Shutter Speed Testing and measurements

[480sparky]

Per request:

Ruler markings are 1/16"

 

[bergytone]

Thanks sparky, That really is a photocell, not a phototransistor or photodiode. I'm not sure how they get the speed they get out of it. Now I've got to try hooking one up to my test fixture and see how fast they are. i know I have one laying around somewhere. How far set back would you say it is?

I'm with you grahamp, checking consistancy may be most important. I think I have to ask myself what I'm trying to do with this gadget? I'm not trying to make a lab grade tester. Just a cheap handheld thing that you don't have to hook up to your laptop and run audacity to check your shutter.

If you look at my previous post with the old Voightlander.. it's supposed to be 1/100 sec , but it measures out closer to 1/150. That is the kind of thing I'd like to know before taking it out and burning through a roll of film.

 

[Jim Jones]

In the early 1970s I tried a CDS cell like Sparky shows to measure shutter speeds with an oscilloscope. The cell's response in a high impedance circuit was awfully slow until was shunted with a resistor.

 

[480sparky]

The original photocell was lost, so I just used whatever I had handy. It's set back in the brass tube about ¼".

 

[John Koehrer]

For "average" exposure a single cell works OK.
The Kyoritsu(sp) tester I've used way back when I actually worked for a living used three. One central, two in diagonally opposite corners to measure consistency(see #25). The edge sensors will also catch bounce and capping.
The diagonal placement is for use with vertical and horizontal travel shutters.

 

[Chan Tran]

Chan Tran, I'm glad you brought this up. If you want to precisely measure the speed of the curtains, you need to have a pinpoint sensor. But film in the camera is not a pinpoint. I would think in all reality, you'd want a sensor as big as the film to get a true average of the light hitting the film. But this is not practical. So the expensive device will give an absolute precise time that the curtain is open, but is that a true representation of the light hitting the film? I would think for it to work properly, the sensor must be almost touching the shutter. Do you agree?

The sensor really should be as small as a point. The exposure is for any given point on the film plane to have the duration of light shining on it. If you use a sensor the same size as the film and testing a focal plane shutter with 1/125 sec max flash sync speed and the shutter is set for 1/1000 sec you would see the duration of approximately 250ms of low level output. You don't have any point that receive full output because the sensor never fully exposed to the light.

 

[Chan Tran]

I meant 8 milisecond not 250.

 

[bergytone]

I'm moving this discussion to the DIY- smoking soldering iron group. If you are interested in following my development, check the thread there...
Thanks for the input everyone!

 

[Jon Gilchrist]

...Those top speeds should be checked at the aperture used for photography. Checking them at the widest aperture erroneously suggests slower speeds.

I'm currently working with a shutter speed tester on leaf shutter lenses and have found that top speeds register slower than I would expect, often a full stop (1/500 reads as 1/256). I measure at full aperture. Can you explain the reason for your comment? I'm trying to understand this phenomenon.

 

[AgX]

The crucial term is "Shutter Efficiency"

-) At leaf shutters there is a time spent before the shutter is completely closed, the same at opening again.

-) At open aperture both these times are of effect already, in partially blocking the light.
-) At minum aperture both these times nearly completely pass without any effect, as the diaphragm still blocks what the moving leaves are obstructing resp. giving free.