Purpose of camera lens with a fixed focal length?

H

HeyAndy

Active member
This may seem like a silly question but what is the purpose of using a camera lens with a fixed focal length as opposed to using a camera lens with changeable focal lengths? For example if one was to continually take shots of a scene or subject at 50mm couldn't they just, for example, set an 18-55mm lens to 50mm and leave it at?
 
Sure. If the Aperture is the same. But an 18-55 3.5-5.6 is extremely slow. The "nifty 50" is $100 and the aperture is 1.8.Shallower depth of field (much shallower), and much more usability in lower lighting. That kit lens is not usable in my house below 1600 ISO, which is too high, really.
 
Primes also tend to look better and often shaper then Zoom lenses. Some of the really expensive Zooms come close, but it is hard to find a Zoom below f2.8.
 
HeyAndy said:
This may seem like a silly question but what is the purpose of using a camera lens with a fixed focal length as opposed to using a camera lens with changeable focal lengths? For example if one was to continually take shots of a scene or subject at 50mm couldn't they just, for example, set an 18-55mm lens to 50mm and leave it at?
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Just to clarify - you mean prime lens vs zoom lens?

Optically, primes are often better quality and will have faster F or T stops. It's often a compromise to use a zoom, as it might not match the glass quality of a prime lens.

In cine lens terms, back in 'ye olde days' (LOL), zooms might not be as reliable as a fixed prime... whilst you'd get the same shot with a zoom at 50mm vs a prime at 50mm, there'd be a lot of drawbacks (old cine zooms were heavy and all mechanical - not like the electronic versions we have today).

So, it's a combination of faster & higher quality glass in primes that makes them (generally) first choice... of course, it also depends on the zoom - some can be as good as primes.
 
Because with super 35mm size sensor you are usually shooting f.2.8 minimum and all lenses are sharper when stopped down few stops. Fastest zooms are f2.8 and the sharpness of f1.4 or f1.8 prime lens stopped down to f2.8 is alot better. If you have 5D MKII you can use zoom lenses without that much of quality differences because f2.8 zoom stopped down to f4 on 5D has same ammount of DOF as f2.8 lens on crop cameras, but the f4 on 5D is a lot sharper than f2.8 on crop.
 
It is easier to engineer a contant focal length lens than a zoom lens - you are not having to compromise on anything and all the glass elements are optimised for a single focal length. With a zoom lens you have more variables, more complicated designs, and more engineering hurdles.

The end-user result is that Prime lenses are generally faster and sharper than zoom lenses.
 
Several points:

1. As others have said, for shooting in low light, fixed focal length lenses can be faster (wider apertures) and lower prices than fast zooms.
2. A fast-aperture zoom, as well as being expensive, will be heavier than a prime lens
3. If ultimate image quality is more important than convenience, then fixed focal length lenses are probably the best bet. For video, zooms are often plenty good enough (but see point 1)
4. The gap is narrower than it used to be years ago, but it's still there
 
I'm yet to buy a new prime lens. To me it's a waste of money as an amateur, usage of primes imo is other aspects - coating characteristics/flares; optical "flaws" that make out personality in the image; etc.

If i buy a new lens it's a zoom lens (Canon L). Sharpness/vignette etc on the latest versions of these lenses rarely make out for any flaw discussions relevant to video in silly 1080p; if even, photography at all (let's be bit sober, 95% of the worlds photographers uses zooms....) Photographers using Zeiss primes etc are not the normal bunch, but landscape nerds etc.
 
HeyAndy said:
This may seem like a silly question but what is the purpose of using a camera lens with a fixed focal length as opposed to using a camera lens with changeable focal lengths? For example if one was to continually take shots of a scene or subject at 50mm couldn't they just, for example, set an 18-55mm lens to 50mm and leave it at?
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And to add to everyone else's statements:Using prime lenses also disciplines you into doing more correct compositions as you'd need to move about properly and choose the correct perspective.Generally I've found, with less professional discipline and just general laziness, you get all sorts of weird unco-ordinated close-ups at wide angles mixed with really high/low angles for medium shots and overzealous usage of zooming lol
 
Martin Oller said:
(let's be bit sober, 95% of the worlds photographers uses zooms....) Photographers using Zeiss primes etc are not the normal bunch, but landscape nerds etc.
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It wasn't always this way. I remember the days before zooms got popular, my "kit" (provided by the air force) was a Nikon F2 with a 50mm F1.4 Nikkor, a 35mm F3.5 wide (not really wide, PJs considered it a wide normal), and a 135mm (forget the aperture). My personal "kit" was a Nikon F with 50mm f1.4, 28mm f3.5 Nikkor, 20mm Vivitar (90 degree FOV) in Nikon mount, and a 200mm Nikkor (again can't remember the aperture).

