Question about lens f/stops

foodpoison

foodpoison

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Sean
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I'm not exactly new to photography, and have researched pretty much everything a fair bit, but there are still a few things I'm not quite sure about.

First up, lens f/stops. I'm talking about the number in the name. For example, the 'Canon EF 24-105mm f/4 L IS' lens compared to the 'Canon EF 50mm f/1.2 L USM' lens. I'm assuming, the lower number, the more light the lens can take in, which means you can use higher shutter speeds and aperture because the lens is more sensitive to light/the lens works better in low light conditions?

Second, if I got the above correct, what do f/stops mean when, say for example the 'Canon EF 28-135mm f/3.5-5.6 IS USM' lens, with a 'varying f/stop' number?
I'm assuming at 28mm the f/stop is 3.5 whereas at 135mm the f/stop is 5.6?
 
Joe T said:
Everything you say is correct. :)
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except to be a bit picky, the faster lenses are not more sensitive to light but in effect, take bigger gulps of it in the same amount of time than a slower one. (y)
 
dazzajl said:
except to be a bit picky, the faster lenses are not more sensitive to light but in effect, take bigger gulps of it in the same amount of time than a slower one. (y)
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Not far off then :p

Seems I didn't need to ask the question hehe.

So, third question, why (in general) do telephoto lenses have higher f/stops than wide angle lenses?
 
Not far off then
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Close enough to poke it in the eye with a very short stick. ;)

As for Q3, it's the ratio of lens diameter to length that determines the f stop. Fast teles need huge bits of glass that are only topped in their weight by their cost.
 
To keep it simple - if the lens had a focal length of 6 inches and the diameter of the aperture with the lens wide open was 3" you'd have an f2 lens. If the diameter was 6 inches you'd have an f1 lens.

Obviously the longer the focal length gets the bigger that front lens element needs to be, (to be a 'fast' lens) and the more difficult it gets to pick the thing up, or be able to afford it! :puke:
 
so a 500mm f/2 lens would need an aperture of 25cm
that's a shed load of glass!
 
That's why you just don't get 500mm lenses that fast, the cost, not to mention the weight, starts to get prohibitive.

I think your figures might be out ( I hate metric) 25cm is around 5". I think it would be nearer 8" (To be an f2)
 
CT said:
That's why you just don't get 500mm lenses that fast, the cost, not to mention the weight, starts to get prohibitive.

I think your figures might be out ( I hate metric) 25cm is around 5". I think it would be nearer 8" (To be an f2)
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25cm is about 10" ;)
 
er 500mm/2 is 250mm in anybody's money :thinking:
 
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