From
what I was able to find the EF vs EF-S is purely a cosmetic thing, EF-S lenses are made to sit deeper in the body, Image quality is supposed to be the same. In any case I am perfectly happy with my current EF-S lens, so I see no reason to opt for EF over EF-S if similar options are available.
IIRC, it is more than 'cosmetic': as you point out, the EF-S lenses sit deeper into the body, and the implications of this are that if one tries to use an EF-S on a Full Frame (d)SLR, that is an obstruction that will be hit by the mirror when it tries to flip up to take a photo. This ruins the shot and probably will also damage/break the mirror, thereby trashing that FF body.
Granted, this was far more noteworthy in the early days, when all Canon legacy users were shooting FF because they were on 35mm film.
IMO, the question for photographers today is if they think that they're ever going to buy a Canon FF dSLR ... and ... have a concern for the fact that these EF-S lenses won't be able to be used on that future possible gear. Personally, I own two EF-S lenses myself, and I'm not particularly concerned about them not fitting onto a 5D Mk3 body...nor any of my old 35mm film bodies anymore, either (I already have those focal lengths covered with EF mount lenses).
Edit:
it appears EF-S lenses do not go beyond 250mm
Which makes sense. In a crop body, the 1.6x multiplication factor makes a 250mm equivalent to a 400mm, which is more telephoto reach than what the non-professional generally ever thinks of having. And for those more advanced users that do want more reach, there's longer telephotos in the EF lenses ... the 100m-400m is an example (400mm --> 640mm equivalent).
What do you mean by a comparable opticallity?
The manufacturers have a couple of different product groups, and the characteristics of these products are determined by a couple of factors (cost, physics, target consumer group, etc).
To oversimply, there's cheap stuff, good stuff and high quality stuff.
To make things a bit more complicated, some cheap stuff can be really good optically, such as the classical 50mm prime lens ... its good because from a physics standpoint, it is very simple (not a zoom lens), which is what allows it to be cheap.
Stereotypically, the cheap stuff are the "Kit" lenses that come bundled with cameras ... the manufacturer's temptation is to make them cheap, because doing so helps keep the price point for the camera down, which helps to sell more cameras.
What we need to understand is that the manufacturer isn't trying to make a poor optical quality lens - his motivation is price, and if he has to compromise on optical quality in order to do that, he will. These designs are strongly influenced by the cost to manufacture, so they're (generally) not the best possible optical solution.
FYI, the good news is that with computer-based optics design tools, the optical quality of these lenses are much better today than what their equivalents were ~20 years ago.
The other extreme is the high quality stuff. Figuratively, cost is no object: just make its optics as good as possible (at all levels). These are our Canon "White" L telephotos and the like, and they all carry hefty price tags. When you dig into the technical details of what's inside them, you typically find many more lens elements, fancy coatings, exotic materials, tighter dimensional tolerances on the lens elements' grindings, etc. Simplistically, these are all things that costs more money, to eke out the last 5% of performance.
In the middle is what I've called the "Good stuff" ... this is a middle ground where the manufacturers are seeking to offer products that their consumers who bought the "Kit" lenses would be interested in moving up to, while still not being as expensive as the L glass.
FWIW, if I had to put a KISS rule of thumb, I'd say:
$100 - Kit
$500 - Good
$1000+ - L Glass
What makes any lens recommendation hard is that no one knows where the person asking for advice may end up.
There is a truism in that one can easily "waste money" by buying & selling your way through these different lens quality levels, and it isn't uncommon to hear recommendations to skip buying the lower quality stuff because you'll save money in the long run.
For example, I learned pretty early to skip the "Kit" stuff, but I was quite content for many years with only having "Good" lenses. What began to change it for me was
one telephoto photo that I believe should have come out a lot better than it had, so I'm now buying L glass.
However, let's put that into perspective, I went for years before I was
finally motivated to go spend more money for top quality glass. So while it is technically true that I lost ~$250 on the 'good' lens I had been using, this gets counterbalanced by the fact that I kept that $1000 (for the L glass) in my own pocket for years. If we figuratively say that I kept it in its own savings account at the bank for (back then) 5% interest, after only 5 years it would have earned $250 in interest savings, so I didn't really lose any money afterall by having the less costly 'good' lens. So I'd not worry too much about the "only buy the best" advice: buy just what you think is appropriate for the next couple years...not what you think you might need for the rest of your life.
But I digress (extensively!).
I'll defer to others for comment as I'm not all that too familiar with either the 18-135 IS nor the 55-250mm IS, but based simply on price points (both around $500, much like the 70-300 IS), my gut reaction is that both are "Good" class lenses. As such, I'd guess that with these choices under consideration, you're mostly looking at buying a different focal length more so than you're looking at buying a substantially better "Quality" piece of optics.
When I originally wrote that, the zoom was my main concern, but after having tried it today the latter has also become a concern. It feels a lot less durable then my 18-135mm, which also has a certain resistance to it when zooming whereas the 55-250mm I tried today was just all over the place.
Well, 'durability' is another metric, as is also how well does a lens interact with your camera's autofocus system (potential for excessive 'seeking', etc).
From reading all of the above posts I'm thinking the
Canon EF 70-300mm f/4-5.6 IS USM is the lens for me, although the costlier option... - I was able to find them used all the way down to ~$300 but I am not sure that is a
great idea.
I would still like to hear any thoughts though!
As a former owner of its 75-300 predecessor, I read
this Amazon Review quite carefully. I think that this reviewer does bring up a good point: make sure that any telephoto you get includes its hood (here, its +$40). I'm also very wary of how he says that this lens is still a slow-focuser and soft beyond 200mm...my next step would be to go read the 55-250mm reviews to see if it is sharp or soft at 200mm+ or not, and do some additional research.
FYI, if you're finding examples at $300, I'd be
very careful about the listings to make very sure that it is not the 75-300mm predecessor. I'd even go as far as to explicitly request an eBay seller to reconfirm that in writing.
-hh
