Guillotine / Drop Shutter and Exposure

[Richard K.]

I've now seen a few of the drop type gravity fed shutters on You-Tube etc. They seem to be a cheap reasonably reliable alternative for large shutterless lenses. My question is - how do they avoid uneven exposure from top to bottom of the neg due to acceleration of the slot due to gravity? Or are the prints burned?

The only thing I can think of is that the slot reaches terminal velocity due to friction before it starts crossing the lens path. But it seems that a higher starting point would have been required to reach terminal velocity?

Can someone confirm that the negs are evenly exposed?

Also, anyone know where to get *this shutter, or is it home-made?

* http://www.youtube.com/watch?v=8odb2oeBrJA

[Steven Tribe]

There was an extensive thread a few months back. Starting point was a "new" drop shutter. Suggest you search using "guillotine".

[Robert Hughes]

My question is - how do they avoid uneven exposure from top to bottom of the neg due to acceleration of the slot due to gravity? Or are the prints burned?

Since the drop/guillotine shutter is at the lens, rather than at the film focal plane, the question is moot. Any part of the lens that is exposed transmits light to the entire sheet of film.

For my DYI 8x10 box camera I built a spring-driven two blade guillotine shutter based on the Minox shutter (but bigger and only one speed: 1/30 sec). I don't use it though because I found a "Galli style" sheet of black foamcore with a 1/2" wide slit cut through the middle works just as well.

[BetterSense]

Since the drop/guillotine shutter is at the lens, rather than at the film focal plane, the question is moot. Any part of the lens that is exposed transmits light to the entire sheet of film.

That pretty much explains it. It's a bit weird to think about but the proof is in the working.

[Mark Barendt]

Since the drop/guillotine shutter is at the lens, rather than at the film focal plane, the question is moot. Any part of the lens that is exposed transmits light to the entire sheet of film.

I doubt this explanation. If it were true vignetting would not happen.

My guess it that the distance across the lens is so short that it doesn't matter a lot.

[Robert Hughes]

I doubt this explanation. If it were true vignetting would not happen.

:confused: What would vignetting have to do with it? I don't see the relevance.

I'm seeing some bogus optics talk here. Experts, please chime in.

[Paul Fitzgerald]

"Any part of the lens that is exposed transmits light to the entire sheet of film."

"What would vignetting have to do with it?"

If any part of the lens exposed covers the whole plate, how could vignetting ever happen?

As far as uneven exposure from a drop shutter goes, it does not because there is NO comparison to a focal plane shutter.

A 1/4" slit in a focal plane exposes 1/4" of film, a 1/4" slit with a drop shutter exposes 1 inch or more at the film plane. You would need to do a film test or use a shutter speed tester to gauge exposure from each slit AT EACH EXTENSION LENGTH, the lens extension changes the 'swipe' size. Drop shutters do work BUT were replaced for lots of reasons.

[JRFrench]

This could be easily tested by holding a slit in front of a view camera and looking at the image on the glass.

[jp]

I'm wondering if a slit shutter on the front would increase DOF. That would not be desirable if using a portrait lens or if you are otherwise going for shallow DOF. It doesn't matter where the aperture is based on some of the soft focus designs where the aperture is at the front for a combination iris/lens shade.

[Arne Croell]

Since the drop/guillotine shutter is at the lens, rather than at the film focal plane, the question is moot. Any part of the lens that is exposed transmits light to the entire sheet of film.


That would be correct if the shutter would be close to the aperture stop plane like a leaf shutter. But in front of the lens it is not that close. It is also not near a field stop plane like a focal plane shutter, where a variable speed would have a visible effect. It is somewhere in between, so it acts a bit as aperture stop and a bit as field stop at the same time.

[Mark Barendt]

Since the drop/guillotine shutter is at the lens, rather than at the film focal plane, the question is moot. Any part of the lens that is exposed transmits light to the entire sheet of film.

For my DYI 8x10 box camera I built a spring-driven two blade guillotine shutter based on the Minox shutter (but bigger and only one speed: 1/30 sec). I don't use it though because I found a "Galli style" sheet of black foamcore with a 1/2" wide slit cut through the middle works just as well.

Simple test, on most any non-pinhole camera hold your fingers like Mr. Spock and cover your lens so that the gap in your fingers creates a "slot".

Now look through the camera.

What do you see?

Is light from the scene reaching everywhere on the ground glass or in the viewfinder?


:confused: What would vignetting have to do with it? I don't see the relevance.

I'm seeing some bogus optics talk here. Experts, please chime in.

A vignette is just a shadow, when we use several screw on filters on a wide angle lens, the rings may start blocking the light path and casting their shadow in the corners of the photo.

If what you said "Any part of the lens that is exposed transmits light to the entire sheet of film." was true, vignetting would not happen and our fingers in front of the lens would not create a slot.

[timparkin]

A vignette is just a shadow, when we use several screw on filters on a wide angle lens, the rings may start blocking the light path and casting their shadow in the corners of the photo.

If what you said "Any part of the lens that is exposed transmits light to the entire sheet of film." was true, vignetting would not happen and our fingers in front of the lens would not create a slot.

Most vignetting is a combination of distorted pupil through angle of view or mechanical clipping (which you could call shadow, if you think the aperture also causes a shadow).

I think anything shading the ground glass is a shadow that's OK.. Most people would call it an aperture I think..

IANAP(hysicist) though :-) (nor linguist, nor a particularly knowledgeable photographer for that matter)

Tim

[Richard K.]

Hmmm..OK; I knew that EACH point on the lens images the WHOLE film plane except where there is mechanical vignetting - and that is usually symmetric so darkens corners. So, a falling slit ought to expose each part of the film uniformaly regardless of it variation in speed. The question that now has entered my mind is how then to explain the sky darkening effect of split neutral density filters since we're agreed that light passing through each part of the filter images all parts of the film?

[Mark Barendt]

... since we're agreed that light passing through each part of the filter images all parts of the film?

Not as I understand it.

Back to fingers in front of the lens, cover the bottom half of the lens and the top half of the ground glass goes dark.

If your statement were true fingers wouldn't show up and the ground glass would stay fully lit..

[cowanw]

I am thinking that the light from any point in focus, on the subject, will radiate light in all directions ( toward all the surfaces of the lens) but those rays will approach the lens from different angles and be bent by the lens to focus on the point that is the image.
Thus all the light that travels from a point and that strikes the front of the lens does reach the film but it is all focused on the appropriate image point.
Unfocused points of the subject will reach a larger area on the film and appear diffused.
The more unfocused the image the less is the effect of obstructions like fingers and camera straps.
A fallng shutter would have to fall twice as fast at the bottom compared to the top to make a 1 stop difference.
Compared to the middle the top and bottom would only be a half stop different. Lesser speed differentials would give lesser exposure differences soon getting to the point were we could not see the difference in exposure.
Focusing all the light from a point subject to a point object is what a lens does.
Regards
Bill

[Paul Fitzgerald]

"A fallng shutter would have to fall twice as fast at the bottom compared to the top to make a 1 stop difference."

Gravity is constant so the acceleration is constant at 32 feet-per-second, the difference over any seven inches would be minimal. A 1 second exposure would travel 32 feet. Giving the shutter a running start would just increase the speed, the difference from the top to bottom of the opening would not change. To change the acceleration you could use a spring or rubber band to jump start the slot until it reaches 1g.

If the slot is 2" in front of the aperture and the aperture is 12" in front of the film it has a 1-6 ratio, a .25" slot exposes 1.5" of film, this should work as a buffer for any speed difference. The nice part is it should also automatically adjust for bellows extension.