Page 1 of 2 12 LastLast
Results 1 to 20 of 39

Thread: Aperture of lenses

  1. #1
    Member gymak90's Avatar
    Join Date
    Jan 2008
    Location
    The Far North
    Posts
    1,448

    Default Aperture of lenses

    My first question of the year!

    When seeing through the viewfinder, it is always at the largest aperture setting permitted by the lens. So assuming I'm using 10-20 f4-5.6, I realise at 10mm F4, the view is equally bright as 20 F5.6. And the metering gives identical exposure settings.

    So what does this mean?

    I understand that the aperture circumference is focal length/f number. Am I right?
    If I'm right, at 10 F4, aperture circumference is 2.5mm. While at 20 F5.6, aperture circumference is 3.5714mm. Given that it is bigger, why the viewfinder didn't go brighter, and the metering didnt give a faster shutter speed?

    I heard, that it could be due to more light being lost within the lens barrel as I go from 10mm to 20mm, hence the viewfinder didnt appear brighter. Is this true?

    Thanks in advance for clarifying my doubts!
    The best things in life are free.

  2. #2

    Default Re: Aperture of lenses

    The aperture only close when u press the shutter. So the lens is always at wide open when you look through the viewfinder. Its only when u press the shutter, the aperture will close to the setting that you set.

  3. #3
    Senior Member
    Join Date
    Apr 2008
    Location
    Singapore, east-ish
    Posts
    2,291

    Default Re: Aperture of lenses

    You forgot one big thing...

    the aperture in your EYE




    Your eye automatically compensates for minor changes in lighting

  4. #4
    Moderator catchlights's Avatar
    Join Date
    Sep 2004
    Location
    Punggol, Singapore
    Posts
    21,901

    Default Re: Aperture of lenses

    between f4 and f5.6 are one stop difference, are you sure your camera tell you that the exposure value of 10mm @f4, is the same as 20mm@f5.6? if yes, time to check your camera metering.
    human eye able to adapt to the change of brightness very quickly, so you may not tell the difference between one stops in the viewfinder, you can do a test using Depth of Field Preview Button to see how much difference of one stop.
    Shoot to Live, Live to Shoot
    www.benjaminloo.com | iStock portfolio

  5. #5
    Member gymak90's Avatar
    Join Date
    Jan 2008
    Location
    The Far North
    Posts
    1,448

    Default Re: Aperture of lenses

    Quote Originally Posted by Daedalus Trent View Post
    You forgot one big thing...

    the aperture in your EYE

    Your eye automatically compensates for minor changes in lighting
    Oops! Haha how could I forget that. I guess I wasted my time studying Biology..

    Quote Originally Posted by catchlights View Post
    between f4 and f5.6 are one stop difference, are you sure your camera tell you that the exposure value of 10mm @f4, is the same as 20mm@f5.6? if yes, time to check your camera metering.
    human eye able to adapt to the change of brightness very quickly, so you may not tell the difference between one stops in the viewfinder, you can do a test using Depth of Field Preview Button to see how much difference of one stop.
    Well I mounted the cam on a tripod, and metered at a piece of paper on my wall. Both at 10mm F4 and 20mm F5.6, the shutter speed remained the same at 1/30s. Both at ISO800.

    Ok so if this case, then for 70-200 f2.8 lens, does it mean that the camera metering 200@F2.8 will give a faster shutter speed than 70@F2.8?
    Last edited by gymak90; 1st January 2009 at 05:07 PM.
    The best things in life are free.

  6. #6

    Default Re: Aperture of lenses

    I cant answer all the technical aspects of this, but one thing I should say is the human eye can't adapt THAT fast, when you suddenly change, you will definitely notice the difference. Even though it eventually adapts, your brain will register that this amount of light is darker.

    As for the Aperture part... I've ever asked myself that question too. For instance, a 300mm F5.6, shouldn't the lens be much larger than a 35mm F2? And thus much much brighter?

    I have no confirmed answer, but my best logical guess is that, the F stop figure is a ratio. And even if the rear diameter is indeed bigger, its the ratio that ultimately matters. Heres an example.

