Ikonta: film-plane and checking focus.

[Grytpype]

Recently, I decided to check the infinity focus on my "new" Tessar lensed Ikonta 521/16 and was rather alarmed to find that it appeared to focus infinity at below 15 metres on the scale! I had always assumed that MF cameras were arranged basically the same as 35mm, but bigger, and therefore would have film rails that define the lens register, guide rails to keep the film in position, and a pressure-plate that sits on the guide rails leaving just enough clearance for the film to pass between itself and the film rails.

I've attached a picture of the 521/16. It has ridges forming guide rails, and flat strips inboard of these that I took at first to be equivalent to the film rails in a 35mm camera, and therefore this is where I placed my ground-glass to check focus. These may look as though they could be film-rails, but they are not. The strips are lower than the rollers over which the film passes (by .008" in this camera). The pressure plate sits on the guide ridges, as in a 35mm, but these are .022" above the rollers (checked in 4 cameras), therefore .030" above the flat strips - a lot more than the thickness of the film, which according to my measurements is .010", or perhaps effectively about .012" when the film is not pressed onto the backing paper by a micrometer.

​

So the film passes across the rollers and runs through the space between the flat strip and the pressure plate and is not positively located at all. My second guess at a film plane was the rollers, but I'm sure that this is wrong too. The film will only run straight and flat from roller to roller if it is under tension, which it is not. Coming off the roll it will have curvature, so I think it will tend to rest with the backing paper touching the pressure-plate. If this is the case, the distance from the film emulsion to the pressure-plate will be the .012" estimated as the film thickness and the film emulsion will lie, therefore, about .010" behind the rollers (.022" minus .012"), or .018" behind the flat areas I had originally taken as the film-plane. No wonder the focus scale was a bit off!

I tried out this theory by checking where infinity focus was optimised on my two 521/16s, the one with the Tessar and another with a Novar (both from 1938). The guide ridges (and therefore the pressure-plate) on both were .022" above the rollers. I placed the ground glass on the rollers and I added packing between the glass and the rollers until the focus on the ground glass was correct. Both focused rather further back than I expected, .015" behind the rollers for the Tessar, and .019" for the Novar, apparently behind the emulsion, by .005" and .009" respectively.

However, I was only considering focus at the centre of frame. In practice the lens produces a curved, not a flat, image, so if optimum focus is a little behind the emulsion in the centre, the definition off-centre and towards the edges will be better than it would be if central focus was perfectly optimised at the emulsion, so overall quality will be better. I have often read that the 4-element Tessar is "better corrected" than the 3-element Novar, which I assume means (amongst other things) that the field is flatter, so this could explain why, in my checks, the Novar focused further behind the rollers than the Tessar.

The same considerations, presumably, will apply to other medium format cameras, even where the film-plane is actually well defined by film-rails, and we should then also be setting optimum infinity focus on these at some point behind the film-plane. A lot of the same ground has been covered in this thread on another forum with respect to a Selfix 820. If there is coverage of this subject elsewhere online, I've yet to find it it.

Does this all make sense, and do my measurements seem reasonable? It all seems very imprecise to me.

 

[Sarcophilus Harrisii]

There are always compromises involved in any optical design (whether you want to discuss manufacturing tolerances for lens pieces or assembling the lens board or indeed, an entire camera body). Clearly a classic front cell focus three element configuration is going to require more compromise than some other installations with either better field correction or Eg. using a unitary focus system.

Under the circumstances, I think all you can reasonably do is simulate a target based on the conditions a film loaded at the gate is subjected to, and then adjust overall sharpness across the gate as best you can depending on you own preferences, needs and/or philosophical attitude to these things (Ie. how much of a perfectionist you are!).

