Drop Shutter Calculation

[nano_burger]

Hi,

 

I just constructed a "drop" shutter for my 5x7 FKD camera. After taxing

my physics knowledge, I decided to just cut a 1.5 inch rectangular hole in the

sliding shutter and see what the exposure was. Not owning a shutter tester, I

put some 100 ASA film in and set the aperture to f16 and took a photo on a nice

sunny day.

 

Much to my surprise, the exposure seemed right on, so I Christened this

slider as 1/100 of a second. My question is, will decreasing the hole by half

give me 1/200 and doubling the size of the hole give me 1/50? I can't remember

if these things are linear. I realize that the sliders are accelerating in

earth's gravity, but I'm thinking that it won't matter too much over such a

short length. Looking carefully at my test negative, I can't see a gradation

in density from top to bottom, so that is encouraging.

 

Also, I cut a rectangular hole in the slider...is this the right shape?

I've seen some drop shutters on the internet that have semicircular and

triangular shapes. TIA

 

v/r

Dan

[pico_digoliardi]

Keeping the width of the shutter the same, then doubling (or halving) length from the previous length gives the correct F-type exposure. 2, 4, 8, 16, 32, 64... or 2, 1, .5, .25, .12 works well enough.

 

You can get some mighty short exposures by making a slit smaller than the aperture of the lens.

 

I know what you mean by the shape of some between-the-lens drop shutters, but the right-angle shape is just fine. It works for modern FP shutters.

 

Releasing the shutter can be tricky - it can move the camera. There are many solutions, one being a simple peg that just barely holds the shutter up (like a sliding door lock) with a string to pull it out. If you have an old pneumatic cylinder, there's a more elegant way.

 

Some very long 'in front of lens' drop shutters were used with filters to create colored waterfalls. I mention this because they were so long (with large lenses) that stopping them at the end of the fall caused a shock to the lens, causing some parts to loosen up. Using a longer-yet shutter, but with a solid trailing section behind aperture and then placing a rubber-band so that its slack was taken at the very end of the exposure cushioned the shock.

[Farside]

<p>Reviving an old thread here, but it cropped up when I searched for a drop shutter.<br>

It occurs to me I could have a sliding board with hole, as above. The speeds could be set with pairs of rubber bands, one each side of the board to equalise pull and prevent jamming. One set of bands for basic speed (depending on strength of bands, of course, to set a datum point), two pairs of bands for twice the speed and so on. Speeds can be measured with a degree of accuracy sufficient for adequate prediction in use.<br>

There's nothing new under the Sun, of course, so I expect this has already been though of.</p>