F Stops

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jameslongstrong

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James
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This may seem really obvious to some but I can't see to work it out.

If a lens is say, f1.8 then the apeture is obviously f1.8.

If a lens is say, f3.5-5.6, then why can I set it to f29 when in manual setting?


Is this because of the Shutter speed? For example if the shutter speed is 30 seconds then the f stop would need to be this high to get the shot? If this is the case (which it may not be) then why give a limit of the f stop from f3.5-5.6?
 
The f-number quoted for lenses is the maximum aperture it can achieve, not the only one it can do.

Your f/1.8 lens (I'm guessing a 50mm prime) has a fixed maximum aperture - it can open up as far as f/1.8, but it can probably close down as small as f/22. Depending on what you're doing for a particular photograph, you can choose any aperture in between those two, usually in one-half or one-third stop increments.

Your f/3.5-f/5.6 lens has a variable maximum aperture, dependent on the focal length (zoom) that you have it set at. Very roughly, longer focal length = darker.

Taking the Canon EF 28-135mm f/3.5-5.6 IS lens as an example, it can open up to f/3.5 at 28mm, but when you zoom to 135mm, the widest it can open is f/5.6.

This has a knock-on effect at the other end of the aperture range. At 28mm, the smallest aperture it can achieve is f/22. At 135mm, it can achieve f/36.

the-digital-picture.com has a table of the maximum apertures this lens can open to, together with the focal length ranges applicable in between.

FWIW, for zoom lenses, it is usually the more expensive ones that provide a fixed maximum aperture throughout their zoom range - the benefit being that you know that f/2.8 is always available to use whether you're at 70mm or 200mm, for example.

The shutter speed is irrelevant to your ability to physically dial-in any particular aperture setting on the lens, though it will obviously affect the exposure of the final picture.
 
The numbers stated are the maximum aperture of the lens (the largest it will open to).

THe f1.8 lens can stop down to any aperture from f1.8 to smaller apertures like f16, f22 and maybe f29 (each lenses smallest aperture may be different).

The differece with slower lens you mention is that the max aperture varies depending on the focal length. Lets say it's 18-55. At 18 the max aperture will be f3.5 but will get smaller as you zoom out.

Large apertures like f1.8 or f3.5 mean faster shutter speeds. That's why fast lenses are expensive.

If all you do is shoot landscapes at f16 then fast lenses are not really a concern - quality is.

Does that make sense? Both lenses can stop down.
 
Amazing how much our posts are the same when typing at the same time :)
 
I was just thinking exactly the same thing :)

[I think we'd better stop the echoes there before it gets too eerie]
 
lol - Great minds and all that :)
 
Cheers guys, This has cleared it up for me. The problem I had was when using my camera then I know in my head what does what etc. Unfortunately I was at work yesterday and someone picked my brains, I found I was unable to explain why things happened the way they did although I could explain how to do things (If that makes sense?).

He is basically wanting to do some light painting and was looking for a lens to get with his soon to be purchased 550D. He was thinking that a lens which was say f3.5-5.6 would only achieve this range of apertures. I knew that you could put the 18-55mm canon kit lens up to f22 but couldn't explain to him why, (I even went home and tried it to prove I wasn't going mad!). So when he was asking me which lens to get, to him, unless he could achieve f8 for a slower shutter speed then it would not suit his needs.

I think I'm just going to have to recommend this site to him!
 
YOu should show him your camera.
Put it on Aperture priority mode and then turn the dial that changes the F number, also do it across different focal ranges so he can see the largest aperture achievable but also just how small it will go too.
Heres an example (albeit a crap picture) of the canon kit lens (18-55mm IS) going down to a whopping F36 on a 5 second exposure.
showphoto.php
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This may be a stupid question, however recently I went to visit my parents on the Sussex Coast, south East UK.

HOW do I take the misty shots of the sea correctly.

I know how to slow the camera to get the right effect but it appears to be over-exposing?
 
Adie Donoghue said:
This may be a stupid question, however recently I went to visit my parents on the Sussex Coast, south East UK.

HOW do I take the misty shots of the sea correctly.

I know how to slow the camera to get the right effect but it appears to be over-exposing?
Click to expand...

If you are slowing the shutter speed then it is letting in lots of light.. so you need to narrow the aperture to compensate (ie, f22 or f45)

If you already have a narrow aperture (high number) selected then using a Neutral Density filter will help lower the exposure at those numbers you need for the shot.

HTH
 
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