Interesting test! Could you post some downsized images to show how you determine this? I think others would find it interesting (as well as myself).
Sure... see the attached images.
First, 10pros.F2.6_zoom1x_Tseq.77.474us.jpg is an image taken using a fast sequence (to show you where all the LEDs really are). The bright one in the middle is a reference diode, it's powered on constantly and doesn't blink. To the left of the reference lamp there is another reference diode, which is invisible as it should be, since it's never powered. Note that not all LEDs are equally bright, this is mostly because of the fact that the LEDs are not ideally positioned and their viewing angle is about 30 deg.
10pros.F2.6_zoom1x_Tseq.1007.16us.jpg shows a neat and clean measurement with four bright LEDs and two dim ones: shutter was closed before the one on the left was done with it's blink time and the one on the right probably started to shine just before or while the shutter was opening.
Have you taken into account the LED's own cycle time? As with all devices, they all suffer from a "lag" of one sort or another. You might want to research online the response time of the particular LEDs that you are using. This could help you to determine a little more accuracy by taking their "fudge factor" into account.
Yes, as I said above, I believe the LEDs to be fast enough. But this is the one thing I haven't actually measured. The manufacturer's data sheet promises a 20 ns typical speed of response (exponential time constant), from which I figured they would quite likely be faster than my dead time between consecutive LED blinks (1.3 us).
This is why testing for such high speeds is proving difficult. I still haven't pulled my old silly-scope out of storage (not only because it's -20 F. outside lately) because I am certain that the phosphor lag on the display would again prove useless for a test of this nature.
I did some initial experiments using an old analog scope (not quite as old as yours I think, from the 80's), but the phosphor was way too laggy for any accuracy (a more cunning source signal setup than DC could've helped but I figured it wasn't worth the trouble).
One more thing. Have you tried using long-zoom settings and using the aperture override too, to see if you can obtain even faster speeds at f/11 (or higher?).
I think I already said (at least I ment to
) I didn't try any aperture overrides yet. By long-zoom do you mean that it would be more likely to get the camera go to F8.0 at full 4x zoom than to get past F8.0 at any zoom position?
Also, I'm not sure what you mean when you say, "Especially the shots taken with 100% image size suffer from significant shutter distortion." Since none of these cameras use a focal-plane shutter there are no shutter distortion effects. What are you referring to here?
I sort of assumed the shutter would be a mechanical obstacle even if it isn't positioned in front of the sensor like it conveniently is positioned in cameras where you can change the lense. What led me to think so was the fact that the camera makes noises when taking pictures and that looking up the lense, there clearly is a shutter. But if that's there just for dark-frames and previews and the real shutter just an electrical signal to the sensor, I'm obviously mistaken.
You'll have to forgive me for actually not knowing much of anything about photography or camera optics
In any case, from those initial results it definitely looks like I can capture the LED "motion" more accurately if I take the camera further ie only use the very center of the lense, instead of taking it up close and filling the entire photo. This made me believe that the thing I see in the lense indeed is the shutter, and that it is a hole that rapidly widens, stays open for a defined time and then closes.
Take a look at 100pros.F2.6_zoom1x_Tseq.1007.16us.jpg. It has only one or two bright LEDs, followed by four or three dim ones. But then there's a lonely LED on the upper left corner, which is clearly a little bit active. I'd call that shutter distortion.
(p.s. Have you thought about focusing with the room lights on first? Big Grin The more "point source" your lights the more easy it will be to have their light show up on the CCD's pixels and therefor be more accurate to tell when they are fully lit or only beginning or ending their ramping or decaying light cycle.)
Actually I think I did that in most photos and locked the focus manually to prevent the camera from messing it up once it was dark. Focus wasn't perfect but not the biggest imperfection here.
I really am assuming the LED response time is insignificant. One more thing making me thing it's not a LED lag issue is something that can be seen in the actual images: on images which have more than two dim LEDs, the unexpected dim ones are those that are supposed to shine next, not ones which have just blinked (except the one dim LED before the first bright one, of course). The further in the future their time to shine is, the dimmer they appear.
This is shown in 100pros.F8.0_zoom4x_Tseq.296.22us.jpg: the brightest LED is in top-left, the one that was powered before it is on it's left and is very dim. Then there are about 10 LEDs which were powered after the brightest one, all decreasing in brightness in the photo.
You've probably noticed I didn't actually include pictures of the fastest shutter speed measurement (F8.0 zoom 1x). That's because the LEDs aren't quite bright enough for crystal clear images at those speeds and I figured they wouldn't be good for introductory purposes.
