Hi, my son and I are doing a science project in which we're measuring dye transport
in plant leaves over time. Using a Canon Powershot A40 and Canon's RemoteCapture app to
snap images every 60 seconds.

Unfortunately, through RemoteCapture you can set film speed, but shutter speed
remains in Auto mode. In order to digitally compare each image with the initial
reference image (to determine the amount of dye absorbed), the exposures need
to be the same each time.

Is there a way to compensate for exposure time in a relatively accurate way, say,
see each image as if it were shot in 1/20 sec? I wish I could use the CHDK package,
but the A40 doesn't seem to be supported.

Regards, kinata

Saul, thank you, sir...replies follow.

Is flash mode available in remote capture? If so, perhaps one could be used to obtain a consistent exposure (no AE, and the "shutter speed" effectively is determined by the flash).

Well, our brainstorm was to use a fluorescent dye, without using a flash. The exposure times range from 1 sec to 1/20 sec, ISO 200, f2.8. This wouldn't represent a showstopper, as we're concentrating on the area of each image that contains pixels having that color of dye. So I just need to iterate through the pixels and count those having a certain threshold of a defined RGB value to get the area of the image containing the dye.

Is there a way to compensate for exposure time in a relatively accurate way, say, see each image as if it were shot in 1/20 sec? I wish I could use the CHDK package, but the A40 doesn't seem to be supported.

Perhaps you could include a test pattern in the frame (ie, place a picture of a test pattern next to the plant) and be able to match exposure settings by adjusting the levels of the images such that the two test patterns match (if this works, it could likely be automated).

Now I reveal my true ignorance. I was thinking that I could duplicate a layer containing a source image, play with its opacity, and then merge the layer back into the original source image to arrive at a target "exposure". Might that work?

In a related issue posted elsewhere in this forum, I'm wanting to isolate the dye part of the image: -- convert the RGB triple for the dye color (approximately a cyan) to HSB (HSV?) colorspace,
-- rotate it to (say) green, and then -- extract the green layer of the image. This last is probably off-topic...just being conversational.

Regards,
Kinata