calculating bulb exposures?

[hornetzoo]

Without a light-meter, how can I calculate the required time for a long bulb exposure? Suppose I can meter at 2.8, but then want to take the shot at 32, by what number would I have to multiply the shutter speed? What other ways are there to do it?

[gasrocks]

When you go from 2.8 to 4 it is 1/2 the light so you double the time. 4 to 5.6 same thing, 8, 11, 16, 22, 32. Supposed to be simple math. So, f/2.8 to f/32 is a diff of 128 times (multiply the time by 128.) Make sense?

[Bob_A]

For each full stop increase you need to cut the shutterspeed in half, i.e.,

1/1000 s @ f1.4
1/500 sec @ f/2
1/250 sec @ f/2.8
1/125 sec @ f/4
1/60 sec @ f/5.6
1/30 sec @ f/8
1/15 sec @ f/11
1/8 sec @ f/16
1/4 sec @ f/22
1/2 sec @ f32
1 sec @ f45
2 sec @ f64

So, divide the shutterspeed you needed at f2.8 by 128 and you will get the shutterspeed you need for f32. Note: the above is a bit of an approximation, if you calculate the aperature area for f2.8 and f32 the number you should divide by is actually 130.6

[hornetzoo]

thanks, and how do you know what a full stop is? my camera has other increments that you have not listed such as 3.2, 4.5, etc... Is there an easy way to remember them?

[Bob_A]

Originally Posted by hornetzoo

thanks, and how do you know what a full stop is? my camera has other increments that you have not listed such as 3.2, 4.5, etc... Is there an easy way to remember them?

The following article may help:

http://www.uscoles.com/fstop.htm

The numbers in between are half or third stops.


Edit: On the above webpage the author has a pdf document that could be handy to print out and keep in your camera bag:

http://www.uscoles.com/thirdstops.pdf

[PacAce]

Given t1 = shutter speed at aperture f1, the shutter speed t2 for aperture f2 is (ignore the periods I'm using for spaces):

.. . . . . f2 X f2
t2 = t1 X __________
.. . . . . f1 X f1

[hornetzoo]

does anyone know how long a bulb exposure can be on the 20d without generating an excessive amount of noise?

[Bob_A]

Originally Posted by PacAce

Given t1 = shutter speed at aperture f1, the shutter speed t2 for aperture f2 is (ignore the periods I'm using for spaces):

.. . . . . f2 X f2
t2 = t1 X __________
.. . . . . f1 X f1

Exactly Leo.

[Bob_A]

Originally Posted by hornetzoo

does anyone know how long a bulb exposure can be on the 20d without generating an excessive amount of noise?

I think there have been several discussions about this on this forum. Try seaching for "long exposure noise" or "hot pixels".

Edit: Also, on the 20D there is a custom function to turn long exposure noise reduction on.

[lancea]

On reading this I had a memory flashback, and pulled out my Kodak Master Photoguide (1968 edition). This little book has all sorts of circular calculators, including one for "existing light" photography. It answers the question so well.

Nowdays there's lot of web pages with charts and Java applets. They do work, but they are nowhere near as convenient as this old guide. The Kodak depth of field calculator is as useful as day it was made. Has anyone ever come across an updated version of this guide or similar? Be nice to have one that doesn't use inches

[robertwgross]

Originally Posted by hornetzoo

thanks, and how do you know what a full stop is?

The easy way to remember the numbers is to start with One. Then, for each stop, you multiply by the square root of two. For non-math majors, that is approximately 1.41, and we generally round that off to 1.4, so the numbers progress: 1.0, 1.4, 2.0, 2.8, etc.

---Bob Gross---

[lostdoggy]

On e of the greatest thing about digital photography is that you don't have to wait for the proof prints to comeback from the lab. Bracket the shot -2, 0, & +2. Look at them and see if you are happy. Unless you like to stand there clicking away at the calculator.

[lostdoggy]

One more thing you can also merge the -2 & +2 togehter for a more detail shot.

[ron chappel]

Originally Posted by lancea

Nowdays there's lot of web pages with charts and Java applets. They do work, but they are nowhere near as convenient as this old guide. The Kodak depth of field calculator is as useful as day it was made. Has anyone ever come across an updated version of this guide or similar? Be nice to have one that doesn't use inches

******** YES !!! *******


And not only does this little program work on your computer,it also allows printouts so you can make your own custom circular DOF charts!!!

http://dfleming.ameranet.com/

[lancea]

Excellent! Thanks for that link Ron. And it even does metric - something the Kodak version doesn't do Printed out on glossy paper (or laminated) it would look pretty slick, especially the 4-disc zoom model.

Looks ideal for the next POTN marketing exercise. Depth of field calculators personalised for your lens collection. Uh - I think you'd better go for the Palm computer version Ron ...