EOS 50D, Resolution and Confusion

Sometimes a statement taken in isolation can be the truth, but it's not the whole truth, and in fact in isolation it can be misleading. For example... "Stepping out of a plane flying at 5000ft won't hurt you". That's pretty much true, but it doesn't address the issue of what happens a short time later when you hit the ground at over 100mph. That will hurt you!

What does that have to do with photography you might ask? Well, while reading a number of photography forums I've seen more than just a few participants who have been confused by some recent technical reviews of the EOS 50D. They've picked up on statements made in the reviews which are true, but which then lead them down a path of faulty reasoning to an incorrect conclusion. Let's take a look at a couple of those statements.

#1 - With the 50D, if you stop a lens down past f8, you'll see a decrease in resolution

That certainly is a true statement with many (if not most lenses). I wouldn't dispute it for one moment. However some readers seem to have have taken this statement and concluded (or assumed) that (a) Other cameras don't do that and (b) If you want to shoot landscapes at f16, then the EOS 50D wouldn't be a good camera to chose. Both are incorrect and neither follows from the original statement.

The fact is that most lenses peak in resolution in the center of the frame around f8, and that then leads to maximum image sharpness. Some might peak at f5.6, some might peak at f11 but it's more often f8. The reason is quite simple. As you stop down, you quickly reduce the amount of spherical aberration a lens exhibits, and spherical aberration is the dominant aberration affecting sharpness in the center of the field, so as you stop down and spherical aberration diminishes, sharpness increases. However, as you stop down something else happens - diffraction increases and resolution drops. It just works out that around f8 there's a point when the combined effects of spherical aberration decreasing and diffraction increasing are at a minimum and therefore sharpness is at a maximum. It could be f5.6, it could be f11, depending on the lens, but it's somewhere in that range and the difference between f5.6, f8 and f11 usually isn't very big.

EF 70-300/4-5.6 @ 70mm (EOS 5D)

To the experienced eye, the images above (100% crops) show that resolution does in fact peak at f8 for this lens (on an EOS 5D), but the difference between f5.6, f8 and f11 is pretty small. f4 and f16 are slightly less sharp and f22 is distinctly less sharp.

Since the lens is sharpest at f8, the image recorded by a digital sensor is also sharpest at f8, so stopping down past f8 lowers image resolution with just about all DSLRs. I suppose if the sensor resolution was low enough, it would totally dominate the final image resolution, so such a camera might not show much change until you got to f16 or even f22, but the point there would be that the resolution would be low at all apertures, and at all apertures it would be worse than with a higher resolution sensor. You aren't really concerned about where resolution peaks, but how high the peak is and how high the resolution is at any given aperture. In all cases the higher resolution sensor will yield sharper images.

So, yes, stopping down past f8 with the EOS 50D might well result in lower image resolution. The important omission is that stopping down an EOS 40D, a Digital Rebel XT, a Nikon D300 or a Nikon D90 past f8 would result in exactly the same thing! Not only that, but the camera with the highest resolution would be sharper at f8. So, for example, the 50D image would be sharper than the 40D image.

Stopping down from f8 to f16 when you need depth of field for a landscape is fine. You will lose a small amount of sharpness in the sharpest regions of the image (see above), but you may more than make up for it with increased sharpness of the area within the now extended depth of field. The tradeoff between maximum sharpness at the focus point and extended depth of field is one you have to make with all cameras, and for all cameras of the same format size (crop sensor or full frame), the tradeoff is exactly the same. The ultimate tradeoff is a pinhole camera where the depth of field is huge - but the image is blurred everywhere!

#2 - You need the best lenses to take advantage of the higher resolution of the 50D over the 40D

So here's another statement that has some truth in it but has lead to some confusion. The incorrect extrapolation that some people make is to then suggest that unless you have the best lenses, you won't see the higher resolution of the EOS 50D, so unless you have a bag of "L" series prime lenses, the 40D will be just as good.

Again that thinking is wrong. I've even seen some people suggest that images from the 50D will be less sharp than those of the 40D with most lenses. I'm not quite sure of the "logic" they have used in reaching that conclusion, but they got there somehow!

40D Vs. 50D with EF-S 18-55 kit lens wide open at 55mm

Again these "deductions" are completely wrong. The fact is that the higher resolution of the 50D will result in higher resolution, sharper, images than those from the 40D whatever lens you use. Doesn't matter if it's the pretty average "all plastic kit zoom" shot wide open or a super sharp lens like the EF 135/2.0L shot at f8. The higher resolution sensor of the EOS 50D will result in sharper images in both cases. See the 100% image crops above for a concrete example that proves the point.

If the lens was REALLY bad, such as a pinhole lens or a single element plastic lens, or a regular lens stopped down to f64 (where diffraction would drastically blur the image), you might not see any difference between EOS 40D and EOS 50D sharpness, but the 50D certainly would not be less sharp.

I suppose you could argue that the difference between a "normal" and a "premium" lens might be very slightly greater with a higher resolution sensor, but that certainly does not mean that you don't still see most of that difference with "normal" lenses.

So is this the truth. the whole truth and nothing but the truth, without any room for confusion? No, it's not! The above images and analysis use fairly high contrast black and white resolution test patterns. In the real world, detail isn't usually black and white. It may be low contrast green and red, or light grey and dark grey. However, the higher resolution sensor will still yield results which are at least as good as, and usually better than, a lower resolution sensor. There may indeed be cases where you would need a good lens to exploit the added resolution of the 50D sensor when dealing with low contrast detail - but that certainly does not imply or suggest that there won't also be plenty of cases where the higher resolution of the 50D will be an advantage when using ordinary consumer zooms. In fact most of the time that will probably be the case. So while it's always good to have a better lens, you don't absolutely need a better lens to see the added resolution of the EOS 50D over the EOS 40D, and the 50D image should never be worse.

Conclusion

You have to read highly technical reviews for what they actually say, not for what you think they are trying to say. If you're not conversant with all the factors that are involved in lens and image sharpness (and how the two interact), then be careful of reading things into reviews that aren't really there.