Shutter behind a larger lens

[Ian Leake]

I saw this video on Instagram:

In the captions he writes: "The lens diameter is noticeably larger than the shutter, but this has no effect on the aperture. it only reduces the image circle..."

That statement seems wrong to me. Surely placing a smaller opening behind the lens will also reduce the effective aperture?

 

[Dan Fromm]

I saw this video on Instagram:

In the captions he writes: "The lens diameter is noticeably larger than the shutter, but this has no effect on the aperture. it only reduces the image circle..."

That statement seems wrong to me. Surely placing a smaller opening behind the lens will also reduce the effective aperture?

It depends on the lens and lens-shutter geometry. I've hung barrel lenses in front of shutters and a shutter in front of a lens. Vignetting can be a problem. Loss of speed much less so.

 

[wiltw]

A shutter within the optics of the lens can constrict the diameter (aperture) of the lens, a shutter behind the lens is merely acting similar to any focal plane shutter...affecting timing and/or area exposed (a FF vs. crop frame shutter)

 

[Ian Leake]

A shutter within the optics of the lens can constrict the diameter (aperture) of the lens, a shutter behind the lens is merely acting similar to any focal plane shutter...affecting timing and/or area exposed (a FF vs. crop frame shutter)

Ah, I get it (I think)... Thank you.

 

[OAPOli]

Yes and no. Some cameras (Olympus 35RC and others) use a shutter and aperture that's directly behind the lens. In other cases an aperture behind the lenses induces vignetting (i.e. reduction of image circle). It should depend on the diameter of the restriction but I'm not sure of the optical principles behind the difference.

 

[reddesert]

It's not possible to make a general statement, it depends on the specific lens and shutter.

If you take a lens and open it to full aperture, and look through it front and back, sometimes you will see that the entire glass aperture of the lens is open from your single on-axis viewpoint (example: typical fast normal lens like a 50mm/2 for 35mm format), and sometimes you'll see that the visible clear aperture only takes up a small fraction of the glass area (example: typical wide angle like a 28/2.8). Technically, what I am calling the visible clear aperture is the entrance or exit pupil, and what matters for a behind-the-lens shutter is the amount that the shutter constricts the rays emanating from the exit pupil.

If you were to hang a shutter with a constricted opening just behind the wide angle lens, it would not vignette on-axis, but it might vignette terribly when well off-axis. I used lenses for 35mm as an example because many people have some of those lenses around to look at, but you can see similar issues with LF lenses (compare the pupils of a Tessar type to that of a Super Angulon).

I don't have any experience with Verito lenses, and I guess that the shutter in use here is large enough to not be an issue.

 

[Dan Fromm]

Red,

What does "vignette on-axis" mean?

With respect to ultrawides, well, there are ways. I have a 60/14 Berthiot Perigraphe Ser, VIa. Its glasses are tiny and the cells are very close together. So close that there's no way to insert shutter and diaphragm leaves between them. Its built-in diaphragm is a thin rotating disk with circular holes.

I use it on a 2x3 Speed Graphic. Turns out that the entire lens can be stuffed into the front of an Ilex #3 shutter. A little analysis and fiddling around made it very clear the the shutter's rear tube vignetted the image horribly, as in so much that it wouldn't cover 2x3. The solution? Cut off the shutter's rear tube, drill and tap holes in the rear of the shutter body and use screws to hold the shutter to the lens board. Works a treat.

 

[OAPOli]

Here's a simple illustration. Please correct me if I'm wrong. Blue ellipsoid is the exit pupil, blue line is the image diagonal, red and green lines the exit rays for respectively the full pupil and stopped-down pupil, and the arrows represent the rear shutter.

If the shutter obstructs the exit rays, you get vignetting. Along the dotted line you can have a smaller shutter which doubles as an aperture stop. Or you can use a big shutter + variable aperture.

 

[Ian Grant]

I saw this video on Instagram:

In the captions he writes: "The lens diameter is noticeably larger than the shutter, but this has no effect on the aperture. it only reduces the image circle..."

That statement seems wrong to me. Surely placing a smaller opening behind the lens will also reduce the effective aperture?

It is worth noting the camera in the video is 10"x8", the 18" Verito covers 14"x11" which is a much larger format. Unless you try these combinations, yoy won't know if they work.

Ian

 

[reddesert]

Here's a simple illustration. Please correct me if I'm wrong. Blue ellipsoid is the exit pupil, blue line is the image diagonal, red and green lines the exit rays for respectively the full pupil and stopped-down pupil, and the arrows represent the rear shutter.
View attachment 412458
If the shutter obstructs the exit rays, you get vignetting. Along the dotted line you can have a smaller shutter which doubles as an aperture stop. Or you can use a big shutter + variable aperture.

