Where should the aperture be located?

BetterSense · Dec 4, 2009

I know that for proper performance, the aperture should be located at a certain point in between lens elements. Could an aperture work on the outside of a lens?

I bought a fixed f/5 copy lens on ebay for building a box camera. I think I would like to stop this down to f/16 or so. I can't do it by putting an aperture right up against one of the outer lens elements, can I?

calceman · Dec 4, 2009

Is it from an old overhead projector?

BetterSense · Dec 4, 2009

No, it's a "copy lens", with a featureless barrel.

*bay item number 270491187340 "Ednalite 220mm f5.3 copy lens for 8x10"

I may be able to take the elements out and insert a cardboard stop, but I'd still want to know where to put it.

John Koehrer · Dec 4, 2009

Some folks say you can improve the performance of projector lenses by putting a smaller aperture in front of the lens, so why not.
If you can determine the approximate center of the lens, cut a slot & make your own waterhouse stops.

keithwms · Dec 4, 2009

You really want the aperture at the waist of the beam. I don't see how to define the speed (f/#) otherwise, and an aperture before the front element will simply vignette. If it's an inexpensive lens then I'd say go ahead and do the exploratory surgery and try to install a waterhouse stop. Waterhouse stops have the (tremendous) advantage of giving you a very smooth aperture even when you're not shooting wide open... so the bokeh may delight you. I mean, I ripped the aperture blades out of an anastigmat and I am glad I did.

How would you define lens speed if the aperture is in front of the lens? :?

P.S. I can imagine putting an aperture at the conjugate focus but, offhand I don't think I have seen this done except for microscopy.

P.P.S. Aperturing the front element might enhance contrast. But I don't see any other benefits.

WetMogwai · Dec 4, 2009

As has been suggested, it is probably less than optimal to do it, but there are commercial options for doing just that. The Lens Baby lenses (at least the 3G, which I have) uses a set of aperture rings that are held to the front of the lens by magnets. Also, Cokin makes a holder for cards that come with preshaped non-round apertures for making out of focus points of light come out the shape you want. You can also cut your own for this holder. There are probably other options, but these are the two I have, or rather had. I have the Lens Baby 3G, but I lost the rings, so it is stuck at f/2.

Alex Bishop-Thorpe · Dec 19, 2009

I don't remember the model, but at least one early Kodak camera used a simple lens with the stop in front of the lens to improve performance. Likely a single element lens, but it's a viable option. The same camera had a fake lens (a bit of plain glass) in front of this stop, because the public weren't going to pay for a camera without a lens on it, were they?

Q.G. · Dec 19, 2009

Putting a stop in front or behind the lens will cause image distortion.

Mark Fisher · Dec 19, 2009

The right place is where the light rays come together which is probably in the middle of the lens. Made a simple aperture and try both locations. To get the effective aperture, try meter the image on the ground glass (assuming you have one) with and without the aperture making sure you are metering only the ground glass and the same area each time. Then meter the scene assuming f5.3 and multiply the exposure by the ratio of the meter readings on the ground glass. Disclaimer: I've never read this or tried it, but I'm pretty sure it would work!

Personally, I'd just get some ND film to pull down the exposure to something reasonable and enjoy the wide open look. Stopping down in the wrong place will probably hurt more than help.

BetterSense · Dec 19, 2009

I really need the DOF is this case because it's a simple box camera with really loose tolerances for the film plane, plus I don't think the lens would properly cover 8x10 wide open.

The lens looks symmetrical and I managed to take the retaining right out, and take out the two elements on one side (there are two separateable elements on each side). I punched a hole in some black card with a common hole punch and fitted it right in the middle of the lens. Should be about f/36. I haven't tested the camera yet but I may go even smaller, since I don't have a shutter anyway.

keithwms · Dec 19, 2009

For my homemade apertures, I use developed (but not exposed) E6 film. It is quite nice and black.

BetterSense · Dec 19, 2009

Do you cut a hole in it, or do you take a picture of a circular spot?

Mark Fisher · Dec 19, 2009

If it is a symmetric lens and you can get the aperture in the middle, you are all set. The downside is that it will be tough to focus with the small aperture. If it works out, maybe you can come up with an inexpensive old lens with a variable aperture.

Larry Bullis · Dec 19, 2009

Fleath said:
I don't remember the model, but at least one early Kodak camera used a simple lens with the stop in front of the lens to improve performance. Likely a single element lens, but it's a viable option. ...

Examples I've seen in folding Kodaks, and the Ansco Sure Shot box camera, had simple meniscus lenses, convex side forward. With only one element, where else would you put it?

Some of the Kodak folders had real f/stops, starting (if I recall correctly) at f/16 and getting smaller. In front was a restrictive disk which prevented the camera from being able to operate at wider apertures, although the aperture range was there. I imagine that this was to insure that the user would get sharp pix, because the lens is sharp in the center but has severe aberration if it is used farther out to the edges. I have removed this disk to see what it will do; I got the kind of image that was prized by the blurry pictorialist school around the turn of the 19-20th C's. I've got that camera somewhere still. It's kind of fun.

These had no glass in front.

unclemack · Dec 26, 2009

I don't know a thing about your lens I'm afraid but I can tell you that I have a Mamiya RZ67 and the 180mm. lens has the shutter and diaphragm behind the lens. Was a big surprise to me when I bought it years ago! No glass behind the shutter at all!!

Not sure how much help this is...

Where should the aperture be located?

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