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Thread: What does the mm mean?

  1. #1
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    Default What does the mm mean?

    Kept thinking & thinking... think i got the most newbie question to ask...

    Y is 50mm, a 50mm? how do we define it 50mm?

    Is MF 50mm, LF 50mm same as 35mm's 50mm?
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  2. #2

    Default Re: What does the mm mean?

    the focal length of a lens is independant of the sensor format.

    a 50 mm lens means that the distance between the focal point (the sensor plane if the image is in focus) and the optical centre of the lens is exactly 50 mm.

    that is why for a p&s camera the specifications would read something like 5.8-24mm (cp5400) and not 28-116 (the 35mm equivalent)

  3. #3
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    Default Re: What does the mm mean?

    thanks for explanation...

    but den again, 24 is using inverse so it means althou its far, but it is near? and those long zoom but due to some lens reflex, become shorter than 500 is also normal?

    also, 50mm for 35mm is standard 'eye' view, MF is also 50mm? LF also 50mm? den so if i use MF 50mm on 35mm, is it still 50mm?
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  4. #4

    Default Re: What does the mm mean?

    field of view (FOV) is not the same as focal lenght. the FOV of a 50 mm lens will change depending upon the sensor (MF,FF,APS etc). so yes, in all practical purposes we will see different things when we mount the same lens on different backs. this is FOV.

    the focal length doesnt change. of course in a zoom lens, the focal length changes while zooming

    i am a newbie too, so i have no idea what you are talking of when you say 24mm inverse and lens reflex. could you elaborate??

  5. #5
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    Default Re: What does the mm mean?

    Quote Originally Posted by vkashi
    field of view (FOV) is not the same as focal lenght. the FOV of a 50 mm lens will change depending upon the sensor (MF,FF,APS etc). so yes, in all practical purposes we will see different things when we mount the same lens on different backs. this is FOV.

    the focal length doesnt change. of course in a zoom lens, the focal length changes while zooming

    i am a newbie too, so i have no idea what you are talking of when you say 24mm inverse and lens reflex. could you elaborate??
    was thinking... like say 50mm is like 50mm from the focal plane, but if we see, from the focal plane to the mount ring is roughly like 30mm or more... so when we say 24mm, how come minimum already more than that, its still able to be 24?
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  6. #6

    Default Re: What does the mm mean?

    unfortunately, the optical centre of the lens could be anywhere. i believe there are cases where the optical centre of the lens mechanism lies outside the lens itself! so you cannot correlate the physical dimension of the lens with its focal length.

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    Default Re: What does the mm mean?

    Quote Originally Posted by Del_CtrlnoAlt
    Is MF 50mm, LF 50mm same as 35mm's 50mm?
    A millimetre is a unit of length. Physical units are the same, regardless what film format one uses. (One does not even have to take photos at all.)

    Otherwise, I would see an interesting business opportunity in buying and reselling flats with magically increasing floor spaces.

  8. #8
    Senior Member icarus's Avatar
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    Default Re: What does the mm mean?

    Quote Originally Posted by Del_CtrlnoAlt

    also, 50mm for 35mm is standard 'eye' view, MF is also 50mm? LF also 50mm? den so if i use MF 50mm on 35mm, is it still 50mm?

    Due to the larger film size of the MF... 50mm on MF cameras = 24mm on 35mm cameras
    Yngwie J. Malmsteen - "...I've never considered myself a fast guitar player..."

  9. #9

    Default Re: What does the mm mean?

    Quote Originally Posted by icarus
    Due to the larger film size of the MF... 50mm on MF cameras = 24mm on 35mm cameras
    although i understand what you are saying, it is technically incorrect to say that. 50mm on MF is 50mm on FF camera! the FOV of 24mm on MF is equivalent to 50mm on FF camera

  10. #10
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    Default Re: What does the mm mean?

    getting more & more chim...

    think there is a few mm that i jumble up... ok solve 1 thing 1st...

    FOV of a 50mm on a 35mm is it same as 50mm on a MF?
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  11. #11

    Default Re: What does the mm mean?

    FOV- field of vision is dependant on lens focal length, film/sensor format type and subject distance.

    for e.g. if we have a 50mm lens and a subject standing at 5 meters from the sensor plane.

    with 6cm X 4.5cm format system we can cover an area of approx. 5.9mtr wide and 4.5mtr tall at 5mtr. so FOV is 5.9 X 4.5. the focal length and the lens still remain 50mm.

    with 35mm format system we can cover an area of approx. 3.5m wide X 2.3m tall at 5mtr. lens focal length still remains 50mm.

    with APS sensor it becomes approx 3m X 1.6m.


    techically speaking FOV is dependant on subject distance. so another measure- the angle of view is used. these angle of view values are independant of subject distance yet dependant on film/sensor format type.

    lens focal length is a property of the lens irrespective of the film/sensor format type. if you mount a 50mm lens on an elephant's eyes, it still remains a 50mm lens

    if your are wondering where i get all those figures from- there is a very nice small free software called f/calc which you can get from http://www.tangentsoft.net/
    just go through that calculator and you can have fun as well as learn about many variables involving camera calculations.

  12. #12

    Default Re: What does the mm mean?

    mm = millimetre = 0.001 m = 0.1cm

  13. #13

    Default Re: What does the mm mean?

    Quote Originally Posted by user111
    mm = millimetre = 0.001 m = 0.1cm
    lol

  14. #14
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    Default Re: What does the mm mean?

    There's certain calculations from a 35mm lens to MF/LF body system. But why want to know when you can't even use most this lenses on the MF/LF body?
    Pros out there do correct me if I'm wrong. Thank you.
    Last edited by Spectrum; 3rd November 2005 at 11:28 AM.

  15. #15
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    Default Re: What does the mm mean?

    In Taiwan and some parts of China, "mm" refers to "model" or "meimei"

  16. #16

    Default Re: What does the mm mean?

    80mm on 1:35 format is equivalent to 50mm on MF

  17. #17

    Default Re: What does the mm mean?

    Quote Originally Posted by user111
    80mm on 1:35 format is equivalent to 50mm on MF
    Firstly there are multiple MF sizes.
    Secondly, even if so I think u got the equivalent field of view comparison backwards.
    80mm on most 645/6x6 MF would roughly be similar to 50mm FOV on 35mm.

  18. #18
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    Default Re: What does the mm mean?

    Quote Originally Posted by Zerstorer
    Firstly there are multiple MF sizes.
    Secondly, even if so I think u got the equivalent field of view comparison backwards.
    80mm on most 645/6x6 MF would roughly be similar to 50mm FOV on 35mm.
    Yah! Something like that dude. Different format of MF/LF also give diff FOV too.

  19. #19
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    Default Re: What does the mm mean?

    Quote Originally Posted by user111
    80mm on 1:35 format is equivalent to 50mm on MF
    you got in in reverse. for hasslebald, 80mm is the standard lens

  20. #20
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    Default Re: What does the mm mean?

    Medium Format uses 120 film, but can be 645, 66, 67, 68, 69, 612 and 617, the last two is a roll film holder uses on large format camera.

    So the Standard lens for each format are different.

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