What do things like "f4" on lenses mean?

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Cookaaaaay

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Liam
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Hi,

I've been looking at lenses and all of them say things like "f3.5" "f4" "f2" and stuff like that.. what does it mean?

Is it something to do with the focus? If so, what's the difference?

Thanks. :thumbs:
 
Its the 'fastest' aperture of the lens, in other words, the widest the aperture will open.

So, for instance, a 24-70 f2.8 means that the widest aperture right across its zoom range [focal lengths] is f2.8, so its quite fast.

a 70-300 f3.5-5.6 [or similar, thats just an example] would mean that the minimum aperture varies as the focal length changes along its zoom....in that example, starting at f3.5 at the 70mm end, closing down to F5.6 at the 300mm end - this variable aperture is generally found on cheaper lenses as its much cheaper to manufacture.


so, the f number is the aperture [the opening in the lens that allows light in, smaller the number, bigger the hole, 'faster' the lens is] and when seen on a lens description, refers to the lenses widest aperture value.

It sounds confusing but is quite simple once you get your head round 'small number, big hole' and vice versa ;)
 
If you are just starting to think about getting a SLR camera then have a read here it explains a lot of things including a bit about lenses.

Short answer is the f number tells you how much light the lens lets in (aperture size). Lower number = bigger aperture (hole for light), more light and usually more money to buy it. Low f number lenses are best in poor light amongst other things.
 
Yv said:
Its the 'fastest' aperture of the lens, in other words, the widest the aperture will open.

So, for instance, a 24-70 f2.8 means that the widest aperture right across its zoom range [focal lengths] is f2.8, so its quite fast.

a 70-300 f3.5-5.6 [or similar, thats just an example] would mean that the minimum aperture varies as the focal length changes along its zoom....usually f3.5 at the 70mm end, closing down to F5.6 at the 300mm end - this variable aperture is generally found on cheaper lenses.


so, the f number is the aperture [the opening in the lens that allows light in, smaller the number, bigger the hole, 'faster' the lens is] and when seen on a lens description, refers to the lenses widest aperture value.

It sounds confusing but is quite simple once you get your head round 'small number, big hole' and vice versa ;)
Click to expand...

RobertP said:
If you are just starting to think about getting a SLR camera then have a read here it explains a lot of things including a bit about lenses.

Short answer is the f number tells you how much light the lens lets in (aperture size). Lower number = bigger aperture (hole for light), more light and usually more money to buy it. Low f number lenses are best in poor light amongst other things.
Click to expand...

NigelPaul said:
This is an excellent read http://www.talkphotography.co.uk/forums/showthread.php?t=99841
Click to expand...


Oh, ok. Thanks guys! :thumbs:

That's really confusing.
I really don't get it, I'll have to research it loads! :lol:

But basically, the lower the "f" the better it is?

I'll take a look at those links, thanks.
 
It can be hard to explain and get the point across when explaining these things in writing.
Hoppy UK, who wrote the first link I posted on this thread is great at explaining the basics, and I reccommend you search some of his threads posted in this section.

Videos, if made well enough are also a great help. The one I found above is the best I could find related to your question. Glad it was of some use. Took me a while to understand things when I first started, and now I know all the basics I do have trouble explaining it to others.
 
NigelPaul said:
It can be hard to explain and get the point across when explaining these things in writing.
Hoppy UK, who wrote the first link I posted on this thread is great at explaining the basics, and I reccommend you search some of his threads posted in this section.

Videos, if made well enough are also a great help. The one I found above is the best I could find related to your question. Glad it was of some use. Took me a while to understand things when I first started, and now I know all the basics I do have trouble explaining it to others.
Click to expand...

I'll have a look now, cheers. :thumbs:
Yeah, explanations in writing are sometimes hard to understand.
Photography is more complex than I first thought.

Thanks. :thumbs:
 
Not all faster lenses are better . F/4 lenses for one thing are often less flare prone than their f/2.8 cousins, and the faster a lens, the bigger it has to be if it is to cover the same focal lengths.
 
trencheel303 said:
Not all faster lenses are better . F/4 lenses for one thing are often less flare prone than their f/2.8 cousins, and the faster a lens, the bigger it has to be if it is to cover the same focal lengths.
Click to expand...

