Quote:
Originally Posted by Renato1
Thanks, but the crop factor - applied to focal length and to f number - describes the field of view and depth of field equivalences/characteristics.
At constant ISO, a 50mm F/4 lens in an APS non-Canon camera gives the same image on the sensor as a 75mm f/6 lens on a full frame camera. And a 50mm f/4 lens on a full frame camera gives the same image as a 33.3mm lens f/2.66 lens on a non Canon APS camera.
And putting each of your cameras in your telescope (which I'm very envious of) and taking pictures of the moon, will give images 1.6X and 2X bigger with the smaller sensor cameras than that with the full frame camera.
Plainly effective focal length has changed, while the lens's actual focal length hasn't.
f number is meant to be a dimensionless number that enables one to get equivalencies between lenses. Without taking effective focal length into account, the equivalencies disappear.
Regards,
Renato
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The crop factor is a way of comparing the relationship between sensor size and focal length. Depth of field is entirely determined by the relationship between distance and focal ratio.
There is no such thing as effective focal length when you're talking about optics, there is only it's actual focal length. This is why in my first post I said it's all a misnomer and it can be easily proven scientifically.
I have a Nikon D810 which is a 36.3MP FF camera but it is also the equivalent of being a 16MP APS-C camera if I crop or a 9MP M4/3.
If I took an image with my D810 and a 16MP APS-C camera with my telescope and compared the images, they'd be near enough to identical. The APS-C camera would NOT have a different focal length. The APS-C sensor would NOT be imaging through a different focal ratio.
Consider that what you're suggesting is that merely cropping an image down changes the effective focal length and changes the focal ratio because there is no difference between cropping a FF to APS-C or M4/3 than actually using one.