Noob question - focal length vs aperture

Samsonite

Senior Member
Hi all, I've had a burning question which I couldn't find the answer on the Internet...

Taking for example a telephoto lens, for simplicity's sake, let's say its a 70 - 200mm F2.8. Even though the Aperture is fixed at 2.8, am I right in saying that you would get more light to the sensor at 70mm than at 200mm? My thinking is the more extended the lens is, the harder it is for light to get to the sensor? For example take a toilet role and a long carton tube of the same diameter, you get more light trough the end of toilet roll coz its shorter... Am I right in think the above? Or completely wrong?

Thanks!




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Rexer John

Senior Member
You are correct sir.
Except where you say the aperture is fixed at 2.8
The aperture size may well be the same size at 70mm as it is at 200mm.
The f number is how much light is getting through the lens. It's not the diameter of the aperture but it is the relative aperture.
So your relative aperture will be less at 200mm, even though the aperture is still the same size.
Set the lens to 200mm and set the aperture as wide as possible, the f number will be displayed in your viewfinder.
Remember that's not the aperture but the lenses light gathering capability at that zoom setting expessed as a number (f).

P.S. If you can get down to 2.8 at 200mm I'm guessing the aperture is able to open further at longer zoom to maintain the same f number.
Someone more knowledgeable than me will reply soon.
 
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Dave_W

The Dude
I would phrase it this way, at 200mm you're sampling less of the world and than at 70mm. As a result the sheer quantity of photons entering your lens is less at 200mm than at 70mm due to a smaller sample area, hence less light.
 

Samsonite

Senior Member
Cheers guys, thanks for your explanations, it was just something I was thinking about this morning whilst on the throne :) still trying to get my head around lenses, f stops, focal lengths and ISOs! :)


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WayneF

Senior Member
Taking for example a telephoto lens, for simplicity's sake, let's say its a 70 - 200mm F2.8. Even though the Aperture is fixed at 2.8, am I right in saying that you would get more light to the sensor at 70mm than at 200mm? My thinking is the more extended the lens is, the harder it is for light to get to the sensor? For example take a toilet role and a long carton tube of the same diameter, you get more light trough the end of toilet roll coz its shorter... Am I right in think the above? Or completely wrong?

Right about the tubes, but wrong about aperture.

The overriding idea to know is that the design is so that f/2.8 always lets the same amount of light in, regardless of focal length.

The definition of f/stop is f/number = focal length / aperture diameter

This means at 70mm and f/2.8, the effective aperture diameter is 70mm / 2.8 = 25mm

At 200mm and f/2.8, the effective aperture diameter is 200mm / 2.8 = 71mm

So at 200mm, the tube might be 200/70 = 2.8 longer,
but the aperture is 71/25 = 2.8 wider, which exactly offsets length.

These numbers here are just approximations, because f/2.8 is technically and precisely 2.828, and the focal length varies slightly with focus distance. The marked focal length is true when focused at infinity.

The effective aperture is the diameter seen through the magnification of the front lens elements (same as the scene sees). Simply look into the front of this 70-200 mm lens as you rotate zoom from 70 to 200 mm, and you will see it grow much larger (to stay the same light transmission).
 
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Horoscope Fish

Senior Member
... the design is so that f/2.8 always lets the same amount of light in, regardless of focal length.

The definition of f/stop is f/number = focal length/aperture diameter ...

*phew*

Thank you... Your explanation coincides with my understanding and I've been watching this thread to figure out if I've been totally misconstruing this whole concept.
 
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