# diaphragm

Started by herwin, Jan 31 2003 05:43 AM

10 replies to this topic

### #1

Posted 31 January 2003 - 05:43 AM

when in manual mode, it is kind of convenient to know about diaphragm; doubling the shutter value requires half of the diaphragma; so F8 with 1/200 seconds, can be changed into F11 with 1/100 seconds (more depht.) so the half of F22 is F16, half of F16 is F11, half of F11 is F8. but then ?? i want to know up to F2. how can i calculate?

### #2

Posted 31 January 2003 - 06:24 AM

http://www.davis.k12...oto/Fstops.html

Remeber, it is half the area, not necessarily half the number. Above link is a nice article on your question.

f/1 f/1.4 f/2 f/2.8 f/4 f/5.6 f/8 f/11 f/16 f/22 f/32 and on to infinity and beyond. For those who don't want to look up the site.

Remeber, it is half the area, not necessarily half the number. Above link is a nice article on your question.

f/1 f/1.4 f/2 f/2.8 f/4 f/5.6 f/8 f/11 f/16 f/22 f/32 and on to infinity and beyond. For those who don't want to look up the site.

### #3

Posted 31 January 2003 - 07:06 AM

Remember, even the smallest of holes in a diaphram can lead to unwanted artifacts.

### #4

Posted 31 January 2003 - 07:08 AM

I wasn't even going to touch that one...

### #5

Posted 31 January 2003 - 07:08 AM

Would that be a technique or a tip, Bob?

*I love it when a plan comes together.*

- Col. John "Hannibal" Smith

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### #6

Posted 31 January 2003 - 07:11 AM

Deffinately a tip!

### #7

Posted 31 January 2003 - 07:19 AM

wow, now theres a scary thought!

### #8

Posted 31 January 2003 - 11:35 AM

thanks for the answer!

### #9

Posted 08 February 2003 - 09:12 AM

Here's an answer from someone who's pretty good in math but not necessarily all that knowledgable about cameras. So

Someone said above, "remember it's half the area, not necessarily half the number." Certainly true, but that doesn't directly answer the question, "How can I calculate?"

The first little problem is that as the f/stop number gets bigger, the amount of light getting through is less. A help here is to think of the f/stop number as the denominator of a fraction that relates to the diameter of the diaphragm, like f/2 is a diameter of 1/2, f/8 is a diameter of 1/8, etc. Smaller diameter, less light.

Now, the amount of light getting through is proportional to the area of the diaphragm opening. To double the area and light (to multiply it by 2), you have to multiply the diameter by the square root of 2 (or 1.414-ish).

So each full f/stop is derived from the previous one by multiplying by 1.414-ish and rounding a bit. Just remember that multiplying gives you a bigger number so you're really reducing the amount of light by 2x, not increasing.

f/4 x 1.414 = f/5.656 (mfr round to 5.6) = half the exposure of f/4

f/8 x 1.414 = f/11.312 (mfr round to 11) = half the exposure of f/8

etc.

*don't*hammer me if I get a camera detail wrong, OK?Someone said above, "remember it's half the area, not necessarily half the number." Certainly true, but that doesn't directly answer the question, "How can I calculate?"

The first little problem is that as the f/stop number gets bigger, the amount of light getting through is less. A help here is to think of the f/stop number as the denominator of a fraction that relates to the diameter of the diaphragm, like f/2 is a diameter of 1/2, f/8 is a diameter of 1/8, etc. Smaller diameter, less light.

Now, the amount of light getting through is proportional to the area of the diaphragm opening. To double the area and light (to multiply it by 2), you have to multiply the diameter by the square root of 2 (or 1.414-ish).

So each full f/stop is derived from the previous one by multiplying by 1.414-ish and rounding a bit. Just remember that multiplying gives you a bigger number so you're really reducing the amount of light by 2x, not increasing.

f/4 x 1.414 = f/5.656 (mfr round to 5.6) = half the exposure of f/4

f/8 x 1.414 = f/11.312 (mfr round to 11) = half the exposure of f/8

etc.

### #10

Posted 08 February 2003 - 03:36 PM

I never knew the correlation of the numbers. The only problem is that we are dealing with reciprocals and as the number goes up the area is going down. So I believe you are multiplying it by 1/1.414 . F4 is 1/4 multiply that by 1/1.414 = +1/5.6 which is F5.6. It still works but it's just a bass ackwards way of thinking about it.

The math works, are we correct in all this?

The math works, are we correct in all this?

### #11

Posted 09 February 2003 - 08:19 PM

Yep... Good posts!

Cheers

James

Cheers

James

Canon 1DsMkIII - Seacam Housing

Dual Ikelite Strobes

Photo site - www.reefpix.org

Dual Ikelite Strobes

Photo site - www.reefpix.org