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Thread: Aperture of lenses

  1. #21
    Senior Member giantcanopy's Avatar
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    Default Re: Aperture of lenses

    catchlights +1

    The metering will definitely change with different apertures. The last I shot with my 18-200mm, I had to constantly change shutter speed when I am using the 5.6 mm end of the 200mm for the same scene.

    to T/S are you using spot metering ?

    The thing about fixed aperture zooms do not work exactly like calculated ( e.g you cannot calculate the so called aperture of both ends of say a 70-200mm/2.8 with your N = f/D ). To simplify, if I am not mistaken the 70-200mm/2.8 uses a sort of "magnifier" within its construct to recreate the telephoto 200mm end, while able to maintain a big aperture to work.

    Ryan
    Last edited by giantcanopy; 1st January 2009 at 09:19 PM.

  2. #22

    Default Re: Aperture of lenses

    Quote Originally Posted by Daedalus Trent View Post
    'THAT fast'. Now now, thats speculate not backed by facts isn't it? Unfortunately, I myself have yet to find any studies/papers that mention the speed an average human eye takes to change the size of its aperture
    True, we shall not speculate without empirical support. However, I didn't quite find it logical that dark adaptation has no noticeable latency. If there were no latency, we won't be able to tell bright from dark at all... such as when a cloud passes over us, we won't pick up the difference.

    I'm under the impression that if we can't tell the difference, its not due to the aperture of our eyes changing, but rather the change does not hit the difference threshold of a 'Just noticeable difference' - ie. Weber's Law.

    Anyways I tried doing a search, no good results yet...

    and... I'm not sure if TS is still interested or are we running off topic?
    Looking for Canon 100mm F2 USM :)

  3. #23
    Member gymak90's Avatar
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    Default Re: Aperture of lenses

    Quote Originally Posted by Daedalus Trent View Post
    Yes, indeed. FOV plays a very big role!

    Ok, here's a simple analogy:


    You have a football field(the scenery) covered in grass(light). Now, you use a lawnmower to scrape off a small circle in the centre of the field (a telephoto lens with a small field of view )

    You don't get much grass/light yea?

    Now, you use an enormous lawnmover the size of the entire field and turn it on. (this is akin to an ultra-wide angle lens with a huge FoV)

    Now you'll have a lot more grass, yes?


    This is pretty much how the FoV principle works Since the sensor size is always the same, using a wide angle allows you to pack more 'grass' in the same sensor than a telephoto with only can capture a small patch of light in the scene
    Lucky I play football so I can understand
    Oh so the missing link is FoV... Thanks a lot!

    Quote Originally Posted by giantcanopy View Post
    catchlights +1

    The metering will definitely change with different apertures. The last I shot with my 18-200mm, I had to constantly change shutter speed when I am using the 5.6 mm end of the 200mm for the same scene.

    to T/S are you using spot metering ?

    The thing about fixed aperture zooms do not work exactly like calculated ( e.g you cannot calculate the so called aperture of both ends of say a 70-200mm/2.8 with your N = f/D ). To simplify, if I am not mistaken the 70-200mm/2.8 uses a sort of "magnifier" within its construct to recreate the telephoto 200mm end, while able to maintain a big aperture to work.

    Ryan
    Yep. Was using spot metering.

    If N=f/D doesn't apply, then the whole idea of ratio seems to be wrong. But well if you're right, that means, the aperture circumference remained the same, but the view became 200mm thanks to a magnifying effect?

    Quote Originally Posted by ombre View Post
    Anyways I tried doing a search, no good results yet...

    and... I'm not sure if TS is still interested or are we running off topic?
    Yea same here too. Maybe no researcher bothered to find out. Or maybe no researcher is a photographer with the same question as me. =D
    Sure I'm interested in anything dealing with Biology
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  4. #24

    Default Re: Aperture of lenses

    Quote Originally Posted by gymak90 View Post
    Lucky I play football so I can understand
    Oh so the missing link is FoV... Thanks a lot!


    Yep. Was using spot metering.

    If N=f/D doesn't apply, then the whole idea of ratio seems to be wrong. But well if you're right, that means, the aperture circumference remained the same, but the view became 200mm thanks to a magnifying effect?
    I think you're confusing yourself! Haha.

    Maybe lets just come up with a new loose definition. By right the figures below should all be in rate of change, but lets just consider an instantaneous moment.

    C = light collected to the sensor by lens.
    C = summation (light collected by lens)
    C = summation (light bounced off the subjects that converges into the sensor)
    C = summation (light bounced off the subjects in FOV)

    This is of course for the sake of overall metering only. It may sound like using a bigger FOV = more light... but if you have one dark subject against a bright background, theres no difference between the tele view and a wide view =P

    And anyway, this is more primitive than using F stop ratio haha, but its probably how it was derived anyway.