Ohhh, ok. Thanks.:thumbs:
 
Cookaaaaay - getting to know f-stops is two-fold:

Firstly, the aperture (f-stop/number) is what lets light through the lens. The lower the number, the wider the aperture hole so more light is let in to expose the film/sensor.

Secondly, the lower the f-stop/number, the narrower the depth of field, which relates to the amount of in focus area of the shot.

The f-stop is balanced against the shutter speed and both these are dictated overall by the sensitivity of the film/sensor, the ISO.

For example:

If the f-stop is f/4 and the shutter speed is 1/500th then that's you're exposure.

Alternatively....

If the f-stop is f/8 then the shutter speed should be 1/125th.

This is a pretty good chart to show you how exposures are worked out in steps (generally referred to as stops, relating to f-stops):

Ken-Storch-The-Lists--ISO-Shutter-Speeds-Fstops.gif


Massively complicated subject but soooooo rewarding when you get your head around it.
 
specialman said:
Cookaaaaay - getting to know f-stops is two-fold:

Firstly, the aperture (f-stop/number) is what lets light through the lens. The lower the number, the wider the aperture hole so more light is let in to expose the film/sensor.

Secondly, the lower the f-stop/number, the narrower the depth of field, which relates to the amount of in focus area of the shot.

The f-stop is balanced against the shutter speed and both these are dictated overall by the sensitivity of the film/sensor, the ISO.

For example:

If the f-stop is f/4 and the shutter speed is 1/500th then that's you're exposure.

Alternatively....

If the f-stop is f/8 then the shutter speed should be 1/125th.

This is a pretty good chart to show you how exposures are worked out in steps (generally referred to as stops, relating to f-stops):

Ken-Storch-The-Lists--ISO-Shutter-Speeds-Fstops.gif


Massively complicated subject but soooooo rewarding when you get your head around it.
Click to expand...

Well.. that just confused me again. :lol:

Dredd123 said:
IMHO the OP could do a lot worse than get hold of the ubiquitous "Understanding Exposure"

http://www.amazon.co.uk/Understandi...9390/ref=sr_1_1?ie=UTF8&qid=1291426800&sr=8-1
Click to expand...

Thanks, I'll take a look.

NigelPaul said:
I like Bryan's videos more. Here's a useful one: http://www.youtube.com/watch?v=Q8NEa-ghHbo
Click to expand...

Thanks, that's great! I'll go and watch it now. :D
 
You could do a lot worse than watch a lot of Bryan Peterson's you tube videos.
 
Cookaaaay - apologies. Just looked at it again and gobeldy-gook without and context or reference point :D
 
Liam, I know it's confusing at first. It was for me, but after some time I just "got" it - and I am really really poor at maths. You'll eventually begin to realise what means what and subconciously work out the numbers. There's a rule called Sunny 16 which basically stipulates that on a sunny day, at f/16, your shutter speed should be reciprocal of your ISO speed, so if you are at f/16 ISO 100 - your shutter speed should be at 1/100 to give a good exposure in sunlight - and it mostly works.

If we keep the ISO the same, but change the f/ stop number, we will have to change the shutter speed to compensate. So...

at ISO 100, f/16 we should have 1/100 shutter speed.
at ISO 100, f/11 we should have 1/200 shutter speed.
at ISO 100, f/8 we should have 1/400 shutter speed.
at ISO 100, f/5.6 we should have 1/800 shutter speed.
at ISO 100, f/4 we should have 1/1600 shutter speed.

All of the factors, ISO, shutter speed and lens aperture work in relation to one another. Think of it as a triangle - you change one of the numbers, and in order to keep the exposure the same, you MUST change at least one of the other numbers, or if you want to you can change both.

Knowing this becomes important in situations where you need explicit control over not just the exposure, but the depth of field (which is controlled by the lens aperture) or whether you want to freeze (or indeed blur) subject motion, which is related to shutter speed. If I'm out in the sun and I want a shallow depth of field, I know I'll want to open my lens up, but that can potentially create problems as you will have to really ramp up your shutter speed into the thousands. Luckily both my cameras allow a max shutter speed of 1/8000, so I can usually get away with shooting even at f/2 in bright sunlight (refer to the sunny 16 rule to check that).

If I wanted to freeze the movement of people, I'd have to increase my shutter speed - normally a minimum of 1/200 ish does it for me. Depending on the light, I'd have to open up my lens aperture, up my ISO, or both.

