Hi all.


I have to say, it's so difficult to figure out the "tone" everyone is using when there is no sound with the words


My comment for this topic is that the other side of the Rayleigh equation needs to be stated in full:


sin(theta) = 1.22(lambda)/D


The DLA for each camera is calculated based on its angular resolution, which is different for each camera due to each camera having different sized pixels. The angular resolution can be defined as:


sin(theta) = L/f where L is the spatial resolution and f is the focal length of the lens.


So these are all lens parameters and have nothing to do with the sensor. But, if you use the definition that L = 1 pixel width (probably should be at least 2, but we'll leave that for another discussion), then the DLA becomes:


D/f = 1.22(lambda)/(L) which is f-# = (L)/(1.22(lambda))


So as L gets smaller (higher resolution for a given sensor size i.e. APS-C), the minimum f-# becomes smaller.


Remember, this is for an ideal lens with no aberrations. If you add aberrations, then all bets are off as it becomes difficult to separate the two phenomena without getting specific MTFs (shudder, shudder).


The DLA is useful for one thing and one thing only, at what aperture can you expect diffraction to become an important factor in image quality. It makes no statements on the quality of the lens (direct correlation to how well aberrations are controlled), or the actual focal length.


Last thing, I looked at the DLA before I got my 7D and all that it told me was that if I could stay below the DLA, I should try. But, fireworks, misty water, large depth of field and sunlight say use whatever f-# you need to get the image and DLA be damned