A question about f-stops and aperture


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artery

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Mar 13, 2009
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#1
Hi.

I recently read that you calculate f-stops by dividing the lens focal length with the aperture optical diameter. Meaning, f/2 = 100mm focal length/ 50mm aperture diameter

What I dun understand is that how come, for example, a 28mm lens with a 10mm aperture diameter has a f/2.8 but 80mm lens with the same 10mm aperture diameter yield a f-stop of 8. Cos I thought both supposedly have the same aperture diameter that allows same amount of light in.

Can someone explain the theory why? Thanks.
 

unclesam

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Dec 3, 2008
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#2
Hi.

I recently read that you calculate f-stops by dividing the lens focal length with the aperture optical diameter. Meaning, f/2 = 100mm focal length/ 50mm aperture diameter

What I dun understand is that how come, for example, a 28mm lens with a 10mm aperture diameter has a f/2.8 but 80mm lens with the same 10mm aperture diameter yield a f-stop of 8. Cos I thought both supposedly have the same aperture diameter that allows same amount of light in.

Can someone explain the theory why? Thanks.
Artery, you may want to read this:

http://en.wikipedia.org/wiki/F_stops
 

night86mare

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#3
I recently read that you calculate f-stops by dividing the lens focal length with the aperture optical diameter. Meaning, f/2 = 100mm focal length/ 50mm aperture diameter

What I dun understand is that how come, for example, a 28mm lens with a 10mm aperture diameter has a f/2.8 but 80mm lens with the same 10mm aperture diameter yield a f-stop of 8. Cos I thought both supposedly have the same aperture diameter that allows same amount of light in.

Can someone explain the theory why? Thanks.
you don't really need to care about that, even if it is true.
 

giantcanopy

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Feb 11, 2007
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#4
Comparison of absolute sizes of the aperture between lenses of different focal lengths will yield no meaningful comparisons. A 10mm physical aperture for a telephoto will collect less light than a wide angle lens.

The geometric value of the calculated f stop gives more meaningful comparisons across lenses of different focal lengths

Ryan
 

Michael

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Apr 5, 2005
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#5
Hi.

I recently read that you calculate f-stops by dividing the lens focal length with the aperture optical diameter. Meaning, f/2 = 100mm focal length/ 50mm aperture diameter

What I dun understand is that how come, for example, a 28mm lens with a 10mm aperture diameter has a f/2.8 but 80mm lens with the same 10mm aperture diameter yield a f-stop of 8. Cos I thought both supposedly have the same aperture diameter that allows same amount of light in.

Can someone explain the theory why? Thanks.
friend you said it above f-stop is defined as focal length / aperture diameter....

so 28/10 = 2.8
and 80/10 = 8

they have the same whole but the collect different amounts of light... think of the focal length as a funnel, the 28mm lens has a wider funnel and hence collects more light than the 80.... they then send the light collected through the same pipe (aperture) but it is still a different amount
 

artery

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#6
Thanks unclesam.

Great reference there. Provided the answer i need.

For those interested also, I copy the 'answer' from wiki..

The common assumption in photography that the pupil diameter is equal to the aperture diameter is not correct for many types of camera lens, because of the magnifying effect of lens elements in front of the aperture.

A 100 mm lens with an aperture setting of f/4 will have a pupil diameter of 25 mm. A 135 mm lens with a setting of f/4 will have a pupil diameter of about 33.8 mm. The 135 mm lens' f/4 opening is larger than that of the 100 mm lens but both will transmit the same amount of light to the film or sensor
 

artery

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Mar 13, 2009
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#7
friend you said it above f-stop is defined as focal length / aperture diameter....

so 28/10 = 2.8
and 80/10 = 8

they have the same whole but the collect different amounts of light... think of the focal length as a funnel, the 28mm lens has a wider funnel and hence collects more light than the 80.... they then send the light collected through the same pipe (aperture) but it is still a different amount

Thanks Michael.

I guess what you are trying to say is what wiki said 'magnifying effect of lens elements', is it?
 

Michael

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#8
Thanks Michael.

I guess what you are trying to say is what wiki said 'magnifying effect of lens elements', is it?
mmm i guess you could call it magnifying effect, yes, as with the 28mm lens you capture light from a wider area but project on the same area (sensor)
 

shootjutsu

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Jun 7, 2009
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#10
I agree.
Such things you won't even remember while shooting.

You will 1st look out for other things while shooting.
haha yup yup agree. i dont even understand this thread but after the explanation from wiki, i think i got it somehow. thanks for the thread too, TS. gambatte!
 

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