Alpa 12SWA Digitar Focusing Problem

[Eric Staudenmaier]

I am using 24mm, 35mm, and 60mm digitars on an ALPA 12SWA with the LEAF Aptus 22 (unitil the 75 comes in). The lenses are not focusing properly. A shim Kit came with the H1A adapter with clear instructions on use. The 35mm lens requires all shims removed ( -.03mm) to acheive focus. This causes the other two lenses to backfocus. The shim kit is meant to adjust the Digiback to the camera mount. It is assumed the lenses would all be calibrated identically before they are sold.

It appears each lens is not calibrated to the same focus point, and each may need shims in the lens mounts. Has anyone ever heard of this?

Thanks,

Eric

[Jack Flesher]

Are you saying that when your 24 and 65 show focus on the GG they are also focused properly on the back, but to do the same with your 35 requires you remove shims from the mount???

If so, this simply makes no sense... The shims set the back to the same plane as the GG so that focus is the same. Changing a lens should not -- heck, I'll even say can not -- alter the spacing between GG and back at all...

[Eric Staudenmaier]

I am not using a ground glass to focus. I am under the assumption that once the H1 adapter is shimmed so that one lens focuses at the proper marks, then all lenses will focus properly.

[Oren Grad]

Eric, how are you determining that the focus is off?

[Eric Staudenmaier]

I am using a tape measure to determine the distance between the film plane and the subject. I then set the lens to that distance and shoot tethered from the Aptus 22 to a computer. This process is meant to adjust the shims in the Digiback adapter to place the sensor at the right distance from the lens. The assumption is that all the lenses will focus perfectly once the Digiback is in the right position. The Adapter is adjustable from +.3mm to -.3mm. My 35mm lens only works at the -.3mm position (all the shims removed). This means that the other two lenses will hyperfocus at infinity(nothing is in focus).

The instructions for the H1 adapter specify shooting a staight edge (or anything with marks that will easlily show focus) on the floor with the camera angeled down. Use a separate tape measure to measure distance from the film plane to the a central spot on the target. Set the lens to the same distance. You can then see if the lens is backfocusing or frontfocusing.

Here are Alpa's instructions:

http://www.alpa.ch/en/faq/knowhow/ALPA_DigitalFocusCal.pdf

[Oren Grad]

A tape measure is unlikely to be accurate enough to shim focus to this level of precision. You really should be using a properly mounted ground glass with a loupe to check fine focus.

That said, I definitely would not expect different lenses to require different shims. At the prices Alpa charges, you should expect the lens mounts to be adjusted to within specification when they leave the factory.

If you still have a problem when adjusting focus critically with a ground glass, Alpa should be willing to take the kit back and set it up properly for you.

[Eric Staudenmaier]

Using a tape measure is working well because I am very able to measure accurately within a 1/4". At a subject distance of 3' the margin of error for my three lenses is 3 to 6 inches. When shooting at 10' one lens might focus as much as 18" differently than another. On the Alpa PDF using a ground glass or Digiback are interchangable. I agree that, for the prices, this system should show up perfectly adjusted. I work on many fashion/beauty shoots where we combine many seperate H1's with many Aptus 22's and P25's. The shots are alway Eyelash, Lizard Skin, Freaky Pores sharp on the 17 year old models. I don't know why my back adapter should require a user custom adjustment.

[Oren Grad]

If you shim for infinity focus, rather than close focus, what happens then to apparent focus agreement of the lenses at close range?

[Eric Staudenmaier]

Each lens has a different shim which works for infinity. I started out with this method. The dealer told me to work on the close focus distances first. I think it is much easier to be more accurate calibrating close focus. It is very easy with a large straight edge to see excactly were the focus is falling. The dealer said that when the close focus was calibrated that the infinity setting would fall into place. This was the case with my 35mm lens. It has not been the case with my 24mm and 60mm lenses.

[Gordon Moat]

I used the close focus method for a couple cameras I built. The variation I used was to measure two distances, and adjust for the best result at both distances. After that infinity focus did work out to be a proper setting. Just to be clear, this was not on an ALPA.

Production variances should be very close to specifications. One thing that might help is knowing the actual flange to focal plane specifications. You should be able to measure that with a dial caliper with the lens in place and without a back on the camera. Tolerances do end up out of specification, so if you really think your 35mm is off, then return it for another replacement lens.

You might also try contacting Andre Oldani at http://www.alpavision.ch who has tested the Leaf and ALPA combination. He would probably be one of the best people to give you some advice on this.

Ciao!

Gordon Moat

[archivue]

considering the price of that kit, and the reputation that alpa try hard to have... you should contact alpa diretly !
please keep us inform !

[Eric Staudenmaier]

I agree. They should simply be the most accurate lenses I've ever worked with. I am very happy not to be shooting with my D2X anymore for the architecture work. Here is a list of things I don't have to correct anymore: Barrel distortion, Perspective, Chromatic abberation, Banding in Hightlights, and color fringing at strong highlight transistions (that purple fringe around the edge of bright windows). The lenses are so sharp with the Aptus 22 that I have exchanged those problems for some moray patterns and jagged diagonal lines. Theses are not hard to correct. I think the Aptus 75 will really improve on those issues when it arrives.