Focal length confusion...


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benbean

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Feb 4, 2005
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#1
I gave up this hobby years ago and only recently awaken again...
Suddenly the world of film is gone and welcome the digital era :)

Now what is confusing to me is the 1.5x FOV vs image magnification.

For example, a 50mm lense to me is same magnification what our eyes see, and a 200mm should be 4x magnification ratio. With digital, as always mentioned, 200mm should equate to 300mm in film. But 300mm to me is always a 6x magnification ratio..

Am I the only one having problem with the conversion?:dunno:
 

ExplorerZ

Senior Member
Jan 9, 2006
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hkchew03.deviantart.com
#2
benbean said:
I gave up this hobby years ago and only recently awaken again...
Suddenly the world of film is gone and welcome the digital era :)

Now what is confusing to me is the 1.5x FOV vs image magnification.

For example, a 50mm lense to me is same magnification what our eyes see, and a 200mm should be 4x magnification ratio. With digital, as always mentioned, 200mm should equate to 300mm in film. But 300mm to me is always a 6x magnification ratio..

Am I the only one having problem with the conversion?:dunno:
quite right to say that 300mm is 6x compared to our eyes. but for if you compared it to a 50mm lens on a 1.5x FOV it will be 75mm and 300mm will still be 4x of 75mm. just like 200mm is 4x of 50mm.
i think the problem lies with you took 1.5x FOV in to account for a 200mm lens and not 50mm lens thus you get 6x magification
 

Michael

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Apr 5, 2005
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#3
Benbean, the problem is an artificial one. First, a 50mm lens is a 50mm lens, on film or on digital. The problem arises with the film/sensor format you are using and what you are doing later with the negative/file, how much you enlarge it.
What makes the 50mm a "normal lense" for 35mm film is not the magnification factor but the perspective of achieved photo. A lense that has a focal length approximately equal to the diagonal of the negative/sensor produces a picture with similar perspective as what we see with our eyes. So for a medium format camera "normal" is about 80mm and for DSLR with APS size sensor normal is about 30mm and for the PS digital cameras its even less.
However, a 50mm or 200mm lense will give you always the same magnification, provided you enlarge the photo to the same size and that means a photo taken by an APS senser will yield a slightly smaller positive than with a full frame (35mm) sensor.
 

benbean

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Feb 4, 2005
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#4
hmmmm interesting answer Michael. The key word here is perspective. :)
Your answer helps, now I just need to get accustomed to the new numbering...
Thanks Bro.
 

benbean

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Feb 4, 2005
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Upper East Coast
#5
Thanks for the answer. When coming from old school of thought, it will take time to get accustomed to aps size sensor with 1.5x FOV.
Cheers.
Andy
 

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