    For the sake of a simple demonstration, all the specifics shall be taken out. Assume we have 2 lenses, one a Tele lens, one a wide lens. Both have the same aperture, say their rear diameter = 1cm. The wide lens will have say... 120 degrees FOV of light coming into the lens, but the Tele lens only has about 30 degrees FOV of light converging on the same spot. Therefore, even though their rear diameters are equal, technically in this case the wide angle is brighter. Of course if you measure up their focal lengths, the F stop on the wide will be larger.

    This crossed my mind when I was wondering if a 85mm @ F2 lens was slower than 100mm F2 lens. Hah.
    Looking for Canon 100mm F2 USM :)

  7. #7

    Default Re: Aperture of lenses

    Quote Originally Posted by gymak90 View Post
    Well I mounted the cam on a tripod, and metered at a piece of paper on my wall. Both at 10mm F4 and 20mm F5.6, the shutter speed remained the same at 1/30s. Both at ISO800
    The 'scene' as per what the camera sees in these two cases would be different.

    What the cam is trying to do is to get to an EV value of 0.0. That being the case, there would be a difference in what the cam, and your eye through the VF, see at 10mm and 20mm. At 20mm there is some 'cropping' being done to the 10mm...

  8. #8

    Default Re: Aperture of lenses

    Quote Originally Posted by ombre View Post

    As for the Aperture part... I've ever asked myself that question too. For instance, a 300mm F5.6, shouldn't the lens be much larger than a 35mm F2? And thus much much brighter?

    I have no confirmed answer, but my best logical guess is that, the F stop figure is a ratio. And even if the rear diameter is indeed bigger, its the ratio that ultimately matters. Heres an example.

    For the sake of a simple demonstration, all the specifics shall be taken out. Assume we have 2 lenses, one a Tele lens, one a wide lens. Both have the same aperture, say their rear diameter = 1cm. The wide lens will have say... 120 degrees FOV of light coming into the lens, but the Tele lens only has about 30 degrees FOV of light converging on the same spot. Therefore, even though their rear diameters are equal, technically in this case the wide angle is brighter. Of course if you measure up their focal lengths, the F stop on the wide will be larger.

    This crossed my mind when I was wondering if a 85mm @ F2 lens was slower than 100mm F2 lens. Hah.
    http://www.clubsnap.com/forums/showthread.php?t=455168

    this might help..not sure if this is your confusion...in any case, the f stop is a ratio

  9. #9

    Default Re: Aperture of lenses

    Quote Originally Posted by gymak90 View Post
    Well I mounted the cam on a tripod, and metered at a piece of paper on my wall. Both at 10mm F4 and 20mm F5.6, the shutter speed remained the same at 1/30s. Both at ISO800.
    Crossed my mind that perhaps the lens has a 'fake' aperture rating?

    The Sigma 30mm F1.4 is one such lens, though rated at F1.4 it never reaches F1.4. The rear diameter is actually 1.4 times smaller than the focal length only, but it seems like those extra glass does little to nothing to converge light onto the sensor.

    There was this guy who performed some tests (I think its on fredmiranda's site for 30/F1.4 review), and there were quite a handful who agreed. At the same scene, the Sigma F1.4 vs another lens at F1.8 metered the same numbers, even though above F1.8, the Sigma's metering was accurate. When manually adjusted to the shutter speed that 'should have been' correct for F1.4, the Sigma underexposed badly.
    Looking for Canon 100mm F2 USM :)

  10. #10
    Member gymak90's Avatar
    Join Date
    Jan 2008
    Location
    The Far North
    Posts
    1,448

    Default Re: Aperture of lenses

    Quote Originally Posted by ombre View Post
    I cant answer all the technical aspects of this, but one thing I should say is the human eye can't adapt THAT fast, when you suddenly change, you will definitely notice the difference. Even though it eventually adapts, your brain will register that this amount of light is darker.
    Trent and Catchlights should answer to this part.
    Actually I agree with Ombre. Yes I've forgot about the eye's iris. But then if the iris really adjusts so quickly, then whenever we press the DoF preview button, we might not notice any difference?

