F stop

jackdaw

Hi
Im a little confused over F stops ..

you could have a lens for example 18-55mm , and F4/5.6 ??

yet you can still set the F number from 3 to 32 !!!
while taking shots ?

is this the same F value (Aperture) ?
as it specifies on the lens ?

pete4130

If the lens is an 18-55 /4-5.6 then that means the widest aperture it can have at 18mm is f4.0 and as you zoom in towards 55mm the widest aperture will get smaller reaching f5.6 at 55mm. You can still stop down the lens (make the aperture smaller such as f32) at any focal length.

Alun

pete4130 wrote: »

You can still stop down the lens (make the aperture smaller such as f32) at any focal length.

But not f3 as the OP stated!

jackdaw

Alun wrote: »

But not f3 as the OP stated!

no F3 was just an example, so my question has been answered ..

this is the same value , thanks ...

I was thinking it must be different ... so the f/4/5.6 are the maximum values at the 18-55 values ?

so a good lens would be something like f4/5.6 as it can go to 5.6 at 55mm ?
where with bad lenses they may be only able to reach f8 at 55mm ??
(for example) ??

Paulw

A fast lens would go down to f/2.8 (or lower), but a slow lens would be f/5.6

Most lenses would go to f/22 but some as far as f/32.

It really depends on what you want or need.

jackdaw

so to be 110% sure this F stop is the same value on the lens
that you can also toggle taking photos ??


and why then does it not give its ranges at 18 and 55 ??

for example at 18 f4 - f32

at 55 f5 - f22 ??

Paulw

The lens will give only the lowest values possible. So, you can have a prime lens, like 50mm f/1.4 or a zoom lens like 70-200mm f/2.8 or another zoom such as 100-400mm f/4.5-5.6. All of these lenses will open up to f/22 or greater, but this value will not be listed on the lens. The lowest value will only be displayed on the lens.

For your lens, at 18mm you will get f/4 and zoomed in to 55mm will only give you f/5.6, but you can still open the lens to f/22 at either range.

Clearer??

pete4130

Expensive lenses have wider/bigger apertures, for example a 70-200mm f2.8 lens that can have an aperture of 2.8 when at 70mm all the way through to 200mm. These lenses are more expensive as they require much higher quality glass. Generally kit lenses such as the 18-55 f4-5.6 lens is ok for general use.
As a rule, cheaper lenses will have a smaller aperture when they are working at a longer focal length (55mm on the 18-55mm lens for example).

A reason why people would need f2.8 at 200mm would be to maximise the amount of light coming through the lens, so they would get faster shutter speeds at the longer focal lengths (as camera shake when handheld gets amplified at longer focal lenghts). The faster shutter speed would mean that the image would be frozen and camera shake would not cause the image to be lurred.

And yes, the F numbers on the lens are the same as theones you can toggle though on the camera. Older more manual cameras you would have had to change the aperture on the lens physically. With modern cameras they keep the aperture as wide open as possible (to help autofocus and composition), and you set the aperture through your camera. When you go to take the picture the camera & lens electronically closes the diaphram in the lens to the aperture you had set just before the shutter is released.

Hope this has cleared things up for you?

Pete.

jackdaw

pete4130 wrote: »

Expensive lenses have wider/bigger apertures, for example a 70-200mm f2.8 lens that can have an aperture of 2.8 when at 70mm all the way through to 200mm. These lenses are more expensive as they require much higher quality glass. Generally kit lenses such as the 18-55 f4-5.6 lens is ok for general use.
As a rule, cheaper lenses will have a smaller aperture when they are working at a longer focal length (55mm on the 18-55mm lens for example).

A reason why people would need f2.8 at 200mm would be to maximise the amount of light coming through the lens, so they would get faster shutter speeds at the longer focal lengths (as camera shake when handheld gets amplified at longer focal lenghts). The faster shutter speed would mean that the image would be frozen and camera shake would not cause the image to be lurred.

And yes, the F numbers on the lens are the same as theones you can toggle though on the camera. Older more manual cameras you would have had to change the aperture on the lens physically. With modern cameras they keep the aperture as wide open as possible (to help autofocus and composition), and you set the aperture through your camera. When you go to take the picture the camera & lens electronically closes the diaphram in the lens to the aperture you had set just before the shutter is released.

Hope this has cleared things up for you?



Pete.

Crystal !! Thanks guys !!!

to be honest I have been embarassed to ask people since I have an EOS 400D
I take good photos but so far havent been understanding a lot of the settings
so have been getting the results through trial & error...
now above info is really helpful

ike

Never be too embarrased to ask! - we all have to start learning somewhere. The great thing i find about photography is that other photographers always seem willing to pass on knowledge

magicbastarder

pete4130 wrote: »

As a rule, cheaper lenses will have a smaller aperture when they are working at a longer focal length (55mm on the 18-55mm lens for example).

to expand on this, the reason the maximum aperture changes is that f-stop is the relationship between focal length and actual aperture size - so f4 at 55mm is actually a physically wider aperture than f4 at 18mm. so its easier to manufacture a wide aperture in a given lens at its wide end than it is at its long end.