Photography types and machinist types.. help/input please.

[AJ Dual]

As most of you have probably seen, I'm working on a semi-DIY night vision scope here: http://www.armedpolitesociety.com/index.php?topic=33491.0

I've purchased a 75mm f1.4 C-mount lens for the objective.

A great Australian guy has done lots of these projects with surplus night vision tubes, and has some drawings for parts and adapters to use off-the-shelf lenses.

http://anvf.blackice.com.au/userfiles/david-allthree.pdf  Since I've sourced the actual military-surplus eyepiece that was made for this tube, I don't need anything for the back-end phosphor screen that delivers the image, however, I do need to get the C-mount lens, with a 75mm focal length mounted and spaced properly in front of the photocathode objective of the tube.

This is the lens I've got. http://www.ebay.com/itm/330688152748?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649#ht_2012wt_1344 (let me know if  you can't see the auction, since it's sold...)

What I need help with is figuring out the spacing, and what I'd need to do to get this machined up for me. Would these plans have enough range of adjustment to allow me to get a proper focus? The 75mm focal length is measured from the center of the first primary lens in the front correct? So this does not need to sit on a long spacer tube. So maybe it does, however I'd want other eyes/brains on this so I'm sure I know what I'm doing. 

[zahc]

Pentax C mount is probably an SLR lens.

The optical focal length of the lens is not necessarily related to its physical dimensions. For example, most SLR lenses are retrofocus designs, so that they are longer than their optical focal length in order to provide room for the mirror box.

In fact, my 28mm and 50mm lenses for my OM1 have the same physical focal length (the exit pupil is the same distance from the focal plane).

You can test how far from the lens mount the focal plane is for a given lens just by pointing it toward a scene at infinity and holding a card up to the rear of it until the image projected on the card is sharp. It works well at night, with distant lights.

That lens is only going to throw about a 1" image circle. Is that enough for your tube?

[AJ Dual]

Pentax C mount is probably an SLR lens.

The optical focal length of the lens is not necessarily related to its physical dimensions. For example, most SLR lenses are retrofocus designs, so that they are longer than their optical focal length in order to provide room for the mirror box.

In fact, my 28mm and 50mm lenses for my OM1 have the same physical focal length (the exit pupil is the same distance from the focal plane).

You can test how far from the lens mount the focal plane is for a given lens just by pointing it toward a scene at infinity and holding a card up to the rear of it until the image projected on the card is sharp. It works well at night, with distant lights.

That lens is only going to throw about a 1" image circle. Is that enough for your tube?

Ah, okay, so the physical focal length might be "folded up" somewhat, depending on what kind of camera it was for. So there's a focal length the light path travels, and a practical one if you stuck it on a ruler. This was for a CCTV or television camera/professional grade camcorder kind of thing.

I think 1" should be really good for the tube. More is better, otherwise I think it would (vignette?) around the edges of the tube's output to the eyepiece screen. Actually, I believe the input fiber optic bundle for the photocathode actually is 25mm so 1" would be about perfect. From all my reading on DIY builds, they stress you can always dial the aperture down on the light, but when you need to be maxed out, like on a moonless overcast night, you can't magically add more.

A question, what do they mean by "format" when they say a lens is 3/4 format? I'm not sure what this lens would be. Does that have anything to do with the image circle at focus?

[zahc]

That's exactly what it means.  Lenses have 3 basic properties...max aperture, (optical) focal length, and image circle. For example, newfangled digital lenses (like the lens you bought) only cover dinky digital camera sensors, and won't cover 35mm film, which requires a 44mm image circle. The micro 4/3 format uses a sensor about the size of a chiclet. There image circle is always a bit bigger than the diagonal sensor size, but my 1" estimate might actually have been optimistic.

[AJ Dual]

Okay. As long as it's even 3/4, that should do well.

I think it should do. Most folks still get acceptable results with a 50mm f1.8, although no one seems to recommend going below that. Or anything below 1/2 format.