Difference between a lens kit 18-55mm and a 50mm or 35mm

camera64

With my 18-55mm I can adjust it to 50mm or 35mm. The difference I found would be the f number, but is there any other reason why someone would want a 35mm or 50mm (they have to walk back and forward)? Thanks.

ohyeahar

The f number refers to the maximum aperture of the lens. That is indeed typically the main difference between a prime lens and a zoom lens. It is a huge deal. Larger aperture means better low light performance and shallower depth of field.

At 35mm with the kit lens, your maximum aperture is f/5. For example, you take a shot where the optimal exposure is f/5, 1/100 sec, ISO 3200. If you use the 35mm f/1.8 prime lens, you would be able to take the same shot at f/1.8, 1/100 sec, ISO 400.

Prime lenses are also typically sharper than zooms and have less distortion.

Also, moving back and forth with a prime lens is not the same as zooming. See here:
http://jasoncheung123.tumblr.com/post/88441197298/just-a-simple-illustration-on-how-you-cant-zoom

bruto

As said above, these days the kit lenses can be very sharp indeed, so you may not notice an essential difference there when conditions are good.

One other factor that often appears is resistance to flare. Zoom lenses, by necessity, have many elements inside and are generally more susceptible to flare, which cannot always be recognized and may manifest itself as just a sort of haziness or washed-out look. A prime lens may be sharp and contrasty under conditions that a zoom has trouble with. In addition, since most lenses work their best a couple of stops below maximum aperture, a faster prime will not only give you more low light options, but may well be sharper at apertures equivalent to the wide-open zoom.

"Zoom with your feet" is a simplistic phrase to be sure. A fixed focal length lens will produce a different perspective than a zoom at other focal lengths, because the distance to an object changes. Film students are often urged to use a single length lens and zoom with their feet, in part to train them to see the world in a normal perspective, and to avoid the laziness of framing an image without regard to perspective. There are plenty of times when the change in perspective is just what you want, or when obstacles make longer or shorter lenses necessary.

Zooms are mighty handy, especially for walking around not knowing what you might encounter. Once you start shooting with a good prime lens, you might find that you like it best. It does enforce a certain rigor in visualizing a scene, and in return for the reduced versatility you will get a performance boost that often is visible.