Sliding backs and helical mounts - some remarks

ALPA has never made a sliding back and we have no future plans to do so. Why? Not because we do not or cannot understand the advantages of such sliding backs (e.g. easier/faster workflow, less danger for dust getting to the sensor), but because we do not only see the advantages but also the disadvantages of sliding backs. Of course there are better and less well designed/manufactured sliding backs, but none of those that we have seen, is as it should be. In our estimation, the negative points clearly outweigh the positive points. This judgment is even more justified than ever given the fact that digital backs are getting more and more sensitive (e.g. with their small and smaller pixels, etc.). To illustrate this, here the comment of an American camera dealer: "... I have sold a variety of sliding adapters over the years made by (...). In all cases it is exactly as you say, the ground glass is always in the wrong place compared to the CCD. They limit the wide lenses you can use and they don't last more than a few years. It is also interesting to me that the most experienced photographers (...) don't ask for a sliding back...".

Arguments against the use of a sliding back:

• A sliding back should be very stable and precise - not just when it is new but also after some serious professional use. Seriously, there are sliding backs on the market which are not useable even when they are brand new: "... I have a (...) for 2 image stitch and groundglass focusing... it is awful because: the sliding back does not hold the digiback securely, the groundglass is out of reference to the sensor, so you cannot focus...".
  
  For being stable and precise the material used for the frame, as well as for the sliding element (as well as the camera body) must have certain dimensions. This is not possible in combination with extreme wide-angle lenses when the sliding back is not an integrated part of the whole camera design. But of course such a camera with an integrated sliding back cannot be used handheld. Most interchangeable sliding backs do not offer the stability we think to be necessary for serious work with high-end digital backs (in the worst cases, the camera body is not stable enough because of the necessity to use thin material to reach a short flange focal distance needed for making the addition of a sliding back).
  
  Then there is the question of what might be the right bearings – the proper bedding – for a sliding back, which will ensure precise movement of the sliding back over the lifetime of the camera. One thing’s for certain: the use of simple friction is not the answer!

• In an ideal case, the relevant surface of the groundglass and the relevant surface of the digital back must be exactly on the same level (going exactly with the flange focal distance). This is not the case with many, if not most sliding backs (see also the citation above) and it is not the case with many if not most groundglasses and it is not the case with many if not most digital backs. That's why we make our groundglass-holders and the groundglasses themselves as precise as technically possible. And that's why (nobody else does) we make adjustable back adapters (adjustable in a range of +/- 3/10mm in steps down to 1/100mm). An adjustment on the lens-side may help to correct either tolerances on a misplaced back sensor but at the same time it brings the groundglass out of line; you need ONE point of reference (ALPA: the groundglass) and then the sensor must be brought in line - otherwise you have to correct every single lens, etc. In other words: this (adjustable adapters - combined with a reliable reference point [ALPA: the groundglass]) is the ONLY way to bring everything to the necessary consistency. Here the comment of an ALPA user on a forum illustrating our point: "... To be honest I always thought that all the talk about 0.01 mm was marketing talk and I thought with a smile on the face, that the fussiness about so small tolerances had something to do with the water they drink in Switzerland :-) - that was until I got a new back that had a bit different focus plane than the old. I took the adjustable ALPA V back adapter and started to shim it. I don't smile anymore at the Swiss, only at my own ignorance... 0.1 mm (millimeter) change the focus a lot and yes you can see a difference in 0.01 mm - it does not ruin the image, but there is a difference... ". And here the experience of another ALPA customer: "... I could attend an ALPA shimming workhop about two weeks ago. I could not believe it, but it is true. A shim of just 5/100 mm or even smaller for an adjustable digital back adapter does make a big difference in sharpness of the raw image. I was very surprised, but believe me, it is true, no doubt about this...".
  
  As the tolerances between different camera adaptions for any digital back
  are shurely different , you can not be shure to get the same raw IQ for any of your possible camera combinations with your digital back(s) .

• The dust-discussion: the risk of getting dust on the sensor during the change groundglass/back has received a new drive by the following remark by an ALPA user: "... As I have gone through my images from (...), it turns out that my ALPA images show no dust, while the images taken with (...) do show dust spots. I think this is because I am usually very meticulous whenever I put the (...) back on the ALPA and whenever I change lenses. I take a small hand held blower with me and I always give the back and the lenses a quick blow before mounting. With the (...) camera, the back is mounted once and stays on the camera all day..It appears that it was pretty easy for dust to get into the camera while a lens was removed (...). The result is that as the day went by more and more dust accumulated on the sensor and by the afternoon there were plenty of dust spots on the images...". Obviously things are even in this case not as simple in practice as they seem to be in theory.

• On a small surface like 37x37mm or 36x48mm or 37x49mm or 40x54mm you cannot easily do any critical focusing - with nearly all groundglasses the use of a loupe is of no help. Why? Because all you are enlarging with a loupe is the grain of the groundglass but not what you would like to see. This fact has led us to design and produce the ALPA PG (ProGrid) groundglasses. For many cognoscenti these ALPA PG (ProGrid) groundglasses are the best in existence for "technical cameras" - not because they are the brightest but because of their fine grain and their extremely fine grid. With an ALPA PG (ProGrid) groundglass you can focus with a loupe as well as you can do correct framing - all in spite of the small size of the picture on the groundglass.

• What about the importance of the precision of a helical mount? We don't talk here about the precision of most rack and pinion focusing mechanisms. Some of these constructions stem anyway from the LF era when focusing on a groundglass was easier and less rewarding mechanically - here just one aspect: 40x54 mm = 2'160 mm2; 4x5 inch = 96x120mm = 11'520 mm2 = more than 5 times the surface of 40x54 mm = quite a difference! But even brand new rack and pinion solutions are commentated by some users e.g. as follows: "... it is horrible to focus..." or "... difficult to focus for digital uses, I prefer by far the helicon mounts for architecture and landscape purposes. This seems to me much more easy, more reliable (if calibrated well) and faster to use...".

