lens speed

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Hi is lens speed more than just having a big aperture? a while back i tried a 5D with a 70 - 200 is F2.8 77mm lens on a cloudy day and i remember thinking that it was giving me significantly higher trigger values for a given aperture than my little old pentax film dslr. i was thinking about this and realised that it could be that one lens has 58mm diameter and the other 77mm thus concentrating more light into the same area and therefor making the lens faster in that respect. so is it fair to say lens speed is more than just having a big aperture and that it is influenced by lens diameter also? and are there any other factors that influence lens speed?

thanks,
 
From my experience, Im pretty sure its just the aperture.
Very interested if there are other reasons though.

Your ISO may have been different between your filmSLR cam and digSLR cam?
 
The speed of a lens is due to the widest the lens's aperture is, f/2.8 is faster than f/4 etc.

...So a f/2.8 lens is described as being faster than an f/4 lens.
 
The maxiumum aperture of any lens is quite simply calculated thus:

Focal length of lens, divided by the diamater of the maximum aperture (usually a function of the diameter of the front element/barrel width), which is why most large aperture lenses have much bigger lens diameters.

There are no other variables. So if the focal length of the lens is equal to the diameter of the aperture, you’d have an f/1. If the focal length of the lens is 8 times longer than the diameter of the aperture, you’d have an f/8.

Any F2.8 lens, no matter what the focal length or maximum aperture, requires the same amount of light, no more, no less than any other F2.8 lens.
 
dougdarter said:
The maxiumum aperture of any lens is quite simply calculated thus:

Focal length of lens, divided by the diamater of the maximum aperture (usually a function of the diameter of the front element/barrel width), which is why most large aperture lenses have much bigger lens diameters.

There are no other variables. So if the focal length of the lens is equal to the diameter of the aperture, you’d have an f/1. If the focal length of the lens is 8 times longer than the diameter of the aperture, you’d have an f/8.

Any F2.8 lens, no matter what the focal length or maximum aperture, requires the same amount of light, no more, no less than any other F2.8 lens.
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Yes, that's all true. But the question is, will certain lenses allow more light in and therefore allow a higher shutter speed for the same aperture.
 
fabs said:
Yes, that's all true. But the question is, will certain lenses allow more light in and therefore allow a higher shutter speed for the same aperture.
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No, there are no other variables. an F number equates to an exposure or light value, which are not affected by anything other than light entering the lens. Aperture and shutter speeds are reciprocals, and also not subject to variations.

The only outside factor which would have the effect of increasing the shutter speed for any given aperture, would be an increase of the ISO.

This is the reason why exposure meters work. They measure light, nothing more, and ALL f2.8 lenses allow the same amount of light to enter the lens.
 
dougdarter said:
No, there are no other variables. an F number equates to an exposure or light value, which are not affected by anything other than light entering the lens. Aperture and shutter speeds are reciprocals, and also not subject to variations.

The only outside factor which would have the effect of increasing the shutter speed for any given aperture, would be an increase of the ISO.

This is the reason why exposure meters work. They measure light, nothing more, and ALL f2.8 lenses allow the same amount of light to enter the lens.
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That answers the question then. :thumbs:
 
Sorry BW, what is your sketch trying to say? I can't understand it at all.
 
StewartR said:
100% correct. End of.
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No - its not.

Have a little read about "T-Stops" - Google is your friend.

I can give a hard example - a Sigma 100-300 f/4 @ "f/4" always gave me 1/500 vs a Nikkor 300mm f/4 AF-S @ "f/4" gave 1/640, meaning that essentially while it was markes as f/4 it was just little slower.

Quite often you can see differing brightnesses even in lens marked with the same aperture.

Have a read and see whether you think the discussion is still closed :)
 
puddleduck said:
Have a little read about "T-Stops" - Google is your friend.

Have a read and see whether you think the discussion is still closed :)
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I bow to your superior knowledge, Andy.

But that's not what the OP was asking about, and you know it. We're trying to help him, not confuse him. ;)
 
BrownWarrior said:
Stewart, what i was trying to show is that for a given focal length, aperture diameter and f number you could have multiple elements that actually focus light from a larger area in some lenses than in others thus making the power of the beam greater and making the lens faster for a given aperture diameter, focal length and f number than in another lens that collects light from a smaller area and has a weaker beam!
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Maybe I'm missing your point here but I'll have a go......

