Stupid question time - apertures. (Sorry!)
Discussion
When you talk about an aperture of (say) F9, is that, as a physical size of the aperture, always the same irrespective of lens, camera, focal length, whatever? Or are the fractions involved based on the lens size (so F9 on a huge lens would be physically bigger than F9 on a small lens)?
In other words, lets say I had a 35mm prime lens on my D7000, and the bloke beside me had a 600mm lens on a full frame D4 or Canon 5D. If, in order to get a perfect exposure at f9 I needed 1/1,000 sec at ISO 200, would the exact same be true for him?
Or would he have a bigger aperture at F9 than me because his lens is bigger and it's a fraction of the lens, so he could maybe shoot at 1/1,600 at ISO 200, for instance?
Probably a very dumb question so my apologies, but a search on the subject isn't yielding me much in the way of an answer to this one.
In other words, lets say I had a 35mm prime lens on my D7000, and the bloke beside me had a 600mm lens on a full frame D4 or Canon 5D. If, in order to get a perfect exposure at f9 I needed 1/1,000 sec at ISO 200, would the exact same be true for him?
Or would he have a bigger aperture at F9 than me because his lens is bigger and it's a fraction of the lens, so he could maybe shoot at 1/1,600 at ISO 200, for instance?
Probably a very dumb question so my apologies, but a search on the subject isn't yielding me much in the way of an answer to this one.
It is linked to the focal length of the lens. The precise way to describe the aperture is f/x, eg f/2.8. f is the focal length of the lens, so for a longer focal length the physical size of the aperture will need to be larger to maintain the same f/x value.
To use your example about exposure, yes you both need to shoot at 1/1000, f/9 and ISO 200 for the correct exposure no matter what focal length you are using. However the aperture will physically be larger on the longer lens.
To use your example about exposure, yes you both need to shoot at 1/1000, f/9 and ISO 200 for the correct exposure no matter what focal length you are using. However the aperture will physically be larger on the longer lens.
All straight forward, although deeper thinking may lead to further questions. I will pose a simple scenario. A lens of focal length 100mm has an iris diameter of 50mm. This gives an aperture of f/2 (100/50). For a lens of focal length 200mm to have the same aperture, the iris diameter will need to be 100mm. This is where the further questions start to arise, or at least they did for me when I was trying to get my head around apertures.
If we have metered a scene and discovered that with an ISO setting of 100 and a shutter speed of 1/250s we need to use f/2, both the lenses in my sample above can cope with the situation perfectly well. But then you think, hang on, the 200mm lens has an iris diameter of 100mm at f/2 so surely that is letting in way more light then the 100mm lens at the same aperture, which only has an iris diameter of 50mm at f/2.
Well, yes it does. But the 200mm lens is also magnifying the scene more than the 100mm lens, which results in less light reaching the sensor (inverse square law) so it all balances out, and makes things a lot easier for us photographers and light meter manufacturers.
If we have metered a scene and discovered that with an ISO setting of 100 and a shutter speed of 1/250s we need to use f/2, both the lenses in my sample above can cope with the situation perfectly well. But then you think, hang on, the 200mm lens has an iris diameter of 100mm at f/2 so surely that is letting in way more light then the 100mm lens at the same aperture, which only has an iris diameter of 50mm at f/2.
Well, yes it does. But the 200mm lens is also magnifying the scene more than the 100mm lens, which results in less light reaching the sensor (inverse square law) so it all balances out, and makes things a lot easier for us photographers and light meter manufacturers.
mojitomax said:
How can a lens have an iris aperture of 100mm if the physical diameter of the lens is less than this?
Do these lenses have TARDIS tech built in?
Mirror lenses used to be used to overcome this, the light travelled through a series of mirrors to emulate a longer focal length.Do these lenses have TARDIS tech built in?
They weren't very good and the bokeh (damn I hate that word) produced by them was unusual.
markmullen said:
mojitomax said:
How can a lens have an iris aperture of 100mm if the physical diameter of the lens is less than this?
Do these lenses have TARDIS tech built in?
Mirror lenses used to be used to overcome this, the light travelled through a series of mirrors to emulate a longer focal length.Do these lenses have TARDIS tech built in?
They weren't very good and the bokeh (damn I hate that word) produced by them was unusual.
It's not really the iris diameter. The iris is just a mechanism for shutting the lens down from wide open. Best to take this as a guide to brightness rather than 100% literal thing.
When it comes to longer lenses, they have a reduced angle of view, which in turn means less light on the sensor.
When it comes to longer lenses, they have a reduced angle of view, which in turn means less light on the sensor.
I was trying to keep it simple without using jargon, and thought iris, whilst technically not correct, would convey what I meant.
Mojitomax, you can't have an opening of 100mm in a lens that is physically smaller in diameter. That's why long telephotos that have a wide maximum aperture are so expensive, they are physically quite big diameter wise, with correspondingly large glass elements inside.
Mirror optics allow you to have long focal lengths in relatively short lenses, as the light rays entering the lens are bounced from back to front and then back again. The downside of mirror lenses are that they are usually fixed aperture, and fairly small apertures at that. Popular mirror lenses from a few years ago were typically 500mm with a fixed aperture of f/8.
Mojitomax, you can't have an opening of 100mm in a lens that is physically smaller in diameter. That's why long telephotos that have a wide maximum aperture are so expensive, they are physically quite big diameter wise, with correspondingly large glass elements inside.
Mirror optics allow you to have long focal lengths in relatively short lenses, as the light rays entering the lens are bounced from back to front and then back again. The downside of mirror lenses are that they are usually fixed aperture, and fairly small apertures at that. Popular mirror lenses from a few years ago were typically 500mm with a fixed aperture of f/8.
DIW35 said:
All straight forward, although deeper thinking may lead to further questions. I will pose a simple scenario. A lens of focal length 100mm has an iris diameter of 50mm. This gives an aperture of f/2 (100/50). For a lens of focal length 200mm to have the same aperture, the iris diameter will need to be 100mm. This is where the further questions start to arise, or at least they did for me when I was trying to get my head around apertures.
If we have metered a scene and discovered that with an ISO setting of 100 and a shutter speed of 1/250s we need to use f/2, both the lenses in my sample above can cope with the situation perfectly well. But then you think, hang on, the 200mm lens has an iris diameter of 100mm at f/2 so surely that is letting in way more light then the 100mm lens at the same aperture, which only has an iris diameter of 50mm at f/2.
Well, yes it does. But the 200mm lens is also magnifying the scene more than the 100mm lens, which results in less light reaching the sensor (inverse square law) so it all balances out, and makes things a lot easier for us photographers and light meter manufacturers.
Took me a couple of reads, but got there in the end! If we have metered a scene and discovered that with an ISO setting of 100 and a shutter speed of 1/250s we need to use f/2, both the lenses in my sample above can cope with the situation perfectly well. But then you think, hang on, the 200mm lens has an iris diameter of 100mm at f/2 so surely that is letting in way more light then the 100mm lens at the same aperture, which only has an iris diameter of 50mm at f/2.
Well, yes it does. But the 200mm lens is also magnifying the scene more than the 100mm lens, which results in less light reaching the sensor (inverse square law) so it all balances out, and makes things a lot easier for us photographers and light meter manufacturers.
Helpful, thank you.
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