James,
I think this is a valid alternative. It might be called "sensitometric" testing.
I think that the Materials and Processes book by Stroebel at al refers to this
method but using film. Doing this with a digital camera would be a bit faster
and more efficient.
Thanks for the input! I think I will forward it to the instructors that teach
out of that book and maybe they will incorporate the method in their course.
Cheers,
Andy
James Schenken wrote:
Andy:
Since the sensors for digital cameras are linear in recording
information ( is this true ), then for a particular aperture, the
density of a white spot on a black background should be a function of
shutter speed alone. So . Take a series of photos of such an object
at set shutter speeds of 1/30th, 1/50th, etc., select the density value
for the shutter speed to be that which has been measured by the spinning
disk method ( that get's a baseline density value for say 1/30th second
) and then calculate the other shutter speed values from there.
This should get you values up to about 1/200th second or so.
Then reset the density value to the higher shutter speed and repeat
farther up the speed scale.
Repeat as often as necessary to complete the speed scale for the
particular camera's shutter speed range.
To check on linearity issues, a single shutter speed and varying
apertures should suffice to get the curve data.
To get the lighting value right, keep adjusting until the density value
( 8 bit .JPG image ) of the white spot is something like 255 for each
iteration of the process.
Cheers,
James
At 09:13 PM 11/17/2006 -0800, you wrote:
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
James Schenken