Given this I would be inclined to use a decent length piece of unexposed and developed film that may be wound across the rollers, with the addition of a suitable thickness clear substitute for the paper backing. There are various possibilities, but perhaps even a length of plain white paper of correct thickness may still pass enough light (with a strong light source behind it) to enable inspection to be carried out. As far as the pressure plate is concerned, if you have established the surface on which the actual pressure plate bears, I can see no reason why a suitably sized piece of clear glass might not be mounted to support your testing film. Providing the glass is of the correct dimensions sitting on the proper perches it will make no difference to the film if it is made of glass or painted metal.

Having done this you are probably going to have to test your hypotheses, perhaps by setting optimum centre infinity focus and checking exactly how bad the edge sharpness is at this point. Naturally it becomes harder to view the target film as the lens is stopped down, but if you conduct at least your initial tests wide open, you would have to reasonably expect that overall sharpness will improve, not deteriorate, as the lens is progressively closed down.

It's an interesting topic and your points are well taken, unfortunately, though, without access to an optical bench I can see no easy way of definitively verifying the effects of the variables, so inevitably any substitute method without such equipment, whilst potentially able to yield satisfactory results, is likely to be largely empirical.

I presume you would be well aware of the procedure that involves using a SLR as a form of automcollimator to magnify the film target? There are several well documented references to this on the web including some examples of it being employed here at RFF.

I appreciate your post because, like many of the best ones, it makes one pause and re-assess a familiar topic from new perspectives. Eg. using a really good SLR to collimate another camera is all very well, but it certainly begs the question: What if the lens attached to your testing SLR isn't particularly good? Apparent deviations from optimum focus at your film target might, then, potentially be partly or wholly attributable to the poor performance of that optic. It's food for thought.

Given this, it makes me pleased that I've settled on my Hasselblad and its lenses as my preferred testing apparatus. They're not perfect by any means but as mine is in good adjustment it has to be as good an option as most other types available for magnifying the film target. The larger than 35mm focus screen also makes for a bigger view of the target which is a plus.

I'd be interested to hear Hans Kerensky's views on the topic. Unlike most of us mere mortals, Hans uses a Gokosha auto collimator for setting up camera focus, and is likely in a position to make measurements of both greater accuracy and consistency more easily than the rest of us, and he has also attended to a few Ikontas of various types.

I'll be interested to read further posts on the topic, thank you for airing it.
Cheers
Brett

 

[citizen99]

Some interesting points discussed here, which prompted me to look at some folders that I have to hand ...

The Bessa Rangefinder, Ensign Autorange-220, and Ensign Ranger II have no structure on/of the film gate which could be interpreted as 'guide rails' for determining the in-plane location of the film. On these, the film vertical registration is determined solely by the vertical positioning of the spools in the camera body.

The Kodak 66 II has what look like 'guide' rails standing 'proud' on the film gate, but on which the film rests, (similarly to those in the OP's picture of the Ikonta), thus determining the film plane distance from the lens. Again, the film vertical registration is determined solely by the vertical positioning of the spools in the camera body.

Whereas, in the Kershaw 450, the film does travel in a slight recess in the film gate (the depth of the recess corresponds to the thickness of the film + backing paper). The vertical position of the recess of course corresponds to the position of the film spools.

 

[hanskerensky]

Surely interesting. Will have a look at my Ikonta's in the near future.

It is true that many times the focus is not in the centre of the film but a bit to the corners. I have observed this with my auto-collimator many times both on MF as well as 35mm. For instance, just yesterday I had a look at a MC Rokkor 55mm 1:1.7 lens on my Minolta XE-5 and observed the same. By tilting the camera underneath the auto-collimator I could see that the infinity focus is correct at some distance from the center. The auto-collimator showed that at the centre the lens would have to be corrected +0.15mm (further away from filmplane) to get sharp focus.
I saw the same recently with a MD Rokkor 50mm on a XD-7.

btw I always use my auto-collimator on a film loaded in the camera. Not, as usual, on a mirror against the filmgate, as I observed that there were deviations showing on images taken with the cameras. No doubt due to the working of the pressure plate and the tension of the film.

 

[Grytpype]

Thanks for the interesting comments.

I presume you would be well aware of the procedure that involves using a SLR as a form of automcollimator to magnify the film target?

