Why not have the aperture in the lens body

[markIVantony]

As I was sitting here debating on taking an old Tamron lens apart, I had an idea about camera/lens design.  The idea is to put the aperture diaphragm inside the camera body near the lens mount, instead of the rear of the lens.  Has there ever been a camera/lens system designed this way?

pros:
- diaphragm cost goes only once with the camera, not duplicated with every lens
- future-proof lens designs (think of Nikon's "G" lenses on manual-focus bodies)
- continued aperture control even when using lens adapters (ex: EOS->FD, EOS->Nikon, etc)

cons:
- with a diaphragm malfunction, you can no longer swap on a new lens and keep shooting (but I always have a backup body for critical shoots).  I've never had this happen anyway.
- other?

Just wondering why they aren't designed this way from the beginning.

[Minnesota Nice]

I theory it's a good idea. 

I'm not aware of any DSLR's that have an in-body aperture, would someone care to confirm that or enlighten me?

[dougkerr]

hI, M,

As I was sitting here debating on taking an old Tamron lens apart, I had an idea about camera/lens design.  The idea is to put the aperture diaphragm inside the camera body near the lens mount, instead of the rear of the lens.

The physical aperture needs to be at a place in the optical train substantially away from where the rays from a point on the object converge.

For example, to take it to an extreme, suppose the aperture were put just in front of the sensor. Then it would not affect the illuminance across the image (the exposure), but would merely adjustably mask the image.

Best regards,

Doug

[neuroanatomist]

+1, Doug.

Nice idea, but impractical in most cases, if not impossible, due to optical constraints.

[gmrza]

hI, M,

As I was sitting here debating on taking an old Tamron lens apart, I had an idea about camera/lens design.  The idea is to put the aperture diaphragm inside the camera body near the lens mount, instead of the rear of the lens.

The physical aperture needs to be at a place in the optical train substantially away from where the rays from a point on the object converge.

For example, to take it to an extreme, suppose the aperture were put just in front of the sensor. Then it would not affect the illuminance across the image (the exposure), but would merely adjustably mask the image.

Best regards,

Doug

This would apply to some degree at any point from the flange (i.e. inside the body).  Especially for telephoto lenses, the aperture diaphragm needs to be somewhere near the front of the lens.  (My understanding is that the exact position required depends on the lens design.)

Another point to consider - to amplify Doug's point, is to think of how large the aperture needs to be.  On an 85mm f/1.2 lens, the aperture is 70.8mm in diameter when open. - Anywhere close to the focal plane it would no effect.
On a 17-40mm f/4 lens, and 17mm the aperture is 4.25mm in diameter when completely open.  Anywhere near the focal plane, that would just act as a mask, as Doug mentions.
Now, for a 400mm f/2.8 lens the aperture is 142.86mm in diameter when completely open!

[LetTheRightLensIn]

As I was sitting here debating on taking an old Tamron lens apart, I had an idea about camera/lens design.  The idea is to put the aperture diaphragm inside the camera body near the lens mount, instead of the rear of the lens.  Has there ever been a camera/lens system designed this way?

pros:
- diaphragm cost goes only once with the camera, not duplicated with every lens
- future-proof lens designs (think of Nikon's "G" lenses on manual-focus bodies)
- continued aperture control even when using lens adapters (ex: EOS->FD, EOS->Nikon, etc)

cons:
- with a diaphragm malfunction, you can no longer swap on a new lens and keep shooting (but I always have a backup body for critical shoots).  I've never had this happen anyway.
- other?

Just wondering why they aren't designed this way from the beginning.

it has to be much deeper within the optical pathway
you want to block the rays from the element edges from building an image not block the edges of the image

[Mt Spokane Photography]

There are adapters available for using EF lenses on micro 4/3, NEX, etc  that have a aperture built-in.  I was suprised that it would work at all, so close to the body.  It will cause vigenetting at small apertures, and the bokah may not be very good.  It likely works only due to the smaller sensor format that does not see vignetting as soon when the aperture is closed down.

Here is one example:  http://www.amazon.com/Fotodiox-Adapter-Aperture-NEX-VG10-Camcorder/dp/B005ODK5LC

[mb66energy]

In a simple lens with only one element/group the diaphragm should be placed as near as possible to that element/group - to regulate the effective area of that lens.

Think about a virtual single lens which substitutes the 10 or 15 lenses of a modern zoom - the aperture diaphragm should be near that virtual lens. This virtual lens is in between the 10 or 15 lenses except in ultra wides: These lenses shift the virtual lens inside the mirror box to allow the mirror to flip up without colliding with a real lens.

I heard about the fact that non-optimal aperture diaphragm positions lead also to distortions - that might be another reason for in-lens-aperture-diaphragms.

So the "aperture-control-in-the-body"-concept is not viable but in my opinion the lens diaphragm is an element of moderate cost - compared to aspheric or special dispersion glass lenses. Perhaps sometimes Canon brings back in-body aperture control by a ring? It should be a legal "copy-and-paste" from the S-series compact cams!

[markIVantony]

Just realized that I forgot to reply.  Thanks for all your technical feedback.  I am an engineer at heart; just not an optical engineer :-)  Everything you guys shared makes sense.

[jcns]

size would be an issue