Basic shutter testing methods for digital cameras

[ADavidhazy temp in Seattle <andpph@xxxxxxx>]

I was asked the following question and I replied below. I thought some of you 
might be interested in the reply. I recall that Bob Talbot was testing shutters 
in related ways.


> So,  I am wondering how you would go about testing the shutter on a digital 
camera. I am assuming they are plagued with the same shutter issues that our 
film cameras are, but we only have pixel values to deal with.


Good point. It is something that I thought about but not for long as I must have 
gotten distracted by other things. Anyway, now that you ask ...

The shutter testers that depend on a clear light path through the camera body 
obviously will not work as digital cameras do not provide this capability. 
Sooooo ...

1. The simplest test would be to test as done in my high speed class. Photograph 
a disc rotating at a known speed and measure that resulting arc and divide that 
by the rotation rate in degrees per second. We use a regular (LP vinyl) 
turntable and measure up to about 1/100 second. But after about 1/30 the test 
becomes quite imprecise.

2. An alternative is to photograph a flashing light source such as the display 
generated by some LED displays ... or even standard neon signs ... by panning 
the camera while keeping the LED visible in the finder. Counting the number of 
flashes of the image of the subject by the rate the subject was flashing at 
(about 120 per second for the neon tube and up to maybe 400 per second for the 
LED display (set to the number 1 best) gives the exposure time.

3. An alternative to the above alternative is to use a calibrated stroboscope to 
make an image of the slit (assuming a DSLR) of the shutter ... without a lens on 
the body ... and at this point there are two ways to go.

3a. Measure the distance one edge of the slit moves between consecutive 
positions of the slit's image. Measure the size of the slit. Divide the size of 
the slit by the rate the slit's edge moves per second and that gives you the 
exposure time. The drawback is you need a good stroboscope.

3b. Pick one of the slit images somewhere in the middle of the frame (keeping 
magnification into account if measuring an enlarged version of the cameras' 
frame size divided into the corresponding value of the enlarged version) and 
determine its size in comparison to the width or height (depending on which 
direction the slit moves) and that ratio multiplied by the X sync speed of the 
camera will give you an approximate exposure time. This can be refined (as I do 
in my class) to get more accurate measurements.

Obviosuly you always need a "standard of measurement" that you don't question. 
Turntable rotation rate, LED flashing rate, stroboscope flashing rate, etc.

Yet another way is to photographn linear motion (as I do in my high speed class) 
such as a car traveling at a known rate (you must assume the speedometer is 
accurate!) and then, placing a small light (such as a bare bulb car taillight) 
on the side of the car, photograph the moving vehicle. The lamp's filament will 
leave a blur on the record. Divide the length of this blur (again relying on 
knowing the magnification of the print you are using) by the rate of motion of 
the car.

4. Finally there is yet another method that depends on the use of an 
oscilloscope but not looking through the camera but, instaed, monitoring the X 
sync connection timewise. In the X sync mode the sync contact closes immediately 
upon the exposure starting and the scope will detect this. The X sync contact 
opens again once the second curtain ends the exposure. The time base of the 
scope is set so as to produce a square wave "image" of the time elapsed between 
the closing and opening of the X sync contacts.

I am sure there are several additional methods! BTW, these approaches can be 
used backwards also by assuming one knows the exposure time (I usually just 
assume the shutter has been first calibrated) I mostly just assume this since 
students tend not to bring the same camera to school, etc. ... and precision in 
my high speed class is less important than the related thinking process).

I hope this is of some use ... if you discover another approach or if I made a 
mistake in my suggestions please let me know. BTW, electronically controlled 
shutters tend to be much more accurate than the older all mechanical ones. I 
would trust their timing and, in fact, would use them as precision timepieces 
for certain applications.

cheerio,
andy