Probably a dumb question lens/aperture question . . .

[Markham941]

I'm doing some research on this now but I figured I'd ask here in case someone had an answer.

In some experimentation I have used aperture cards with large and non-traditional optics But, I wish to mount a mechanical iris as it is much easier to more fluidly change the F number.


I was curious to know the effect (if any) of positioning a mechanical iris / aperture in front of a lens instead of behind it (inside the barrel).

Also, if there is any visual effect if the distance between the aperture and the optic element is (too greatly) increased.


Essentially,
I hear that barrel lens are a mixed bag (optically) but also due to some being difficult to fit with an aperture. Exposure time is not a factor in my project (Scanner camera) so it seems easier and cheaper to make a mount where an iris / aperture would sit in front of any lens / optics that did not already have a working aperture / shutter.

I would in turn also mount lenses (mostly barrel) more recessed or flush to the board for better camera balance.

Thoughts?

(any lens/optics physics-technical answers are fine. I'l try to follow along and look up the parts that i don't fully grasp)

[Two23]

I don't know the answer, but some of my older lenses (e.g. 1850s) had apertures placed in front of the lens. I haven't seen any lenses where it was placed behind.


Kent in SD

[ottluuk]

The results will greatly depend on the kind of lens you are using. Placing the aperture in front of the lens is a normal procedure for single meniscus/achromat lenses like the landscape lenses of 19th century and various soft-focus lenses. Have a look here for a clean and simple take on the classic meniscus lens (note the placement of slots for Waterhouse stops): http://re-inventedphotoequip.com/Formats.html

The aperture in front (or rear) of lens situation also happens with convertible lenses such as when using a half of an old Symmar.

But the common designs in modern lenses need to have the stop between lens elements. If you put a stop in front of them, you'll simply end up vignetting – obstructing the image from the edges. Take any camera with through-the-lens viewing, look through the viewfinder and hold a piece of card with a small hole in front of the lens and move it back and forth...

"Barrel lenses" is a near-useless term for this discussion. You will either need to find out the optical construction of a particular lens for a theoretical solution – or experiment as you've done before.

[Markham941]

The results will greatly depend on the kind of lens you are using. Placing the aperture in front of the lens is a normal procedure for single meniscus/achromat lenses like the landscape lenses of 19th century and various soft-focus lenses. Have a look here for a clean and simple take on the classic meniscus lens (note the placement of slots for Waterhouse stops): http://re-inventedphotoequip.com/Formats.html

The aperture in front (or rear) of lens situation also happens with convertible lenses such as when using a half of an old Symmar.

But the common designs in modern lenses need to have the stop between lens elements. If you put a stop in front of them, you'll simply end up vignetting – obstructing the image from the edges. Take any camera with through-the-lens viewing, look through the viewfinder and hold a piece of card with a small hole in front of the lens and move it back and forth...

"Barrel lenses" is a near-useless term for this discussion. You will either need to find out the optical construction of a particular lens for a theoretical solution – or experiment as you've done before.

Thank you for your reply. It has helped me sharpen my search inquiries and how to approach any lens mounting and modifications.

For those like me who would like to learn more (or spend some free time melting their brain with physics explanations of lens optics)
Below are some links that better explain lens optics as well as some relevant forum posts.


Why does a small aperture increase depth of field - Physics explanation
https://physicssoup.wordpress.com/2012/05/18/why-does-a-small-aperture-increase-depth-of-field/


General explanation of stops, aperture, etc.
http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/stop.html

Modern camera with iris in front of lens
https://www.japancamerahunter.com/2012/02/ms-optical-perar-28mm-f4-super-triplet/

Manual of Photography
By Ralph Jacobson, Sidney Ray, Geoffrey G (EXCEPTS)

Ch.5 The Photometry of Image Formation
https://books.google.co.jp/books?id=3VEdAAAAQBAJ&pg=PA61&dq=diaphragm+stop+location&hl=en&sa=X&redir_esc=y#v=onepage&q=diaphragm%20stop%20location&f=false


Very good explanation on aperture placement

http://www.largeformatphotography.info/forum/showthread.php?60087-Aperture-placement

[Dan Fromm]

I don't know the answer, but some of my older lenses (e.g. 1850s) had apertures placed in front of the lens. I haven't seen any lenses where it was placed behind.


Kent in SD

Look at a 200/4 MicroNikkor AIS.