Tech Camera Focus Fine Tuning

[stephengilbert]

Mal,

There are details about the rings, as well as an installation video here: http://www.alpa.ch/en/products/lenses/alpa-hpf/alpa-hpf-alpaschneider.html

Steve

 

[GrahamWelland]

If you get the very latest HPF rings they also come with extra stubs that aid with quickly recognizing the position of the ring and also make it easier to turn with your fingers.

 

[PeterA]

For landscape shooting I guess infinity is the best calibration target. For anything less than infinity it may not be - precisely because of what Bob mentioned above.

Now the more resolution you have in your back - the greater the problem you may have - regarding getting the very best possible resolution possible..

from a practical point of view- each new lens introduces more measurement error potential - so yes Bob again in jocular form id correct.

My personal solution is to purposefully limit the amount of resolution I care to use - the lest of my concern being issues to do with focusing

Also I like using a glass and loupe for closer focusing purposes - as in all things there are compromises to be made and work arounds - shooting at F11 gives one a fair amount of real world latitude

 

[Woody Campbell]

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

Thanks - I missed that. There doesn't seem to be a problem but I'll take a close look next week.

 

[vjbelle]

Woody,

You can find that information at http://www.alpa.ch/en/glossary/phot...-the-adjustable-adpaters-for-digital-use.html.

Victor

 

[Thierry]

I just want to intervene to add some details about the right lens(es) to take for shimming.

An ideal focal length would be around 70mm, e.g. the Schneider Apo-Digitar 5,6/72mm L

It is correct that the short focal length lenses are not ideal for shimming, because they are not corrected for "flatness".

Short focal length lenses, although these are very sensitive to the position of the sensor, respectively to the focus, make it difficult to judge the focus differences at infinity

The longer focal length, understand above 100mm, make it as well difficult to shim correctly, because they are less sensitive to focus differences with a few 1/100th differences.

Thierry

 

[vjbelle]

The longer focal length, understand above 100mm, make it as well difficult to shim correctly, because they are less sensitive to focus differences with a few 1/100th differences.

Thierry

Thierry,

This makes no sense to me. I could see the focus line move when using my 150mm lens and the thinnest shim possible (0.01) when focusing for infinity. In fact when I fine tuned with a lensalign for shorter distances the 0.01 shim proved to be too thick to REALLY fine tune the focus.

Victor

 

[Thierry]

Victor, certainly, you are right here. Nevertheless, it is simply not as obvious and easy as with a lens in the focal range of around 70mm.
In any case, we do not recommend to go higher than 100mm.

Thierry

I could see the focus line move when using my 150mm lens and the thinnest shim possible (0.01) when focusing for infinity.
Victor

 

[jlm]

Thierry:

the advice seems to be to set the shims based on focus at inf. Any reason not to to it at some intermediate distance that could be accurately measured and accurately set to the right lens marking? like 10 meters, for example?

 

[Wayne Fox]

Thierry:

the advice seems to be to set the shims based on focus at inf. Any reason not to to it at some intermediate distance that could be accurately measured and accurately set to the right lens marking? like 10 meters, for example?

The only 100% accurately repeatable stop is the hard stop at infinity so using some other setting on the lens introduces error. You just can't set the lens to an incremental stop with perfect accuracy. Once the back is calibrated at infinity however the accuracy of the HPF rings is quite remarkable ... especially at close distances such as 10 meters.

 

[delled]

Wayne --

That sounds logical, but what if the infinity stop isn't set correctly, so the lens, when hard against the stop, either back- or front-focuses. Or maybe it's a very hot or cold day, so the lens has expanded or contracted.

It seems that a better reference might be a fixed and repeatable point near the rotational center of the lens helix. Perhaps with the aid of an added vernier scale and a tape or laser rangefinder.

Then, as Jack pointed out, whatever rotational offset is best at this reference point can be applied at other distances.

Seems there may be more to it than meets the eye.

Dave.

 

[GrahamWelland]

The point about front/back focusing is certainly valid but really the best solution to accomodate for that is to have your lenses adjusted correctly in the first place. My SK 47XL APO Digitar had a very slight back focus at infinity and my dealer (Paul @ Optechs) was able to adjust it using their collimator. This made it possible to ensure that all my lenses work the same and that no adjustment factor needs to be applied to each one.

Btw, it is much more convenient to have the infinity hard stop - one of the challenges I have with my 35 XL is that it doesn't have a hard stop at infinity as it has to allow for lens curvature. With the infinity stop it's easy to quickly set the lens for focus distance without having to look around or down at the lens.