    Quote Originally Posted by ombre View Post
    For the sake of a simple demonstration, all the specifics shall be taken out. Assume we have 2 lenses, one a Tele lens, one a wide lens. Both have the same aperture, say their rear diameter = 1cm. The wide lens will have say... 120 degrees FOV of light coming into the lens, but the Tele lens only has about 30 degrees FOV of light converging on the same spot. Therefore, even though their rear diameters are equal, technically in this case the wide angle is brighter. Of course if you measure up their focal lengths, the F stop on the wide will be larger.
    Oh interesting point. Is this true? Does the FOV play a role?
    But aren't exposure settings based on light intensity? To me, FOV has got nothing to do with light intensity.
    The best things in life are free.

  11. #11
    Moderator catchlights's Avatar
    Join Date
    Sep 2004
    Location
    Punggol, Singapore
    Posts
    21,901

    Default Re: Aperture of lenses

    Quote Originally Posted by gymak90 View Post
    Oops! Haha how could I forget that. I guess I wasted my time studying Biology..



    Well I mounted the cam on a tripod, and metered at a piece of paper on my wall. Both at 10mm F4 and 20mm F5.6, the shutter speed remained the same at 1/30s. Both at ISO800.

    Ok so if this case, then for 70-200 f2.8 lens, does it mean that the camera metering 200@F2.8 will give a faster shutter speed than 70@F2.8?
    how can a camera meter says f5.6 and f4 are the same EV? something is wrong here.
    btw, you say using a piece of paper, that piece of paper must be very big, able to fill up the whole frame when the lens is at 10mm.
    Shoot to Live, Live to Shoot
    www.benjaminloo.com | iStock portfolio

  12. #12
    Member gymak90's Avatar
    Join Date
    Jan 2008
    Location
    The Far North
    Posts
    1,448

    Default Re: Aperture of lenses

    Quote Originally Posted by catchlights View Post
    how can a camera meter says f5.6 and f4 are the same EV? something is wrong here.
    btw, you say using a piece of paper, that piece of paper must be very big, able to fill up the whole frame when the lens is at 10mm.
    Well it happened. 1/30s for 10@F4, and also 1/30s for 20@F5.6. Blame the D70?
    I did spot metering on a poster. Theoretically, spot metering should meter only at the selected AF point.
    The best things in life are free.

  13. #13
    Moderator catchlights's Avatar
    Join Date
    Sep 2004
    Location
    Punggol, Singapore
    Posts
    21,901

    Default Re: Aperture of lenses

    Quote Originally Posted by gymak90 View Post
    Well it happened. 1/30s for 10@F4, and also 1/30s for 20@F5.6. Blame the D70?
    I did spot metering on a poster. Theoretically, spot metering should meter only at the selected AF point.
    shoot a test, load the images in computer to see, also see the Exif data..
    Shoot to Live, Live to Shoot
    www.benjaminloo.com | iStock portfolio

  14. #14
    Member gymak90's Avatar
    Join Date
    Jan 2008
    Location
    The Far North
    Posts
    1,448

    Default Re: Aperture of lenses

    Quote Originally Posted by catchlights View Post
    shoot a test, load the images in computer to see, also see the Exif data..
    Ok shall do a re-test. If something is not repeatable, then it is not Science

    Anyway another question, what about fixed aperture zoom lenses, like 70-200 F2.8. Will more light be entering at 200mm@F2.8 compared to 70mm@F2.8?
    The best things in life are free.

  15. #15
    Moderator catchlights's Avatar
    Join Date
    Sep 2004
    Location
    Punggol, Singapore
    Posts
    21,901

    Default Re: Aperture of lenses

    Quote Originally Posted by gymak90 View Post
    Ok shall do a re-test. If something is not repeatable, then it is not Science

    Anyway another question, what about fixed aperture zoom lenses, like 70-200 F2.8. Will more light be entering at 200mm@F2.8 compared to 70mm@F2.8?
    no difference on how much light enter the lens, as long the external condition is the same.

    and it should be the same amount of light reach the camera sensor, if the EV remain the same.
    Shoot to Live, Live to Shoot
    www.benjaminloo.com | iStock portfolio

