what does the f number mean ?

MrcValets

hey guys

i understand shutter speed but i cant understand teh f number . what would be the difference between shooting with a f5 and say a f18 ??

thanks

marty

charybdis

I appreciate that you're trying to learn, but this is really the sort of thing you should use Google for.

GavinZac

It's a ratio: 1:1.7 means that 4/5ths (roughly) of the light gets into the sensor - 1:14 means only 1/14th of the light is getting in. It's controlled by the aperture, which looks like this:



Thus, the more open the aperture, the lower (closer to one) the ratio gets.

An effect this has is that with a wider aperture (closer to one) there is more light coming in and it is "harder" to focus on everything. This results in a nice effect where only the subject you're focused on is in focus, everything else melts away.


This shot had an aperture of 1:4; not very short but you can see how only her face is really in focus. You can make out the guys behind her, and the rest is blur (bokeh). If I had used my 1:1.7 prime lens, it's quite likely the guys would also be blur, and maybe even far parts of her face would be out of focus.

sebastianlieken

charybdis wrote: »

I appreciate that you're trying to learn, but this is really the sort of thing you should use Google for.

google? bah! why?

this is a question I myself never would really have bothered to investigate, but now that I know the answer, I'm happy!

charybdis

GavinZac wrote: »

It's a ratio: 1:1.7 means that 4/5ths (roughly) of the light gets into the sensor - 1:14 means only 1/14th of the light is getting in. It's controlled by the aperture, which looks like this:



Thus, the more open the aperture, the lower (closer to one) the ratio gets.

An effect this has is that with a wider aperture (closer to one) there is more light coming in and it is "harder" to focus on everything. This results in a nice effect where only the subject you're focused on is in focus, everything else melts away.


This shot had an aperture of 1:4; not very short but you can see how only her face is really in focus. You can make out the guys behind her, and the rest is blur (bokeh). If I had used my 1:1.7 prime lens, it's quite likely the guys would also be blur, and maybe even far parts of her face would be out of focus.

Most of this is completely wrong (I'm not trying to cause offence, it just is).

sebastianlieken wrote: »

google? bah! why?

this is a question I myself never would really have bothered to investigate, but now that I know the answer, I'm happy!

Well, if an answer is worth knowing then it's probably worth looking for and, as I've pointed out above, you don't know the answer, you have had one person give you a mistaken and wrong account and have now accepted it as fact. That's why you should use Google.

GavinZac

charybdis wrote: »

Most of this is completely wrong (I'm not trying to cause offence, it just is).

Well then you have a decent audience to explain to. To be honest, most of it is a diagram...

That's why you should use Google.

Where every account is from an optics engineer?

To clarify, I'm genuinely requesting a better explanation as this is my working knowledge. I've been on forums long enough to know better than to take offense to anything

Kbeg3

http://tinyurl.com/32w92zq

charybdis

GavinZac wrote: »

Well then you have a decent audience to explain to. To be honest, most of it is a diagram...


Where every account is from an optics engineer?

To clarify, I'm genuinely requesting a better explanation as this is my working knowledge. I've been on forums long enough to know better than to take offense to anything

The diagram is fine (except for how it says "f2" instead of "f/2") but while the gist of the accompanying text is sort-of right, it's really far too inaccurate to be useful.

F-number expresses a ratio between the focal length of a lens and the size of the opening that admits light (or "aperture"). It's expressed as a fraction so that the size of the number is inversely proportional to the size of the denominator, e.g.: f/2 > f/4. F-numbers are usually expressed as a series of approximate powers of the square root of two in the series 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 where each number represents an aperture that admits half as much as the number that preceded it and twice as much as the number that succeeds it. This is useful because it allows us to deal with aperture in terms of stops and establishes a common way we can express changes in exposure between the different exposure parameters.

The difference between f/5 and f/18 is that a lens set to f/5 will admit 12.7 times the amount of light it would when set to f/18, or a difference of 3 & 2/3 stops.

Differences in aperture also influence depth-of-field where, typically (all other things being equal), a larger aperture will produce a shallower depth-of-field, but in reality there are many variables that influence depth-of-field and it must be understood in terms of all of these rather than just aperture.

magicbastarder

charybdis wrote: »

The diagram is fine (except for how it says "f2" instead of "f/2") but while the gist of the accompanying text is sort-of right, it's really far too inaccurate to be useful.

