F-stops Explained

[TCampbell]

When I explain f-stops (aperture), I sometimes point out that while it does seem confusing that the dial has these funny and somewhat arbitrary progression of values (1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22 ... ) there is a reason for them that makes sense once you know it. It might seem easier to just use values like 1, 2, 3, 4... but those would actually be arbitrary values that don't have any true meaning.

Sparky is right in that the numbers indicate the focal ratio... the ratio of the effective aperture divided into the effective focal length (and we say "effective" and not "physical" because modern lens optics allows for effective focal lengths that don't match physical lengths.)

But Josh is right in that reason we don't just pick *any* arbitrary ratios is because those particular ratios all happen to allow twice (or half) of the light through because the area of the opening is changing by that amount. Josh did make the error in that he multiplied the square root of 2, but it's actually being raised by "powers".

√2 is approximately 1.4.
√2[SUP]2[/SUP] is simply "2" again.

In photography, the values are all rounded for simplicity.

The progression is:
√2[SUP]0[/SUP] = 1
√2[SUP]1[/SUP] = 1.4
√2[SUP]2[/SUP] = 2
√2[SUP]3[/SUP] = 2.8
√2[SUP]4[/SUP] = 4
√2[SUP]5[/SUP] = 5.6
√2[SUP]6[/SUP] = 8
√2[SUP]7[/SUP] = 11 (really it'd be 11.2 but it's rounded because they never use more than 2 numerals in an f-stop)
√2[SUP]8[/SUP] = 16
√2[SUP]9[/SUP] = 22

You can keep going... large view cameras often have f-stops all the way to f/64. Most DSLR lenses go up to at least f/22. Some go up to f/32. I usually don't see f/44 or f/64 on a camera unless it's a large format camera.

 

[NickStevens]

I must be missing something huge here can somebody explain why it matters them being numbered like that.

I mean is that their actualy a bigger difference between say f11 to f16 than their is between 7.1 and 8......
Does that matter...... I mean I think to myself that I'm gonna stop down by two stops two clicks of the dial is two stops...is that all that's important?

Is it doing to improve my photography any by knowing why the numbers are like that?

 

[shefjr]

Well 2 clicks isn't two stops. Most cameras also allow for half stops and thirds of a stop.
For instance, 2.8-4 is a full stop
2.8-3.2 is 1/3 a stop 2.8-3.5 is 2/3 and 2.8-3.4 is 1/2 a stop. At least for Nikon
So that is actually four clicks for one full stop.

 

[NickStevens]

Ummm ok then we'll I still don't get it.... But I don't think it's ever affected my photos lol

 

[480sparky]

..........Is it doing to improve my photography any by knowing why the numbers are like that?

Did anyone ever claim it would?

 

[Helen B]

Sparky is right in that the numbers indicate the focal ratio... the ratio of the effective aperture divided into the effective focal length (and we say "effective" and not "physical" because modern lens optics allows for effective focal lengths that don't match physical lengths.).

In case this causes confusion, 'effective focal length' is just another way of saying 'focal length'. They are the same thing in photography. The 'effective aperture' Tim refers to is actually the entrance pupil diameter, not the effective aperture, which is something quite different. The effective aperture is the aperture (calculated from the focal length and the entrance pupil diameter) with an adjustment for image magnification.

 

[NickStevens]

..........Is it doing to improve my photography any by knowing why the numbers are like that?

Did anyone ever claim it would?

Nope they didn't...... If theirs any benefit I'll try to learn it, if not then I'd rather read a thread where I will learn things

 

[LungFish]

I must be missing something huge here can somebody explain why it matters them being numbered like that.

I mean is that their actualy a bigger difference between say f11 to f16 than their is between 7.1 and 8......

Yes, that's why on most cameras (all?) it would take a bigger adjustment to go from 11 to 16. I actually think it is important to know the difference between big changes and small changes, saves you a lot of time in settings changes.

When I first read Sparkys post I actually got a bit lost because he starts by asking a question about why the common f-stops aren't multiples of 2, then heads in a different direction about what an f-stop actually means. I was expecting something a bit shorter like "area is proportional to the square of the diameter". I read through the rest of the thread nodding at the responses which said it was too complicated. I then went back to read the first post again, and it actually does make perfect sense and isn't that complicated, you've just got to read it without anticipating what the next sentence will be.

 

[480sparky]

Nope they didn't...... If theirs any benefit I'll try to learn it, if not then I'd rather read a thread where I will learn things

Ever wonder why pianists learn and practice scales?

 

[JayJeep]

..........Is it doing to improve my photography any by knowing why the numbers are like that?

Did anyone ever claim it would?

Nope they didn't...... If there's any benefit I'll try to learn it, if not then I'd rather read a thread where I will learn things

Well I liked reading through it. It basically comes down whether or not you want to know where the f stop numbers come from. If you didn't know and don't care to know, don't read it. I've always wondered where the numbers came from and now I know. Does it change anything about how I take photos? No not really, but now I know what the numbers mean. Just because it doesn't interest you doesn't mean you should jump all over the OP about posting something you don't care about.

 

[IByte]

I may have a terrible memory, but I'm fairly certain I never stated that it's required to know this in order to operate a camera. There's not going to be a pop quiz tomorrow. No test at the end of the month. No Finals in order to 'graduate'.

But I studied sooo hard >.<

 

[480sparky]

I may have a terrible memory, but I'm fairly certain I never stated that it's required to know this in order to operate a camera. There's not going to be a pop quiz tomorrow. No test at the end of the month. No Finals in order to 'graduate'.

But I studied sooo hard >.<

OK.. pop quiz just for you.

1. What is the capital of Cleveland?
2. Is there ever a time mattresses are NOT on sale?
3. Where does the white go when the snow melts?
4. When do fish sleep?
5. How come we never hear father-in-law jokes?
6. Why is Greenland called Greenland when it's mostly covered with ice, and Iceland is called Iceland when it's mostly green?
7. If a tree falls in the forest, and it&#8217;s not posted on MySpace, YouTube, Facebook or Twitter, did it really happen?
8. Is it true that the only difference between a yard sale and a trash pickup is how close to the street the stuff is placed?
9. Why is it that when you transport something by truck, it's called a shipment, and when you transport it by ship, it's called a cargo?
10. If "practice makes perfect" and "nobody is perfect", why bother practicing?

 

[thunderkyss]

I think understanding the math behind f-stops helps understand why an image taken with a 200mm lens would look different on a 100mm lens even if they were framed the same. Knowing the math also helps me understand depth of field. It also gives me something else to consider when shopping for lenses instead of the, "the more expensive one is better" or, "Nikon lenses are just better" line of thinking.

 

[480sparky]

.......It also gives me something else to consider when shopping for lenses instead of the, "the more expensive one is better" or, "Nikon lenses are just better" line of thinking.

Maybe I'm missing something here...... What does the math behind f-stop have to do with lens quality?

 

[thunderkyss]

.......It also gives me something else to consider when shopping for lenses instead of the, "the more expensive one is better" or, "Nikon lenses are just better" line of thinking.

Maybe I'm missing something here...... What does the math behind f-stop have to do with lens quality?

I didn't say quality.

But understanding f-stop I know why an f1.8 55mm lens would cost more than an f3.5 55mm lens. Sure, knowing the f1.8 lets in more light than the f3.5 should be enough. But I also know how difficult it would be, & why, to build a 100mm f1.8 or f3.5