Really basic question!

magicmonkey · Feb 22, 2006

Now I know this has been asked and answered a few times before, I have searched the forums and other web sites as well but I've never found an explanation that made all that much sense to me so I was hopeing that someone here might be able to give it a try.

The problem is that I don't really understand f stops. Firstly, what does the 'f' actually stand for? Secondly, how does it work and what does it effect? Thirdly, how is this related to the aperture?:blushing:

I am approaching digital from a very non technical (photography wise) background and it's turning into a bit of a steep learning curve, even more so than paintshop! Any help much appreciated and I apologise for repeating a question you've all been asked before:blushing:

darich · Feb 22, 2006

first things first - the f stop IS the aperture so they're not only related, they're one in the same!!
I don't know what it stands for so i can't help on that front.

The effect it has on photos is quite important and can make or break a photo.

The smaller the f number eg 2.8, 3.5 etc then the shallower the depth of field. in other words the less of the image will be in focus. this can work well for portraits or product shots but no use for landscapes (who wants a landscape with sharp hills in the background and blurred trees in the foreground?)
A large f number means more of the photo is in focus eg f32 or f36. ideal for landscapes but not so good for portraits.

When a small f number is used the aperture is very large. this means that a lot of light can reach the sensor or film in a short time. Ideal for sports or action shots. The aperture has to be very fast to open very wide and close again in a short time eg 1/1000 hence a lens with f2.8 ability is called "fast" Check the sports pages of your local paper - you'll probably find that the players will be sharp and the background very blurred. (large aperture)

When a large f number is used the aperture is small so the exposure must be longer to allow more light to reach the film or sensor. for shots using apertures like f32 you'll most likely need a tripod because the exposure will be longer than you can hand hold - could be several seconds long. Check posters for sale with landscapes - they'll be sharp from forground right through to the back because the f stop will have been large (small aperture)

Hope this is helpful...but if anyone can tell me what "f" stands for i'd be interested to know!!!

cecilc · Feb 22, 2006

magicmonkey said:
Firstly, what does the 'f' actually stand for? Secondly, how does it work and what does it effect?

F-stop is the focal length divided by the diameter of the lens (basically, it turns into a math problem). For example, a 200mm f/4 lens will be 50mm wide. A 200mm/50mm = f/4. That is why f-stop is typically written as F/4, meaning "focal-length over 4" or "focal-length divided by four".

So, a 200mm f4 lens will need a 50mm filter ..... a 400mm f5.6 will need a 72mm filter ....

It's all in the numbers .....

magicmonkey · Feb 22, 2006

thanks for the explanations guys! Yet another question though, What does it actually do? I mean physically inside the lens, what moves, how does it move and in what way has that affected the light going through the lens? Getting a bit more technical now!

darich · Feb 22, 2006

i already described what it does

it adjusts the size of the aperture. The smaller the number the larger the aperture. And the larger the number the smaller the aperture.

That's how it affects the photo

Rob · Feb 22, 2006

cecilc said:
So, a 200mm f4 lens will need a 50mm filter ..... a 400mm f5.6 will need a 72mm filter ....

Although you are quite correct in theory, in practice filter sizes are not very close to the calculated numbers, especially with zooms as the outside of the barrel is much bigger than the actual glass itself.

A 50mm f1.8 lens should have a width of 27.7mm, but in reality the filter size will be closer to 52mm for 1.8 and 58mm for 1.4 always check the label first, that's what the crossed o is on the inside of the lens cap or the edge of a filter.

Rob

Rob · Feb 22, 2006

magicmonkey said:
thanks for the explanations guys! Yet another question though, What does it actually do? I mean physically inside the lens, what moves, how does it move and in what way has that affected the light going through the lens? Getting a bit more technical now!

It moves blades, or apertures to narrow/widen the opening - it looks a lot like your eye. Get a manual lens and look at it off the camera and move the aperture ring and watch it in action.

Rob

magicmonkey · Feb 22, 2006

Just found an explanation which made a some sense to me, god bless google! Here it is if anyone will find it useful:

Finally the last technical piece for the lens we need to be aware of is the aperture. This is the size of the opening in the lens. It turns out that as the focal length gets larger this opening also needs to get larger in order to let in enough light to expose our image. This is because the amount of light gathered in by a telephoto lens is spread over a larger area thus making the brightness less.

I stop asking so many questions some time soon!

cecilc · Feb 22, 2006

Rob said:
Although you are quite correct in theory, in practice filter sizes are not very close to the calculated numbers, especially with zooms as the outside of the barrel is much bigger than the actual glass itself.

Point taken, Rob .....

But I am a very "theoretical" photographer, you know ....

In "theory", I should take good photos, but in "practice" .....

PlasticSpanner · Feb 22, 2006

cecilc said:
F-stop is the focal length divided by the diameter of the lens (basically, it turns into a math problem). For example, a 200mm f/4 lens will be 50mm wide. A 200mm/50mm = f/4. That is why f-stop is typically written as F/4, meaning "focal-length over 4" or "focal-length divided by four".

So, a 200mm f4 lens will need a 50mm filter ..... a 400mm f5.6 will need a 72mm filter ....

It's all in the numbers .....

This is sort of correct apart from the very last bit!

The F means the focal ratio and is dependant ot the focal legnth ot the lens. The F ratio is controlled by the aperture blades inside the lens.

A 50mm lens at Focal Ratio 1.8 (F/1.8) has a hole (aperture) through the lens created by the aperture blades, of 27.7mm.

The same 50mm lens at F/4 has an aperture through the lens of 12.5mm

At F/8 the aperture is 6.25mm. There are more f/stops between these and you could quite easily calculate them yourself in an Excel spreadsheet for any legnth lens.

Unfortunately, the focal ration of a lens has little to do with the filter size exceptthat the larger the aperture inside the lense, the larger the glass has to be at the front of the lens and so the bigger filter it will need.

Also a fast lens is called such because it lets light in at a fast rate and has nothing to do with how fast the aperture blades close since the aperture is fully open for metering and focussing then closes for the shot, whic at a low F number, isn't much movement! They obviously have to move at quite a snappy speed for smaller apertures like F/22 since the blades have to reduce the aperture by a significant amount in the same amount of available time.