What does the mm mean?


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Del_CtrlnoAlt

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#1
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?
 

#2
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)
 

Del_CtrlnoAlt

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#3
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?
 

#4
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??
 

Del_CtrlnoAlt

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#5
vkashi said:
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?
 

#6
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.
 

LittleWolf

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#7
Del_CtrlnoAlt said:
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.
 

icarus

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#8
Del_CtrlnoAlt said:
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
;)
 

#9
icarus said:
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
 

#11
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.
 

user111

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#12
mm = millimetre = 0.001 m = 0.1cm
 

Spectrum

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#14
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.:)
 

user111

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#16
80mm on 1:35 format is equivalent to 50mm on MF
 

Zerstorer

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#17
user111 said:
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.
 

Spectrum

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#18
Zerstorer said:
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.:confused:
 

catchlights

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#20
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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