I notice that my faster lenses seem to make better photos even when shooting at f8 or higher. Or is it just because of the quality of the glass (faster lenses are usually more expensive) or am I just fooling myself?
Results depend on the specific lenses you are using and your expectations. Some people may not care as much about some lens characteristics. So equipment one person considers acceptable may be considered poor by another, and vice versa.
Faster lenses are often, but not always, of higher quality than slower lenses because manufacturers tend to put more care into manufacturing them. But some slower lenses may be more suitable for different tasks.
Also, as you push the aperture smaller (F8 and higher), diffraction degrades image quality. Better-made lenses (which often have faster apertures and cost more) tend to handle it better.
Examples of lenses that make different trade-offs with respect to speed, image quality, and price.
- Canon EF-S 24/2.8 STM – "Slow", but sharp and inexpensive.
- FujiFilm XF 35/1.4 – Fast, sharp, and 5x the cost of a Neewer lens.
- Minolta AF 50/2.8 Macro – "Slow", but sharp with low distortion. Focuses from infinity to 1:1.
- Neewer 35/1.2 – Fast, but not sharp and poor flare handling. What to expect from such a cheap lens?
Yes, faster lenses are generally higher quality.
There's another aspect too - when autofocusing, the lens is generally opened up to its widest aperture. This narrows the DOF which can affect the accuracy of the focus.
Any broken bottle of f/4 or higher will be great stopped down to f8.
There is certainly a difference when it comes to phenomena like flare, coma, spherical aberration, spherochromatism, low contrast etc which haunt, especially non-aspherical, primes when wide open. They will often be better corrected in eg an f/1.4 50mm at f/2 than a "native" f/2 50mm. Of course, it always depends to some degree at what you are comparing.
Also, you can focus with the lens wide open, which will make AF performance better for many systems, and will make MF easier (or get you a usable finder image AT ALL in very low light situations!).
Mind that some effects that depend on there being an aperture-stop-shaped aperture stop (eg sunstars, many bokeh factors) will also be different...
Focusing accuracy of low f lenses doesn’t have anything to do with DOF, but with the higher luminosity (light that goes thru the glass) of the faster lenses –what you see in the viewfinder of a DSLR with a pentaprism is always the lowest f-stop or larger aperture available in that lens and focusing is done thru the lenses at this f-stop– compared with slower one specially noticeable in low light situations.
If you are buying brand name fast lenses, f-stops below f1:2.0, you are not probably buying a zoom lens but your single focal distance and you are also paying top dollars for them. 90% of that translates in much better lens design and manufacturing than what you would find in cheaper lenses.
Anyways the sweet aperture (best result) of most, if not all, fast lenses are obtained when they are stopped down to f1:5.6 or f1:8 but almost never when you approach any of the extremes f1:1.4 or f1:22.