  16. #16
    Member gymak90's Avatar
    Join Date
    Jan 2008
    Location
    The Far North
    Posts
    1,448

    Default Re: Aperture of lenses

    Quote Originally Posted by catchlights View Post
    no difference on how much light enter the lens, as long the external condition is the same.

    and it should be the same amount of light reach the camera sensor, if the EV remain the same.
    Oh really? How come? My understanding is that the aperture circumference is bigger with 200mm@F2.8 than 70mm@F2.8, so more light can enter the lens. Because 200/2.8 = 71.428 as compared to 70/2.8 = 25.
    The best things in life are free.

  17. #17

    Default Re: Aperture of lenses

    Quote Originally Posted by gymak90 View Post
    Trent and Catchlights should answer to this part.
    Actually I agree with Ombre. Yes I've forgot about the eye's iris. But then if the iris really adjusts so quickly, then whenever we press the DoF preview button, we might not notice any difference?


    Oh interesting point. Is this true? Does the FOV play a role?
    But aren't exposure settings based on light intensity? To me, FOV has got nothing to do with light intensity.
    Like I said, I don't have the correct answer. But I strongly believe FOV should play a role... but of course, this role is offset if you use F-stops to calculate, since thats a ratio. But if you go plainly by lens diameter and focal length, yea the FOV will come into picture.

    Light intensity is basically a summation of all the light waves in one particular spot? If you have a larger 'area' of light, converging in one spot, the intensity is naturally higher since the waveforms will ... urm... (whats the word!?), lets just use reinforce each other... sorry been out of school for awhile... haha.

    Its just like longer shutter is also allowing more light energy to take effect on the film... (nearly said sensor but the concept is slightly different).


    Its interesting though, that when you press the DOF preview button, you see the difference, but when you zoom, you don't notice it as much. Probably a unique case of monocular disparity.


    Anyway I did test it after reading the post, no funny metering problems like you raised. F4 at 55mm is one stop faster than F5.6 at 250mm. using a 55-250mm F4-5.6 lens.
    Looking for Canon 100mm F2 USM :)

  18. #18

    Default Re: Aperture of lenses

    Quote Originally Posted by gymak90 View Post
    Oh really? How come? My understanding is that the aperture circumference is bigger with 200mm@F2.8 than 70mm@F2.8, so more light can enter the lens. Because 200/2.8 = 71.428 as compared to 70/2.8 = 25.
    This is where the FOV comes in by the way.

    The FOV ratio between the two should be 71.428 : 25

    =)
    Looking for Canon 100mm F2 USM :)

  19. #19
    Senior Member
    Join Date
    Apr 2008
    Location
    Singapore, east-ish
    Posts
    2,291

    Default Re: Aperture of lenses

    Quote Originally Posted by ombre View Post
    I cant answer all the technical aspects of this, but one thing I should say is the human eye can't adapt THAT fast, when you suddenly change, you will definitely notice the difference. Even though it eventually adapts, your brain will register that this amount of light is darker.
    'THAT fast'. Now now, thats speculation 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

    Quote Originally Posted by ombre View Post
    I have no confirmed answer, but my best logical guess is that, the F stop figure is a ratio. And even if the rear diameter is indeed bigger, its the ratio that ultimately matters.
    uh...why are you guessing? F-stops are a ratio , represented by:

    N = f/D

    where N is the F-number
    f is the focal length of a lens
    D is the diameter of the lens aperture
    Last edited by Daedalus Trent; 2nd January 2009 at 07:03 AM.

  20. #20
    Senior Member
    Join Date
    Apr 2008
    Location
    Singapore, east-ish
    Posts
    2,291

    Default Re: Aperture of lenses

    Quote Originally Posted by gymak90 View Post
    Oh interesting point. Is this true? Does the FOV play a role?
    But aren't exposure settings based on light intensity? To me, FOV has got nothing to do with light intensity.
    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

    Then you may ask, how come a 20mm f5.6 lens collects less light than a 200mm f2.8 lens? It's now to do with the size of the aperture. But thats another story

Page 1 of 2 12 LastLast

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •