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Old Oct 20, 2007, 8:21 AM   #11
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Harj - you probably already know this but for those folks who don't,the reason the relationship between f-numbers (f-stops)and shutter speeds or ISOs tends to be a bit confusing, is because the f-number of a lens is related to its diameter (actually it is the ratio of the diameter to the focal length). Because the amount of light captured by a lens is related to its area and area is pi-r-squared, the f-number sequence for reducing the amount of light in half doesn't progress by doubling the f-number; because of the radius-squared relationship, doubling the f-number reduces the amount of light to a fourth.

It turns out that if you do the math, the following sequence of f-numbersreduces the light in half sequentially: 1.4, 2.0, 2.8, 4, 5.6, 8, 11, 16, 22. There are other sequences of f-numbers that would do the same thing but thissequence is the one that has traditionally been used.

Sooo, going from an f/2 lens to an f/2.8 lenscuts the full-aperture light gathering in half and thus for the same exposure requires the ISOto be doubled or the shutter speed to be cut in half (i.e. the shutter stays open twice as long).

Ted
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Old Oct 20, 2007, 10:18 AM   #12
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Hi Jim/Ted

thanks for that - i was never sure how it worked but thats been really helpful indeed.

Cheers

HarjTT

:O :?


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Old Oct 20, 2007, 10:34 AM   #13
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Here's an online exposure calculator you may find handy. You can check an options box to give you 1/3 and 1/2 stop increments, too. Film Speed in the calculator is the same thing as ISO Speed. Light probably runs at an EV of around 7 in a high school gym with good lighting.

http://www.robert-barrett.com/photo/...alculator.html

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Old Oct 20, 2007, 11:11 AM   #14
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You're welcome, Harj. And if you want to see the actual calculations (and more explanation) about f-stops go here:

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

BTW, all of this would indicate that for low light imaging, the upcoming 14-35mmF/2 ZD would seem to be attractive. And f/2 always is, but28-70mm (equivalent) isn't nearly as useful a focal range as my marvelous Vario-Elmarit 14-50mm f/2.8-3.5 and to get that range in an f/2 lens will make the lens much larger, heavier, and way more expensive. As I said, the best bet is to stay away from low-light imaging...

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