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Which lens is faster 70-200 f2.8 or 100-400 f4?
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Originally talking about taking pictures of players on a field under flood light conditions which lens would shoot faster. People automatically think that it’s the f2.8 coz f2.8 opens wider than an f4, but only on like lenses. f2.8 and f4 are factors of the lens length, so if the length changes then so must the aperture.
Let’s do a little copy and paste from the wrong thread that inspired the question for those that don’t often wander into the sports department.
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The f2.8 is a factor of say 200mm, thereby making the aperture 56mm. The f4 of the 400mm gives an aperture of 100mm. So 2 things, 1, a 100mm hole seems to be a lot of light, a lot more than 56mm despite the Fnumber. And 2, 100mm seems like a very big bleedin’ hole for something that takes a 77mm filter – hence some of my confusion. I’m wondering have I got it completely wrong or is it an ‘effective’ 100mm due to multiple elements that make up a lens? But ‘effective’ or not we’re still saying that it’s a bigger hole right? Or no?
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2 people so far have disagreed but I’m not convinced by a “it just is” type answer. I’d want to put these lenses side by side in a controlled environment, say a basket ball court at night with lights only on (no daylight), set the cam to Av and then see what shutter they return. Unfortunately for me I don’t have the 100-400 so I’m asking can anyone else clear this up?
i am not sure i completely understand your question
if your talking about traditional expression faster with reguards to lenes , the apreture rating is whats used
therefor the lens with the 2.8 apreture would be classed as the faster
but sometimes people ask about faster in terms of autofocus, and i am wondering if thins is what you mean
I am not sure i understand your statement about apretures and lens length either, for example the 16:35 f2.8 can and f4 17:40 are not much different both in physical size and focal length,
are you saing that as the focal length of a lens is changed the physical apreture must change?
I’m asking; when wide open under the same light which will give the fastest shutter speed?… and if you go ahead and say the 2.8 feel free to explain it to me.
Think about it this way –
A light meter doesn’t take into consideration the type of lens you are using… it’s totally irrelevant.
F2.8 will give a faster shutter speed than F4/F5.6 and the light meter won’t care about the lens length.
nfl-fan wrote:
Think about it this way –
A light meter doesn’t take into consideration the type of lens you are using… it’s totally irrelevant.
F2.8 will give a faster shutter speed than F4/F5.6 and the light meter won’t care about the lens length.
hm? thinking..!
Right fair point with the light meter. The light is what it is on the subject and the metering is the same. If the meter give you 2.8 then you go with that regardless of the lens – check!
Still though there is a question that I can’t quite clarify (never mind the answer). The fact remains that the f4 400mm has a greater aperture / bigger hole therefore lets in more light. So what’s my question? Better start at the basics – Bigger hole = more light right? So I expect an aperture size of 100mm must have a performance gain over 56mm right? Not so obvioulsy wrong.
So what is happening over the 400mm that a 100mm aperture lets in less light than a 56mm aperture over 200mm?
Nossie – I think you are confusing the diamater of the lens (100mm) with the apperture size (f2.8 or f4 ) These are two separate things.
The same aperture e.g. f2.8 will always give the same exposure, irrespective of the lens, and assuming other factors are equal (shutter speed, ISO etc).
Looking at a sample calculation – I am assuming f4 and 100mm and 200mm lenses for simplicity :
100mm lens : f4 gives a diameter of 25mm. Total area of the ‘hole’ = 490.9375 sq. mm
200mm lens : f4 gives a diameter of 50mm. Total area of the ‘hole’ = 1963.75 sq mm (or 4 times bigger)(area = pie*radius squared)
So, you’re correct, the hole is much bigger.
However, now the inverse square law comes into play. So, the 200mm lens is twice as long as the 100mm and therefore only 1/4 of the light reaches the sensor (or film) as for a 100mm with the same ‘hole’ size. So, it exactly balances the larger size of the ‘hole’ or aperture. Four times bigger but twice as far away = same amount of light.On a related point, more expensive zoom lenses (e.g. Canon ‘L’ series) will vary the size of the aperture to maintain the same ratio as the lens is zoomed. Cheaper zoom lenses often express the aperture as a range e.g. f3.5-f5.6, as the effective aperture will change as the lens is zoomed.
All very simple really :)
i am not sure where you get the 100mm from
as i can think of two lenes with similar focal lengths and big differences in physical size,
also if you look at things like the 400l DO lens from canon is physically much smaller than the version without DO.
isnt the apreture rating giving you the size of the hole, or at least one of the things that contributes to it?
Daky wrote:
Nossie – I think you are confusing the diamater of the lens (100mm) with the apperture size (f2.8 or f4 ) These are two separate things.
100mm in this context is the focal length of the lens, rather than the diameter.
(It can be confused with the filter size which is the diameter of the front of the lens)
Some reading –
http://www.uscoles.com/fstop.htm
Just to be clear, is f/8 the same on my 20mm as it is on my 50?
Yes. That’s actually kind of the point. Through all these focal lengths, aperture areas and even film formats, the brightness of the illumination reaching the film plane or sensor is identical for any given f/stop. That’s why handheld light meters work. They don’t know if you’re shooting digital, film or movies, if it’s a DX or FX format sensor or 35mm, 120 or 4X5 inch film. It doesn’t matter. f/8 is f/8, f/11 is f/11, regardless of the lens or film.
FrankC wrote:
Daky wrote:
Nossie – I think you are confusing the diamater of the lens (100mm) with the apperture size (f2.8 or f4 ) These are two separate things.
100mm in this context is the focal length of the lens, rather than the diameter.
(It can be confused with the filter size which is the diameter of the front of the lens)
Nossie wrote:
So I expect an aperture size of 100mm must have a performance gain over 56mm right? Not so obvioulsy wrong.
I assumed when talking about the 56mm and aperture Noissie was talking about the diamter of the lens (100mm is obviously the focal length, so I used a bad example, although the same one as Nossie) so maybe there is confusion on a lot of fronts.
Nossie wrote:
So what is happening over the 400mm that a 100mm aperture lets in less light than a 56mm aperture over 200mm?
FrankC I went off to watch the rugby and the whole thing came together in my head but you beat me to it. The light does of course “fall off” and the 400mm is 2x as far away than the 200mm so the inverse square law comes into play. Nice one, that’s that mess addressed in my nut anyway. Serves me right for never putting my mind to it but I do have a knack for getting yas going ;0)
And the related point; eventually even the L Lens must give way to variance as nfl_fan pointed out on the 100-400L it is in fact 4.5-5.6stcstc Originally lenses where basically nothing more than a convex lens at the end of a tube. A 1000 mm lens would be a convex lens at the end of a 1000mm tube. Later techniques were developed to use multiple elements in a lens to make the effective focal length of a lens much longer than its real length (example: a 1000mm lens could now be made 250mm long). Mirror lens are another example commonly seen in telescopes. That’s why I noted the word ‘effective’ along the way. How could on paper a 100mm aperture fit into a 77mm lens?
daky Only talking aperture size here. f2.8 on a 200mm gives an aperture of 56mm.
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