 2.00m when your Disto says 2.00m.

Everything is now registered, except you have to remember when focusing at infinity not to just turn the helical to its hard stop. You have to turn it to the infinity mark on the HPF ring, which is now just before the infinity stop on the lens.

Of course you have to do the same thing for all your HPF rings on all your lenses! The good news is your GG was off the right way. If it was off the other way, you would have to mess with the lens helical to keep infinity in focus since, as you point out, you can't shim the GG. Messing with lens helicals is not near as fun as simply rotating the HPF rings to compensate.

Dave

PS: Every time I think this through I get it backwards six times... Hopefully I didn't this time!
:loco:
@algrove: my Hassy back is CCD, thus live view isn't available at all in the field. And when tethered, live view is B&W and refreshes about once per second. Not viable as the sole means of focusing.

@dchew: Absolutely -- thank you. I actually hit upon more or less this same solution as I was going to sleep last night, based on the same realization that I could just reposition the HPF ring. But is your Step 1 necessary? Here's the process I came up with last night: 1) Reinstall the HPF ring so its infinity mark matches up with infinity as focused on the ground glass. The HPF should now be accurate on ground glass at all distances. 2) Re-shim digital back to match newly placed HPF. For example, set the sensor 2.00m away from an object, focus the HPF at 2.00m, and now shim until the object is optimally sharp.

Do you see any flaws in that process? Is this fundamentally any different from yours?

Very much appreciated!
 

jotloob

Subscriber Member
A few years ago , I could watch an ALPA technician to shim my two digital back adapters .
I fact only one adapter needed shimming .
The first he did was to place the HPF ring in the correct position . He used my HRDIGARON-w 4/40mm
The adapters were shimmed to real infinity setting , not just 50m but real infinity and also not for 2m as is described .
 

dchew

Well-known member
@algrove: my Hassy back is CCD, thus live view isn't available at all in the field. And when tethered, live view is B&W and refreshes about once per second. Not viable as the sole means of focusing.

@dchew: Absolutely -- thank you. I actually hit upon more or less this same solution as I was going to sleep last night, based on the same realization that I could just reposition the HPF ring. But is your Step 1 necessary? Here's the process I came up with last night: 1) Reinstall the HPF ring so its infinity mark matches up with infinity as focused on the ground glass. The HPF should now be accurate on ground glass at all distances. 2) Re-shim digital back to match newly placed HPF. For example, set the sensor 2.00m away from an object, focus the HPF at 2.00m, and now shim until the object is optimally sharp.

Do you see any flaws in that process? Is this fundamentally any different from yours?

Very much appreciated!
Well, my step 1 is your step 2, so yes it has to be done, but no I don’t see any flaws in your process, just a different order.

Dave
 
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