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Lens calibrated for digital


Pjay

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A recent purchase of M6 Travelers Kit included a 50mm Summilux that has been calibrated for digital, at a cost of nearly US$400.  I'm not sure what that really involves, and whether it then impacts in any way on the use of the lens on film bodies.

 

Any explanation would be much appreciated.

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A recent purchase of M6 Travelers Kit included a 50mm Summilux that has been calibrated for digital, at a cost of nearly US$400.  I'm not sure what that really involves, and whether it then impacts in any way on the use of the lens on film bodies.

 

Any explanation would be much appreciated.

IT'S BULLSH*T.

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Well, that's getting to the point!

I did wonder about the BS possibility, but thought that Fotopia was a reliable source...

Being relatively new to the world of Leica might make me a little more gullible, I guess.

This from Gilbert: "The lens was calibrated to digital in Leica Germany, it costs USD390."

Edited by Pjay
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I admit that I agree with pico - though perhaps you like a longer version:

 

Film has a certain thickness. If the focus of a lens is „off“ for a certain degree, it still may be in the layer of the film material. This allowed larger tolerances.

 

A digital sensor has a much „thinner“ layer where it is photosensitive, so tolerances are much tighter.

 

And most important: pixel peeping immediately after the shot was unknown during film days. User were more tolerant.

 

This leads to a much higher user‘s demand for their cameras and lenses being calibrated. Some people call this „calibrate for digital“.

 

There is something else which is not directly related to digital sensors or film, but it has gotten much more critical in „digital times“.

All lenses have some sort of focus shift, i.e. when you stop it down the focus changes. This is inherent in the optical design and has nothing to do with digital or film. For most lenses the focus shift is very, very small, so you almost won’t see it. But some lenses have considerable focus shift. With film - thicker photosensitive layers - most of the focus shift happened in the thickness of the film material. On a digital sensor there is no room to cover the focus shift, so it will easier be noticed.

 

During film times it was custom to calibrate a lens with noticeable focus shift at a smaller stop, since it was thought that the lens was used most of the times with this opening. A Summilux would be calibrated at f/2.8, a Summicron perhaps at f/4. So at f1.4 or f/2 it might be slightly off.

 

Now comes digital and people want to see their fully opened lenses’ results with extreme maginfication on the monitor. Therefore customs for calbration have changed: they do it, to assure best focus at largest opening - allowing it to be slight off due to focus shift at smaller stops.

 

But why is pico still right? I have many more old lenses than modern ones. They aren’t off - even when I try a lot with focus peaking and the EVF. My 35mm Summilux asph 1.Vers. which shows quite a lot of focus shift, was calibrated to f1.4 when it was coded for the M8 ten years ago. Since then it is off most times I use it - I had to learn to compensate.

 

The 50mm Summilux Pre asph I bought with the M6 is still spot on focus. I never realized whether they calibrated it „for digital“ when it was coded - may be they did but I don’t notice it.

 

Certainly you wont notice anything of all this when you use your lens with film.

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IT'S BULLSH*T.

Not really. Lenses that were calibrated for film were adjusted to a fairly wide tolerance, which sufficed for film. Film has a thickness, is never 100% flat, and the emulsion exhibits dispersion of the light. A digital sensor is zero thickness in the focal plane, completely flat, and controls the incident light through microlenses. That means that a lens must be adjusted to a more narrow tolerance for digital use. When Leica ran into this problem in 2007 they invested in an elaborate adjustment rig to be able to narrow the tolerance span.

 

A "film" lens may, of course, fall into the parameters needed for digital use. This is, however, a bit of a lottery. It is always wise to have an older lens checked out by Leica, or by one of the reputed third-party repair shops, who have developed their own precision adjustment workflows.

 

All this is apart from the focus shift aspect, which Uli mentions.

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I don't know about lenses, but Sherry Krauter (well respected third party repairer) told me that she doesn't calibrate digital M's because the cost of the required calibration gear would be cost prohibitive. She instead sticks with film Ms and refers others to Leica Service.

 

Jeff

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That is not quite true, other technicians manage quite well, but it does indicate the difference.

Possibly she refers to the adjustment of sensors, which requires laser interference measurement.

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Each of my film lenses needing 6-bit coding has been calibrated for digital and gained focus accuracy on digital Ms afterwards. I did not use them on film Ms since then but i've never heard of lenses accurate on digital and inaccurate on film so far.

