Folder distance scale origin point

[bernard_L]

As we know, for "proper" cameras the distance scale is measured from the film plane. "Proper" cameras include all(?) reflex cameras and possibly all rangefinders as well.

I have this "new" Perkeo I 6x6 folder with 80/3.5 color-Skopar, front-element focusing. Went straight ahead with a test film. Several pictures include a foreground object intended to be sharp against blurred background. Distance was measured using a Voigtlander accessory rangefinder mounted on the camera's cold shoe.

Turns out the plane of best focus seems to be systematically farther than intended, by maybe 10-20cm (hard to be more precise absent a dedicated target). Checked infinity focus of the camera, using the method proposed (if memory serves well) by Rick Oleson: Scratch piece of film, place in film gate with scratch illuminated by red window; camera closed; look into the Skopar (shutter open) with a known good reflex camera having a split-prism center spot. Infinity focus is OK to an accuracy as good or better than the difference between infinity and 20m settings. Checked the accessory rangefinder: infinity is OK, and aiming an object at then end of a 1m ruler, I obtain exactly 1m (0.98<d<1.02) distance on the rangefinder scale, so rangefinder is OK as well.

Which leaves me wondering whether the distance scale on the front element of the Skopar (to which I transfer the reading of the rangefinder) is not meant to be from that front element rather than from the film plane. And my question: Does anybody have solid information (traceable documentation, experimental evidence) on which cameras, if any, have their distance scale measured from the front element? OTOH, that would be a bit odd, since Perkeo, Nettar, Isolette, have close relatives with a built-in rangefinder located more or less in the film plane.

Why don't I check myself the focus plane at close distance, using a ground glass? Because, as discussed in this thread, the film plane position is uncertain when the camera back is open. Which leaves me with the more time-consuming option of shooting a dedicated focus target.

 

[R.Gould]

I have several folders and fixed lens cameras and I use a watameter shoe rangefinder for focus setting, and they are all spot on AFIK, the focus measurement is always from the focal plane, have you checked that your rangefinder is focusing correctly, they can go out of focus and need checking from time to time, the best way I have ever come across for checking focus in any camera is some ground glass held over the focal plane of the camera with the camera on a tripod, check infinity and closer focus, if the image is sharp on the focus screen the it will be sharp on the film, I have checked a number of cameras this way and only rarely found one wrong, normally if the front element has been removed to get to the shutter and then not collimated correctly, but certainly every camera of this type, folders especialy, are all checked as I get them, and every one is focused from the film plane, and I must have checked at least 50, all in my collection and all used . so I suggest that you check your rangefinder, then check that the lens is collimated correctly

 

[bernard_L]

so I suggest that you check your rangefinder

I thought I had stated this clearly; at the end of the third paragraph of the original post.

the best way I have ever come across for checking focus...

did you have a look at the thread to which I provide a link in the last paragraph of my original post, especially the paragraph in italics by flavio81?

Then again, the safest method for a rollfilm camera (as opposed to a 135-format camera where the plane of the film is unambiguously defined) is in actual working conditions: take a pic of a focus target, i.e. a series of labeled targets spanning from closer to farther than the nominal distance.

 

[Alan9940]

FWIW, I have a Zeiss Super Ikonta III that had a very similar issue when I ran the first test roll through it. I checked focus using a small piece of ground glass that I made; this camera has obvious "rails" that the film rides on. I checked rangefinder accuracy using an external rangefinder and via tape measure, and it was good. Long story short, I sent it to Ken Ruth (he's retired now) and it turns out that the entire front shutter/lens assembly was not parallel or otherwise out-a-whack with the film plane. Took six months to get it back, but it now focuses exactly where I set it via the rangefinder!

 

[bernard_L]

this camera has obvious "rails" that the film rides on

I just checked another of my folders (Nettar). Indeed, there are "rails", but they are below the level of the rollers. Ditto on my super-Ikonta A. So, the pressure plate hits the rollers before the rails. I guess that position of the pressure plate (resting on the rollers) plus the natural concavity of the film just unwound from the supply spool, define the film plane, above the rails. So, testing with a ground glass on the "rails" carries some error. Similar issues are discussed in the link that I gave.

the entire front shutter/lens assembly was not parallel or otherwise out-a-whack with the film plane

Thanks for the head-up. I need to check that parallelism. The required accuracy I estimate as 1/300 radian approx 1/5 deg, corresponding to 0.1mm deviation from center to edge (30mm).

