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Tech Camera Focus Fine Tuning

vjbelle

Well-known member
All of the instructional data available for adjusting a tech camera for focus is biased towards finding focus at infinity. This, for sure, is the starting point. The adjustment process becomes more accurate by using longer lenses (100mm and beyond) but even that is limiting - the DOF is just too deep for an accurate focus adjustment.

The shimming range for infinity adjustment can keep an intended target in focus easily within a 0.03mm deviation. When I used my 150mm lens I found that my target remained in focus with shims ranging in thickness from 0.10 to 0.13 - in fact I probably could have taken that to further extremes but didn't (it was 100 degrees that day). I could see where the focus line in the foreground moved within the shim range but my intended infinity target was always in focus.

I found that after setting the infinity macro adjustment that focus adjustments could be fine tuned with the use of LensAlign and a Leica Disto D5 and, for Alpa users, HPF rings. The process is very simple and, to me, very accurate although it is very important to align the camera and the Lens Align tool properly. I measured my distance from the sensor plane to the LensAlign with the Disto and set my HPF distance to match (this happened to be around 6.5 feet for my 150mm). I started with my infinity shim range which added 0.10 to 0.13 shims to achieve infinity focus. My first shim adjustment which added 0.10mm to the standard Alpa shim resulted in a slightly back focused target. When adding shims to equal 0.13mm the focus shifted to slight front focus. All of this is very easy to see on the target at 100% pixels. The closest to dead on was achieved by adding a total of 0.11 shims. So, even though I had a range of 0.10 to 0.13 shim thickness that resulted in accurate infinity focus I was able to set focus more accurately for closer distances with the aid of LensAlign.

All of this applies to using a Disto and HPF rings but could also be applied to a Cambo if enough helical distance markings are available. Live view will eliminate the need for this but live view isn't always easy to use and is a big time battery dis-charger. Regardless infinity focus, at a minimum, has to be set.

Just thought I would pass this on to anyone who might be interested.

Victor
 

rga

Member
Interesting. Have you now tested other lenses to see if this shim setup works for them?
 

stephengilbert

Active member
Victor,

I love it that you used a Disto to measure 6.5 feet. Not having one, I'd probably use a tape measure. Of course, to put the shimmed camera to use, you'd need a Disto to measure distances. :)

Steve
 

Jack

Sr. Administrator
Staff member
It is perhaps one advantage to the Arca system: once you find your offset for infinity, you apply the same offset to each helical reference number and your focus is spot on for all distances.
 

malmac

Member
Guys

I have a Cambo system which does not have this shimming process. Could someone please explain exactly what this shimming is doing. Is it calibrating the scale on your focus ring with the actual number of metres in reality or is doing a whole lot more?

I would like to understand this a bit better.


Thanks


Mal
 

dchew

Well-known member
Mal,
Generally the shimming process is relative to Alpa's cameras. Yes, it basically calibrates the focus ring so it is precise. Traditionally it is done at infinity, probably because the infinity stop eliminates human positioning error when setting the helical. Focusing at infinity also ensures the back is not too far away, which would mean the lens would not be able to focus at infinity (the stop prevents the lens from moving in close enough).

Arca solves this another way by incorporating an "offset" on the helical. Accomplishes the same thing just in a different way. I have not used a Cambo much, but I think those set screws in the adapter can be adjusted by the factory. Again, similar effect just done in a different way. And they would need your digital back...

My problem with infinity is there just are not very many crisp air days in the midwest during the summer! Too much heat and haze to evaluate an image of something 1 mile away.

Dave
 

jlm

Workshop Member
i have stewed over this. My conclusion is that when your lens is set on the inf end stop, you may need to shim the back (or the lens) so that far objects are in accurate focus, determined by test shots and peeping. This involves some very small distances, a shim thickness of 0.01mm being the smallest increment Alpa uses.

after that, you are at the mercy of the distance accuracy (and resolution) of both the calibrated focus ring, the helicoid, and how you measure your subject, with the last being the least accurate, in general.

What usually happens with a focus ring is that your measured subject distance falls in between the focus markings, here the HPF rings are very handy, as they give a measurement at every 5 degrees and each tick mark spaced at every 1 degree. So for example, the 70mm HPF ring has distances marked at 5.30', 6.21', 6.57', 6.97', 7.43'...with five ticks between each of those readings spaced about 1 mm between tick marks on the barrel. The stock 70mm lens ring has readings at 4', (then about a 19mm spacing) 5' (then about a 30mm spacing), 10'.
 

