DLA is Nonsense! It should be deleted from the reviews.

[Daniel Browning]

if i understand correctly, you define DLA as





A.what f-numbers are capable of the highest sharpness (depending on the lens, of course)





B.the point of diminishing returns (in the context of a multi-camera comparison)

No. Those are two reasons why I like having DLA on the site. I did not say that they were any kind of definition of DLA. I did not think it was necessary to define DLA, because it is stated plainly on every review on this site and you yourself quoted portions of it.

I don't think a ideal lens has the best sharpness at DLA. the largest aperture is clearly influenced less by the diffraction. any aperture smaller has more diffraction problems.

No. The difference in the effects of diffraction at apertures wider than the DLA are so minute that it is practically imperceptible. It has no influence on resolution whatsoever, and the the difference in contrast is barely measurable. See for yourself in the comparison of these center crops (not the corners):


1D3 + 200mm f/2 L IS at f/2.8 vs. 1D3 + 200mm f/2 L IS at f/5.6.


This result is expected because the ratio between the combined intensity of the second minimum and the airy disk gives you an idea of the smallness of the difference in diffraction that can be seen between f-numbers below the DLA.

DLA means nothing in the real multi-camera comparison. because you can't use DLA to calculate when a 18MP sensor have the same resolution or same amount of sharpness loss as a 8MP sensor.

Of course you can't use the DLA for either of those things. You also can't use DLA to find the sharpest f-number of an aberrated lens. Nor can you use it to find the diffraction cutoff frequency. Nor can it be used to do your laundry or predict winning lottery numbers. There are a million things you can't use the DLA for. That doesn't mean it "means nothing in the real multi-camera comparison."





One of the things the DLA is useful for is predicting which f-number is required in order to avoid diminishing returns of a camera upgrade. For example, if a forum member asks "What f-number do I need to use in order to avoid any blurring effect from diffraction when I upgrade from the 6 MP Rebel to the 18 MP 7D?" The answer is the DLA. Or if they ask "Why don't I get the full expected increase in resolution when I upgrad from 6 MP to 18 MP -- the lens has no aberration and I'm using f/22?". The answer is "because you are beyond the DLA and into the territory of diminished returns." Those are just two examples of when the DLA is useful in the context of multiple cameras.