Originally posted by windhorse I am confused by what I understand to be the advice here that using exactly the same settings (f-stop, ss, ISO, lens focal length) on a FF camera as an APS-C camera may result in different amounts of light landing on the sensor??
The amount of light landing on the sensor, for a given f-ratio, also depends on the focal length.
In the thread you referenced, I assumed that whether one uses an APS-C camera or an FF camera, one wants to obtain the same framing (i.e., the same image content). This implies that the lens on the FF camera has a longer focal length. If you used the same focal length on both cameras, the FF camera would show a wider view.
One can calculate the focal length needed for the FF camera by multiplying the focal length of the lens on the APS-C camera with the factor 1.5 (the "crop factor" between the APS-C and the FF formats).
The amount of light landing on the sensor depends on the shutter speed and the aperture diameter. One can
work out the aperture diameter by dividing the focal length by the f-ratio. For instance a 560mm lens at f/5.6 has an aperture diameter of 100mm.
We established before that the APS-C camera and the FF camera need lenses with different focal lengths. Let's assume we want the same shutter speed for both cameras to capture the same motion blur. If we want the same amount of light for both cameras then we need to make sure that the aperture diameters are the same. From this it follows that we cannot use the same f-ratio for both cameras. If we did, the aperture diameter of the lens on the FF camera would be 1.5 times as large as the aperture diameter of the lens on the APS-C camera (because the focal length of the lens on the FF camera is 1.5 times as large).
The above tells us that we need to stop down the lens on the FF camera by a factor of 1.5, if we want the same total amount of light. For instance, if the APS-C lens is stopped down to f/2.8 then the (longer) FF lens needs to be stopped down to ~f/4. This not only achieves the same total amount of light hitting both sensors, but also (by necessity) the same DOF.
Originally posted by Adam The amount of light is the same, so the same exposure settings should be used.
With the same "exposure settings", you achieve the same exposure, i.e., the same amount of light per square millimetre. In other words, you achieve the same light intensity.
However, the same light intensity would create a different DOF and different noise levels.
In order to get the same DOF and the same noise levels, the
total amount of light (the number of all photons) has to be the same, not the
light intensity (the number of photons per square millimetre).
Hence, in order to get the same image from an FF camera/lens combination compared to an APS-C camera/lens combination, on the FF camera one
- must use a longer lens (factor 1.5),
- stop down more (factor 1.5),
- use a higher ISO setting (factor 1.5*1.5).
The higher ISO setting will not negatively impact on the noise levels, because the FF sensor is larger and thus compensates for that (put very briefly).
BTW, there is one sentence in the
"The Crop Factor Unmasked" article that isn't quite complete. The statement
"If you were to mount the 35mm lens on a Pentax Q, you would in fact get the same background blur as with the 35mm on the K-3..."
is only correct if you also crop the image from the K-3 to Q-dimensions. Otherwise, the K-3 images shows a much wider view (which is stated in the article) but also more DOF (by a factor of 3.7). Yes, cropping changes the DOF, as it is equivalent to using a longer focal length (apart from losing pixel resolution).
To the OP: You may want to also read
my post to the "Full Frame: The Real Pros (and Cons?)" thread and perhaps even more from that thread.
In summary, there is no DOF or exposure disadvantage to using an FF camera, as you can just choose the respective settings (one can call them "equivalent" settings, but one does not need to).
Hope all this helps.
Last edited by Class A; 03-01-2015 at 03:45 AM.