Primes ruled the day and rapid lens changes became an acquired skill (as did rapid film changes). We generally knew our lenses and how to work them to fullest advantage.

Yeah, I know. I just dated myself.

But...I've also become somewhat lazy and accustomed to zooms now.
 
I enjoy shooting video and stills with vintage manual fixed focal lenses.
Want to shoot landscape? put on that Vivitar 24mm.
Want a portrait? Bring me that Helios or Jupiter 85mm or Pentacon 135mm.
Lowlight? let's pull that Porst 55mm f1.2 out of the bag. Etc.
I always carry 4-5 lenses on my backpack.
Who needs zooms and autofocus? :) :) heavy lenses - Waste of money :)
I enjoy every single photo, even if I cannot use the riffle function of the camera.
I guess I am not following the trend though...
 
Bruce Foreman said:
But...I've also become somewhat lazy and accustomed to zooms now.
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Well, I'm 23, and honestly have a hard time understanding the automatic-or-nothing culture. I see a lot of people considering themselves photographers, and basicly getting pissed-off by manufacturers like Samyang etc offering great cheap lenses, but that are manual, and people have no clue how to expose or anything. Or, as a Ken Rockwell likes to call it, "back to the 50's" everytime a lens doesn't do everything by itself... (And speaking of that: front page today on swedens biggest photographic forum has the following highlighted article; "Learn to expose without automatic").
 
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Do you guys think a Panny 14-140, a Panny 20mm and a Cannon 50mm is a good kit to start with? Are the 20 and 50 redundant?
 
I started out with the 20 and 14-140. The 20 is redundant in focal length, but not in f-stop! It can open to f/1.7, whereas the 14-140, when zoomed to 20mm, only opens to something like f/4.8 or so. Huge difference.

Also, for a GH2, the 20mm is just a delightful size. The 14-140 is comparatively huge on the little GH2 body. Doesn't matter that much on something bigger like the AF100, but it's really quite a difference on the GH2, it almost means the difference between "pocketable" and "needing a camera bag".
 
Heartfelt said:
Do you guys think a Panny 14-140, a Panny 20mm and a Cannon 50mm is a good kit to start with? Are the 20 and 50 redundant?
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It depends what you want to do. As it happens, I have all of the above.

The 14-140 is a pretty decent all-rounder, especially for video, and it covers enough of a focal length range to allow you to shoot a lot of different kinds of stuff. It's quite big on the GH body (I have a GH1 and a GF1), but it handles well.

But...

1. For indoor/low light shooting, the limited maximum aperture is a limitation. You can bump the in-camera ISO to get footage, but higher ISO = more noise. For some types of shooting style, that's not a problem, but you have to be aware of it.

2. The 20mm and 50mm are excellent focal lengths on the GH for video. The 50mm is great for close-ups, headshots, over-the-shoulder etc. The 20mm is great for "natural" perspective (its perspective is similar to the human eye). They're also great in low light.

3. There's a lot to be said for fixed focal length, especially for video. Makes you think about framing and composition, and plan your shots. It's also a nice discipline for stills shooting, because you have to use your feet to get the shots you want. And if you like run and gun, these lenses are small and light and *very* unobtrusive.

So, all in all, I think it's a decent selection.
 
I've owned borrowed and rented lots of Canon glass over the years. Some very expensive and others very affordable. If your just starting out and you would like to venture into prime lens territory, there is no better place to start than the EF 50mm f/1.8 II. To this day, its one of my favorite lenses in the bag and at just over $100, you can't go wrong.

Luke
 
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