Lens Speed ?

[Alpha_7]

Ok I've been slack recently and haven't been asking my usual newbie questions so I thought I'd ask this one to get a better understanding.

Why can Prime lenses be maded faster (f1.2 / f1.4 / f1. over Zoom lenses ?

Now I can understand that Zoom lenses take a lot more to build, require more moving parts and also suffer as their expected to peform over entire ranges of focal lengths rather then at a fixed focal length, which obviously effects their design...

but what is it exactly that is stopping a zoom lens from have a f1.4 ? I think from memory 2.0 or 2.8 is the best I've seen on a zoom lens.

So is it cost ? There is already expensive lens out their, why not add another teir ?

Is it manufacturing capabilities / physics ? Is it just too hard to make something with those type of optical characterists (or is it impossible?)


(I'm afraid its been 4 years or so since I've done any optics, so that part of my physics education is a little lacking).

[wendellt]

not an expert craig

but i think if you there was a f1.4 zoom over 200mm
the design of the barel woudl have to accomodate a very large lens element like huge, and makign bigger and fater barels may not be cost effective or practical to use

also at f1.4 the depth of field is so shallow that you would have to get so far away from the subbject at 300mm just to get decent depth of field otherwise most of the image even the near focused areaa woudl be blurred

so i guess it's a balance between practicality and need

see the 200mm f2 AFS VR which i would love to get is fat and huge, very nice, but if they made them any bigger it would not be practical to use

either that or jump ship and go with canon's L series lenses

[Alpha_7]

not an expert craig

but i think if you there was a f1.4 zoom over 200mm
the design of the barel woudl have to accomodate a very large lens element like huge, and makign bigger and fater barels may not be cost effective or practical to use

also at f1.4 the depth of field is so shallow that you would have to get so far away from the subbject at 300mm just to get decent depth of field otherwise most of the image even the near focused areaa woudl be blurred

so i guess it's a balance between practicality and need

see the 200mm f2 AFS VR which i would love to get is fat and huge, very nice, but if they made them any bigger it would not be practical to use

either that or jump ship and go with canon's L series lenses

Thanks Wendell, there is no fear of my jumping ship, this is merely, something I'd like to know more about. It seems there are plenty of people here that advocate the use of fast primes, while their speed is great and they are only built for one focal length so therefore 'generally' perform better then zoom...

I just really like the ability to frame/recompose shots differently by just a simple twist of my lens. While admittedly I haven't played with a prime yet so this is merely all out of my head.... they are atleast focal length challenged

[wendellt]

craig

primes are heaps better in terms of optical design and image quality
they are als easy to use

e.g I have a 85mm f1.4 50mm f1.4 as my primes

the 85mm is great for the runway easy point and bang
before the 85 i used t use the 70-200VR it is great for compsoition but more complicated when you change focal length and have to think a little, then focus etc

[Gordon]

Faster zooms could be made, but they would be huge bulky heavy lenses at F2 or faster. The optical surfaces would need more work in order to produce sharp images across the zoom range which makes for more expense. Even so, various aberrations would be evident at the faster speeds, making them less than ideally sharp.
I would think there wouldnt be much of a market for such lenses, so they dont bother making them.
Prime lenses generally are sharper than zooms as less compromises in optical design have to be made.

I'm in the process of replacing my kit zoom with fast prime lenses, 20/1.8, 30/1.4, and a 50/1.4 at some stage in the future. I use my old 85/2.0 manually, it gives much better astro photo results than the zoom at 70mm.

Gordon

[greencardigan]

Zooms have heaps more glass elements compared to primes. So I guess to build a fast zoom, lots of BIG glass elements would be needed.

The smaller the sensor, the smaller the equivalent lense. That's probably why they can make fast zooms for small P&S's.

[Alpha_7]

The smaller the sensor, the smaller the equivalent lense. That's probably why they can make fast zooms for small P&S's.

Makes sense, thanks for bringing that bit to light!

[Steffen]

Two reasons, both boiling down to cost and/or difficulty in design and manufacture:

Long lenses, including zooms at the long end, need bigger front elements, since their absolute aperture (aka entry pupil size, aka apparent size of the diaphragm as seem from the front element) is calculated as focal length divided by relative aperture (the "f-stop"). Hence, a 200mm f/1.4 lens would have to have an entry pupil of about 143mm, and a front element of at least that diameter. Naturally, big chunks of glass are more expensive to make and shape.

Secondly, large apertures aggravate a bunch of different abberations and can also affect contrast due to increased stray light within the lens. Fast primes incorporate counter-measures to that, and thus use more complicated optical designs (compare the number of elements in f/1.8 and f/1.4 primes, for example). Getting those abberations under control is a complex optimisation problem, since the lens is expected to perform well throughout the focus range and across the image area. Having a single focal length to optimise for greatly simplifies the task.

Zooms, on top of all this, are meant to do well across a range of focal lenghts. This makes finding a suitable lens formula a lot more difficult, and the manufacturing of the design much more expensive. Plus, the lens maker has to come up with a design that is relatively insensitive to small manufacturing tolerances, in other words one that can be manufactured at a sufficient yield.

Making zooms slow goes quite a way in fighting those abberations, hence less effort needs to be spent on designing and making the lens.

Cheers
Steffen.

[Alpha_7]

Thanks Steffan, this is the sort of detail and technical answer I was looking for! Cheers!

[Onyx]

Zooms have improved by leaps and bounds with each successive generation that nowadays there's little discernable difference aside from pixel peeping of the differences in image quality between high end zooms and high end primes.

Looking at it from the marketing perspective - imagine if zooms offered equally wide max apertures as primes. Assuming the equal image quality theory, what then would be the goal of prime lenses? If a zoom lens offered the versatility of variable focal length and (big assumption) came with equal image quality and max aperture, primes would be dead!