Practical length of focusing rail

[Kaj Krinsmoe]

Designing a 4 x 5 view camera, the standard bed slides on another bed that slides on dual-rails and can be quick-secured for approximate focusing on the ground glass. The upper standard bed can be moved forth/back by a brass bolt for fine-focusing.

Is an inch forth and back adequate?

How many threads per inch is appropriate?

kkrinsmoe@stofanet.dk

[Leonard Evens]

Perhaps I don't understand your first question. You certainly need much more than an inch or so of bellows extension. Most view cameras will allow the separation of the standards to vary between something like 75 mm or less at the low end to 300 mm or more at the high end.

For fine focusing, my view camera produces 20 mm of movement along the rail from one full rotation of the focusing knob. I've checked some other view cameras and this seems fairly typical. I don't know how many threads per unit length I have, but I can adjust the position to within a about 0.1-0.2 mm using the focusing knob. That will give you some idea of how sensitive it is.

[Darin Cozine]

Kaj, your message was a bit hard to understand, but I had been thinking along the same lines as you for a fine focussing device.

If you are just focussing straight-on, about an inch of fine-focussing would be fine. However once you are using movements, it may be more. And there are many factors that would affect this. Like are you using base tilt or axis tilt? Where is the center of the lens? Are you using rise/fall? Swings and shift can also affect this.

I would want 2-3 inches of fine focus, but I have not actually tested this. Perhaps you can put some tape on your camera and try it out.

[Emmanuel BIGLER]

Kaj. I think I understand your point. For additional fine focusing, plus or minus 25 mm on top of a quick-sliding device might even be too much.
The problem is the trade-off between fast action and precise focusing.
Precise monorail view cameras like Leonard's have a precise rack and pinion system which is much faster than adjusting a longitudinal screw, their sensitivity is in the 0.1 mm range wich is probably enough for 4"x5"
The "cammed" Rolleiflex TLR focusing system moves by about 10mm in one full turn of the main knob for a 75-80 mm lens. Scaling things by a factor two in 4"x5" we get something close to Leonard's evaluation, 20 mm per one full knob rotation. Now imagine that even 10 mm for one full knob turn is acceptable for your longitudinal screw. You will not find a regular thread in the ISO series able to deliver
10 mm of pitch per one turn of a knob except in a 60 mm diameter ! probably you have in mind standard ISO-metric threads like M6x100, M8x125, M10x150 (M-diameter-mm x pitch in 1/100 mm). So a M10x150 thread is 1.5 millimetre per one knob turn.
You could try this, keeping in mind the typical value of conventional depth of focus. Helical mounts like in some old enlargers have a very special kind of fast thread with multiple interlaced threads in order to allow a huge pitch. This is very difficult to machine and not at all available in the ISO series of threads ;-)
If you consider a confusion circle of 50 microns which would appear very stringent for 4"x5", at f/8 the depth of focus is plus or minus 400 microns for a total travel of 800 microns.
Say : about one millimetre. You could try any pitch between 1 and 1.5 mm with a standard metric thread, you'll get something really fine, why not, but this would be quite unusual compared to most precision field or view cameras.
Another problem is that you'll have to reset the fine screw from time to time since you'll have to adjust at random in one direction or another after locking the quick slide.
May be this is the main problem in fact, resetting the fine screw to zero. You could imagine a spring-loaded screw in two halves that would allow a quick zero reset by opening the two halves, such systems exist.
Another idea could be to adapt a folding crank to the knob, exactly like the film-rewinding knob & crank device of 35 mm cameras. Fine tune with the knob, fast action with many turns of the foldable crank. This could be compatible with an easy to find M6x100 to M10x150 screw or the equivalent imperial sizes if you prefer ;-)

[Kaj Krinsmoe]

Normally the focusing axis is at right angle to the movement of the standard, but the focusing bolt I contemplate is parallel, mounted between lower bed and the upper bed on which is placed the standard with the lensboard. Practically this means that if the pitch is 20 threads/inch, then twenty turns are required to move the upper bed one inch, but it also means that the bed cannot move further than to the extent of the turning knob, therefore I like to know what are the pratical limits, so as to have enough lenght on the bolt.

Under the lower bed is a wing nut for quickly tightening, and the whole of this front standard assembly can be manually pushed along the two rails to an approximate focus, yes to the extent of the bellows. The fine focusing is done by moving the upper bed forth and back on the now fixed lower bad.

By this method I hope to achieve a more compact assembly, and less complicated construction.

I'm grateful for the two replies, that both provide useful information.

Kaj Krinsmoe

kkrinsmoe@stofanet.dk

[Emmanuel BIGLER]

Kaj. I'm not sure if I understand well, but you mention : and the whole of this front standard assembly can be manually pushed....The fine focusing is done by moving the upper bed forth and back..

If you intend to add the fine focusing system in front, my objection would be that a fine focusing system is more useful on the rear standard than on the front standard. When taking pictures of distant objects, there no difference, you may focus the front or the rear standard as you like. However the closer to the subject, the less efficient movements of the front standard can be for focusing. In the limit of the 1:1 ratio, image = life-size, front movements become almost useless for focusing. The advantage of fine-setting the rear standard is that the same focusing sensitivity is preserved in the whole range of lens-to-subject distances. So I would keep a coarse setting by sliding the front strandard but I would install, if possible, the fine focusing or the double focusing system at rear.

[Dan Fromm]

Kai, to expand a little on Emmanuel's last point, at relatively high magnification, for example 1:1 as he mentioned, one usually focuses by moving the camera/lens assembly, not by changing magnification. Remember that changing extension, i.e., moving one of the standards, changes magnification. So if you are designing a camera for closeup work, you might want to consider building in a mechanism for moving the entire camera forwards and backwards over the tripod head.

Good luck,

Dan