While we wait for the X1D to arrived maybe someone could explain to me how you calculate the equivalent f-stop between cameras with differently sized sensors?
I know some are not super happy with the slow lenses for the X1D, but the sensor is almost four times as large as what I'm used to on my Fujis so I am curious how big the difference really is when it comes to lens speed.
thanks
Forget the silliness about "four times the sensor, two stops more light" ... that's wrong. An f/2.8 lens on either camera allows the same amount of light light energy per unit time to pass through: there's no difference in actual light gathering power. If this were
not the case, you could not use the same exposure meter for a medium format camera as you would for a 35mm camera; the f/number normalizes the lens opening to ensure that f/2.8 on any lens gives the same exposure as on any other, regardless of format.
The difference is in the evaluation of Field of View and Depth of Field. The smaller format camera will use a shorter focal length lens to achieve the same Field of View, which means that a larger lens opening generates more Depth of Field because the physical size of the lens opening for a given f/number is larger.
Example: The Fuji sensor is 16x24mm, so a normal lens on the Fuji is approximately 35mm focal length. The Hasselblad X1D sensor is 33x44mm, its normal lens on the X1D is 68mm focal length for the same FoV (approximately 44 degrees across the diagonal, discounting format proportion differences). So the Fuji camera has a crop factor of ~1.9x relative to the X1D. This is very similar to the relationship between FourThirds format and 35mm FF format (crop factor ~2x based on diagonal measure), so you would expect that a given f/number on the Fuji gains DoF relative to the X1D similarly to the way a given f/number on an Olympus E-M1 gains DoF relative to a 35mm FF format camera.
Running the numbers through a DoF calculator, the DoF produced by the Fuji 35mm at f/2.8 is achieved by a hypothetical X1D 68mm lens at f/5.6, which is virtually the same difference seen by considering an Olympus E-M1 25mm lens (its normal) against a Leica M-P 50mm lens (its normal).
So the difference in X1D Field of View compared to your Fuji camera is about a crop factor of 1.9x and the difference in DoF for the same field of view lens is about 2 stops. This means the X1D 45mm f/3.5 lens behaves similarly to a 23mm lens on the Fuji with respect to field of view and exposure, and at f/5.6 it produces DoF similar to what that 23mm lens on the Fuji would at f/2.8.
You can run the numbers yourself for the 90mm lens ... ;-)
G
BTW: This difference in depth of field is why 35mm cameras were touted as such an advantage over medium format cameras for hand-held work with active subjects. Because of the smaller format and consequent DoF gain, you could get more DoF with shorter exposure times which, back in the day, was a big deal when film speeds averaged ISO 25 for normal film and ISO 160 for grainy, high-speed film. The problem with larger formats then was seen as too
little DoF or too long an exposure time, which 35mm cameras solved nicely with a 2-stop-plus gain in speed due to the format size change with its shorter lenses.