The "aperture" should more correctly be termed "effective aperture". A 300mm f/4 lens would need an aperture 75mm diameter but it doesn't have this. The aperture diameter is configured to represent an aperture of 75mm IF that aperture was placed at the object lens (front element) before the convergence of the incoming light.

Am I on track?

Bob
 
BrownWarrior said:
Stewart, what i was trying to show is that for a given focal length, aperture diameter and f number you could have multiple elements that actually focus light from a larger area in some lenses than in others thus making the power of the beam greater and making the lens faster for a given aperture diameter, focal length and f number than in another lens that collects light from a smaller area and has a weaker beam!
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No. The area from which the light is collected is the angle of view and it is completely determined by the focal length and the sensor size. The Wikipedia article Angle Of View describes this, and half-way down the article (here) is a diagram to illustrate it.
 
fabs said:
I thinkm the question the OP is asking here though is, can f2.8 fro example be faster on one lens than another based on the diameter of the front element.
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BrownWarrior said:
yes Fabs has got it exactly!
its basically the principle that the bigger the magnifying lens the more powerful the beam that comes from it.
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And hence the physical aperture can be designed to be smaller depending where it's place in the lens groups. The limitation imposed is that by going too small then diffraction would occur earlier.
BrownWarrior said:
Canon Bob i think you must be correct however i don't see how this affects my argument or anybody elses arguments. perhaps you could explain a bit more?
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I thought that you're argument was that f/x on one lens would not be equal in "light gathering capacity" to f/x on a lens of different focal length......it is equal in respect to how it affects exposure. A longer focal length may appear slower as the subject may be darker than the background and it would fill a larger portion of the image.

Closer?

Bob
 
puddleduck said:
No - its not.

Have a little read about "T-Stops" - Google is your friend.

I can give a hard example - a Sigma 100-300 f/4 @ "f/4" always gave me 1/500 vs a Nikkor 300mm f/4 AF-S @ "f/4" gave 1/640, meaning that essentially while it was markes as f/4 it was just little slower.
Quite often you can see differing brightnesses even in lens marked with the same aperture.

Have a read and see whether you think the discussion is still closed :)
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Andy, if the Nikon was giving you 1/640th @ f4, whilst the Sigma zoom was giving you 1/500th @ f4, then the Nikon was slightly faster, not slower:thinking: Let's not confuse Brown Warrior even further:lol:
 
BrownWarrior said:
no. i'm saying that for lenses of equal focal length and equal f number could although gathering light though aperatures of equal relative area (as they may be at different possitions in the lens) have different speeds due to the shapes of the elements magnifying the intensity of the light more in one lens than the other. do you understand what i am saying now?
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BW, did you read my last post and the Wikipedia article? It sounds like you didn't.....
 
BrownWarrior said:
no. i'm saying that for lenses of equal focal length and equal f number could although gathering light though aperatures of equal relative area (as they may be at different possitions in the lens) have different speeds due to the shapes of the elements magnifying the intensity of the light more in one lens than the other. do you understand what i am saying now?
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Okay....
Same focal length, same aperture setting and one appears to let in more light than the other due to its internal construction or front element size.

f/x is fact .....
A manufacturer's f/x may well be rounded up or down slightly. I would also guess that the fewer aperture blades that a lens has then more rounding up or down would be required to keep the gearing of the mechanism simple enough to keep the costs down. (I'm not exactly sure whether the number of blades affects light passage through diffraction losses and, if it does, then has this been catered for in the physical stops)

Bob
 
Canon Bob said:
A manufacturer's f/x may well be rounded up or down slightly. I would also guess that the fewer aperture blades that a lens has then more rounding up or down would be required
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Bob, I think you're getting dragged into angels-on-heads-of-pins territory here...

The original question was whether a lens with a 77mm front element would be faster than a lens (with the same focal length and f number) with a 58mm front element. And the answer is no.

Yes, we know there might be several detailed reasons why the amount of light might not be exactly the same. But that's not what this is about.
 
StewartR said:
Bob, I think you're getting dragged into angels-on-heads-of-pins territory here...

The original question was whether a lens with a 77mm front element would be faster than a lens (with the same focal length and f number) with a 58mm front element. And the answer is no.

Yes, we know there might be several detailed reasons why the amount of light might not be exactly the same. But that's not what this is about.
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Okay...I've missed the point here so I'll sit and watch.

Bob
 
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