SLR Lens question: Is bigger better?

[JSCampbell]

Hiya All,I just bought a Canon 350D SLR. It came with an 18-55mm wide angle lens. I am looking into a telephoto lens but have a couple of questions.Why is the 72mm dia. so much more than the 58mm dia lens?Other than light gathering, is there a difference?Is it worth the difference in price?Thanks in advance

[ZubenElGenubi]

There is absolutely a measurable difference in image quality for a larger lens and worth the price difference.First of all, a 72mm lens has 54% more surface area (light gathering area) than a 58mm lens. That's more glass - and where the cost factor comes in.If the focal lengths are equal (say, 400mm), then the 72 mm lens is also faster by about a half-stop (400mm fl, 72mm lens = f/5.6; 58mm lens = f/6.9). This means you can use shorter exposures.Personally, I wouldn't buy any telephoto lens with an aperture less than 72mm, regardless of focal length.{Edited...fatfingered the surface area!}

[RIT]

Isn't it 54% more surface area - not 154%. But your other points are daed-right. Good points.

[ZubenElGenubi]

DOH! Fixed. Thanks!Also, forgot to point out that lenses with shorter focal lengths, while harder to focus, tend to have sharper images.

[JSCampbell]

Great! Thanks for the replies. I will look into the larger dia. lens then...now how do I convince my wife? lolCheers,JS

[DonB]

The reason the diameter is bigger is probably because it is a faster lens. I have a 70-200mm straight f2.8 lens which I use for concert photography shooting in available light. That lens has a 77mm front glass and weighs a ton. I have another telephoto that I use for casual shooting that is 28-300mm with a variable aperture of f3.5-f6.3 and has a front glass diameter of 62mm. The bottom line is you buy the lens you need for the type of photography you are doing. I wouldn't want to drag the 70-200 around while doing the tourist thing, but on the other hand I couldn't use the 28-300 for concert photography either.