Sort of, but I would draw it differently to illustrate the beam emerging from the exit pupil and how it can be partially obstructed. Here's an example, excuse the slightly clunky Keynote graphics, where the light blue and dark blue ovals are apertures to-scale for an f/2.2 and f/4.5 beam. The lightblue/darkblue and red/magenta ray bundles are the on-axis and off-axis beams emerging from the pupils. For a shutter opening as drawn here (black line), it would vignette the on-axis beam at f/2.2, but not on-axis when stopped down to f/4.5. However, it will vignette either if sufficiently far off-axis (which limits the angle of coverage).

This is just an example, although the beam opening angles are correct for f/2.2 and 4.5, the shutter opening and distance behind the lens are merely something I drew, not taken from any real shutter.

 

[reddesert]

Red,

What does "vignette on-axis" mean?

An image forming beam can be partially obstructed at the outskirts (vignetted) and still form the image, and this can happen for an on-axis beam, as in the light blue cone of rays for the f/2.2 example in my above drawing. We often use "vignetted" to mean light falloff in the image towards the edge, but the beam itself can be vignetted.

IIRC, for those matte box lens hoods that have a slot for black cards with a cutout shape (like a circle, star or heart cutout, as a cheesy studio photography device), the black cards were called "vignettes", but I can't swear that's right. I can't find a reference offhand because portrait studios do that all digitally now in post processing, I think.

 

[Dan Fromm]

An image forming beam can be partially obstructed at the outskirts (vignetted) and still form the image, and this can happen for an on-axis beam, as in the light blue cone of rays for the f/2.2 example in my above drawing. We often use "vignetted" to mean light falloff in the image towards the edge, but the beam itself can be vignetted.

IIRC, for those matte box lens hoods that have a slot for black cards with a cutout shape (like a circle, star or heart cutout, as a cheesy studio photography device), the black cards were called "vignettes", but I can't swear that's right. I can't find a reference offhand because portrait studios do that all digitally now in post processing, I think.

Thanks for explaining. I still don't get it, think you are using an unconventional definition of vignetting.

 

[OAPOli]

When you have an obstruction to a finite (not a point) light source like the exit pupil, there is a penumbra region. Vignetting starts at the penumbra and gradually darkens to a full shadow (umbra). In my diagram the shutter adds zero vignetting to the full pupil. In other words you could use a smaller shutter and still illuminate the whole image but with some vignetting. Since a wide-open lens generally already has some vignetting from the elements it could cause issues.

 

[Ian Grant]

Thanks for explaining. I still don't get it, think you are using an unconventional definition of vignetting.

Seems to be explaining how the Auto Aperture of a Sinar shutter works, (when there is one).

In the case of the f4 18" Verito, the Sinar shutter is very close to the rear elements, with a 3" aperture, effectively limiting the widest aperture to approx f5.6.

Ian

 

[reddesert]

Thanks for explaining. I still don't get it, think you are using an unconventional definition of vignetting.

I'm not sure what the issue is. Perhaps it is that I said the on-axis "beam" is vignetted ? The beam is different from the on-axis ray. The on-axis ray is a single ray of light, but the beam refers to the entire cone of light from a point on the subject to a point on the image. (This beam vs. ray is standard optics terminology.) Vignetting refers to partial obstruction of the light path, typically at the edges. Here's another drawing:

Here I drew the full light beam from the bottom of the green tree to its image, on-axis: the chief ray is the black line, and the beam is the two blue lines (encompassing the rays in between). The off-axis beam from the top of the tree is the magenta lines.

If we interpose a too much under-sized aperture behind or in front of the lens (the vertical black lines), the on-axis chief ray goes straight through, but the aperture will partially obstruct the outskirts of the on-axis beam (the blue lines). This is vignetting the beam. It will form an image of the bottom of the tree, but slightly dimmed. It will vignette the off-axis beam more, of course, so the top of the tree will suffer more light loss.

I am not an optical designer, but I talk to optics people frequently (I'm an astronomer) and this usage of "vignette" is common.

As the diagram suggests, the size of the obstructing aperture or baffle (like a rear shutter) that you may be able to get away with really depends on the angle of view and the geometry of the situation, there's not a simple rule of thumb for it.

 

[Dan Fromm]

Red, thanks for taking the trouble to explain more fully. I still don't see that there are two kinds of vignetting, on- and off-axis.

By the way, I'm very aware of the risk of vignetting when putting any kind of, um, obstacle between subject and lens or lens and sensitized surface. I mean, I have a small heap of barrel lenses that I hang in front of a variety of shutters. When I went 6x12 I had to consider carefully which of my barrel lens-shutter combinations that worked on 6x9 wouldn't work on 6x12. My tandem Graphic's inter-camera coupler was designed with the risk of vignetting in mind.