I lied about the ground glass! It makes things easier to explain. I was in fact using a home-made collimator consisting of an old Exakta VX1000 body without curtains, and a Meyer Trioplan 100mm/f2.8 lens. I seem to get fairly reliable results with it, though I'm conscious of the fact I could be introducing another source of errors.

I have observed this with my auto-collimator many times both on MF as well as 35mm.

Actually, in light of the above, I'd been worrying about whether we should be checking focus in 35mm cameras a little behind the film rails, and if not - why not? I could only think of one MF camera where I had messed about with the focus adjustment, and I've re-set that, but I have quite a lot of 35mm, and I will have checked all of them at some time for central focus at the film-plane and quite possibly adjusted most of them!

 

[sevo]

Anybody who ever wrestled with glassless projection knows that it is not that easy to get even 35mm film to stay flat inside a 24x36mm window - Leitz even made projection lenses whose field curvature followed the average warping of film in glassless paper mount slides. Medium format spans more than twice the unsupported distance, so medium format cameras have to use particularly sophisticated tricks to make the most relevant areas of the film bend into the focal plane. Most modern pro cameras use different pressure plates for the relatively small difference of the missing backing paper in 220, some even use different magazines with different film rails - if it could work by simply pressing the film to the front rail, the same pressure plate would do for both.

 

[hanskerensky]

Rollei even experimented with a vacuum mechanism in a Rolleiflex to get better film flatness ! It never made it to the market.

 

[hanskerensky]

Well, I had a pleasant evening doing some measurements on my own Ikonta 521/16.

Guess I do not have time to publish everything today but can already conclude that the observations by Grytpype are right.

First of all some more insight about the film we are talking about, a type 120 :


Ikonta 521-16 Film Plane Measurements (01) by Hans Kerensky, on Flickr

As you can see the back paper is about 63mm wide. The film itself is a bit smaller, say about 1mm

 

[hanskerensky]

Here you see how the film lies in the camera.
As stated by the OP it runs between the film guides (red arrows) :


Ikonta 521-16 Film Plane Measurements (02) by Hans Kerensky, on Flickr

 

[hanskerensky]

The width just between the Film Guides is about 63mm. So same as the Film itself.


Ikonta 521-16 Film Plane Measurements (03) by Hans Kerensky, on Flickr

 

[hanskerensky]

The width of the Pressure Plate however is about 67.5 mm. Wider then the Film !
So, indeed, the Pressure Plate will rest on the (outer) Film Guides !


Ikonta 521-16 Film Plane Measurements (04) by Hans Kerensky, on Flickr


Oii, have to go now, my wife is calling me for the third time 🙁

 

[tunalegs]

Many box cameras and some folders use the curl of the film against the film gate to push the film against the back of the camera (or against the pressure plate in a folder).

 

[hanskerensky]

Many box cameras and some folders use the curl of the film against the film gate to push the film against the back of the camera (or against the pressure plate in a folder).

This is indeed seen in camera's using (single element) Meniscus lenses. These lenses can not be corrected for a flat filmplane so a curved filmplane is needed to still keep some sharpness towards the corners.

 

[BernardL]

A very informative thread: not only the OP, but also several responders. Wish all threads were as informative.
Also quite timely for me: I need to re-calibrate the focus of my super-Ikonta A, and I had some doubts as to where the ground glass should be resting: ineer guides, outer guides, rollers?

Maybe the focus should be measured in "operating conditions."
- find some way to introduce lighting inside the camera, possibly using a Flex (copper-kapton) wiring to pass between the back and the body; scavenge from a dead digital camera or a dead laptop.
- Sacrifice a piece of film + backing paper: mark some sharp pattern on the emulsion side.
- Use collimation method with a known good SLR. If in doubt, cross-check with several known good SLRs.
With that method, the film is positioned just the way it is when being exposed; no guesswork. Your thoughts?

