Stupid question time
Discussion
Stupid question (probably) but here goes.
If I've got a f2 50mm lens then (presumably) at F2 I have a 25mm aperture.
If so then if I have an f2 80mm lens then at f2 the aperture is 40mm.
So does that mean that because the aperture is larger, the camera is getting more light in, ergo a faster shutter speed?
Or is f2 the same shutter speed regardless of lens size?
If I've got a f2 50mm lens then (presumably) at F2 I have a 25mm aperture.
If so then if I have an f2 80mm lens then at f2 the aperture is 40mm.
So does that mean that because the aperture is larger, the camera is getting more light in, ergo a faster shutter speed?
Or is f2 the same shutter speed regardless of lens size?
The original reason why f-numbers are consistent across focal lengths relates to a single lens element (think of a 5x4 view camera). Focussed at infinity, a 50mm lens will be 50mm from the film plane, and a 25mm aperture will let a certain amount of light through. Now put a 100mm lens on (with the same 25mm aperture): the lens (at infinity) is now 100mm from the film plane. The inverse square law comes into play, meaning that, because the lens is twice as far away, the amount of light hitting the film is 1/4 of that from the 50mm lens. If you use a bellows camera, you have to increase your shutter speed if you focus on something close to, since the lens will be further from the film plane than its focal length.
With the 100mm lens, if you double the diameter of the aperture to 50mm, that lets 4x the light through (The area of the aperture goes up as the square of the diameter, so doubling it increases the area by 4x), bringing it up to the same amount of light as the 50mm lens with a 25mm aperture.
As far as (D)SLRs are concerned, they involve complex optics (retrofocus, telephoto and internal focussing elements) that allow the lens mount to remain at a fixed distance from the image plane, and the lens itself to be a manageable size. The upshot is that the aperture of your 50mm lens is, most likely, not 25mm at f/2 - it's a size that allows the same amount of light through as would a 25mm aperture on a 50mm single element lens 50mm from the image plane. And you don't have to make adjustments for close focussing.
Basically the manufacturers do some complicated maths to make sure that f/2 is the 'same' no matter what the focal length of the lens.
With the 100mm lens, if you double the diameter of the aperture to 50mm, that lets 4x the light through (The area of the aperture goes up as the square of the diameter, so doubling it increases the area by 4x), bringing it up to the same amount of light as the 50mm lens with a 25mm aperture.
As far as (D)SLRs are concerned, they involve complex optics (retrofocus, telephoto and internal focussing elements) that allow the lens mount to remain at a fixed distance from the image plane, and the lens itself to be a manageable size. The upshot is that the aperture of your 50mm lens is, most likely, not 25mm at f/2 - it's a size that allows the same amount of light through as would a 25mm aperture on a 50mm single element lens 50mm from the image plane. And you don't have to make adjustments for close focussing.
Basically the manufacturers do some complicated maths to make sure that f/2 is the 'same' no matter what the focal length of the lens.
Shutter speed is not tied to the aperture of the lens, it's the exposure metering which governs the speed at a given f number, which takes account of the light level present.
Not all camera brands are calibrated the same, although they are all calibrated to 18% grey level.
That's why you will find some cameras under the same lighting conditions, may require an EV comp to prevent under or over exposure.
Some slightly under expose because they can have their shadow level pushed without introducing noise, others can have their highlights pulled back without introducing banding.
Not all camera brands are calibrated the same, although they are all calibrated to 18% grey level.
That's why you will find some cameras under the same lighting conditions, may require an EV comp to prevent under or over exposure.
Some slightly under expose because they can have their shadow level pushed without introducing noise, others can have their highlights pulled back without introducing banding.
Major Bloodnok said:
The original reason why f-numbers are consistent across focal lengths relates to a single lens element (think of a 5x4 view camera). Focussed at infinity, a 50mm lens will be 50mm from the film plane, and a 25mm aperture will let a certain amount of light through. Now put a 100mm lens on (with the same 25mm aperture): the lens (at infinity) is now 100mm from the film plane. The inverse square law comes into play, meaning that, because the lens is twice as far away, the amount of light hitting the film is 1/4 of that from the 50mm lens. If you use a bellows camera, you have to increase your shutter speed if you focus on something close to, since the lens will be further from the film plane than its focal length.
