View Full Version : Backfocus- Using aerial lenses on MF or Viewcameras

21-Jan-2013, 06:42
I have seen aerial lenses perfectly corrected for infinity to 1:200. the have backfocus 21 and 24mm. Since i dont yet really understand backfocus i am asking:
is it the distance from the rear lens to film/sensor?
If yes then a mirror-cam like rz67 or hasselblad 2000 would be out of the game, right? but it cold possibly be used in a mamiya press or not?
The other options are gaorsi, fotoman and dayi.

register versus backfocus-distance
which register has mamiya press and which backfocus distances 50 and 75mm lenses.
I once had a DIY-Linhof Technar-like construction from a friend with mamiya press-lens mount and international back. genious.

From a red-forum:
Flange Depth - The physical distance between the lens mount plate and the focusing surface of the cameraq sensor.

Back Focus - The optical distance that a properly collimated lens is set to so that the image focuses on the correct plane.
So flange depth means register (distance)

21-Jan-2013, 07:01
Back focus is the rearward deviation from proper focus (the forward version is front focus). In rigid camera body systems like a aerial camera it may also mean the deviation from register or some standard focal length (e.g. a 150mm lens might need to be placed 24mm inward of the stock 180mm lens if both are to be adjusted for infinity, assuming that the nodal plane to flange distance difference between both sums up to 6mm).

Both are irrelevant for LF/view cameras, with the one exception of rangefinder calibration on press cameras. You will have to test whether your lenses have any back or front focus issues - Mamiya did not intentionally ship mis-adjusted ones, and where adjustment errors happen, they will not be uniform.

21-Jan-2013, 07:11
back focus, usually defined as the distance from the lens' rear vertex to the film plane when the lens is focused at infinity.

21-Jan-2013, 08:26
Oh well, have your own terminology. But it sure makes it hard figuring out what you actually want to ask...

21-Jan-2013, 09:55
I have seen aerial lenses perfectly corrected for infinity to 1:200. the have backfocus 21 and 24mm.

Infinity is much farther than 1:200 if you mean focal length to object @ infinity. And what lens has 21 AND 24mm distance from the rear lens (trying to find a case that fits.) ? It should be one or the other. (One such aerial lens is a 3" Biogon.) Better, please, what lens are you talking about? If you have it in your possession, there should be a marking that gives the exact focal length. It is sometimes inscribed by hand on the back of the lens. Aerial lenses are shimmed to the exact focal length.

(Aerials are an interesting specialty. For example, at least when I was involved, to be certified a lens had to pass resolution tests and they were required to be shot wide-open even if they had an adjustable aperture.)

Emmanuel BIGLER
21-Jan-2013, 10:27
Hello friom France!

Il the distance of the rear lens vertex to the focal point is 21 or 24 mm, you cannot mount and focus to infinity this lens in front of a series 200/2000 Hasselblad camera since the register distance between the lens bayonet and the film plane is 74.9 mm for all Hasselblad single lens reflex cameras series V.

Mamiya Press cameras having no reflex mirror and a large diameter for the lens mount, it is not impossible that you could mount and focus a lens with a back focal distance of 21 mm on a Mamiya Press ... unfortunately, there is no focal plane shutter in Mamiya Press cameras.
So if there is no leaf shutter in the said aerial lenses, you can still use the lens cap as a manual shutter if you wish ;)

Depending on the diameter of the lens you could, or could not, attempt to mount the lens in front of a Speed Graphic Press camera with a built-in focal-plane shutter ("image-plane" shutter would be a better terminology, but here we are aiming and focusing at infinity, nothing closer ;) )
Or attempt to attach a focal plane shutter salvaged from an old Speed Graphic camera behind a monorail camera.
This is what Jean-Baptiste Maison did ; the article is in French, but the images speak for themselves. However I'm not sure that 24 mm of back focal distance can be reached whith this very-special customized camera. (http://www.galerie-photo.com/metissage-arca-swiss-speed-graphic.html)

Dan Fromm
21-Jan-2013, 11:46
sevo, your concept of back- and front-focus are quite new, came in with autofocus cameras. I was puzzled by the words when I first encountered them. The old, and usual, until the ahistorical small format autofocus ignoramuses came along, back focus concept is the distance from the lens' rear vertex to the focal plane when the lens is focused at infinity. Front focus is an entirely new idea. More instances of ignorant people ruining perfectly good words.

europanorama, it may be possible to use a lens with short back focus on a rangefinder camera such as one of the Mamiya Press cameras, but whether it can be done depends on the lens' other dimensions and on how much you're willing to spend. FWIW, I have a collection of USAF lens datasheets, none more recent than the late 1960s; the majority of lenses described have longer back focus than the ones you mentioned.

Register is an attribute of cameras with fixed body thickness. It is the (fixed) flange-to-film distance. Depending on the lens and camera, back focus can be shorter than the register (rear of lens protrudes into the camera body) or longer (back of lens in front of the camera body.

europanorama, when you ask for help being coy about the details is unhelpful. Which lenses are you asking about? And are they in shutters? Making an aerial camera lens in barrel usable can be difficult.

JAC, the 3" Biogon's back focus is on the order of 39 mm. The 1.5 incher's is roughly 18 mm. What are you thinking of?

22-Jan-2013, 15:20
Dan, I will go through my storage drives for pictures of the 3" mounted. In the meantime see attached. This lens is "the enemy of the good". Remember? :)