lens question

mcdaniel52761 · Jan 29, 2008

I'm sure this is a dumb question, but I honestly don't know the answer. Why do they make fixed (or I think they're called prime) lenses? I'm looking at getting a Nikon D80 and am currently looking at lenses and trying to figure out why I (or anyone) would need say a 50mm lens when you can get 50 mm out of a zoom lens that goes from 14-200 for example. I know there is a difference in the "f" numbers between a fixed and a zoom, but not exactly sure if that has anything to do with it or not. Thanks in advance,

Mike

MarcusM · Jan 29, 2008

Prime lenses make sharper pictures:

http://www.slrphotographyguide.com/camera/lens/zoom-verses-fixedfocal.shtml

fightheheathens · Jan 29, 2008

they make primes lenses because
1.) they are easier to make and therefore less expensive
2.) optically superior. back in the day, prime lenses were vastly superior to a zoom lens in optical quality. Sharper, better contrast, etc etc. nowadays, zoom lenses have come a long way and the good zoom lenses are every bit as sharp as prime lenses, that being said a zoom with the same quality as a fixed lens is going to be much more expensive. nikons 20mm 2.8 lens is something like 400 bucks new. to get the same image quality at 20mm in a zoom you would be looking at something like the 17-35mm f2.8 which is about 1800.
3.) prime lenses can be made with a wider aperture. most camera companies make a 50mm 1.4 lens, some even have a 1.2 lens. i do not know of any zoom lenses that have a maximum aperture larger then 2.8.
for example
nikons 300mm 2.8 VR lens is 4300 bucks on Band H photo
the closes zoom to that i could find was a 200-400mm f4 VR lens that was 5100 bucks. the optical quality at 300mm in both the lenses is probably the same, however, the prime lens is a full stop faster and 800 bucks cheaper...

i would like to own the 17-35 f2.8mm lens, but i cannot afford that so ill probably end up buying a 20mm f2.8 and a 35mm f2.8 prime lens for half the price of the zoom

Helen B · Jan 29, 2008

Yes fixed (or nearly fixed*) focal length lenses are called prime lenses. 'Prime lens' has another meaning as well: the main lens in a combination of lenses, eg a prime plus a close-up lens, or a prime plus a teleconverter.

As you say, fixed focal length lenses have an aperture advantage over zooms. The fastest readily available zooms for full-frame 35 mm (format matters) are usually f/2.8, while f/1.4 and even f/1 primes are available. That two or three stop difference can be a great advantage.

Primes are also lighter - an f/1.4 50 mm is lighter and smaller than, say, a 25-50 mm f/4 and a 35 mm f/1.4 is much smaller and lighter than a 17-35 mm f/2.8. It is easier to design a prime with an optical performance that surpasses that of a zoom, though if you pay enough for a zoom you can match the performance of a prime.

I use primes a lot. I would far rather use two cameras, one with a 35 mm and one with a 70 mm prime, than a single camera with a zoom.

*There are primes that change their focal length as they focus. Most of the current Micro-Nikkor primes shorten their focal length as they are focused more closely, so that they can achieve very close focus without excessive extension. Motion picture lenses do similar thing so that the angle of view stays the same as the focus is changed - otherwise a focus pull would look like a zoom.

Best,
Helen