Interesting digressions. But remember the OP question.

 

[BrianShaw]

I would not expect any camera to have a gap between the pressure plate and the film rails that allows the film to float. Are you sure about that? Seems to defy logic in too many ways.

Unrelated question: is it true that accessory rangefinders measure from scen to rangefinder with no adjustment for the difference between rangefinder and fil plane. Not that it matters, but I’m wondering if my assumption is correct.

 

[Bill Burk]

I don't have any NIST-traceable info regarding the focus engravings reference point. So I'd be interested in what turns up here, but I don't know any camera which calls it as the front of the camera. I thought it was always focal plane.

I also have had trouble confirming a focus to film plane using glass. Seems to me that curvature of the film might be engineered into where the camera manufacturer defines the focal plane.

 

[R.Gould]

I would not expect any camera to have a gap between the pressure plate and the film rails that allows the film to float. Are you sure about that? Seems to defy logic in too many ways.

Unrelated question: is it true that accessory rangefinders measure from scen to rangefinder with no adjustment for the difference between rangefinder and fil plane. Not that it matters, but I’m wondering if my assumption is correct.

yes you are correct,they mesure from the scene with no focal plane adjustment, it makes no difference when the lens is stopped down, but wide open, when an old lens is not operating at it's best it can make a difference at closer distances

 

[BrianShaw]

yes you are correct,they mesure from the scene with no focal plane adjustment, it makes no difference when the lens is stopped down, but wide open, when an old lens is not operating at it's best it can make a difference at closer distances

Thanks very much. As I suspected.

 

[bernard_L]

I would not expect any camera to have a gap between the pressure plate and the film rails that allows the film to float. Are you sure about that?

I checked again earlier today on a Nettar. Machinist ruler resting on the rollers. Daylight is seen between ruler and inner rails (those one would expect the film to rest upon, not the outer ones, raised, that might act as lateral guides. And I'm not the only one who defies logic with facts: see this post: https://www.photrio.com/forum/threa...stment-focus-calibration.126327/#post-1670205

is it true that accessory rangefinders measure from scene to rangefinder with no adjustment for the difference between rangefinder and fil plane.

How would the designer of the accessory rangefinder know what correction to apply for the TBD camera on which it will be mounted? And it's a small correction.

And remember, the question in my OP is not about the few mm between the accessory rangefinder (on the cold shoe) and the film plane, but between the rangefinder and the lens, in case the distance scale of the lens has an origin at the front element.

 

[bernard_L]

Seems to me that curvature of the film might be engineered into where the camera manufacturer defines the focal plane.

This is true for some cheap 6x9cm cameras with a simple (meniscus) lens: the film guides are curved, absorbing the field curvature in at least in one direction.

 

[BrianShaw]

In every camera I’ve ever seen, the rollers are always on a slightly different plane than the film rails. It’s to facilitate film flatness. What you are saying reduces the chance of film flatness to zero.

And to call it a design feature seems very odd to me. There is no standard (or common expectation) for uncontrolled film curvature.

I understand curved guides. While curved there is at last a minimal effort to control the film against the film rail. My first 127 camera was like that... way back when. Fixed focus, fixed aperture, fixed shutter speed, no controls whatsoever. In the end it probably didn’t matter much as the only aperture available produced sufficient DOF.

 

[BrianShaw]

“And remember, the question in my OP is not about the few mm between the accessory rangefinder (on the cold shoe) and the film plane, but ...”

Yes, that’s why I prefaced with “ Unrelated question, ...”.

 

[Bill Burk]

...but I don't know any camera which calls it as the front of the camera. I thought it was always focal plane.

I think I'm wrong here.

On Butkus there's manuals for Perkeo I and Perkeo II which tell how to focus with the Focar accessory lenses and you do measure to the front of the lens.

