Quote:
Originally Posted by bojan
Hmm.. not quite like that.
Full frame lens used with APS sensor will collect exactly the same amout of light, ...
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YES, the lens
will collect exactly the same amout of light, BUT it is incapable of
delivering all of that light to the smaller APSc sensor (
without further optics & a necessarily different flange distance).
The APSc sensor can only "see" a rectangle of 23.5mm x 15.7mm within the circa 43.2mm Full-frame image circle being cast by the Full-frame lens at the focal plane (based on an APSc sensor size of 23.5mm x 15.7mm).
The APSc image sensor therefore sees
25.2% of the area of the circa 43.2mm image circle cast by the Full-frame lens.
The Full-frame sensor on the other hand "sees" a rectangle of 36mm x 24mm within the circa 43.2mm Full-frame image circle being cast by the Full-frame lens at the focal plane (based on a Full-frame sensor size of 36mm x 24mm).
The Full-frame image sensor therefore sees
58.9% of the area of the circa 43.2mm image circle cast by the Full-frame lens.
Given equal flange distances, on an APSc and Full-frame camera, as is typical for Sony, Nikon, etc.. and
not considering the use of reducers, etc on say mirrorless cameras, then
The ratio of those two percentages, 2.33:1 indicates that the Full-frame image sensor receives 2.33 times more total light than the APSc image sensor, or put another way the APSc sensor receives 42.9% of the light received by the full-frame sensor. It should be noted however: that
the light per unit area (of sensor), and hence the exposure, on both sensors
will/can be the same, BUT
the Total light delivered to the sensor, if you could count the number of photons delivered to the sensor, will NOT be.
Quote:
Originally Posted by bojan
Hmm.. not quite like that.
... , and it will still be 35mm FL...
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YES of course, in this example it is physically ALWAYS a 35mm focal length lens, it's just that it behaves differently on a APSc sized sensor compared to the Full-frame sensor for which it was designed.
Best
JA