    Yea same here too. Maybe no researcher bothered to find out. Or maybe no researcher is a photographer with the same question as me. =D
    Sure I'm interested in anything dealing with Biology
    Well I found that it the eyes have a latency of 80ms... I forgot to record the source. But then this isn't the time it takes to change 'aperture', but just the time the cones and rods take to pick up a stimuli. Do you study bio by the way?

    Anyways, if really need be, the pupils are just controlled by a set of muscles, which I forgot the name because it was the last term's work (drats~!)... essentially if the eyes really wanted to 'stop down', its a matter of response time from

    signal detection to signal travelling to occipital lobe (80ms) > signal to frontal cortex > signal back to eye muscles (assuming distance is the same this should be 80ms also)

    Should be less than split second?

    We should do an experiment haha,
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  5. #25
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    Default Re: Aperture of lenses

    When i first read this thread i wanted to try zooming in and out a lens, to see with i could notice a change in brightness!

    Then i realised and my lenses have constant apertures
    Last edited by Daedalus Trent; 2nd January 2009 at 09:27 AM.

  6. #26

    Default Re: Aperture of lenses

    Quote Originally Posted by Daedalus Trent View Post
    When i first read this thread i wanted to try zooming in and out, to see with i could notice a change in brightness

    Then i realised and my lenses have constant apertures
    Oh thats okay... we can swap lenses, any day man!


    I do see a difference though, but its pretty slight, i'm not sure if its a placebo effect... maybe we need one of those F3.5-6.3 to see the difference.
    Last edited by ombre; 2nd January 2009 at 09:29 AM.
    Looking for Canon 100mm F2 USM :)

  7. #27
    Member gymak90's Avatar
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    Default Re: Aperture of lenses

    Quote Originally Posted by ombre View Post
    I think you're confusing yourself! Haha.

    Maybe lets just come up with a new loose definition. By right the figures below should all be in rate of change, but lets just consider an instantaneous moment.

    C = light collected to the sensor by lens.
    C = summation (light collected by lens)
    C = summation (light bounced off the subjects that converges into the sensor)
    C = summation (light bounced off the subjects in FOV)

    This is of course for the sake of overall metering only. It may sound like using a bigger FOV = more light... but if you have one dark subject against a bright background, theres no difference between the tele view and a wide view =P

    And anyway, this is more primitive than using F stop ratio haha, but its probably how it was derived anyway.
    Haha, I'm confusing myself?
    But that was what giantcanopy said
    Ok but my understand is the same as your explanation. Thanks!



    Quote Originally Posted by ombre View Post
    Well I found that it the eyes have a latency of 80ms... I forgot to record the source. But then this isn't the time it takes to change 'aperture', but just the time the cones and rods take to pick up a stimuli. Do you study bio by the way?
    Yep I study bio.
    Latency doesn't seem to be a determining factor. There is also a lag time from the optic fibres to the brain, where the image is formed.

    Quote Originally Posted by ombre View Post
    Anyways, if really need be, the pupils are just controlled by a set of muscles, which I forgot the name because it was the last term's work (drats~!)...
    You mean radial and circular muscles?
    Pupils? I think the aperture is the iris leh.

    Quote Originally Posted by Daedalus Trent View Post
    When i first read this thread i wanted to try zooming in and out a lens, to see with i could notice a change in brightness!

    Then i realised and my lenses have constant apertures

    Haiyo, Trent, you belong to the upper tier la. This question applies for newbies using newbie cheaper lenses
    Last edited by gymak90; 2nd January 2009 at 11:40 PM.
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  8. #28

    Default Re: Aperture of lenses

    Quote Originally Posted by gymak90 View Post
    Yep I study bio.
    Latency doesn't seem to be a determining factor. There is also a lag time from the optic fibres to the brain, where the image is formed.


    You mean radial and circular muscles?
    Pupils? I think the aperture is the iris leh.
    I believe they're called cillary muscles if I didn't recall wrongly, haha. Pupil refers to the opening, Iris is just the surface surrounding the pupils. But do correct me if I'm wrong, haha I don't study bio, so you should know better. I only study psych and its covered in visual perception.

    Anyway about latency... latency of responding to the image is more about consciousness I guess. I believe the closing of aperture in our eyes is an unconscious response and therefore does not require our conscious mind to notice the change before responding.

    Also after some thought, I think our eyes may not respond so drastically to such minor changes...Even if you step into the sun from a dark room, still takes about 5-10 seconds before you adapt don't you? But the reverse is even slower... entering a dark from from even a room light lit room takes a long while to dark adapt... and our rods have to bump up the 'ISO' in our eyes, by combining the signals of all the millions of rods?