So it's highly likely you are expected to measure to the front of the lens. Unless they just do that for close-up and assume it doesn't really matter when you are farther away.

 

[BrianShaw]

Front of lens rather than lens node (approx at aperture often)? No matter, that rather unusual.

 

[bernard_L]

the rollers are always on a slightly different plane than the film rails. It’s to facilitate film flatness

How would that be? I would expect emulsion side of film to rest on the two rollers and on the two side rails, all in one and the same plane. And the pressure plate to ensure that contact on 2x2 segments surrounding the image area, at a distance from the emulsion of film + paper thickness.

And to call it a design feature seems very odd to me.

I did not call it a design feature for the classic folders. Just answering Bill Burk, and restricted to cheapo cameras, often made of plastic or bakelite.

 

[bernard_L]

On Butkus there's manuals for Perkeo I and Perkeo II which tell how to focus with the Focar accessory lenses and you do measure to the front of the lens.

Thank you Bill for pointing out this manual. This is (almost) the kind of info I was hoping for on this forum. Possibly by working on the numbers in the table I might be able to decide about the distance scale without close-up accessories. But i want to do this "off-line" and double-check.

 

[BrianShaw]

How would that be? I would expect emulsion side of film to rest on the two rollers and on the two side rails, all in one and the same plane. And the pressure plate to ensure that contact on 2x2 segments surrounding the image area, at a distance from the emulsion of film + paper thickness.

I did not call it a design feature for the classic folders. Just answering Bill Burk, and restricted to cheapo cameras, often made of plastic or bakelite.

Your planar expectation is not my experience. Rollers are for smooth transport. Film rails are to establish the film plane.

 

[Bill Burk]

I made a point light source, focused my Bessa II on 3.5 feet and put a ground glass on the film rails... Not saying it's accurate within .3 mm but we're talking a significant distance from front of lens to film plane.

I moved forwards and back until the ground glass image looked sharp and then measured, the front of the lens was around 40 inches (42 is 1.5 feet)...

Then I focused at 4 feet, and this time I set the focal plane at 48 inches. The image was distinctly unsharp.
Moved to where the front of the lens was 48 inches and the image was sharp.

I think Voigtlander measured to the fronts of the lens.

 

[shutterfinger]

Looking at page 4 of http://www.cameramanuals.org/voigtlander_pdf/voigtlander_perkeo_i.pdf item 16 is a film roller on the supply side and there does not appear to be one on the take up side, but you say rollers in your check so is the manual picture incorrect?
Item 17 is the pressure plate, is the length of the plate the same as the distance between the film rollers or is it the same as the length from the outside edges or centers of the roller(s)?
Backing paper is .004 inch (4mil) thick. Kodak and Ilford films are 4.5 mil thick, Foma 3 mil, Fuji 3.5 mil.
Is the gap you see between the roller(s) and guides greater than the bp+film sandwich?

 

[BrianShaw]

I made a point light source, focused my Bessa II on 3.5 feet and put a ground glass on the film rails... Not saying it's accurate within .3 mm but we're talking a significant distance from front of lens to film plane.

I moved forwards and back until the ground glass image looked sharp and then measured, the front of the lens was around 40 inches (42 is 1.5 feet)...

Then I focused at 4 feet, and this time I set the focal plane at 48 inches. The image was distinctly unsharp.
Moved to where the front of the lens was 48 inches and the image was sharp.

I think Voigtlander measured to the fronts of the lens.

 

[bernard_L]

Indeed the table in the manual, pointed out by Bill Burk, seems to confirm that plain distances (no close-up lens) are from the front element.

• Take Focar 1; infinity setting of mains lens. "Sharp definition" at 2' 7.5" = 31.5". So power of Focar1 in inch-1 is 1/31.5=0.0317460.
  
• Move to next-to-last line. 4' = 48" main lens setting. If that is from the lens itself the power of the main lens has increased (from infinity setting) by 1/48=0.02083333 inch-1. Add to that the power of the Focar 1, result is 0.02083333+0.0317460=0.05257933inch-1. Take reciprocal, resulting 19.018". Look up corresponding entry for sharp distance with Focar1 (second column, next-to last line: 1' 7" = 19".
  