Thierry

New member
Dear Mal,

Yes, it is basically doing this, "calibrating the scale on one's focus ring" by adjusting the right distance between sensor and lens at a given target distance. In other words, make sure that when one focuses on a point the focus is really at the set point, e.g. at infinity. It is a alignment which can be done in one way or the other, a + or - alignment, bringing the sensor closer or further away. This is done by the so-called shims, in 1/100th of mm steps.

But it does even more, since it is possible to offset also a possible sensor un-flatness on its sensor-board: this happens more than one might be aware, either in a vertical way, or then horizontally, and as well in the diagonal. Shimming can be done the same way, in all directions, or on only part of the plane (e.g in one corner).

There is a nice video by Paul Slotboom (Optechs Digital), explaining this in detail, here:

Alpa adjustable digital back adapters

As well as a Alpa review done by Michael Reichmann, explaining the shimming and its importance, here:

M. Reichmann "Alpa Review" - Importance of a Well Aligned Sensor

Best regards
Thierry

Guys

I have a Cambo system which does not have this shimming process. Could someone please explain exactly what this shimming is doing. Is it calibrating the scale on your focus ring with the actual number of metres in reality or is doing a whole lot more?

I would like to understand this a bit better.


Thanks


Mal
 

Bob

Administrator
Staff member
One might think that sensors are flat, but really, after processing, the wafers that they are cut from resemble potato chips although of course to a much smaller degree.
Sensors are cut (or scribed and broken depending on the fab) from the wafer so each sensor resembles topographically a rectangle cut from a potato chip. Now they might be the flattest thing that you own, but they are not really flat. There is a sub-micron curvature that will not be improved by shimming. 1/100 of a mm is also far too course an adjustment for the kind of curvature I am speaking about. I suspect that there is no significant photographic issue related to the curvature but I thought I might describe it so that folks can have nightmares about it. :ROTFL:
-bob
 

Thierry

New member
Bob, thanks for this picturesque sight.

I will from now on have a different (angle of) view when eating potato chips!

:)

But right, effectively that (a sub-micron curvature) cannot be improved by shimming, BUT is also not of an issue. Only the flatness or lack of parallelity of the sensor due to a wrong or inaccurate assembly is of relevance.

Best
Thierry

... each sensor resembles topographically a rectangle cut from a potato chip.
-bob
 
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vjbelle

Well-known member
Victor,

I love it that you used a Disto to measure 6.5 feet. Not having one, I'd probably use a tape measure. Of course, to put the shimmed camera to use, you'd need a Disto to measure distances. :)

Steve
Yes.... the Disto is very necessary for using the HPF rings. This combination is very fast and accurate.

Victor
 

jlm

Workshop Member
except for inf, sensor planarity is of no impact, unless or course, you are trying to focus on an exact plane
 
Victor, very thoughtful and useful information.

Bob, I've licked my sensor and pretty sure mine was cheese and onion flavoured.
 

Woody Campbell

Workshop Member
I've had no problem with the Alpa. Alpa recommends that you adjust the shims with your widest lens (in my case a 35mm xl) because it has the narrowest depth of focus. .01mm actually makes a visible difference. Having shimmed for the 35 my other lenses (47xl and 72 xl) are dead on.
 

GrahamWelland

Subscriber & Workshop Member
Woody,

That's interesting. My Alpa back/adapter was calibrated by my dealer using my *longest* lenses. So in my case it was with my 90 HRW initially and then the SK 150.
 

vjbelle

Well-known member
Alpa recommends that you adjust the shims with your widest lens (in my case a 35mm xl) because it has the narrowest depth of focus.
This is incorrect. Alpa specifically recommends that the 24XL and 35XL not be used for adapter calibrations. This information is available on their site.

Victor
 

jlm

Workshop Member
my camera system is Cambo and my lenses have the T/S mount. the standard focus rings suck. So i bought alpa HPF rings for the 70 and 43, which or course don't work, as they interfere with the T/S knobs, otherwise seems to be good. A tip for you cambo fans.

However, I think I found a way to mod my T/S knob and the rings so they will work together, so far for the rodie 70, at least.

by the way, for $300 bucks or so, those HPF rings are extremely well made and seem a very good deal
 

malmac

Member
Dave and Thierry in particular


Thanks for the info on the shimming process. I have been to the Alpa site and it is starting to make sense.

John (jlm) as a Cambo user I also feel the focus ring leaves room for improvement - is the HPF ring just an interferance fit over the Cambo (SK or Rodenstock) focus ring? ie just a way of having a scale to work off.

thanks in advance


mal
 

jlm

Workshop Member
the HPF ring simply breaks down the markings into a much, much finer scale. I clamps right around the existing focus ring
 
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