Why are faster lenses sharper in the telephoto range?

While scanning some of the Canon MTF charts I notice that the faster varients of the same lens almost always always sharper via MTF (and the photo archive thread!).

Such as:

200 f/2L versus the 200 f/2.8L

300 f/2.8L verus 300 f/4L

400 f/2.8L versus 400 f/4L

Why is this? You would think that the greater DOF of larger aperture lenses would resolve more contrast and sharpness? I mean, usually you stop down your lenses a stop or so to get them to 'ideal' sharpness at times, so why wouldn't the "slower" lenses be sharper wide open? But this is almost never the case?

Is it due to more advanced lens design or something else? They usually use more specialized elements in the faster lenses to help with CA and other issues, but why could they not shove these same elements in the slower lenses to offer just as sharp images?

Just curious about this... Appreciate the insight!

It is my understanding that large aperture lenses demand better quality glass to be able to deliver good performance at those large apertures (as the lens needs to "use" more (larger) glass to render the image at those large apertures). If you start with better glass to get acceptable results at large apertures, you're going to have better results with smaller apertures where the demands on the glass' optical quality aren't as big.

This is why (again, to my understanding) the 70-200 f/4 IS was a better performer than the 70-200 f/2.8 IS MK 1; the glass that just didn't make the cut for the f/2.8 was reshaped for the f/4, thus giving a high (f/2.8) quality of glass to the f/4 lens.

My guess is that any photographer who has reached a point where the combination of long and fast lenses is important. will also be very concerned about image quality. The lenses tend to appeal to "sophisticated" photographers, who have learned how important a couple of f-stops can be, and who make demands on their equipment in both build quality and image quality that the "average" photographer doesn't. As a consequence, the manufacturers usually make fast lenses using their best technology.

Another consequence, of course, is price.

Invertalon wrote in post #10281137
While scanning some of the Canon MTF charts I notice that the faster varients of the same lens almost always always sharper via MTF (and the photo archive thread!).

Such as:

200 f/2L versus the 200 f/2.8L

300 f/2.8L verus 300 f/4L

400 f/2.8L versus 400 f/4L

Why is this? You would think that the greater DOF of larger aperture lenses would resolve more contrast and sharpness? I mean, usually you stop down your lenses a stop or so to get them to 'ideal' sharpness at times, so why wouldn't the "slower" lenses be sharper wide open? But this is almost never the case?

Is it due to more advanced lens design or something else? They usually use more specialized elements in the faster lenses to help with CA and other issues, but why could they not shove these same elements in the slower lenses to offer just as sharp images?

Just curious about this... Appreciate the insight!

It's basically the lens design and the quality of the elements.

First off, lenses are designed for their specific maximum apertures. A 300mm f/4 is not a stopped down 300mm f/2.8. It was designed as a f/4 from its inception.

Virtually all lenses are sharpest at about two stops down from wide open. This is because lenses are designed for a range of apertures, and anything with any kind of range in its design must have compromises. The 300mm f/2.8, therefore, is sharpest at about f/5.6, and the 300mm f/4 at about f/8.

In theory, a lens could be designed with a single aperture and maximum sharpness at that aperture. Such a lens, however, would be severely limiting in its use. Mirror lenses have this drawback—this plus the fact that they are almost always use very poor glass to keep the price down.

Secondly, large aperture lenses require very primo front elements. These elements are the most critical elements in most lenses, and cannot tolerate aberations. After all, the light passes through these elements first, and, if distorted, remains distorted forever after. These elements are also the largest elements in the lenses. When grinding and polishing glass, the larger the diameter, the harder it is to prevent aberations from creeping in. Large aperture lenses means means super-precision grinding and polishing out to the very edges, which in turn means very high prices.

Take Canon's 300mm "L" lenses as examples:
The EF 300mm f/2.8L IS USM has a front element with an active area about 107mm in diameter and sells for about $4340 at B&H.
The EF 300mm f/4L IS USM has a front element with an active area about 75mm in diameter and sells for about $1269 at B&H.

A lot of effort goes into that 32mm difference.

^ Makes sense, thank you!