F-number expresses a ratio between the focal length of a lens and the size of the opening that admits light (or "aperture"). It's expressed as a fraction so that the size of the number is inversely proportional to the size of the denominator, e.g.: f/2 > f/4. F-numbers are usually expressed as a series of approximate powers of the square root of two in the series 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 where each number represents an aperture that admits half as much as the number that preceded it and twice as much as the number that succeeds it. This is useful because it allows us to deal with aperture in terms of stops and establishes a common way we can express changes in exposure between the different exposure parameters.

The difference between f/5 and f/18 is that a lens set to f/5 will admit 12.7 times the amount of light it would when set to f/18, or a difference of 3 & 2/3 stops.

Differences in aperture also influence depth-of-field where, typically (all other things being equal), a larger aperture will produce a shallower depth-of-field, but in reality there are many variables that influence depth-of-field and it must be understood in terms of all of these rather than just aperture.

in short, as you're learning - small number, lots of light, shallow depth of field.
large number, less light, much greater depth of field.

so if you want faster shutter speeds or need to throw a distracting background out of focus, go for large aperture (small f-stop number).
if you want as much of the image to be in focus as possible, go for a small aperture (large f-stop number)

GavinZac

magicbastarder wrote: »

in short, as you're learning - small number, lots of light, shallow depth of field.
large number, less light, much greater depth of field.

so if you want faster shutter speeds or need to throw a distracting background out of focus, go for large aperture (small f-stop number).
if you want as much of the image to be in focus as possible, go for a small aperture (large f-stop number)

That's essentially what my post above was trying to get across. I've read my manual and wiki and "photography for dummies" and know that there's some complex equations behind the number itself but in terms of the exposure triangle - shutter speed, ISO and aperture - that's it in a nutshell, right? And each corner of the triangle also has a secondary effect as well as affecting the exposure level:

shutter speed = blur or lack thereof
iso = noise or lack thereof
aperture = depth of field or lack thereof.

I appreciate that charybdis is correct and accurate with the science behind it but I wouldn't have answered with a sort of gist-of, results-orientated explanation if I hadnt thought that's what the OP was going for.

charybdis

GavinZac wrote: »

That's essentially what my post above was trying to get across. I've read my manual and wiki and "photography for dummies" and know that there's some complex equations behind the number itself but in terms of the exposure triangle - shutter speed, ISO and aperture - that's it in a nutshell, right? And each corner of the triangle also has a secondary effect as well as affecting the exposure level:

shutter speed = blur or lack thereof
iso = noise or lack thereof
aperture = depth of field or lack thereof.

I appreciate that charybdis is correct and accurate with the science behind it but I wouldn't have answered with a sort of gist-of, results-orientated explanation if I hadnt thought that's what the OP was going for.

What I really should've said above is that f-number represents a relationship between the focal length of a lens and the diameter of the aperture (what I said was effectively true, but this is more correct).

The most helpful thing in learning about exposure is to understand what a stop is. A stop is a measure of a change in the amount of light in terms of double the amount for an increase and half the amount for a decrease. Exposure parameters are usually expressed and put in a scale in terms of stops. Often, theses scales and expressions will include half, third, or other fractional stops, but the scales usually go something like this (for full "integer" stops):

shutter speed: 1/1000, 1/500, 1/250, 1/125, 1/60, 1/30, 1/15, 1/8, 1/4, 1/2, 1
(Exposure increases with each successive value; note how each time is approximately twice as long as the previous one, and half as long as the next.)

aperture: 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
(Exposure decreases with each successive value.)

ISO: ISO 100, ISO 200, ISO 400, ISO 800, ISO 1600
(Exposure increases with each successive value.)

The secondary effects of the three exposure parameters are more-or-less as you've described, but each effect is influenced by more than just the exposure parameters and will have to be understood individually. As in: you can take photographs with slow shutter speeds that don't show motion blur; you can take photographs at high ISO that don't show excessive noise levels; you can take photographs at wide apertures that don't appear to have shallow depth-of-field.

Might be of help;

http://boards.ie/vbulletin/showpost.php?p=66281916&postcount=28


(that whole thread may be useful)

charybdis

Deleted User wrote: »

Might be of help;

http://rapidshare.com/files/361756209/Rome.S02E10.DVDRip.XviD-FoV.avi


(that whole thread may be useful)

I don't think that link is to what you think it is to.

Fionn

Google might not throw this up straight off - it's a simple tutorial for new DSLR owners starting out.

Canon enjoy dslr

and the subject of Aperture & Depth of Field

charybdis

charybdis wrote: »

I don't think that link is to what you think it is to.

Woops...

Changed it. :pac:

Cheers for the heads up