zoom and aperture size

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samsnikon

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Why does the amount I can open the aperture decrease as I increase the zoom.

so on my 28-200 lens which is f/2.8 at 150 its reads something like 4.5 as the lowest I can get to. I don't understand why it cannot go to 2.8

thanks

sam
 
The f number is derived from the focal length of the lens divided by the usable diameter of the lens opening

E.G. if a 100 - 300mm lens had usable lens opening of 50mm the f number at 300mm would be f6 and f2 at 100mm.

Fixed aperture zooms are available and these will have variable usable lens opening I expect and be more expensive.
 
your not making a lot of sense sam you say at 150 it is f2.8 and reads f4.5 ??
so which one is it ,,,cant be both ????
i cant think of any 28 - 200 lens that has a constant f2.8 aperture throughout its entire range ( anyone put me right on that ? )
f3.5 to f5.6 seems to be the most you can get from the nikon one
 
donutagain said:
your not making a lot of sense sam you say at 150 it is f2.8 and reads f4.5 ??
so which one is it ,,,cant be both ????
i cant think of any 28 - 200 lens that has a constant f2.8 aperture throughout its entire range ( anyone put me right on that ? )
f3.5 to f5.6 seems to be the most you can get from the nikon one
Click to expand...

Think Canon do a 70 - 200mm f2.8 constant aperture. Don't know of one with a broader range, though.
 
I think he is saying that it's a 2.8 lens but will only open up to f/4.5 when he is zoomed in to 150mm.

Arclight has it right, but it may be easier to understand if explained slightly differently.

Let us assume your lens is f/2 (it makes the math easier). At 28mm the useable lens diameter would need to be 14mm (28/14=2). To maintain f/2 at the 200mm end of your zoom, the useable diameter would need to increase to 100mm. That's a big lens, and would probably be difficult to manufacturer to give acceptable results, which would make it expensive.

The compromise that manufacturers use is to accept a smaller useable diameter as the lens is zoomed in, giving us a higher f ratio as a result.
 
arclight said:
Think Canon do a 70 - 200mm f2.8 constant aperture. Don't know of one with a broader range, though.
Click to expand...

nikon do one as well Dougie ,,,anyway how come we both posted at the same time and you got in first ???are you bribing someone with cookies ??:LOL:
 
donutagain said:
nikon do one as well Dougie ,,,anyway how come we both posted at the same time and you got in first ???are you bribing someone with cookies ??:LOL:
Click to expand...

Happens to me a lot. Type a reply and by the time it is posted 6 others have got in first.(y)
 
Thanks for clearing it up.

Its the first time I noticed it on the weekend and could not understand it

sam
 
I was using on the weekend was the 18-105VR.

Its the first time I have used a zoom in about 2 years having just been using my prime and so I noticed it.

sam
 
the 18 - 105 vr ( and the 18 -200 ) are both f3.5 at their widest , so you wouldnt have seen f2.8 at 150 ( mm? )
 
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