    Okay I'm throwing out everything by memory, do correct me where I am wrong. =)
    Looking for Canon 100mm F2 USM :)

  9. #29

    Default Re: Aperture of lenses

    No one mentioned this for am I wrong ? I read this somewhere, maybe not entirely in topic, but for most focusing screen in most DSLR, you cannot observe any brighter than f2.8 You put on a 50mm f1.4, aperture switch to f2.8, observe thru the viewfinder and depress the DOF button, there will not be any visible changes. Correct me if I'm wrong
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  10. #30
    Member gymak90's Avatar
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    Default Re: Aperture of lenses

    Quote Originally Posted by ombre View Post
    I believe they're called cillary muscles if I didn't recall wrongly, haha. Pupil refers to the opening, Iris is just the surface surrounding the pupils. But do correct me if I'm wrong, haha I don't study bio, so you should know better. I only study psych and its covered in visual perception.
    If I'm not wrong, iris is the aperture. While yes pupil is the opening. The size of the pupil is determined by the iris loh.
    Cillary muscles, I believe are the muscles controlling the focus of the lens. Cillary muscles are connected to the suspendary ligaments that bind to the lens. So if the muscles contract or relax, the ligaments will become taut or slack respectively. Then this changes the shape of the lens in the eye. And by doing so, the eye can focus on objects of different distances.
    I also go by memory..

    Quote Originally Posted by David Kwok View Post
    No one mentioned this for am I wrong ? I read this somewhere, maybe not entirely in topic, but for most focusing screen in most DSLR, you cannot observe any brighter than f2.8 You put on a 50mm f1.4, aperture switch to f2.8, observe thru the viewfinder and depress the DOF button, there will not be any visible changes. Correct me if I'm wrong
    Really got such thing? I don't have a f1.4 to try. But isn't the focusing screen also a mirror that will just reflect all the light received?
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  11. #31

    Default Re: Aperture of lenses

    Quote Originally Posted by David Kwok View Post
    No one mentioned this for am I wrong ? I read this somewhere, maybe not entirely in topic, but for most focusing screen in most DSLR, you cannot observe any brighter than f2.8 You put on a 50mm f1.4, aperture switch to f2.8, observe thru the viewfinder and depress the DOF button, there will not be any visible changes. Correct me if I'm wrong
    Sounds too familiar, I believe he's right. I've read this somewhere. But TS is in 4-5.6 so don't entirely apply, but good to know anyways.

    Anyway gymak, is the issue still bothering you? Lens still metering incorrectly?
    Looking for Canon 100mm F2 USM :)

  12. #32

    Default Re: Aperture of lenses

    Just tested David's point with an 85mm F1.8. Indeed there is no change from F1.8 to F2.8.. once at F3.2 the change is obvious. Whats the explanation behind this though? kind of weird.
    Looking for Canon 100mm F2 USM :)

  13. #33
    Member gymak90's Avatar
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    Default Re: Aperture of lenses

    Quote Originally Posted by ombre View Post
    Sounds too familiar, I believe he's right. I've read this somewhere. But TS is in 4-5.6 so don't entirely apply, but good to know anyways.

    Anyway gymak, is the issue still bothering you? Lens still metering incorrectly?
    I see. Well this issue comes sometimes if I pay careful attention, other wise I think it's acceptable. Can't really be helped, or can it?
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  14. #34

    Default Re: Aperture of lenses

    Quote Originally Posted by gymak90 View Post
    I see. Well this issue comes sometimes if I pay careful attention, other wise I think it's acceptable. Can't really be helped, or can it?
    The ground glass used for the the focus screen. You need special focus screen to observe aperture changes larger than f2.8, but then it will make your f2.8 dimmer than now

    I also would like to know what is so special about the ground glass used to observe changes. Any PRO out there have good reason for it ?
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  15. #35

    Default Re: Aperture of lenses

    Anyway, TS can ride on your thread to ask one relavent to aperture question? Haha, maybe we can discuss.

    Just wondering. Say you have a 70-300mm F4-F5.6

    @ the 100mm Mark, I am still able to meter at F4, is this essentially a smaller aperture that the camera is unable to detect, perhaps because its not small enough to be F4.5 yet. Or really is it truly F4 still @ the 100mm mark?
    Looking for Canon 100mm F2 USM :)

  16. #36
    Member gymak90's Avatar
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    Default Re: Aperture of lenses

    Quote Originally Posted by David Kwok View Post
    The ground glass used for the the focus screen. You need special focus screen to observe aperture changes larger than f2.8, but then it will make your f2.8 dimmer than now

    I also would like to know what is so special about the ground glass used to observe changes. Any PRO out there have good reason for it ?
    Come in PROS! I haven't heard of ground glass la.
    Quote Originally Posted by ombre View Post
    Anyway, TS can ride on your thread to ask one relavent to aperture question? Haha, maybe we can discuss.

