Behind the lens Iris? Effects thereof?

Early Riser · Mar 28, 2016

I'm thinking of getting an adapter what would allow me to use some of my Rolleiflex 6008i lenses on a Nikon. The thing is that the Rollei uses electronics to close the diaphragm so the adapter comes with a built in iris to compensate. The thing is that this set up puts the iris behind the entire lens. I know that vignetting could be an issue if stopped down too much, but I wonder what other effect it would have optically such as DOF, distortion, resolution, etc. The whole point in getting this adapter is to get some of the wonderful qualities of these Rollei lenses on anion body, seems pointless if the behind the lens iris ruins the optical qualities.

Sirius Glass · Mar 28, 2016

The location of the iris is part of the design of the lens. Placing the iris behind the lens may or may not effect the photographic results. One would have to do some testing to know.

itsdoable · Mar 28, 2016

The diaphragm/iris reduces the aperture by letting the light enter from only the central portion of the glass elements. To do this, it needs to be at, or close to the nodal point of the lens to function correctly. The nodal point is typically between the front and rear groups of the lens elements (- however, lenses have been designed with all elements in-front, or behind the nodal point). As you move out towards to the edge of the image circle, the further away the diaphragm is from the nodal point, the more off-center that cone of light is, with respect to the optical axis.

So what does that mean?

The off-center ray-paths typically contribute more to spherical aberrations and vignetting (which is why stopping down reduces these artefacts). Thus stopping down a diaphragm that is behind the lens (and presumably a distance from the nodal point) means that you won't be reducing veil (spherical aberrations) and vignetting as much towards the edge of your picture frame.

There is a group working on an adapter to control the shutter and aperture of Rollei lenses, but that is sure to be expensive.

Early Riser · Mar 29, 2016

Sirius Glass said:
The location of the iris is part of the design of the lens. Placing the iris behind the lens may or may not effect the photographic results. One would have to do some testing to know.

itsdoable said:
The diaphragm/iris reduces the aperture by letting the light enter from only the central portion of the glass elements. To do this, it needs to be at, or close to the nodal point of the lens to function correctly. The nodal point is typically between the front and rear groups of the lens elements (- however, lenses have been designed with all elements in-front, or behind the nodal point). As you move out towards to the edge of the image circle, the further away the diaphragm is from the nodal point, the more off-center that cone of light is, with respect to the optical axis.

So what does that mean?

The off-center ray-paths typically contribute more to spherical aberrations and vignetting (which is why stopping down reduces these artefacts). Thus stopping down a diaphragm that is behind the lens (and presumably a distance from the nodal point) means that you won't be reducing veil (spherical aberrations) and vignetting as much towards the edge of your picture frame.

There is a group working on an adapter to control the shutter and aperture of Rollei lenses, but that is sure to be expensive.

I knew that they were designed to be at or near the nodal point so I assumed there'd be a change in the qualities of the lens, I just didn't know to what extent. A for a group working on a Rollei adapter that would allow the use pf the lenses, that would be something that I'd be very interest in as I have a lot of Rollei glass. Can you give me their names?

itsdoable · Mar 29, 2016

Early Riser said:
I knew that they were designed to be at or near the nodal point so I assumed there'd be a change in the qualities of the lens, I just didn't know to what extent. A for a group working on a Rollei adapter that would allow the use pf the lenses, that would be something that I'd be very interest in as I have a lot of Rollei glass. Can you give me their names?

As you move further from the central axis of the lens, the behind the lens diaphragm stopped down image looks more like the wide open IQ, but with better DoF. Until the shifted cone hits the image circle, and vignettes.

I believe it was hcam that mentioned they might work on a Rollei adapter: (http://www.getdpi.com/forum/medium-...-lenses-wide-open-80mp-sensor.html#post336696) - keeping in mind this was for technical cameras (but if they can make it for a technical/view camera, then it's pretty simple to add an SLR mount).

There was also: http://www.john.banister.name/Rollei_lens_adapter.html

The diaphragm and leaf shutter in the Rollei lenses are driven by 1970's technology linear motors, which require a lot of current, thus these adapters use their own power supply.

Early Riser · Mar 29, 2016

itsdoable said:
As you move further from the central axis of the lens, the behind the lens diaphragm stopped down image looks more like the wide open IQ, but with better DoF. Until the shifted cone hits the image circle, and vignettes.

I believe it was hcam that mentioned they might work on a Rollei adapter: (http://www.getdpi.com/forum/medium-...-lenses-wide-open-80mp-sensor.html#post336696) - keeping in mind this was for technical cameras (but if they can make it for a technical/view camera, then it's pretty simple to add an SLR mount).

There was also: http://www.john.banister.name/Rollei_lens_adapter.html

The diaphragm and leaf shutter in the Rollei lenses are driven by 1970's technology linear motors, which require a lot of current, thus these adapters use their own power supply.

Thanks!

Behind the lens Iris? Effects thereof?

Early Riser

Sirius Glass

itsdoable

Early Riser

itsdoable

Early Riser

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