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That is not quite true, other technicians manage quite well, but it does indicate the difference.

Possibly she refers to the adjustment of sensors, which requires laser interference measurement.

This is what she told me. If I get a chance, I'll ask DAG, the guy I trust the most, even more than company service reps.

 

Jeff

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Each of my film lenses needing 6-bit coding has been calibrated for digital and gained focus accuracy on digital Ms afterwards. I did not use them on film Ms since then but i've never heard of lenses accurate on digital and inaccurate on film so far.

 

That's mostly my experience as well. Especially with longer, faster lenses (DoF can hide a lot of loose calibration with a 28mm @ f/2.8 ;) ). I'm not sure I'd see the difference with a 50mm - but with a 75 f/1.4 or 90mm f/2 or 135 f/3.4, every one* I've bought with 6-bit added focused more accurately on my M9 than the uncoded ones.

 

The wording "calibrated for digital" can be misleading - it just means "Calibrated/adjusted to tighter tolerances than when built in 1954-2006." It'll be more accurate on a film body as well - but you may not notice it on film.

 

Years ago, when Leica was first charging "only" $125 to add 6-bit coding to older lenses, my Leica rep to me a story. She had talked with Gunther, the lead serviceman at Leica NJ, and he was shaking his head and laughing over that price. "Management just doesn't understand the work involved in re-collimating and testing a lens after changing the lens mount! We're charging $125 - and my labor alone costs over $200 per lens!"

________________

 

*love-hate" relationships ;)

Edited by adan
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If the focus can be anywhere within the layer of the film material how does the scanner or enlarger know where to get the image data from?

 

The answer is - they don't "know."

 

Enlargers - photographer/printmaker manually focuses image onto paper (a substitute "focusing sheet" to avoid fogging the final print). With or without a grain magnifier.

 

AF dedicated film scanners - contrast-detection: the film, or lens, or sensor, or a mirror system, moves back and forth to find point of highest image contrast. You can hear the focus motor "whirr" as it adjusts distances to focus.

 

Fixed-focus flat-bed scanners (and to some extent all of these devices) - count on depth of field to cover the full thickness of the film gelatin.

 

The original Nikon Coolscan LS-10 was manual-focus - the operator made repeated small-area sample scans, and twiddled a thumb-roller on the front until seeing maximum grain/image sharpness.

 

https://d1t4l16dpbiwrj.cloudfront.net/vuescan/nikon_ls_10.jpg

 

In addition, of course, it is not as though there is only one focus plane recorded within film gelatin. There is one plane of best focus within the gelatin - but the film records layered images (sharp or less focused) throughout the whole depth of the gelatin/silver layer (on the order of 8 microns deep for B&W, 3x that for color films). What you capture is "pretty sharp" layered in between "not so sharp." And they all result in exposed silver - you can't pick and choose.

 

https://rodgermarion.files.wordpress.com/2017/02/diagram-depth-of-focus.png

Edited by adan
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[...] I'm not sure I'd see the difference with a 50mm [...]

 

You certainly would if your 50 was suffering from back or front focusing before calibration. Happened to me with a 50/1.4 asph from 2005. Lens calibration can't seem to fix focus shift though.

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Lenses that were calibrated for film were adjusted to a fairly wide tolerance, which sufficed for film.

I'm not convinced of this at all myself. My suspicion is that for the most part we are discussing older lenses which may well have been within "tolerance" when new, but which have been used for many years since and consequently probably need a service and re-adjustment to bring them back into "tolerance". I've had a few M lenses which were out. Three 90mm Summicrons (decades old) which were all off, three 135s which were all off focus too (the older E39 design which at least one thread on the forum suggested are difficult to adjust for film or digital ;) - my later E46 version is accurate), and a 75mm Summicron which was well out. I also have a 75mm Summilux which simply wouldn't focus at all and which was serviced by Leica and is not as precise as I could want it to be, even wide-open.

 

Its interesting to see that these are all longer lenses, the 90mm Summicrons were all at least 30 years old, the 135s have a potential adjustment issue and the 75mm Summicron is a complex mechanical design. With the exception of the 135s, I suspect that all the lenses were simply showing the signs of age and use - the 135s were always tricky to use well (I had one in the early 1980s which was always iffy wide-open). So calibrated for film or digital, or simply correctly adjusted - I'd go for the latter. Tolerances are tight on digital certainly, but they the are tight on film too and always have been.

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