View Full Version : Exposure value in the centre

20-Feb-2012, 02:35
OK ...

So I have a lens. It is mounted in the usual way but I place a black card with a hole in the centre behind the lens inside the bellows. I still have enough of an image circle for my film of choice with a little room for movements.

What have I changed about this photographic setup ? The only thing I can't do is swing, shift and tilt anymore right ?? I have to take the photo straight on.

Have I changed the bokeh ? Have I changed the f-stop exposure ?

If it were a wide angle f4.5 lens it is still an f4.5 lens right but without the wide angle ...

Comments please ...



Doremus Scudder
20-Feb-2012, 03:19
You are a) changing the aperture of the lens (assuming the lens is wide open) and b) vignetting the exit cone of the lens, thereby cutting down its covering power. Better to just use the built-in iris for stopping down, then you don't alter coverage.

If your lens doesn't have an aperture mechanism, what you are doing is providing a makeshift one. Most lenses with no iris have a slot between lens elements for "Waterhouse stops," which are slides with various sized openings, to provide apertures. Optically, an aperture between elements is better.

If you are using a makeshift/homemade/repurposed lens, then your solution could be the only way to stop down. Finding which aperture you are using is a different story...



20-Feb-2012, 07:41
The usual location for waterhouse stops is between the front and rear cells. I'm not sure what ill effects an aperture behind the rear cell will cause. Since you have the camera and setup, maybe you can tell us what happens.

20-Feb-2012, 08:38

as long as you don't need any movements..you are doing nothing..blocking stray internal reflections maybe

Jim Jones
20-Feb-2012, 09:52
Controlling exposure by an aperture behind or in front of the lens may have a strong affect on image quality with some lenses, depending on their design. For example, the aperture for the traditional simple landscape lens is placed in front of the lens, and the distance from the lens is selected to control some of the deficiencies of the lens.

20-Feb-2012, 12:31
Thanks for the thoughts all.

I'm still with Dr Tang at this stage but am still open to change... If the hole in the card were pretty large (it would have to be to allow full coverage) I still can't see it blocking anything but internal reflections for a straight-on shot with no movements.

I struggle with thinking intuitively about how light and lenses work sometimes. (make that most-times!)

Doremus Scudder
21-Feb-2012, 02:56
... Maybe I was a bit hasty. I thought, as indicated in my post, that you were using a hole smaller than the lens elements in an attempt to add an aperture/control exposure. If the hole in the card you are using does not impinge on the cone of light that exits the lens, then, as Dr. Tang says, you are doing nothing except maybe adding some internal baffling inside the bellows, which may, or more likely, may not catch some stray internal reflections.

If you are blocking some of the lens exit cone, but still covering your film, you probably are causing a little light fall-off at the edges of the film due to the penumbra of the shadow of the hole. In order not to do this, it needs to let in a lot more image circle than just enough to cover the film.

If you are trying to limit reflections and flare, a lens hood would be a much better choice, and a compendium shade the best choice.



23-Feb-2012, 01:29
Thanks Doremus.

I'm not trying to limit flare or any such thing ... I'm thinking of front mounting a lens on a shutter that is too small for it :-)

Although the shutter be quite large I am worried that the experiment will fail. Some might give it a go but these things are all costly.

Cheers all.