LCHIEN
The focal length of a lens is the focal length regardless of anything else. The equivalent field angle of acceptance of a lens used for digital photograhpy is often expressed as being equivalent to a 35mm (film size) lens.
Lets say hypothetically at some distance D a 50 mm lens fills the 24x36mm negative of a 35-mm film camera with a 6-foot man. e.g. his image is 24mm (approx an inch) tall.
on a Digital camera with a 50mm lens, at the same distance D, the image will still be 24mm tall, BUT the image sensor for a digital camera is typically smaller than the 24x36mm frame size of the film camera.
So if its 12x18mm say, then you'll have effective magnification of a 100 mm focal length 35mm film camera. So in this case, its 2:1. Actual image sensor sizes are not standardized - film had to be because it was a physical medium but digital cameras, once the image is recorded, its virtual 1600x 1200 pixels or whatever.
The trade-offs of a digital image sensor size are
cost (smaller=better)
number of pixels (more=better)
noise (larger=better)
sensitivity (larger = better)
dust effect (larger=better)
camera compact size (smaller=better)
lens cost and size (smaller=better, because the image doesn't need to be good as far out)
etc. so you can see how this leads to many different compromises.
So every camera maker has different sized sensors. probably ranging from about 1:1 to 1:4 compared to 35mm image size.
BTW, I have a drawer full of OM-system lenses. Probably about 6 or more
(28mm 50mmx2 135 200 75-150z 35-200z 55mmmacro) I've looked with interest at Olympus' 2/3 digital SLR cameras (the sensor is 2/3 of the 35mm image size) but I don't think I'm ready for that...
Lets say hypothetically at some distance D a 50 mm lens fills the 24x36mm negative of a 35-mm film camera with a 6-foot man. e.g. his image is 24mm (approx an inch) tall.
on a Digital camera with a 50mm lens, at the same distance D, the image will still be 24mm tall, BUT the image sensor for a digital camera is typically smaller than the 24x36mm frame size of the film camera.
So if its 12x18mm say, then you'll have effective magnification of a 100 mm focal length 35mm film camera. So in this case, its 2:1. Actual image sensor sizes are not standardized - film had to be because it was a physical medium but digital cameras, once the image is recorded, its virtual 1600x 1200 pixels or whatever.
The trade-offs of a digital image sensor size are
cost (smaller=better)
number of pixels (more=better)
noise (larger=better)
sensitivity (larger = better)
dust effect (larger=better)
camera compact size (smaller=better)
lens cost and size (smaller=better, because the image doesn't need to be good as far out)
etc. so you can see how this leads to many different compromises.
So every camera maker has different sized sensors. probably ranging from about 1:1 to 1:4 compared to 35mm image size.
BTW, I have a drawer full of OM-system lenses. Probably about 6 or more
(28mm 50mmx2 135 200 75-150z 35-200z 55mmmacro) I've looked with interest at Olympus' 2/3 digital SLR cameras (the sensor is 2/3 of the 35mm image size) but I don't think I'm ready for that...
Loring in Katy, TX USA
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