IQ of cheap prime vs nice zoom?

I guess the most relevant comparison for me would be, how do the inexpensive Canon primes such as the 28mm 1.8, 35mm f/2, and the 50mm 1.8 do at f/2.8 compared to say the 17-55 f/2.8 wide open at each comparable focal length?

my 17-55 was pretty sharp wide open, my my 50 1.8 was razor sharp at the same aperture.

Yea my nifty still blows me away with the sharpness it can produce up against my $3000 70-200 f/2.8, and it certainly does not get neglected against my other L's either. I am considering buying an 85 f/1.8 just because of how good the nifty is.

It depends.....

Purely looking at optical performance, most of the affordable Canon primes are quite good once stopped down to f/2.8. I'd guess they would (as a group) hold their own against many of the nicer zooms within the range at similar apertures.

That said, there are three reasons you might prefer one of the better zooms if you do find that f/2.8 is fast enough for you and optical performance and budget are both important:

1) The AF motors on many of these affordable primes suck. The 35/2 and 50/1.8 are particularly egregious examples, but the micro-motor USM of the 50/1.4 is nothing to write home about either. Only the true ring-USM motor equipped lenses like the 28/1.8 and 85/1.8 are up to the same performance of a nice zoom like the 17-55 or 24-70. Even the slower 24-105 beats the pants off the non-ring-USM primes for AF.
2) If you really need the range of focal lengths provided by one of these zooms, the 'budget' primes lose their 'budget' feel pretty damn fast. As a real, real stretch you could replace a 24-70 zoom lens with a 24mm/28mm and 50mm prime set. That saves a bit. But then.....cropping from 24mm all the way to 49mm and cropping from 50mm all the way to 70mm is a little bit of a stretch. Once you decide to go for three primes you are sunk on any savings.
3) Finally....if you have a 1.6X format sensor you will not find any affordable primes that can replace the 15mm to 17mm end of the most popular zooms like the EF-S 15-85 or EF-S 17-55. There are no cheap, fast 17mm primes out there.

at comparable apertures the sharpness of the 50/1.8 is better than the 50L. but the less than desirable bokeh, slow AF, build quality, etc, hurts it, but sharpness and image degradation on a FF at the edges, the 50/1.8 owns it.

They are all reasonably sharp, zooms and primes.
The real difference is as it was mentioned above in bokeh and colors plus contrast.
This is where L, Zeiss and some old primes prevails.

50/1.8 is one of canons sharpest lenses and usually as sharp for the same aperture as the other 50 primes.

I think I had a good 17-55, it was certainly sharp for a zoom but you could tell a prime shot from ones with it.

I've not personally tried the 28/35 tho.

I assume you are talking about center sharpness because if you look at the rest of the image the cheap primes don't perform very well compared to the high quality zooms

35mm f2 @ f2.8 vs 24-70L mk1 at 35mm f2.8
http://www.the-digital-picture.com …omp=0&FLIComp=2​&APIComp=0

50mm f1.8II @ f2.8 vs 24-70 mk1 at 50mm f2.8
http://www.the-digital-picture.com …omp=0&FLIComp=3​&APIComp=0

100mm f2 @ f2.8 vs 70-200 f2.8 II at 100mmm f2.8
http://www.the-digital-picture.com …omp=0&FLIComp=1​&APIComp=0

and all those primes were stopped down at lease one stop...

Comparing my 17-50 f2.8 and 50mm f1.8, the 50mm1.8 is a tad bit sharper @f2.8 then the 17-50 but you would have to zoom in a lot on the photo to see it.

JeffreyG wrote in post #15080959
Even the slower 24-105 beats the pants off the non-ring-USM primes for AF.

Just a note of caution here. Some cameras have autofocus (AF) points which work better with faster (larger aperture) lenses. f/2.8 is one of the cutoffs. So those primes have some AF advantages as well, e.g. when focusing in low light. Exactly which body/lens combination will focus faster and more accurately may depend on a particular combination of camera body, lens, lighting, and subject.

For a more complete explaination check out the "Lines, crosses and double-crosses" section of http://www.the-digital-picture.com …-Autofocus-Explained.aspx.

I primarily shoot landscape and most decent lenses are sharp at f/8-f/11. Doubt I could tell a difference between a prime and a zoom at those apertures.

melauer wrote in post #15084257
Just a note of caution here. Some cameras have autofocus (AF) points which work better with faster (larger aperture) lenses. f/2.8 is one of the cutoffs.

You bring up good points, but at least to me what you wrote could be slightly misunderstood. The cutoff being f/2.8 actually means that no fast primes have an advantage in focusing over a zoom that is f/2.8 over the range. The cutoff is for lenses slower than f/2.8.

And to be more clear, the difference between ring-USM and everything else is much bigger than the difference between the cross type AF points and the non-cross points.

Finally, several of the f/4 "L" series zooms get to use the full array of cross type points even though they are slower than the supposed f/2.8 cutoff. Canon notes this for the 24-105L and 17-40L for example at least up to the 5D3 and 1DX, where things change all over again.

But back to my first point......I'd rather have ring-USM and no cross type points than AFD and cross points.

If we're just talking about sharpness then the nifty fifty is sharper than the 17-55/2.8, but the others, well, it's a little more even (of the ones I have: 24/2.8, 35/2 and 50/1.8). All have harsher bokeh than the 17-55, and to my eye the 17-55 renders colour rather more nicely. AF is much faster and more accurate with the 17-55, and it's USM focus works better with ETTL-II flash. The primes work on my 5Dc, the EF-S zoom doesn't. The primes are smaller and lighter to pack and carry, and are less obvious and "intimidating" when you point them at people.

I hardly ever use the 50/1.8 - but that's only because I have the 50/1.2L which just about lives on my 5Dc. I use the 24/2.8 and 35/2 a lot on crop bodies (the 24 gives a good 45ish equivalent field of view, which I like, the 35 makes a nice fast long-ish normal) and on full-frame (my 5Dc goes most places with me, either by itself - often with the 35 - or in a small pouch with camera, 5Dc+50/1.2 and 24/2.8). Alternatively, I might take my 50D in a camera bag which (just) fits the camera+100-400mm lens, and throw the 24/2.8 in an external pocket, just in case I need something that isn't telephoto.

I use the 17-55/2.8 more when I'm planning to photograph something that I know it will be suited to, rather than as a casual "just walking around" lens.

Just some random thoughts on how I use these various lenses. Maybe it'll help, maybe not.

...Mike