• That would not work out if the 4' setting was from the film plane, resulting in (approx) 3'9" from the front lens.

You will have to make do with the numerology. If I would try to explain, half a dozen people would prove me wrong, at least until someone else with established optics expertise (e.g; Nodda Duma) would chime in.

So, as far as I'm concerned, the case is closed.

Thank you again Bill Burk for providing the key information.

 

[DWThomas]

I was about to investigate my Perkeo II in regard to some of these questions and discovered I had it loaded with film and am too lazy to unload it ("next time"). But one observation I thought was interesting: besides measuring from the front of the lens, the chart for Focars shows with the focus scale set to infinity the Focar 1 is focused at 2' 7.5" (80 cm) and the Focar 2 at 1' 5.5".(44.5 cm). Now normally a +1 diopter should focus at 1 meter, the +2 at a half meter. So maybe the chart is tweaked to compensate for measuring from the front -- but using that assumption the delta sounds too long for the focal plane to lens front. So maybe the Focar 1 is not exactly 1 diopter? I have seen some Zeiss closeup lenses at 0.7 diopter and the like. Ah well!

In my Ercona II (East German Zeiss Ikonta) it appears the pressure plate does not extend over the rollers on either side of the film gate/mask; I shall check the Perkeo one of these days, but I'd be surprised if it's not similar. The manual picture of the Perkeo II interior shows shiny rails above and below the film gate which would normally be the bearing surface contacting the edges of the film to establish the film plane. The Ercona II has a little row of bumps instead of rails; it also accepted a mask (which I don't have) to shoot 6x6 instead of 6x9.

Over the last 1.5 centuries there have probably been several dozen manufacturers of folders, some with more than a dozen models, so I would be reluctant to make too many blanket statements.

Alas, so far -- the more I learn, the less I know. Ver-r-r-ry interesting

 

[bernard_L]

@DWThomas.
Thank you for checking your Ercona. As concerns the Perkeo, see below. Indeed, Focar1,2... are just names.

@shutterfinger.
Checked my Perkeo I. There are rollers on both sides; the one on the take up side has a larger diameter. The distance between rollers is 70mm (give or take 2mm); the width of the pressure plate is 80mm. So the pressure plate also presses against the rollers. The rollers appear to be in the same plane as the film guides, so that guides+rollers+plate properly define the film plane. And, in the case of the Perkeo, my previous objection against checking focus with a ground glass is unfounded.

But, for a reason known only to them, the designers at Zeiss Ikon decided otherwise. I repeat/confirm that on both my 6x6 Nettar and my Super Ikonta A, the rollers stand higher than the guides, and the pressure plate is wide enough to press also against the rollers.

Backing paper is .004 inch (4mil) thick. Kodak and Ilford films are 4.5 mil thick, Foma 3 mil, Fuji 3.5 mil.
Is the gap you see between the roller(s) and guides greater than the bp+film sandwich?

I do not think that is relevant: film and backing paper are between the guides and the pressure plate; you can make the same statement replacing "guides" with "rollers". In both cases the pressure plate is at the back, so rollers and guides "should" be in the same plane irrespective of the combined thickness film+paper.

Actually, I need to re-set the focus of my baby Ikonta. I think I'll press the ground glass against the rollers, and after that do a film check.

 

[shutterfinger]

Some cameras and roll film holders for 2x3 and larger sheet film cameras have 2 sets of guide rails the outer most for the backing paper and the inner ones for the actual film. Guide rollers are for improving film flatness and ensuring smooth film transport.

But, for a reason known only to them, the designers at Zeiss Ikon decided otherwise. I repeat/confirm that on both my 6x6 Nettar and my Super Ikonta A, the rollers stand higher than the guides, and the pressure plate is wide enough to press also against the rollers.

Tension from the film winding will or should hold the film flat against the pressure plate making the image/film plane at the outer edge of the rollers.
Anyone's guess as to why they did it this way. If only one could search the archives of Zeiss provided there are some.