As my figures have been challenged - here is the basis for my arument over the amount of film to lens distance error vs the focusing scale on a 50mm lens. If we assume that any 50mm lens will require the same amount of film to optic distance to focus then here is how I worked it out; Test 1, measure the flange to filter ring distance with a pair of vernier calipers ( mine are accurate to .02mm) - one measurement at infinity and another at between the 3 and 5m setting on the scale. Do 10 seperate measurements and avarage the results to account for measuring error. distance at infinity - 53.50mm distance at 3-5m - 54.25mm total movement .75mm

The other calculation involved using the same method to measure the amount of lens element travel for exactly 1/2 the total rotation of the focusing ring. Multiplying this by 2 will give the thread movement i.e the amount of lens extension for 1 complete rotation of the lens focusing ring ( if it theoreticaly moved this far) This gives you a ratio and some simple math(s) enables you to calculate the amount of focusing ring rotation required to produce a definate amount of lens movement . This calculation confirms the first practical measurement. When you combine the measured results from the report and subtract these from the Leica spec's you get the figure of .75mm ( 27.95-28.7=0.75mm) I hope this clears things up.

Johann

-- Johann Fuller (johannfuller@hotmail.com), May 20, 2002

Answers

Johann; Usually the lenses are not exactly 50mm focal length. That is a marketing number. Many times the nominal is 52 or 53mm. Many times in the past Leitz made several different helical pitches for each lens model.. The lens block assemblies were assembled and measured; and matched to a specific helical lens assembly tube..The exact with usually 0.2mm focal length was marked on each lens sometimes with a special code.. Typically a "50mm" lens has its entire lens assembly move as one solid block in its helix..Using the lens formula is one way to calculate the lens block movement during focusing.. 1/a + 1/b + 1/c ..The standard Physics book lens equation.. For a 50mm lens and a 5000 mm ( 5 meters ) focus the movement is 0.5050505 . . . mm For a 50 mm lens and a 3000 mm ( 3 meters ) focus the movement is 0.84745762 mm

-- Kelly Flanigan (zorki3c@netscape.net), May 20, 2002.

Ah at last - I'm not alone!!!!

-- Johann Fuller (johannfuller@hotmail.com), May 20, 2002.

I'll play devil's advocate here (but I admire your thinking and creativity!)

You have _vernier_ calipers are accurate to .02mm, or less than .001 inches? Anything meaningful with that resolutoin ought to be done under controlled temperture conditions. That filter ring to flange test is a devil of a thing to measure accurately, due to even the slightest tilt adding seperation to the calipers jaws. Really... your smallest measurement there, of the 10 would have been the closest to correct. A height gauge calibrated to pair of blocks from a surface plate would have been better to make those measurements, but who has that stuff laying around (I dont anymore dammit!)

The second issue is the engraving accuracy. I've always thought those numerical distance engravings were approximations at best.

-- Charles (cbarcellona@telocity.com), May 20, 2002.

Very good points Charles - and I'm always open to improvements in method and to scrutiny thereof!- it's the only way we can eliminate errors and variables in this area and get to the truth. Yes the slightest angular change will cause errors and so to minimise this it is possible to mate the flat edge of the caliper jaws with the face of the flange. In addition it is not the actual distance measured which is of primary importance but the difference in the two (averaged) measurements. As kelly has calculated, I was estimating a value of 4m as the rotational distance (as measured on the circumference) was between the 3 and 5m scale on my lens. As for lens scale engraving accuracy I have found it very precise at near distances but obviously at the longer distances there is no way to accuratley match an engraved number with a fixed line. I still have no idea if there is a Leica - Hexar ( or Voigtlander) incompatiblity I have only sought to highlight an obvious error in the published figures - either a result of a misprint or flawed measurement.

-- Johann Fuller (johannfuller@hotmail.com), May 20, 2002.

Yup, now it all makes sense to me: I can now see HCB sitting around at night running these calculations. No wonder he was a loner. No time for a casual beer: mathematics first! The 10 separate calculations cinched it for me. Wow. Thanks for the creative insights.

