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Thread: "Light" of WA and TC.

  1. #1

    Default "Light" of WA and TC.

    For film based telecovertors, i heard that there is a drop in stops of light i.e. if i use a 2X tele convertor, my effective aperature will drop by 1 stop. wondered is it true that if a 2X TC is used it will drop by 1 stops. If a 1.4 X TC it will drop by 1/2 stop? Since "F stop=(focal length/diameter of opening of lens)"

    How about those convertors used in digital cameras, which is installed at the front of the lens. Do they exhibit the same phenomema?

    How about wide angle convertors? do they increase the amount of light? ( quite lame right? having more elements will increase light? )

  2. #2

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    for SLR teleconverters (those put between body and lens), a 2X will cause 2 stops loss of light (not 1), and 1.4x will cause 1 stop loss.

    For TCs that add to the front of the lenses, loss of light is quite minimal, can say that there is no loss, for TCs that are well-designed. Same goes for WAs, no loss, and definitely no addition in light. Cheers!

  3. #3
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    Default Re: "Light" of WA and TC.

    Originally posted by sweat100
    For film based telecovertors, i heard that there is a drop in stops of light i.e. if i use a 2X tele convertor, my effective aperature will drop by 1 stop. wondered is it true that if a 2X TC is used it will drop by 1 stops. If a 1.4 X TC it will drop by 1/2 stop? Since "F stop=(focal length/diameter of opening of lens)"

    How about those convertors used in digital cameras, which is installed at the front of the lens. Do they exhibit the same phenomema?

    How about wide angle convertors? do they increase the amount of light? ( quite lame right? having more elements will increase light? )
    For SLR TCs, a 1.4x will drop by 1 stop, 2x will drop by 2 stops. Every stop has a factor of 1.4 or squareroot of 2.

    For TCs and WAs which goes in front of the lens (the ones most DCs use), there's usually no appreciable loss of light as long as the front element is big enough.

    Regards
    CK

  4. #4

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    So the formula does not apply here?
    For example if a 50mm lens that is at F1.4. If i add on a 2X TC, its effective focal length will be 100mm ( with the camera still set to F1.4) from the formula, it will be F2.8. Isnt that 1 stop? (or i got the definition of "Stop" mixed up?)

    All along i thought, if u add TC infront, you will use the effect focal length and divide by the diameter of the opening. Since the focal length is increased and the diameter remains the same, so by right it will change the F number?

    Wouldnt it be better if i am using a film SLR and use a digital camera TC so i woundnt get a reduction in the amount of light?

  5. #5
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    Default

    Originally posted by sweat100
    So the formula does not apply here?
    For example if a 50mm lens that is at F1.4. If i add on a 2X TC, its effective focal length will be 100mm ( with the camera still set to F1.4) from the formula, it will be F2.8. Isnt that 1 stop? (or i got the definition of "Stop" mixed up?)

    All along i thought, if u add TC infront, you will use the effect focal length and divide by the diameter of the opening. Since the focal length is increased and the diameter remains the same, so by right it will change the F number?

    Wouldnt it be better if i am using a film SLR and use a digital camera TC so i woundnt get a reduction in the amount of light?
    In your example, your 50/1.4 with a 2x TC becomes a 100/2.8, that's 2, not one stop from 1.4. Every stop has a 1.4 factor, so the f-stop scale goes like 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32 and so forth.

    For those TC's in front, the formula of aperture = focal length/f-number no longer applies.

    For film cameras, if you try to use a digital camera TC/WC, you might see some softness, especially at the edges.

    Regards
    CK

  6. #6
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    Default

    Originally posted by sweat100
    So the formula does not apply here?
    For example if a 50mm lens that is at F1.4. If i add on a 2X TC, its effective focal length will be 100mm ( with the camera still set to F1.4) from the formula, it will be F2.8. Isnt that 1 stop? (or i got the definition of "Stop" mixed up?)

    All along i thought, if u add TC infront, you will use the effect focal length and divide by the diameter of the opening. Since the focal length is increased and the diameter remains the same, so by right it will change the F number?

    Wouldnt it be better if i am using a film SLR and use a digital camera TC so i woundnt get a reduction in the amount of light?
    the light drops by one stop, so 1.4 (1/2) -> 2.0 (1/4)
    the maths sort of fails here cos the TC is more like a magnifying glass rather than a full lens extension/conversion...
    Last edited by denizenx; 28th December 2002 at 12:01 AM.
    "I'm... dreaming... of a wide... angle~
    Just like the ones I used to know~"

  7. #7
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    Default

    Anyway, to add on, for the f-stop scale, you just need to remember 2 numbers : 1 and 1.4 (root2)

    So here's how it's derived.

    1
    1 x 1.4 = 1.4
    1.4 x 1.4 = 2
    2 x 1.4 = 2.8
    2.8 x 1.4 = 4
    4 x 1.4 = 5.6

    and so forth.

    Remember that 1.4 is really square root of 2, which is 1.41421356.....

    Regards
    CK

  8. #8

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    Icic... No wonder i tried doing some maths and cant really get things straightened out. Thanks everyone for clearifying me.

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