Aperture iris in front of lenses (in object space)

Question

This question discusses the iris placed in camera body and this question does not reveal too much details about external iris.

I recall from scattered information that I have read that iris placed in front of front lens has some application. For example, hobbyists owning objectives without internal aperture make external aperture stop.

I also recall a compact film camera having an external iris.

My questions:

• what cameras used external iris? Compact ones in particular
• what is the good source to read about placing iris in front of lenses? I searched the webs for quite long but found nothing significant

Answer 1

I am looking for such lenses as well. In general putting the aperture at the front or outside is avoided because not good; best is to put the aperture more or less in the center of the objective. With the aperture in front you'll have more aberrations and/or need bigger and more expensive design. Only in two cases the front aperture design is used:

1. if the lens is very small and simple, like, really just one group (see the Kodak vest camera and the Triplet), here you cannot put the aperture in the middle! Choose, in the front or in the back.
2. Otherwise, you put the aperture in front only if you are absolutely forced to do that, typically for coupling with other optical systems.

Small & simple: The Wollaston meniscus is the oldest/simplest photographic lens, just one lens. It is the best single-lens performer for wide angle. Aperture is in the front; the Kodak vest is a version of it. Mobile phone lenses; to make them so compact, the aperture is usually placed at the brim of the front lens. Not really outside, but on the brim.

Forced: Pinholes and many probe objectives, that need to peep from an hole; the hole is the natural aperture and the lens must be built to use all the light that goes through such hole; putting another aperture will cause vignetting. Nice example the SO spy lenses by Zeiss Jena: Marco Cavina or see the catalog of Marshall Electronics.

Non-photographic lenses:

• Laser scanning lenses, called F-theta Rogonar; the laser light is coming form the aperture position.
• All eyepieces: the aperture is well outside, at the "exit pupil", so I can place my eye with my iris in this place.

If you are curious about aberrations etc, the best short introduction I've found are those slides from Jena: Gross Jena 2017; lecture 11/3, stop position.

Edit: I discovered that the one of first photographic lens ever, the Wollaston meniscus, has front aperture! The Kodak pocket vest is an achromatic version of this lens. The aperture optimal position is really there, few mm in front of the lens.

Answer 2

In answer to your question about specific cameras, the only models I can recall seeing with an external aperture in front of the lens are a few of the oldest Kodak Autographics and Kodak Vest Pocket Cameras from approximately 1913-15 that featured a 'meniscus achromat' lens.. Here's an example of some..

When I first saw these, I assumed the front element had simply fallen out, as they were simple lenses and not apparently constructed as well as the more famous lenses. It didn't make sense that the aperture was exposed to the elements, but upon inspection you'll see they're designed this way, with sturdy fixed circular apertures, much like Waterhouse stops.

Answer 3

See textbook “Optics The Technique of Definition” by Arthur Cox 1945. Excerpts from: The fact that different parts of the lens of the glass is used when the stop is in front of the lens, from that that is used when the stop is ---- the aberrations are different ------ this is the utmost importance in establishing a lens constriction that will give good definition and holds whether the is simple or complicated as to construction.

Answer 4

The MS SuperTriplet Perar 24mm F4 and MS SuperTriplet Perar 28mm F4 were objectives which I tried to recall. They are M-mount objectives not tied to a specific camera. Photographs suggests the iris is in front.

Here is an article which says it straightly.

Answer 5

The older (50's-60's) large format Schneider Symmars that are "convertible" fit this description when used with the front group removed.

My 135/235 isn't wonderful used as a 235 with the front group removed, but it's not awful either, at least by f16.

Suspect there are other convertibles (Cooke?) that this is also true for.

Answer 6

I have a commercial Canon lens, 125 mm, f/2.6 with dual counter-rotating irises in front of the lens glass. It came from a 35 mm high-speed x-ray cine camera, Canon #XI 125 mm, no. 78928.

It has a focal distance back through the lens of just over 8 inches, and it is a beast weighing 2 pounds.

I cannot find out any info about it other than I have said. At f/8 the iris is 5/8" wide; at f/2.5 it is 2".