Would Like To Verify Exposure Duration

Started 3 months ago | Questions thread
CharlesPhillips
OP CharlesPhillips Forum Member • Posts: 79
Re: A simple WWW app

ProfHankD wrote:

J A C S wrote:

CharlesPhillips wrote:

ProfHankD wrote:

CharlesPhillips wrote:

All -

After solving several other difficult questions, now another one comes up. I have a Nikon D200 and use it to take photos of satellites, using mostly 10 second exposures. But it appears that the exposures are not exactly 10 seconds.

I am trying to think of way to measure the duration, I'd like to know to the tenth of a second. I think that the exposures are a little long.

I am cross posting to the Nikon discussions.

Easy check: photograph a changing computer display. Most computer displays refresh at least 24 times a second, usually 30 or 60, so you just need a program that simply outputs a character in a new position every 1/10 second. Better still, here's a javascript page I just threw together to do it for you:

http://aggregate.org/DIT/shutspeed.html

Just load that page, photograph the screen, count how many # images you see, and divide by 10 to get the number of seconds for the exposure. A complete line is 1 second. It might work better if you invert the text and background colors for white text on a black background.

That is a great idea, let me try it (tomorrow). I'll report back.

The image you will get will look almost entirely white, I guess. This is basically a DR test of your monitor with small detail.

Not a very harsh test. 10s @ 1/10s per character displayed means you need 1:100 contrast captured, or less than 7 stops. Typical LCD monitors do at least 1:2000. Typical JPEGs represent at least 9-10 stops, and better cameras can capture 13-15. As for the "small detail," well, that's a matter of how big you set the scaling on your browser window and how close-up you make the photo.

That said, it usually will work better with white text on black background than the reverse colors. I just switched it to that so you don't need to mess with browser settings. I also switched it to 2 seconds (20 symbols) per line. I made the font bigger too and have it showing a digit sequence to help those you can subtract easier than they can count.

Let me "oops" my reply! After trying the test several times I realized that I had not read this above note - I had wondered why the app looked different. This is sadly not the first time I have felt foolish.

It took me several tries before I got the camera set up to get a reasonable, readable image and got a fairly good start of the camera at the same time as the app.

I forgot to mention that I did try the simplest answer first - used the stopwatch feature on my iPhone! That indicated that the exposure time was pretty close, I wondered why that disagreed with the "app" test. Oops. I was looking for a much more accurate answer, one that did not rely on my older reflexes too much.

I do want to do a better test of the exposure time to see if it is reliably a few tenths of a second short or maybe long. Even a few tenths of a second makes a difference.

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Charles
Software Writer, Orbital Mechanics Guy, Amateur Astronomer

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