Question that got stuck in my head...

Discussion in 'Digital Discussion & Q&A' started by iskoos, Mar 16, 2010.

  1. iskoos

    iskoos TPF Noob!

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    How is it possible that as you go from one of the the lowests f-stops (like 1.4) to all the way to f/16, f/22, f/32 or whatever, the image circle size doesn't change causing vignetting and/or FOV doesn't get smaller?
    (Assume fixed focal length please...)

    I just cannot come up with an explanation:confused:
     
  2. iskoos

    iskoos TPF Noob!

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    He he... I wasn't expecting an immediate answer to this question.
    Looks like I am not the only one that puzzled with it:)
     
  3. Hardrock

    Hardrock TPF Noob!

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    Im not a hundred percent sure but I will give it a try. Just because you change the F-stop doesn't mean you change the focal length. By changing focal length the fov changes but when changing the F-stop the entire image is still going through the lens and hitting the sensor. The F-stop only changes the amount of light and Dof. I know my explanation sucks and may be wrong but that is the best I could come up with.
     
  4. KmH

    KmH Helping photographers learn to fish Supporting Member

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    Because the lenses bend the light.

    The APS-C size image sensor is only 15.8mm high and 23.6mm wide (Nikon) so it's just under 1" wide. 1" = 25.4 mm
     
  5. iskoos

    iskoos TPF Noob!

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    All of the above saying is true but I didn't understand what it has (even remotely) anything to do with what I am asking:)
    Plus I already said "Assume fixed focal length please...) so why chaging focal length. Sorry you tried but not even close...:)

    KmH, what you are saying is true as well but it is not the answer to my question.

    My question is not a cropped sensor body question. It is a generic questions that can apply to any SLR camera.

    Say you have a 50mm prime lens (CAN'T change the Focal Length!..)
    at f/1.0 your aperture diameter is 50mm (a huge opening). You take a picture. Then you stop down to smallest opening say f/22 (or maybe smaller doesn't matter). At f/22, all you have is an opening that is less then 3mm. All other parameters same. You take another picture and you have the SAME FOV... (don't get into DOF, I am NOT even talking about it) you get no vignetting on the sensor. Same picture on both images (with different DOF of course) but same FOV!...
    This I cannot explain. The only one explanation I can come up with is that if the image circle is large enough to cover the entire image sensor at smallest aperture (the highest f number ), then it is alright. If this is true then I know that at any f-stop, I will have an image circle that will cover the entire sensor.
    But if not, then I cannot explain how we get the same FOV when chaging the opening that lets the light enter and reach the image sensor(or film).
    I am hoping that I am able explain myself. If not, then I will have to sketch it up next time.

    So it sounds complex but actually is not when you think about it. And knowing the answer to this will not make me a better photographer. But I will feel better.

    As I think about it now, I feel that I should have posted this in "Beyond the basic" forum... Any moderator reading this:)
     
  6. PhotoXopher

    PhotoXopher TPF Noob!

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    Because it's not external to the lens, much like the pupil of your eye. When your pupil dilates your FOV doesn't change either. If the aperture blades were outside the lens it would be different.
     
  7. iskoos

    iskoos TPF Noob!

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    Good point!.. it seems it is not external to the lens but it sits inside the lens. The aperture blades sits in front of the rear glass-somewhere inside the lens barrel... So it definitely does NOT block the light that hits the front element, but it does block the light that hits the rear element(glass).
     
  8. PhotoXopher

    PhotoXopher TPF Noob!

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    I prefer the term controlling vs blocking - which is exactly what it's supposed to do.
     
  9. KmH

    KmH Helping photographers learn to fish Supporting Member

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    Ok.

    Light has wave-like properties as well as particle-like properties. The famous duality of light.

    Now, consider how light from way off on the sides, top, and bottom of a scene gets onto the image sensor at any lens opening.

    With a wide angle I can make an image that shows a foreground 30 feet wide or with a telephoto a foreground 300 feet wide, all with a lens that is less than 6 inches wide.

    Hummmmm!
     
  10. JimmyO

    JimmyO TPF Noob!

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    Not something im very well read on but you cant think of the image traveling through the lens like a tube, it doesn't work like that.
     
  11. iskoos

    iskoos TPF Noob!

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    I prefer the term controlling vs blocking - which is exactly what it's supposed to do. Today 04:37 PMI prefer the term controlling vs blocking - which is exactly what it's supposed to do. Today 04:37 PM
    That's okay. When I said blocking I really meant partial blocking. Term "Controlling" would make more sense. Agreed.

    When I looked at the other comments, I feel that I couldn't explain myself well.
    I will see if I can sketch this up on a paper. Actually if I had a lens that I can change the aperture manually, I could probably do a few test but I don't have any lens of that kind...
     
  12. mdtusz

    mdtusz TPF Noob!

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    Poke a hole through a sheet of paper then shine light through it and move the paper wider and farther away from a wall. The circle on the wall will get bigger and smaller. Although light travels in "straight" lines, it doesn't really, especially with cameras. Think of the scene as a bunch of little flashlight beams. They all hit the lens then bend different directions, ultimately, 'landing' on the image sensor upside down and backwards. If you were to cut this into cross sections (perpendicular to the lens axis) it would be a series of concentric circles of light, then eccentric circles of light until they are projected onto the sensor. Look at this picture.

    http://dpnow.com/images/5339/lens-angles1.png

    The aperture just so happens to lie close enough to the focal plane (intersection of the lines) that all the light passes through it.

    I think.
     

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