[ncallender]

Hey all - here's a fun little rabbit trail. My curiosity got the best of me and an old m lens and so I took it apart to figure out how the helical focusing worked in it, which was fun. However, I got a great look at the aperture controls and I realized it's actually pretty amazing that many of the newer third part lenses actually exist for the pentax mount considering the mechanical linkages to the aperture (which is in the middle of the lens most of the time) from the lens mount and into the camera. So, I was wondering, have there been camera systems that place the aperture in the body instead of the lenses, and what would it require, if possible at all, to move the aperture into the body?

[tacticdesigns]

The Pentax A110 had the aperture in the body, and it acted as both the aperture (stop?) and the shutter.

Pentax Auto 110 - Wikipedia, the free encyclopedia

Take care,
Glen

[Nick Siebers]

Pentax Auto 110 had the stop down mechanism in the body, part of the reason the lenses are so small. I don't know of any others, but there may well be. I have found that the preset Takumars, with the apertures towards the front, seem to have very nice optics - not sure if there is a connection.

[RioRico]

I think the iris is placed between lens elements because that's optimal for controlling light. Typically the front element(s) gather and focus light to the rear element(s) which disperse the rays to cover the frame. An iris in between acts as a gateway without impeding that dispersal. A behind-the-lens iris would act as a choke, not an optical diaphragm. A choke that closes down to any extent would cause vignetting and probably diffraction.

Try it: Cut a plastic disc, punch a little hole in it, place it just behind a lens, and see what happens. Actually, I have a project in mind that does just that. I got a small batch of Ilex Paragon iris assemblies, real cheap, for 5.5 inch f/4.5 enlarger lenses. I'm trying to find some glass (or plastic, I'm not choosy!) to set in there -- maybe a projector or copy lens. That should be fun!

EDIT: Yes, some interchangeable-lens cameras have the iris and shutter inside the body. But I think the interchangeable lenses are front elements only, with the rear elements behind the mechanisms inside the body.

EDIT2: Further reading shows that they're called FRONT CELL SYSTEMS. The Lens Collector's Vade Mecum says: "In the 1950's several makers designed systems where the front components were exchanged to alter the focus..."

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Last edited by RioRico; 06-27-2011 at 09:56 PM.

[Lowell Goudge]

I think the aperture is in a strategic place between the elements where the light path is similar in size for most lenses so they can achieve a reliable and standard design that has the blades small enough to be fast, limited blades to reduce complexity, but a big enough hole that the dimensional tolerances don't lead to exposure errors at small apertures

If you consider starting from presets, with millions of blades close tothe front, to auto apertures which, to reduce linkage, moved the aperture to the rear, along with reduced blade counts you will see what I am getting at

[bdery]

Being an optical designer by day, I can answer and build on some of the comments so far.

There is no reason why you couldn't place the iris anywhere. However, there are some physical limitations that you have to work with.

The most important of these limitations is that you have to control light rays at the area called the "aperture stop". In short, the aperture stop is the area hat limits the angle that a light ray can have (in relation to the lens axis) and still be part of the final image. Not controling the lights rays at the aperture stop means you essentially risk egrading your image quality because of stray light rays (it's a tad more complicated in fact).

When you place the iris at the aperture stop (which is often the smallest diameter in the light path) you control very easily those stray rays.

If you want to place the iris elsewhere, you have to design it so its size corrsponds to the image the aperture stop would make at that spot. And that's the part complicated to understand if you have zero background in optics. In other words, you have to ensure that you're still controlling the stray light rays at that particular spot.

Doing this usually means a larger iris, with the problem this entails (parts more expensive, take more space, etc).

Does that help?

[tacticdesigns]

bdery,

Thanks for the explanation. Although I'm sure I don't fully understand what you just said, I find it absolutely fascinating!

Take care,
Glen (TacticDesigns)

[ncallender]

bdery - thanks for the explanation. So, what you are saying is that if you move the aperture you need to know how to recalculate what the max size of the stop would be at that new location, and then set the aperture to that max size as well? That seems like it could be easy enough. Does the aperture have to be a certain distance from the image plane (film/sensor) for it to work properly?

[Blue]

There were some unique 135 systems in the 50s. One was the Kodak Retina Reflexes. The rear element of the lens stayed on the body and the front elements were changed out.

Retina Reflex - Camerapedia

[RioRico]

There were some unique 135 systems in the 50s. One was the Kodak Retina Reflexes. The rear element of the lens stayed on the body and the front elements were changed out.

That was fairly common with German SLRs. Zeiss, Contax, Voigtlander (I think), Kodak, all had leaf-shutter SLRs with in-body rear elements and interchangeable front element plus aperture assemblies. The Sockel system was one such, I think. Very complicated, prone to failure -- their shorts were eaten by Pentax!

[bdery]

So, what you are saying is that if you move the aperture you need to know how to recalculate what the max size of the stop would be at that new location, and then set the aperture to that max size as well? That seems like it could be easy enough. Does the aperture have to be a certain distance from the image plane (film/sensor) for it to work properly?

In theory it' easy enough... but you have to calculate the size of the iris by taking into account the angle the light rays have at that place (more correctly speaking on that optical plane), and making THAT correspond to what you want to get at the aperture stop. It's quite feasible and with modern lens design softwares it's not that hard, but it's often a lot of work for no real gain.

I imagine there could be advantages to moving the iris that I don't know of. Photographic lens design is not my personal expertize.

[LayerCake]

Sorry for bringing up an old thread but I'm hoping someone could help me out with a project I'm doing.

I use a GF1 camera and I want to convert an Eiki 50mm F1.2 Projector lens to be able to mount onto the camera, I have already started the project and I'm using a nikon 50mm 1.8 lens to create a helicoid but I want to also add an aperture system to it.

With the positioning of the helix and how close the lens will be to the sensor it leave only one position to place it but I'm a bit worried that it will act more like a choke than a proper aperture system.

Here's a diagram of how the lens is constructed

the first line on the diagram after the first 2 elements is where I could place the aperture system but I suspect the second set of lines after the 3rd element would be the ideal positioning as it is where I would suspect the axis would be.

any help would be appreciated

[bdery]

Hi,

looking at the diagram, I would tend to agree that the aperture stop is where the second set of lines is (to the right). As I wrote some years ago, you can place the iris at the first set of lines, you will simply have to experiment to get properly matching aperture values, plus you could see some image degradation because you're not controlling stray light (off-axis rays if you prefer) as well.

[LayerCake]

Thanks bdery, in truth I was hoping that you would make a comment on it as you seem to have much knowledge of such matter, I have a couple of 50mm 1.4 lenses that I use mostly but 1.2 are a bit rich for my liking!! the one thing I really like about the Eiki is that it gives a bit of a petzvalish type effect to the images but it's also quite glowy at 1.2 and that was the main reasoning for wanting to place an aperture system into it but if its only going to degrade the image quality instead of restricting stray light then I might just keep it without an aperture system.

[grahame]

There is no reason why you couldn't place the iris anywhere.

Hi bdery,

I don't have any optical background, but I always thought there must be a certain place in a lens that you can put aperture to control the light, but not cause black ring in the image. I suspect if you put the aperture right under the front element of a lens, and if you close the aperture to F16-22, you should see a small hole of light. I am always amazed that the lens designer can find a good place for aperture, to make it controls light, but not reduce the image circle size.

Maybe I was wrong?

Thanks

Graham