There are camera manufacturers who are very proud e.g. of their homemade 1'800° helical mount (5x 360° = five full turns from the shortest focusing distance to "infinity"). Indeed: such a 1'800° helical mount (one for all lenses) is certainly very precise due to the fine thread. The practicability of "5x 360°" and of the corresponding special system where you have to "translate" meters/feet in a peculiar division system from e.g. 1 to 34 is another matter. One thing is absolutely correct: a precise helical mount is important - no doubt about this (do you remember the Chinese made "FOTOMAN" helical mounts? - a nightmare concerning their precision!). The original helical mounts made by Rodenstock and Schneider-Kreuznach for their lenses and used e.g. by ALPA, CAMBO, HORSEMAN and others are perfectly precise enough for their tasks and they have proved this with ten thousands of lenses. What is another matter is the precision of the DISTANCE SCALES (in m or ft) and of the DEPTH OF FIELD SCALES of the helical mounts made by Rodenstock and Schneider-Kreuznach: in some cases they are indeed not as exact - e.g. not offering the fine division - as we would like. We are actually thinking about an ALPA-specific solution for this - hope we will be ready until the Photokina 2010. If a photographer is working (focusing/framing) with an ALPA PG (ProGrid) groundglass and a loupe, the not fully satisfying precision of some distance scales/depth of field scales can be widely ignored. If somebody is focusing by "guesstimation" (as he e.g. did for decades with his Hasselblad SWC or Linhof Technorama) this is also a sufficently precise solution for many applications. If somebody is focusing with the help of a laser rangefinder (e.g. the Leica Geosystems Disto D5) it might indeed be something that should - as said before - be ameliorated in the future (and which will be ameliorated as soon as possible with our own solution that - as we hope - can be retrofitted to all ALPA lenses featuring size no. 0 shutters). But this is, in our eyes, easier to overcome than if you have e.g. to use a 5x360° helical mount freehand. In any case it does not legitimate prophecies of doom like this one: "... Please, please, please, test focusing accuracy with something like a Leica D5 , cheap laser distance finder (remark by ALPA: "Leica Disto D5" = made by Leica Geosystems, Switzerland - we are happy the author does recommend this great laser rangefinder and not a monstrosity like e.g. an ultrasonic rangefinder featuring a 10 - 20° angle of measurement which is said to really exist...) or, worst case, measuring tape. Transfer the measured distance to the helical mount and don't use the ground glass for focus. Ideally do it with a number of lenses at different distances/apertures. This will test what distinguishes this camera from every other on the market. If it works as advertised, all the Cambos, Silvestris, Sinars and even the beautiful Alpas are left for dust...".

Oh, sancta simplicitas! Fact is that a camera/sliding-back/camera-combo not offering an adjusting possibility for the back will never ever leave ALPA "for dust". There have already been several cameras in the last ten years ballyhooed as being the definitive "ALPA killer" - they have already passed away or remain unsold on the dealer's storage racks like bars of lead.

The difficulties to make extreme wide-angle lenses work properly with sliding backs and within the unavoidably restricted space are illustrated by more or less funny extension tubes (in general not exactly a contribution to top precision) and on the other hand the difficulties to make long focal length tele-lenses work properly with integrated tilt mechanisms are illustrated by the repeated re-design of such mechanism and/or by the restriction to e.g. just 135mm focal length.

A last side note: correct stitching (mainly on short distances [interiors, works of art, etc.] not to speak about even shorter distances) is only possible if the lens remains stationary - and just the back is moved vertically as well as horizontally. Why? Because moving the lens means changing the perspective. As far as we know there are even very recent "technical cameras" which cannot offer this feature (stitching without moving the lens) in both horizontal and vertical directions.

Conclusions and outlook:

If you cannot accept sliding backs as valid solutions these are our recommendations:

1. If you are working in the studio and on a tripod (or under ideal condition also outdoor): work tethered
    
2. If you are working outdoor and on a tripod:
   We recommend to use the ALPA solution (groundglass-holder/back have to be interchanged for focusing/framing resp. for taking a shot = this is not top convenient and not very fast plus you have to look out for dust, BUT: it comes up to the requested precision. It's up to the user to decide what is of importance to him (WE have taken our decision: quality comes first).
    
3. We hope we can soon recommend using digital backs or other devices offering true live-view (on what technical basis ever - e.g. CMOS or CCD with tethered tablet PCs). When such devices will at last appear, they will indeed be the definitive end of ALL sliding backs and they might also be the dream solution for handheld use (see 4], right below). With a bit of luck we do not have to wait for another couple of years for such a solution: "... expect a number of interesting display / transfer / process applications from various companies over the next 6-9 months. Photokina this September will be particularly interesting...".
    
4. If you are working freehand: Use a viewfinder for framing and your eyes/experience to guess the distances as you did/do e.g. with your SWC or/and Technorama. Or use a viewfinder for framing and a laser rangefinder (our recommendation: the Leica Geosystems Disto D5) to measure the distances. The laser rangefinder will be much more precise than what you can set on any existing helical mount's scale (try to focus exactly on e.g. 2.735 m ....). This is not ideal but its not at all the tragedy some are evoking (for whatever reasons) - at least as long as you have retained your photographic common sense.

We hope we can soon recommend to use digital backs or other devices offering true live-view (see 3],  above).

_{rev. 10 Feb 2010}