Exposure With Barrel Lenses

[John Bartley]

Joe,

Thanks for bursting our gravity bubble with the "friction" skewer .

When I first heard about a guillotine shutter, I also thought about a spring loaded version as my concern was a lack of positive shutter action. I discounted the idea due to the problem of movement/shake at shutter startup. Your idea of "opposite action" twin blades solves that problem by making the action inertias cancel each other, but requires an actuating and release mechanism (also easily built).

However, the friction problem caused by tilt can also be solved by a simple weight, string and pulley where the weight is heavier then the the shutter and instead of the shutter sliding "down", it gets pulled "up" when the weight is dropped .... or .... the weight can pull the shutter across the lens just as well.

cheers

 

[John Bartley]

If your guillotine shutter is of the type that has a falling blade with a horizontal slit that travels (falls) across the face of the lens for exposure, then the way to increase exposure (slower speed) is to make the slit wider and to decrease exposure (faster speed) is to narrow the slit. The downside of this is that you need to have a guillotine knife with a different width of slit for each different speed.

So, I did a bit of remedial reading up on gravity and the formulas (formulae? ) associated with it and here's the chart I came up with for the size of the slit in ANY slit shutter assuming that the start speed is zero and the slit starts just outside the edge of the lens viewing area. The sizes are "almost" manageable up to about 1/5'th of a second, but what I haven't reconciled yet is how the acceleration affects the exposure from bottom to top. As the shutter finishes its trip, it will be moving faster so the bottom will be underexposed and the top will be overexposed.

Just some thoughts ...

cheers

 

[JLP]

A front mounted Luc shutter is quite handy too. They do occasionally come up on the bay, i found one a few months ago, a 65mm and after taking it apart for cleaning and lubing it fires consistently around 1/25s It is easy to take apart for cleaning.

jan