The relevance of EA at high magnification is that it tells you how much shutter speed you need for given light, which could be of interest to film photographers I suppose

If you want to know how it affects resolution, you go to the

oracle (or do the math by hand

). You tell the oracle your camera model, or crop factor and megapixel count. You set "lens aperture" to the calculated EA number. The oracle shows you how aperture affects your pixels. You remember that only half of your pixels are green and only a quarter is red and that there is an anti-alias filter too. (The oracle helpfully reminds you not to worry if blur is about 2px because at that level you are screwed by your sensor anyway.) You also consider how much you intend to scale the image down for publication.

Without crop factor and without megapixel count of the final, published image those EA numbers are meaningless. It is not surprising to find an order of magnitude spread between different people, gear and purposes.

EA is very convenient in ordinary photography, because it is equal to the F-number at infinity focus, 10% worse at 0.1 magnification and 25% worse at 0.25 magnification. You do the blur calculation once for any combination of camera body and F-number and you are set for life, as long as you stay away from macro.

For the same reason, it's an ass-backwards way to deal with high-mag. Since the dawn of ages and long before digital imaging, microscopists talked about

object-side aperture. You can think about it in two ways. One is to calculate "input referred" blur, which is the sensor side blur divided by magnification. It tells you how the blur compares to magnified features on the object. The object-side aperture formula is: F·(1+M)/M. Or you can imagine that your sensor is the object and your object is the sensor (easy to imagine if the lens is reverse-mounted) and trace the rays backwards and then calculate EA that way, assuming magnification 1/M. The object-side aperture formula is: F·(1+1/M). Observe that the two formulas really are the same and that magnification is almost irrelevant as long as it is far from unity, just like in ordinary photography.

So you go to the oracle again. You tell the oracle your object-side aperture. The oracle shows you how many microns of blur you get on your object. If your object is 1200µ in size and the calculated blur is 1.2µ then you know it's 0.1% of your object. You don't care about the sensor at all, unless pixels are too coarse to resolve features that aren't blurred by the optics - then it's time to increase magnification or get a better camera.

Only at close to unity magnification both the macroscopic and the microscopic approximations are equally wrong by a factor of 2x. Then you are screwed and you have to do the math precisely - such is the sad life of bug shooters and that's why they worry about EA.

Often that´s a good thing but getting more contrast out of the die is always better.

I sometime use ISO10 trick. I take 10 identical shots at ISO100 and average them with GraphicsMagick. ISO noise is greatly reduced and more contrast enhancement can be applied. Lens flare remains

edit

When stacking lenses, diffraction surely occurs in both lenses so in theory you should do both calculations and combine the results. It could be interesting to see what comes out and whether the telephoto lens turns out to be insignificant in practice or whether (maybe) some of the combinations that people recommend aren't really as good as they think because of the aperture of the tele lens. Again, all of that is messy calculations involving pixel pitch and sensor size. Perhaps somebody has already figured it out.

It is generally recommended to use the tele wide open. In my experience with imaging the Chinese opamps (reversed webcam lens stacked onto a point and shoot) stopping down the camera's lens made no improvement and caused bad chromatic aberration

edit edit

Actually, it's trivial. The tele lens is operating normally and focused at infinity. So as long as it isn't some diffraction-bottlenecked megazoom piece of junk, it should be good.