-- Patrick (pg@patrickgarner.com), May 20, 2002.

look -- i don't know what you are smoking, but all anybody has to do is remove their 50mm summicoron and watchthe rear element in relation to the lens flange as the focus from 3m to infinity. you don't need calipers to see that the movement is on the order of several mm, not a tiny fraction of a mm. likewise, if you merely watch the increase in the width of the band between the knurled focus ring and the knurled aperture ring, you see that the movement is WAY more than any .4 mm (or .7 or whatever). could someone else just look at their lens for a second and confirm that something is seriously out of whack here by a factor of ten or so!! anyone who has ever used a view camera will immediately realize these tiny numbers are way off.

-- roger michel (michel@tcn.org), May 20, 2002.

Roger, Read kelly's calculation based on the laws of optics, read my calculations and methodology, have a look again at your own 50mm lens (with some calipers if possible) and if you still think the lens elements move several mm then I want some of the stuff you are smoking! I use a view camera nearly every day - with my 47mm and a 58mm lenses the amount of travel to move from a close object and infinity is tiny.

-- Johann Fuller (johannfuller@hotmail.com), May 20, 2002.

a few further points: (1) i just popped my 65mm angulon into my wide angle focus device and thence onto my linhof tech. the measured distance from infinity to approx 3m for this lens is nearly 9 mm. a 50mm would not be an order of mag less; (2) due to the log relation between distance and focus movement, if your estimate of .5mm or so (depending on which or your figures one uses) as the total movement between 3m and inf were accurate, that would mean at the long end of the focus scale, there would be mere hundredths of a mm between focus points. the mechanical helical is not that accurate by any means, nor could the feeler arm detect movements of that order; (3) kelley, what are the variables in your equation, and what is the complete equation. it bears no relation to the one merklinger supplies in his "focusing the view camera" or to others i have seen for determining focus spread; (4) again, i ask people to just eyeball the movement of the rear element of their cron as you focus from 3m to inf. a fraction of a mm is really too small a movement to even see. even a casual observation of the rear element (in relation to the flange) will tell you that it's moving a MUCH greater distance.

-- roger michel (michel@tcn.org), May 20, 2002.

thanks for reminding me about the 47mm -- i have one in my brooks veriwide. i just measured the movement of the rear element. it is approx 5 or 6mm from 4m to inf. this is indeed a tiny movement. and indeed when i use the 65mm on the linhof the movements are tiny. however, theyare not microscopic as .4 (as in 4/10ths) of a mm would be. it would not even be possible to make such an adjustment without the most finely geared movements (and even then i'm not sure). could someone else just please actually look at their lens and confirm that we are not talking about hundredths of an inch!

-- roger michel (michel@tcn.org), May 20, 2002.

kelly -- using your formula, what woud the spread be for a 90mm from 3m to inf?? thanks.

-- roger michel (michel@tcn.org), May 20, 2002.

Roger, This is getting out of hand and I'm starting to laugh - I looked at my Nikon 50mm (just to confirm I'm not going insane) and it moves just under 1mm from 3m to infinity. I have just mounted my 58mm and focused on my window which is 2m away and the distance and the movement is 'about' 1mm - certainly no more. I also looked at my 15mm Heliar and this takes less than 1mm to move from .3m (12 inches) and infinity.

-- Johann Fuller (johannfuller@hotmail.com), May 20, 2002.

Johann and Kelly:

Roger is right.

I did as Roger requested and just measured my new version 50 Summilux. I used dial calipers which are calibrated to .001 inches, or 1/1000th of an inch. Total movement from .7m to infinity on my Summilux is .198 inches, or 198 thousandths of an inch, or a little over 5mm -- 5.029mm to be exact.

Cheers,

-- Jack Flesher (jbflesher@msn.com), May 20, 2002.

Roger,
I just performed an empirical study as you suggested on a 50mm Summicron s/n #23403xx.
I do not have ultra precise tools, but the tool I used confirms that the movement between 4m and infinity is indeed less than 1 (one) mm.

