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09/22/2005 10:43:49 AM · #1 |
could someone advise me why does the aperture setting of most point and shoot cameras (just the basic ones. cameras w/o aperture priority mode, etc.) change whenever I use my zoom??? at the widest its 2.8 then goes to 4.0 when i zoom it.
hope to hear your comments! thank you |
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09/22/2005 10:48:42 AM · #2 |
2.8 is the fastest you can shoot with your camera zoomed in and 4.0 would be the fastest you can shoot with it zoomed all the way out. I'm not sure why but thats just the way it is. lol
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09/22/2005 10:52:43 AM · #3 |
thanks. yeah i noticed that. i just want to know the logic behind it. anyone???
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09/22/2005 11:00:15 AM · #4 |
It's the construction of the lens. F stop of 2.8 means focal length/2.8 = size of hole. In the "zoom" lenses the actual size of the hole is the same - but the focal length changes - that means the f number has to increase.
For example if you have a 50mm f2.8, the size of the max apeture is 50/2.8 = 17.85. If that same lens zoomed to 100mm, the max hole size is the same (17.85), so working backwards the f number is now:
100/f = 17.85 => f= 100/17.85 = 5.6
Some zoom lenses do not have a range - they do not expand and contract - and they are usually more expensive.
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09/22/2005 12:25:41 PM · #5 |
thanks joebok.
say i have the 18-50mm lens of sigma with f/3.5-5.6
how do you apply your formula to this?
sorry for being so pesky. just want to understand how this works.
thanks! |
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09/22/2005 01:11:09 PM · #6 |
I have found this thread informative.
May I add a question to the mix that might help unravel yet more the mysteries of F?
What I had heard regarding Aperture is that it itself is a ratio of [the distance from the center of the lens (rear element?) to the sensory plane] to [the width of the hole itself].
Would it be ok if someone clarified this definition as mine is spotty and likely inaccurate.
This might also help the general purpose of the thread.
Thanks |
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09/22/2005 01:19:39 PM · #7 |
I'm sure it has to do with the construction of the lens - if we look at the "telephoto" end we get an apeture of 50/5.6 or 8.9mm. If we applied that at the telephoto end we would get 18/8.9, which is about 2. But the lens says the max apeture is only 3.5 (which is a smaller physical hole).
This implies to me that if you could set f/2 on that lens at the wide angle you would get some kind of vingetting effects - probably from the housing of the lens. So they have to close down the max apeture at the wide end.
If you have the chance, look at a "fixed f lens" versus a "variable f lens" - I happen to have a 28-200 f3.5-5.6 and a 70-200 f2.8. The 28-200 is small, lightweight, telescopes and was a LOT cheaper. The 70-200 f2.8 is HUGE and heavy, it does not change length. Look into them as you zoom in and out - the elements inside operate differently.
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09/22/2005 01:27:02 PM · #8 |
Originally posted by eschelar:
I have found this thread informative.
May I add a question to the mix that might help unravel yet more the mysteries of F?
What I had heard regarding Aperture is that it itself is a ratio of [the distance from the center of the lens (rear element?) to the sensory plane] to [the width of the hole itself].
Would it be ok if someone clarified this definition as mine is spotty and likely inaccurate.
This might also help the general purpose of the thread.
Thanks |
f is the focal length of the lens which is the distance from the lens to the point of focus - which is usually the film plane or where the sensor is. (If it isn't, then you've got a problem!)
An aperture is a hole - in this context the "hole" in the lens that allows the light through - assumed to be a circle. For purposes of computing exposure it is conveneint to measure it in relation to the focal length of the lens. An f-stop of 2 means focal length / 2 = diamater of the aperture.
Doing it this way means that if your light meter says use f 5.6 you use 5.6 on a 15 mm lens and a 400 mm lens - each will let the proper amount of light through (even though the physical sizes of the apertures in each lens would be different) to properly expose the film or sensor.
It's just a mathematical trick of convenience. |
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09/22/2005 11:53:45 PM · #9 |
Thanks Joebok. I'm glad there are guys who truly understand things and can explain them like this :)
Putting the two peices of information (relationship of apterture to focal distance and the effects of zoom on max aperture) helped me to understand it better even though I'm not the OP.
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