Same image on a 35mm camera vs a digital camera (smaller sensor) has a different focal length. Thus, an aperture of f4.0 is a different size hole for both ... and thus allowing in a different amount of light. How can a hand held light meter be accurate for both? Good answers. But my confusion remains ... if you have a 35 mm camera with a 40mm lens f4.0 would be a 10mm hole. If you have the same angle of view on a little digital, 40mm equivalent would be SAY 8mm and f4.0 would be a 2mm hole. (The smaller sensor results in a much shorter focal length for the same angle of view)
If you used the hand held meter (for an incident reading which the camera can't do!) and you got 1/60th f4.0 ... how could this possibly be correct for the two cameras ... the 10mm hole is bigger than the entire other camera's lens! Maybe thinking about it this way might help:
Say you have a digital sensor 2/3 the length and width of the standard 35mm film, and the correct exposure will be achieved for the film at f4. If you are using a 40mm lens on the film camera, this will be an average iris diameter of 10mm, or a iris area of 314mm2.
The digital sensor is 4/9ths (2/3 x 2/3) the area of the film, so it will need only need an iris area of 139mm2 to collect the light required for the same exposure at the same sensitivity, or an iris diameter of 6.67mm. But the lens focal length to achieve the same field of view is only 26.7mm. For the 26.7mm lens, an iris diameter of 6.67mm is 26.7/6.67 or f4. (Note that the sum only works as conveniently at f4, other f stops required more complex calculations but yield the same results.)
So to achieve the correct exposure, an aperture of f4 is required no matter what the lens focal length or sensor size, which is exactly what the incident meter indicated, and is part of the beauty of how f stops are calculated. you are confused, exposure is exposure
F4 on a lens is the same if you put it on a DSLR or a SLR
as are all the other appetures,
trust the meter not your logic above
go hard
ps you thumb downers go hard boo hoo oh boo hoo You have a good point.
The focal length of the lens is the same no matter how large the sensor is. A 400mm f/2.8 is still a 400mm f/2.8 whether used on a 35mm camera or DSLR. As you know, f/stops are a mathematical ratio between focal length and the aperture of the lens. The 1.5x focal length multiplier is only used to help the photographer determine the field of view of any lens they may be using (in this case similar to a 600mm f/2.8 which doesn't exist at this time), but does not effect the actual focal length of the lens.
Since most modern cameras now have not only excellent meters, but choices in how they meter light (matrix, center weighted or spot), there are very few instances one would need to use an external light meter. | 