 

[Grytpype]

You may find that the Super Ikonta is arranged rather differently, Bernard. I don't have an 'A', but I have the remains of a 531/2 ('C') and on this one the guides are formed differently, and the flat areas in the 521/16 are replaced by a series of slanting lumps (see image). I don't have my tools to hand, but as far as I can tell with a ruler, these are at the same height as the rollers. The pressure plate is small enough to go between the guides, and between the rollers, so it is actually pressing the film on to these lumps, and in this case they are film-rails, and the film plane is well defined. The same focus-behind-the-emulsion principle presumably applies, though. (I think the design of the 'lumps' is to allow airflow, to prevent the film being drawn forward by suction when the bellows open)

​

As well as the 521/16 I have one other Tessar lensed Ikonta, a 520/2, and in both these cameras the three grub screws fixing the focus-ring to the front element go into drillings in the front element housing, so it is obviously not intended that focus adjustments should ever be made by rotating the focus ring relative to the front element, and the focus should never change unless the camera is damaged or very worn. I assume that other Tessar lensed examples are the same, and I think that in this case, as long as the camera is reassembled after any dismantling with the grub screws in their original holes (and the helical thread in the correct 'start'), there should be no need to worry about resetting the focus. When I defungused the 520/2 I replaced the grub screws in their drillings and was very satisfied with the definition when I put film through.

On Novar lensed examples however, the focus ring can be adjusted freely, so the focus will need re-setting if the lens has been fully dismantled for cleaning or helical re-greasing. I'd suggest that, before dismantling these, assuming that there is no reason to suppose the focus is wrong to start with, the position of the focus ring relative to the front element is marked, if possible, or that the position of optimum central infinity focus is checked, so it can be reset on assembly.

 

[hanskerensky]

Had no time yesterday because we went to La Traviata, but let's go on.

I used my caliper as a precision ruler and placed it on the Rollers.

Ikonta 521-16 Film Plane Measurements (05) by Hans Kerensky, on Flickr



Using a mirror and a strong light source a slit shows up, an indication that the Rollers are (very) slightly positioned above the (inner) Film Rails.

Ikonta 521-16 Film Plane Measurements (06) by Hans Kerensky, on Flickr

 

[BernardL]

@ Grytpype. My S-Ikonta A has the film channel like the 521/16 in your OP. (Must be one of the more recent types, has viewfinder with collimated bright frame.) And, as seen by hanskerensky, the rollers are slightly above the inner rails.

 

[hanskerensky]

Measurement of distance between the outer edges of the Rollers. About 74mm.

Ikonta 521-16 Film Plane Measurements (09) by Hans Kerensky, on Flickr

This measurement brought to the Pressure Plate. It is now clear that this Plate extends till beyond the Rollers.

Ikonta 521-16 Film Plane Measurements (10) by Hans Kerensky, on Flickr

And also explains why the shaped ends of the Pressure Plate are no problem.
btw See that handwritten 1948 ? Maybe the production year.

Ikonta 521-16 Film Plane Measurements (08) by Hans Kerensky, on Flickr

 

[hanskerensky]

Finally I decided to cut a piece of stiff paper exactly to the height of the Pressure Plate and placed it in the Film chamber.
As you can see it rests clearly on the (outer) Film Guides.

Ikonta 521-16 Film Plane Measurements (11) by Hans Kerensky, on Flickr

 

[hanskerensky]

These measurements did indeed show that the (inner) Film Rails can NOT be used as a reference for the Filmplane.

But neither can the (outer) Film Guides !

My guess is that there is a room for the Film between Pressure Plate and (inner) Film Rails of about 0.5mm.
Now, the Film is about 0.2mm thick so that leaves still about 0.3mm freedom for the Film to roam freely around the focal plane.

Not what I would call precise but consistent with my earlier auto-collimator measurements using film in this particular Ikonta type.

For those who use a Ground Glass to check infinity on this Ikonta I would advice to first place 0,15mm thick strips on the (inner) Film Rails and press the Ground Glass against those.

A definite check always has to be done by shooting a test film