Kit Lens comparison f/3.5 vs f/4.0, which is better?

[S230]

I have a question regarding Lens quality and Lighting. An example is an advertisment Henry's (Photography store) www.henrys.com.

I always thought that the lower the f/stop on the lens the better because it let in more light.

Listed is the the original Digital Rebel Kit's lens is a 18-55mm Lens and has an f-stop of 3.5 - 5.6

But in the same ad it also list the new lens kit is a 17-80mm f.4.
"This kit includes a Canon EF-S 17-85MM4.0-5.6 IS USM Lens"

Shouldn't the original Kit Lens "3.5" be better? I know that the IS and USM is more expensive but does it make up for the light?

[Medic1]

While it is true that the kit lens that originally came with the 300D does have a wider maximum aperture, the lens really is not a quality lens. You notice the difference right away as soon as you buy another lens for your collection. First off, as soon as you pick it up you can tell it is very very light compared to all other lenses, even one of the same focal length and aperture. I think that with the popularity of the Rebel they had to start looking at giving consumers a more quality piece of equipment with the kit. While it is true that the 18-55 is a slighty faster lens....probably not even noticable, you just simply cannot compare it to the quality of the 17-85. To compare max. apertures only on these two lenses does not do the comparison justice.

The Nikon D70 kit lens retails at $500 CDN. The 300D kit lens does not even compare to the quality of the Nikon kit lens. My guess is Canon had to do something to keep par with Nikon. You also have to look at the price......$1799 CDN for the 17-85 kit. That lens retails at Henrys for $899 CDN, compared with $299 CDN for the 18-55 .

[Jon]

A faster f stop doesn't automatically equate to a better lens, although better lenses commonly are faster. It's possible to make a really poor fast lens (and in fact it's harder to fully correct a fast lens than a slower one). The 17-85 has image stabilization, which gives you, more or less, a two step boost in hand-held shutter speed. There's only 1/3 stop difference between f/3.5 and f/4 (wide open), while the 17-85 hits f/5.6 at 85 mm, not at 55 mm. If the aperture change is linear, that says the 17-85 will be faster at 55 mm than the 18-55. But again, it's only about 1/3 stop. USM in this lens really only means that it'll focus more quietly, and doesn't affect the optical quality. The real virtue of the 17-85 is that it gives you a more useful range of focal lengths for normal, day-to-day use (a smidgen on the wide end, and a whole bunch on the tele end).

[hk29]

Also be wary that some lenses with low aperture are actually pretty soft. So even though you can shoot more pictures with low light, they might not be sharp (of course still better than no picture at all).

Bottomline, don't base a lense quality by its specifications alone.

[S230]

Thanks. More info to my internal database (thing in my head).. :)