-- Niels H. S. Nielsen (nhsn@ruc.dk), May 20, 2002.

johann -- i think this may be just a decima point thing. if you go to tuan's large format website there is a list of tables that sets out focus spreads for various focal lengths (focus is identical regardless of format for a given f). you will see that your numbers appear to be off by a factor 10. if you are a view camera user, you know this must be the case. certainly you have used a 90mm many times and know that the spreads are many many mm, not fractions of a mm. also, i don't know where kelly got that formula, or what it relates to, but the actual formula for focus spread that i am looking at in merklinger is a long log formula. certainly focus spread is not a simple arithmetic relation as kelly seems to indicate. a look at the very non-linear focus scale on any lens will tell you that. anyway, i am happy to contnue this via private email. i am sure we have totally bored everybody. AND THANK GOD FOR JACK -- I THOUGHT I WAS LIVING IN A PARALLEL (but slitely diff vis-a-vis focus spread) UNIVERSE!!

-- roger michel (michel@tcn.org), May 20, 2002.

What is happening here?
I checked again:
4m to infinity= less than 1mm movement.
I also checked on a 50mm f 1.8 nikkor = same result.
Johann is correct!

0.7m to infinity= around 5mm is also correct -but it is not what Johann is talking about.


-- Niels H. S. Nielsen (nhsn@ruc.dk), May 20, 2002.

Jack, Congratulations correctly measuring the lens extension from .7m to infinity - now confirm that the distance we are actually talking about (3/4/5m to infinity) is as Kelly and I have calculated and measured. Roger and I are now going to continue our discussions via private e- mail - colective sigh of relief!

-- Johann Fuller (johannfuller@hotmail.com), May 20, 2002.

Have Leica kept the same tooling for many years? when your talking about these sort of tolerances is there any camera that is bang on the button? even SLR`s have to rely on the mirror being correctly positioned. Maybe we are all putting up with dof to make our images appear sharp?

Dave c

-- D J Chilvers (davechilvers@btinternet.com), May 20, 2002.

Your methods are flawed.

Lens group movement is not the same for every 50mm lens. Focal length is the distance between the lens nodal point and the focus plane for an oject located an infinete distances away. The nodel point can be in fron of the lens group, within the lens group, or behid the lens group. That's why some lenses are called telephoto (nodal point in front of the lens) and parafocal (nodeal point behind the lens , like a 24mm lens in an SLR with the rear element 35-40mm in front of the film plane).

Granted, 50mm lenses are rarely telephoto or parafocal, but, each design places the nodal point at different locations between the elements. this affects the actual travel distance of the lens elements required to adjust focus from infinity to some closer distance. The helical placement (relative to the nodal point) and the helical rate are designed to account for these differences. Therefore, comparing focus movements of lens groups (or focus ring angle of rotation) to assess focus accuracy is highly suspect.

-- Dan Brown (brpatent@swbell.net), May 20, 2002.

Granted, different lenses may have the nodal point in different places relative to the elements, but surely the amount of movement of the nodal point (compared to the film plane) will still be the same as the movement of the lens group as a whole. The nodal point for a particular lens doesn't move relative to the lens elements at different focussed distances does it?

However, I don't see the need for (possibly error prone) physical measurements. A simple calculation (1/u + 1/v = 1/f) shows that the lens to film distance changes by 0.63mm when focussing between 4m and infinity, and by 3.85mm when focussing between 0.7m and infinity.

Nigel

-- Nigel Bowley (nigel.bowley@btinternet.com), May 20, 2002.

Dan, I am well aware of the issues you have refered to - that is why I am confining the test to around the infinity setting of a 50mm lens where any variation in nodal point will have least influence and not a wide angle or retro focus lens design. I am merely using a few simple and hopefully repeatable and understandable measurements to show the error in the original published test. Flawed - if used to describe the behaviour and principle behind all lens focusing actions but accurate enough to reveal the absurd notion that two bodies can possible measure with a .75mm difference in a critical area (and that figure is not my calculation but as published if you do the maths).

-- Johann Fuller (johannfuller@hotmail.com), May 20, 2002.

Johann, I think that the margin of error is very small, because I think most modern double-gauss 50mm lenses (6 or 7 element) are darn close to the same in terms of nodal point placement. The rangefinder lenses may have the nodal point a little farther out, as a class of lens.

-- Dan Brown (brpatent@swbell.net), May 20, 2002.

Jeepers all this lens stuff makes me wonder my I was the lead optical engineer on a project 23 years ago...All my love for the 35mm started with the Argus A2.....................Kelly

-- Kelly Flanigan (zorki3c@netscape.net), May 20, 2002.