With the 100mm lens, if you double the diameter of the aperture to 50mm, that lets 4x the light through (The area of the aperture goes up as the square of the diameter, so doubling it increases the area by 4x), bringing it up to the same amount of light as the 50mm lens with a 25mm aperture.
As far as (D)SLRs are concerned, they involve complex optics (retrofocus, telephoto and internal focussing elements) that allow the lens mount to remain at a fixed distance from the image plane, and the lens itself to be a manageable size. The upshot is that the aperture of your 50mm lens is, most likely, not 25mm at f/2 - it's a size that allows the same amount of light through as would a 25mm aperture on a 50mm single element lens 50mm from the image plane. And you don't have to make adjustments for close focussing.
Basically the manufacturers do some complicated maths to make sure that f/2 is the 'same' no matter what the focal length of the lens.
I'm really pizzed, but I think he's right,With the 100mm lens, if you double the diameter of the aperture to 50mm, that lets 4x the light through (The area of the aperture goes up as the square of the diameter, so doubling it increases the area by 4x), bringing it up to the same amount of light as the 50mm lens with a 25mm aperture.
As far as (D)SLRs are concerned, they involve complex optics (retrofocus, telephoto and internal focussing elements) that allow the lens mount to remain at a fixed distance from the image plane, and the lens itself to be a manageable size. The upshot is that the aperture of your 50mm lens is, most likely, not 25mm at f/2 - it's a size that allows the same amount of light through as would a 25mm aperture on a 50mm single element lens 50mm from the image plane. And you don't have to make adjustments for close focussing.
Basically the manufacturers do some complicated maths to make sure that f/2 is the 'same' no matter what the focal length of the lens.
The same aperture value but different actually physical sized hole because with the longer lens you are cutting down the amount of light.
Why? Smaller angle of view.
Think of a window as the 50mm. Block off half for 100mm now the room is darker you need some way of making it lighter.
F numbers are the way we achieve that simply.
Why? Smaller angle of view.
Think of a window as the 50mm. Block off half for 100mm now the room is darker you need some way of making it lighter.
F numbers are the way we achieve that simply.
Major Bloodnok said:
The original reason why f-numbers are consistent across focal lengths relates to a single lens element (think of a 5x4 view camera). Focussed at infinity, a 50mm lens will be 50mm from the film plane, and a 25mm aperture will let a certain amount of light through. Now put a 100mm lens on (with the same 25mm aperture): the lens (at infinity) is now 100mm from the film plane. The inverse square law comes into play, meaning that, because the lens is twice as far away, the amount of light hitting the film is 1/4 of that from the 50mm lens. If you use a bellows camera, you have to increase your shutter speed if you focus on something close to, since the lens will be further from the film plane than its focal length.
With the 100mm lens, if you double the diameter of the aperture to 50mm, that lets 4x the light through (The area of the aperture goes up as the square of the diameter, so doubling it increases the area by 4x), bringing it up to the same amount of light as the 50mm lens with a 25mm aperture.
As far as (D)SLRs are concerned, they involve complex optics (retrofocus, telephoto and internal focussing elements) that allow the lens mount to remain at a fixed distance from the image plane, and the lens itself to be a manageable size. The upshot is that the aperture of your 50mm lens is, most likely, not 25mm at f/2 - it's a size that allows the same amount of light through as would a 25mm aperture on a 50mm single element lens 50mm from the image plane. And you don't have to make adjustments for close focussing.
Basically the manufacturers do some complicated maths to make sure that f/2 is the 'same' no matter what the focal length of the lens.
That's the most comprehensive reply I think I've ever seen, thank you! With the 100mm lens, if you double the diameter of the aperture to 50mm, that lets 4x the light through (The area of the aperture goes up as the square of the diameter, so doubling it increases the area by 4x), bringing it up to the same amount of light as the 50mm lens with a 25mm aperture.
As far as (D)SLRs are concerned, they involve complex optics (retrofocus, telephoto and internal focussing elements) that allow the lens mount to remain at a fixed distance from the image plane, and the lens itself to be a manageable size. The upshot is that the aperture of your 50mm lens is, most likely, not 25mm at f/2 - it's a size that allows the same amount of light through as would a 25mm aperture on a 50mm single element lens 50mm from the image plane. And you don't have to make adjustments for close focussing.
Basically the manufacturers do some complicated maths to make sure that f/2 is the 'same' no matter what the focal length of the lens.
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