Different lens sizes

[r2160]

Hi all

Just thinking . . .

Why would somebody buy say, a 50mm lens when an 18-70 lens will cover that size lens?

I mean my next lens will probably be an 80-400 VR (the ever amazing Birddog let me have a play with his!) but I always thought that a lens with a range would be more versatile.

I figure that there is obviously a reason that I have no doubt I am about to find out.

thanks
Glenn

[gstark]

Any number of reasons.

The prime may be faster, sharper, lighter, smaller ....

[birddog114]

And more expensive in comparison with zoom in the same range

[mudder]

G'day,

I'm certainly not very knowledgeable when it somes to photographic equipment, I still consider myself a newbie in that stuff, but for example...

I assume a prime (ie; non-zooming 50mm) can be built to specialise for its intended use, whereas I assume a zoom needs to be a compromise over it's zoom range... The 50mm F1.8 for example can be bought for less than $200, the F1.4 less than $400, but try to get a zoom over the 50mm range at that aperture range for that. The "kit lens" appears to be a very good example for a zoom lens but it starts at F3.5 (at 18mm) and goes to F5.6 (I think) at 70mm. The F-stop making the lens very "fast", good in low light, also very light etc...

Dunno if that helps, other more experienced guys will chip in...

Cheers,
Mudder

[johndec]

Hehe, Gary said in 4 words what I was going to say in 4 paragraphs

I'm still very much in newbie status, but to expand on what Gary said:

Faster: It's all about aperture,aperture and aperture. I just checked with my kit lens and it won't go below f4.5 at 50mm. Buying a prime lens (50mm,85mm, etc) allows you to let 3-4 times as much light in for a given shutter speed. This has two benefits, either you can shoot in much lower light without a flash or you can use that wide aperture to create a pretty out of focus background that portrait photographers get all moist and excited about

Sharper: Because the lens is set up for only one focal length, there are no comprimises made to allow for bits of glass moving about such as occurs in a zoom lens.

Lighter and Smaller: See above. That is, no need to allow for moving bits of glass and the different physical focal lengths they require.