 

[Wayne Fox]

Wayne --

That sounds logical, but what if the infinity stop isn't set correctly, so the lens, when hard against the stop, either back- or front-focuses. .

If infinity is incorrect, then so are all of the other measured points on the lens. In addition you are trying to adjust the sensor to a tolerance of .01mm (theoretically at least) ... I don't think you repeat that accuracy with any other point on the lens. The only possible way to use another point on the lens would be to lock it down so it could not move at all between the various tests.

But then it is quite possible you have set yourself up for a situation where infinity is no longer in focus ... you may easily bias your sensor for front or back focusing if you choose to calibrate with some other distance.

 

[dchew]

... I don't think you repeat that accuracy with any other point on the lens.

Yes, exactly. According to the Alpa distance tables, the helicals move the lens about 30 microns per degree of rotation (Rodi's a little more, SK's a little less). So each minor tick mark on the hpf ring is about a 0.03 shim. I don't know about your eyes, but even using a loupe to stare at the hpf ring it is tough for me to divide those 1 degree marks into thirds for the shimming process down to 0.01mm.

As long as you have a good clear infinity view, it seems more robust to use the infinity stop. The key for me is to start "under-shimmed" so I know which direction I need to go.

Dave

 

[malmac]

Mal,

There are details about the rings, as well as an installation video here: http://www.alpa.ch/en/products/lense...schneider.html

Steve

Steve

Have you seen these rings fitted to lens used on a Cambo camera?


Mal

 

[vjbelle]

If infinity is incorrect, then so are all of the other measured points on the lens.

Wayne,

Even with long lenses an infinity target will remain in focus with numerous shim thicknesses resulting in either back or front focus. Infinity, by itself, is not an accurate enough visual measurement for correct shimming. Focus accuracy can be enhanced by close focus calibrating as I mentioned in previous posts. Obviously the only 'true' way to determine correct focus is with live view..... and even that can be tricky.

Victor

 

[jlm]

reason I asked was that in the 10 foot range, with a 70mm lens, a +/- distance measurement delta of .9 ft, or 11", (9.3ft and 10.2ft) represents five degrees lens rotation (taken off the HPF scale)

however near infinity, 97ft and inf represent 5 degrees lens rotation.

I would think we want the smallest image distance delta per degree of lens rotation to set shims accurately, assuming of course the HPF scale is right on (and we have to, right?) and we can accurately measure the distances

 

[Thierry]

Dear John,

There is no other reason than a practical and financial one to do this at infinity.

Of course one can do this at any distance. Alpa does the shimming for instance at 2-3 meters, allowing therefore to do it inside and not be dependent of the weather. But then it needs special tools like a laser-pointer and a special target telling how much to shim. Also, a special lens is used, calibrated for a distance of 2 m and a maximum opening of f 2.2.
This equipment costs a couple of thousands CHF.
This makes no economical sense for a photographer, therefore the solution to shim at infinity. In any case, we do as well a test at infinity, once the back has been shimmed at 2 m.

Best regards
Thierry

Thierry:

the advice seems to be to set the shims based on focus at inf. Any reason not to to it at some intermediate distance that could be accurately measured and accurately set to the right lens marking? like 10 meters, for example?

 

[Thierry]

That's another reason, to do it at infinity. However, one can imagine a setup where one focuses the lenses at a certain distance on the scale/HPF ring (e.g. on the 10m mark) and then not longer touch this focus, while doing the shots and checking/shimming, until the focus is at the correct place, like you said it bellow.

Also, doing it at infinity or at any other distance, there is of course a pre-condition, as mentioned by Graham already: to have adjusted lenses.

Thierry

If infinity is incorrect, then so are all of the other measured points on the lens. In addition you are trying to adjust the sensor to a tolerance of .01mm (theoretically at least) ... I don't think you repeat that accuracy with any other point on the lens. The only possible way to use another point on the lens would be to lock it down so it could not move at all between the various tests.

But then it is quite possible you have set yourself up for a situation where infinity is no longer in focus ... you may easily bias your sensor for front or back focusing if you choose to calibrate with some other distance.

 

[Thierry]

Victor,

That's true, that infinity is not a measurable distance, and that's why Alpa gives a infinity distance = 20 x Focal Length in Meters. That's the minimum which should be taken for infinity, when calibrating the back.
I have seen photographers using simply the focal length in meters.

Thierry

Infinity, by itself, is not an accurate enough visual measurement for correct shimming. Focus accuracy can be enhanced by close focus calibrating as I mentioned in previous posts.
Victor