Originally posted by elho_cid It provides decent IQ with high ISO and yet you can use f1.1 lens and have the DOF of f5.6. That's nothing short of amazing.
That's not amazing (and no advantage) at all. The impressive "f/1.1" designation purely can be made because the sensor is tiny and the lens hence only has to gather small amounts of lights in order to achieve the same exposure as a lens for an FF sensor.
Originally posted by snostorm That statement considers only the total amount of light entering the lens, not the light intensity at the sensor needed for a given exposure which is how the concept of f-stop is used by photographers.
The original question was whether the lens retains the "light gathering" abilities of an "f/1.1" lens on FF (while producing the DOF of a "f/5.6" lens only).
Whether my or your answer is correct depends on how you define "light gathering". You are correct in stating that the lens gathers sufficient light to achieve the same
exposure (light density, e.g., cd/mm^2).
I am correct in stating that the
total light gathered by the lens is only a fraction of that light that would be gathered by an f/1.1 on an FF sensor. The total light is "sensor-size * light density", hence the lens for the larger sensor needs to gather much more light.
I believe it is wrong to equate "light gathering ability" with "
exposure" because what matters for image quality is the
total light gathered. That's why the equivalent f-stop for an FF camera is f/5.6, otherwise it would be allowed to collect a lot more light, resulting in much less DOF and more total light. Obviously, f/5.6 results in less exposure which has to be compensated by choosing a higher ISO setting on the FF sensor (->
Falk Lumo: Camera equivalence).
N.B., your thought experiment where parts of the sensor gets masked neglects the fact that the image from the small unmasked part needs to be scaled up more than the original image. If you print your Q images at size 1 x 0.7 (instead of 6 x 4) and look at that tiny print from the same distance as you would look at a 6 x 4 print then you could argue that the resulting quality / light gathering is comparable to the result of an f/1.1 used on FF.
Last edited by Class A; 02-24-2012 at 04:21 PM.