May old portrait cameras (and lenses) have rack-and-pinion focusing. That might be worth looking into?
Considering the money that has been spent on advertising "internal focusing" in 35mm SLR lenses, it is a little too simple to make a blanket statement that "helical focusing on 35mm SLR moves the whole lens". It does for most lenses, but far from all.
Ok, zooms and some more special lenses (achromats) have the internal focusing but, as you say, most of "normal" SLR lenses move the whole assembly. Sometimes it's difficult to see as the lens can be burried deeply in the lens tube.
Richard Ide wrote "With 35mm and MF lenses, various elements in the lens move in relation to each other when you rotate the focussing ring."
In general, stuff and nonsense, but there are some exceptions. I think they're irrelevant.
You're right for some crappy 35 mm, 6x6, and 6x9 cameras with fixed lenses that have front element focusing. This is widely recognized as a compromise between cost (= complexity) and quality.
In more "serious" gear, the first lens to market with a floating element, if I'm not mistaken, was the 24/2.8 Nikkor in Nikon F mount. And in that lens the floating element is there to reduce aberrations, mainly coma, at close focusing distances.
Internal focusing is a relatively new development. Lenses for LF cameras are all unit focusing.
Bill, lenses for LF cameras are designed to work best when the cells are screwed tight into the shutter. When you change cell spacing, especially with wide angle lenses, what you mainly do is add aberrations. Another way of looking at focusing is that what it really does is change magnification. Either way, something has to move to change the distance between the lens and the film plane.
Bill,
View camera lenses focus, as you have discovered, by moving the entire lens towards or away from the film. That is the purpose of a bellows on a view camera. It lets you move the standards towards or away from each other.
That is also the purpose for a view camera lens or an enlarging lens helicoid. It moves the entire lens torwards or away from the film.
There is also a thrid and simpler way to do this. That would require two tube, one that simply slides front to back inside the other. You mount your shutter to the front of inner tube and you mount your camera to the back of the larger tube. Then you simply slide the lens in or out till the image is in focus. To lock it in place you tighten a set screw.
Drawbacks to this system? Ain't as smooth and accurate as moving two standards on a rail or in and out via helicoid and parallisim between the lens and film is harder to maintain so sharpness overall can be effected.
Caveat - the tubes both have to be at least as large in diameter as the rear elemnet or the lens won't fit.
One more thing - just because your " focus requirments" are so small (0.3 mm indeed) you cannot rely on a simple straightforward movement guided by your fingers. You could focus for hours in this way without being sure where you really focus. The helical focusing ring is there to multiply those 0.3 mm on a longer scale - that of the circumference of the helical focus. Useless to try to reinvent the wheel.
Your introductory statement mentioned a 5x7 camera back. The
47 mm and 55 mm lenses won't cover the format... a 72 mm SSA XL
will.
If you choose the helical mount (commercial or homemade) to insure
focusing ease and accuracy, the centering of the lens optical axis
with the focusing movement and the alignment of the mount to the film
plane must be precise.
The deviation from planarity of the perpendicular line from
the lens axis to the corner should be on the order of 0.03 mm or
0.001".
You can not rely on depth of focus to "correct" that problem.
The depth of focus formula (N*c) assumes acceptable focus on a plane.
Using that recommendation, you might find that two opposing corners are
sharp, and the others are focused in front of and behind the plane, or
that one edge is focused in front of the plane, and the other behind it
when the center is in sharp focus. After mounting the lens, shim the lens
board so focus is sharp across the entire field.
If you are using a helical mount, there is a further restriction that focus
is sharp at EVERY MAGNIFICATION. In order to do that, the lens axis must be
precisely centered in the mount. Otherwise while focusing, the axis will
wander in a circle on the film plane forming a "cone" making alignment
lens board alignment impossible at different magnifications. The commercial
helical mounts provide the required lens axis alignment, though the lens board
may require shims in a homemade box camera.
If you want to use wide lenses in the manner you've suggested, the
"alignment specs" that are required are similar or should exceed those
of a good medium format camera. The expensive but easiest and most reliable
approach is to buy a special purpose camera like a Silvestri which exceeds
the requirements, or start experiments with a commercial mount or one made
by a good machine shop.
There still will be issues such as film flatness, etc. which will require
that the lens be stopped down.
Hi there,
Bill:
("Paul... "A helical slot thru the outer tube and the set screw to the inner would provide movement, no gear train needed."
I keep reading this but don't get it. What's wrong with a simple threaded fitting going through the lens board.... a simple twist of the lens, maybe one revolution, will give me the full .3mm focus adjustment I need? Is your concept just a simple push/pull, like telescoping rail? Sorry I am not seeing the advantage, can you please elaborate... TYIA")
Close to push-pull but the helical slot reduces the movement, easier to focus. The tube-in-tube setup is easier to machine properly, no wobble when you change focus. A helix mount is VERY expensive to build precisely.
You could look for a Kodak Medalist; MedalistII shell for the focus mount, but you would have to band-saw the body away. The tube ID is approx.55mm..
You can just build it and set the focus at 30 feet and use depth of field.
On front-cell-focus folders, it only moved the very front lens element NOT the whole front cell, thats why it didn't work for you.
Have fun with it.
Something to consider - all you need to do is change the distance between the lens and the film. It doesn't matter whether the lens moves, or the film moves.
Can you just have a set of shims for the back of the camera, different thicknesses for different focus distances?
Zone VI at one time offered an arrangement for aligning the lens on earlier Beseler enlargers. This involved two lensboards separated by a piece of neoprene and held together with thre equally spaced screws. Tightening and/or loosening the screws changed the position of the lensboard. Not a precision arrangement but workable with GG viewing?
I purchased a broken Contarex on Ebay that suffered from a bout with the Yugoslavian border guards where they had tried to unscrew a bayonet lens to check for boo. It was in the early 70 and most cameras in eastern Europe apparently still had screw mount lenses.
What they had actually done was force the helical mount past it's stop.
After about 20 tries and 20 squared cuss words I got it back together. There are four sets of threads, nested within each other.
What I learned is that the point of that type of mount seems to be to have an adjustable screw mount that allows the user to change the distance between the lens and the film without the lens body turning. I.E. your F stops and focus ring always stay on top.
A helical mount consists of a male set of threads inside a tube with female threads on the inside and another set of male threads on the outside, which then engages a set of female threads on the part that you turn.
The whole thing functions like a planetary gear set where if you hold one element and turn another the third turns in the opposite direction.
If you don't care if your lens turns when you screw it in and out, I see no reason that you need a "helical" mount just a normal screw mount with very course threads.
Keep in mind this is all "like triangles” a little bit of movement goes a long way on a 50mm lens but you need a lot more movement on a large format lens to do the same thing.
I suppose that any screw mount would be a “helical mount” but I think the term has come to mean what is described above at least when speaking of lens mounts.
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