Sometimes, there are situations where you just don't have enough light no matter how much you up your ISO or open up your lens, and this is where flash can be useful. Then there are situations where in bright light you want to slow down your shutter speed more than is possible, and for this you can use ND filters, which is essentially sunglasses for the camera.

Sunny 16 is a good one to remember, but most importantly don't forget the relationship between all 3 of the exposure factors.
 
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trencheel303 said:
Liam, I know it's confusing at first. It was for me, but after some time I just "got" it - and I am really really poor at maths. You'll eventually begin to realise what means what and subconciously work out the numbers. There's a rule called Sunny 16 which basically stipulates that on a sunny day, at f/16, your shutter speed should be reciprocal of your ISO speed, so if you are at f/16 ISO 100 - your shutter speed should be at 1/100 to give a good exposure in sunlight - and it mostly works.

If we keep the ISO the same, but change the f/ stop number, we will have to change the shutter speed to compensate. So...

at ISO 100, f/16 we should have 1/100 shutter speed.
at ISO 100, f/11 we should have 1/200 shutter speed.
at ISO 100, f/8 we should have 1/400 shutter speed.
at ISO 100, f/5.6 we should have 1/800 shutter speed.
at ISO 100, f/4 we should have 1/1600 shutter speed.

All of the factors, ISO, shutter speed and lens aperture work in relation to one another. Think of it as a triangle - you change one of the numbers, and in order to keep the exposure the same, you MUST change at least one of the other numbers, or if you want to you can change both.

Knowing this becomes important in situations where you need explicit control over not just the exposure, but the depth of field (which is controlled by the lens aperture) or whether you want to freeze (or indeed blur) subject motion, which is related to shutter speed. If I'm out in the sun and I want a shallow depth of field, I know I'll want to open my lens up, but that can potentially create problems as you will have to really ramp up your shutter speed into the thousands. Luckily both my cameras allow a max shutter speed of 1/8000, so I can usually get away with shooting even at f/2 in bright sunlight (refer to the sunny 16 rule to check that).

If I wanted to freeze the movement of people, I'd have to increase my shutter speed - normally a minimum of 1/200 ish does it for me. Depending on the light, I'd have to open up my lens aperture, up my ISO, or both.

Sometimes, there are situations where you just don't have enough light no matter how much you up your ISO or open up your lens, and this is where flash can be useful. Then there are situations where in bright light you want to slow down your shutter speed more than is possible, and for this you can use ND filters, which is essentially sunglasses for the camera.

Sunny 16 is a good one to remember, but most importantly don't forget the relationship between all 3 of the exposure factors.
Click to expand...

Cheers. :thumbs:
 
EdBray said:
You could do a lot worse than watch a lot of Bryan Peterson's you tube videos.
Click to expand...

Read this and watched a few last night. Very good. :thumbs:

Was he in the rolling stones at any point...? :lol:
 
i don't know if this helps at all.....

Imagine you have a container within which there is a fixed volume of liquid.

That is the equivalent of the "correct exposure".

If you change one dimension of the container - say by doubling it - then to maintain the same volume of liquid, you have two alternatives - to change one or other of the other two dimensions, halving one or the other. You have a choice.

It is the same with exposure. If you double the shutter speed, you can halve either the aperture or the "film speed" (now "sensitivity") to maintain correct exposure.

:)
 
Ooo i quite like that simulator , I can see me having a good long play around with that .
 
It's a ratio - the focal lenght divided by the f number which gives you the size of the hole (aperture)letting light in, so on a 50mm lens for example f4 would be 1/4 of 50 = 12.5mm hole for light to go through, and an f2 version would be a 25mm hole for light to go through, which is why they are better in low light as you can allow more light in. It also explains why a 300mm f2.8 is massive (over 100mm!), because the hole is so big the lens obviously has to be bigger, and are therefore more expensive.

Turning the aperture ring opens and closes the hole, so you can adjust the amount of light coming in to suit the conditions.

On modern cameras the aperture is held wide open till you press the shutter, regardless what size you want it to be eventually, as more light means easier focussing for the camera. For this reason you'll find that cheap lenses are poor in low light conditions as they just can't let enough light in for the camera to focus...
 
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