    Just wondering. Say you have a 70-300mm F4-F5.6
    Sure no problem. No I do have a 70-300 F4-5.6

    Quote Originally Posted by ombre View Post
    @ the 100mm Mark, I am still able to meter at F4, is this essentially a smaller aperture that the camera is unable to detect, perhaps because its not small enough to be F4.5 yet. Or really is it truly F4 still @ the 100mm mark?
    Actually I believe at 100mm, it is smaller than F4. Because when you zoom, the ratio keeps changing. And the aperture size would gradually change. However, the camera display just doesn't show it. You don't really see F4...4.1...4.2... I think the display jumps to show the stops.
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  17. #37

    Default Re: Aperture of lenses

    Quote Originally Posted by gymak90 View Post
    Come in PROS! I haven't heard of ground glass la.
    http://en.wikipedia.org/wiki/Ground-glass
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  18. #38

    Default Re: Aperture of lenses

    Hi gymak, you seem to be needlessly confusing yourself

    First of all, the aperture (which means opening) of a lens is the effective diameter of it's front element, NOT its circumference. That is something you really do not need to know to calculate exposure. The iris in the lens can 'stop-down' the lens to a smaller effective aperture, to let less light in.

    We use f-numbers, or relative apertures, to make exposure calculations simpler across a range of lenses. This next part is a little hard to explain without diagrams (and I can't seem to find a good one on the web), but I'll give it a shot.

    Take 2 simple lenses, one with a focal length (F) of 100mm, and the other 50mm. Both have an aperture of 10mm (diameter of the lens).
    Now both are aimed at the same object -- say for example, a spotlit painting in an otherwise dark room. The image is focused on a screen behind them (a piece of paper will do).
    The same amount of light from the subject (the lit painting) enters each lens. But the image projected by the 100mm lens is twice as large (and 4 times the area) as the image from the 50mm lens. Since the image is formed by the light entering the lens, and the amount of light entering each is the same, the larger image produced by the 100mm less is 4 times less intense i.e. same amount of light spread over 4x the area.

    To get the same image intensity (and therefore the same required shutter speed, if we're taking about cameras), we need to increase the amount of light captured by 4x. This would mean doubling the diameter of the 100mm lens, to 20mm.

    Conveniently, a 100mm lens with an aperture of 20mm is an f5 lens (100/20), the same as a 50mm lens with an aperture of 10mm (50/10).

    When photographers talk about aperture, we usually mean the f-number, not the diameter of the lens.

    As for your odd exposure readings, I can't really say without being there, but suggest you try the same experiment aiming at an evenly lit wall. I just did, and my exposure changed as I zoomed and went from f4 to f5.6.

    Sadly, understanding all this will probably not improve your photographs .

  19. #39
    Member gymak90's Avatar
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    Default Re: Aperture of lenses

    Quote Originally Posted by Edwin Francis View Post
    Hi gymak, you seem to be needlessly confusing yourself

    First of all, the aperture (which means opening) of a lens is the effective diameter of it's front element, NOT its circumference. That is something you really do not need to know to calculate exposure. The iris in the lens can 'stop-down' the lens to a smaller effective aperture, to let less light in.

    We use f-numbers, or relative apertures, to make exposure calculations simpler across a range of lenses. This next part is a little hard to explain without diagrams (and I can't seem to find a good one on the web), but I'll give it a shot.

    Take 2 simple lenses, one with a focal length (F) of 100mm, and the other 50mm. Both have an aperture of 10mm (diameter of the lens).
    Now both are aimed at the same object -- say for example, a spotlit painting in an otherwise dark room. The image is focused on a screen behind them (a piece of paper will do).
    The same amount of light from the subject (the lit painting) enters each lens. But the image projected by the 100mm lens is twice as large (and 4 times the area) as the image from the 50mm lens. Since the image is formed by the light entering the lens, and the amount of light entering each is the same, the larger image produced by the 100mm less is 4 times less intense i.e. same amount of light spread over 4x the area.

    To get the same image intensity (and therefore the same required shutter speed, if we're taking about cameras), we need to increase the amount of light captured by 4x. This would mean doubling the diameter of the 100mm lens, to 20mm.

    Conveniently, a 100mm lens with an aperture of 20mm is an f5 lens (100/20), the same as a 50mm lens with an aperture of 10mm (50/10).

    When photographers talk about aperture, we usually mean the f-number, not the diameter of the lens.

    As for your odd exposure readings, I can't really say without being there, but suggest you try the same experiment aiming at an evenly lit wall. I just did, and my exposure changed as I zoomed and went from f4 to f5.6.

    Sadly, understanding all this will probably not improve your photographs .
    Wow this is more complicated than I thought
    Ok I guess I understood, it is not the circumference that matters?

    Eh maybe I can improve my photographs by buying the correct lens
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