# Thread: Need help on f-stop calculations

1. ## Need help on f-stop calculations

Hi... was puzzled on how people calculate f-stop value?
For example a f5.6 + 2 stops equals to wat no.? what is the formula used?

3. ## Re: Need help on f-stop calculations

thx for the infor...

4. ## Re: Need help on f-stop calculations

Originally Posted by Keiichi
Hi... was puzzled on how people calculate f-stop value?
For example a f5.6 + 2 stops equals to wat no.? what is the formula used?
Every +2 stops is x2 to the f-number. Every +1 stop is roughly x1.4 (more exactly x square-root of 2) to the f-number. After a while, the progression (+1 stop each time) 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22 should come naturally to you.

5. ## Re: Need help on f-stop calculations

Originally Posted by Keiichi
Hi... was puzzled on how people calculate f-stop value?
For example a f5.6 + 2 stops equals to wat no.? what is the formula used?

I guess for stuff like F-stops, the easiest way to get around them is to memorize the whole range? It'll come to you after a while

6. ## Re: Need help on f-stop calculations

When people say '+' I usually think to open up, not stop down. So my answer would be f2.8. You should clarify your '+' is to let in more or less light, cheers.

7. ## Re: Need help on f-stop calculations

Originally Posted by foxtwo
When people say '+' I usually think to open up, not stop down. So my answer would be f2.8. You should clarify your '+' is to let in more or less light, cheers.
hmm...good point

8. ## Re: Need help on f-stop calculations

Originally Posted by foxtwo
When people say '+' I usually think to open up, not stop down. So my answer would be f2.8. You should clarify your '+' is to let in more or less light, cheers.
strobe power is metered in f-stops as well. f11 would be 'correct' in that case - two stops more output. really depends on context.

9. ## Re: Need help on f-stop calculations

Originally Posted by Daedalus Trent

I guess for stuff like F-stops, the easiest way to get around them is to memorize the whole range? It'll come to you after a while
Then there's stuff like 1/2 stops or 1/3 stops in camera settings, which makes it even more confusing.

If I'm not wrong, for 1/3 stops, it goes something like this: (correct me if I'm wrong)

f1, 1.1, 1.2, f1.4, 1.6, 1.8, f2.0, 2.2, 2.5, f2.8, 3.2, 3.5, f4, 4.5, 5.0, f5.6, 6.3, 7.1, f8, 9, 10, f11, 13, 14, f16, 18, 19, f22

10. ## Re: Need help on f-stop calculations

Originally Posted by Garion
Then there's stuff like 1/2 stops or 1/3 stops in camera settings, which makes it even more confusing.

If I'm not wrong, for 1/3 stops, it goes something like this: (correct me if I'm wrong)

f1, 1.1, 1.2, f1.4, 1.6, 1.8, f2.0, 2.2, 2.5, f2.8, 3.2, 3.5, f4, 4.5, 5.0, f5.6, 6.3, 7.1, f8, 9, 10, f11, 13, 14, f16, 18, 19, f22
Garion, I was reading and following this thread. Your this information is very enlightening to me. I have never thought of it in such an exact way. Thank you.
I have a question here: do these terms 'increase by 1/3 stop, 1 stop, 2 stops' appply to , say, shutter speed and ISO too?

11. ## Re: Need help on f-stop calculations

Originally Posted by limwhow
I have a question here: do these terms 'increase by 1/3 stop, 1 stop, 2 stops' appply to , say, shutter speed and ISO too?
yes. for ISO settings, "one stop faster" is equivalent to doubling the ISO number, eg. 400->800. for shutter speed, "one stop faster" means you should double the exposure time, eg. 1/30s to 1/15s.

12. ## Re: Need help on f-stop calculations

Originally Posted by varf
yes. for ISO settings, "one stop faster" is equivalent to doubling the ISO number, eg. 400->800. for shutter speed, "one stop faster" means you should double the exposure time, eg. 1/30s to 1/15s.
I see. So in summary, 'Increasing by 1 stop' means essentially doubling it for shutter speed and ISO, but in the case of aperture, it means multiply by 1.4. Have I got it correct?

13. ## Re: Need help on f-stop calculations

Originally Posted by Garion
Then there's stuff like 1/2 stops or 1/3 stops in camera settings, which makes it even more confusing.

If I'm not wrong, for 1/3 stops, it goes something like this: (correct me if I'm wrong)

f1, 1.1, 1.2, f1.4, 1.6, 1.8, f2.0, 2.2, 2.5, f2.8, 3.2, 3.5, f4, 4.5, 5.0, f5.6, 6.3, 7.1, f8, 9, 10, f11, 13, 14, f16, 18, 19, f22
yea! Totally!

I think just memorizing the full-stops can already lah

14. ## Re: Need help on f-stop calculations

actually just rem the FULL stops can liaoz.. 1/2 stops usually not necessary and typically only "occur" if you use AUTO.

15. ## Re: Need help on f-stop calculations

Basically F stop number is focus distance / diameter of aperture

This math works in a way which if you increase a stop, the diameter of the aperture doubles and more light enters.
Example f1.4 - F2.0

This is also the reason why photographer uses stop for exposure. Increase 1 stop of light means double the amount of light.

16. ## Re: Need help on f-stop calculations

To put it in a physical context, everytime you go UP a stop, you are letting in twice as much light (with regards to aperture or shutter speed).
That is, you are opening the shutter twice as long (1/30s --> 1/15s) or doubling the size of the aperture (f/2 --> f/2.8)

ahhh, but why 2 --> 2.8? Shouldn't it be 4?

the formula for area of a circle is PI - r squared. [secondary, or was it primary school maths?]
Multiply PI x 2(squared) vs PI x 2.8(squared) and one result is double the other.

So the diameter (or radius, no difference in this context) of the circle increases by a multiple of approximately 1.4, but the area of the circle doubles.

hope that helps.

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