'Stop' Calculations / Units??

locostbob

locostbob

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Hey up...

i understand 'stop' as in a measurement of light. as i see it a 1 +stop is double the light and a 1 - stop is half the light...

i am hoping to shoot some stair trails and low light landscapes this weekend as in scotland so not too much light poloution [rain may be an issue tho!]

i have read around and still none the wiser... i have found a few tables to show f. stops so f4 to f5.6 is one stop but nothing to show me shutter and iso stops... could somebody point me in the direction of stop measurements etc..

what i want to do is shoot at say iso 3200, f4 30 sec as a bench mark to gague and adjust for exposure, composition etc, and then work out the stops to give me the correct exposure at say iso 100.. if im hitting 10min exposures the usual trial and error method will take me hours and hours!!:lol:

many thanks

bob :)
 
I don't have any real experience with star trails, but calculating stops for shutter speed and ISO is simple enough.
An exposure that is twice as long will have double the amount of light, therefore double the exposure time (e.g. 1/100 to 1/50) is +1 stop, half the exposure time (e.g. 1/100 to 1/200) is -1 stop.
Similarly, ISO is a linear scale so ISO 200 allows twice as much light as ISO 100.
Double ISO is +1 stop, half ISO is -1 stop

What I don't know is how well this applies to star trails, because each of the stars is a moving (albeit very slowly) point of light, therefore I guess a different part of the sensor is being 'exposed' as time moves on. It's one I've wondered about for a while... My guess would be that rather than exposing for the full length of the exposure, you're really exposing for the amount of time it takes a star to move from one 'point' to the next 'point' in the sky, but then you have to account for the background light as well - it's a mystery to me. Would like to be corrected on this :p Maybe you can just expose for the background and stars are bright enough that you can assume they will always leave a trail..
Note to self: stop waffling
 
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there are no real units its a relative thing, a stop is twice the light, so 1/100 lets through twice the light of 1/200 because its open twice as long, iso works the same way

aperture is a ratio of area so works on a factor of 2^1/2 (the square route of two) which for all intents and purposes is 1.4, hence f4 lets through twice the light as f5.6 (4x1.4=5.6)

if the light level is iso 3200 f4 30 sec and you want to shoot at iso 100 f8 you need to compensate for 5 stops of iso (3200/2=1600 /2=800 /2=400 /2=200 /2=100) and 2 stops of aperture (4x1.4=5.6 x1.4=8). This means 7 stops of extra shutter speed to give 3,840 seconds or 64 minutes (30x2=60 x2=120 x2=240 x2=480 x2=960 x2=1920 x2=3840)

so the best part of an hour :D
 
Shoot at a lower ISO (say, 200 or 400), wide open f4 for 2-3 minutes and repeat until you've got as long an overall exposure as you want. Then stick the lens cap on and shoot 5-6 dark frames for reference.

Download startrails.exe and import the image jpegs and dark frames into that, click build and away you go. Finally, PP to your heart's content :)

Make sure your batteries are fully charged - the cold will affect them.

There's an article on here somewhere that gives a bit more information - my version is a bit rough and ready but it works.
 
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