Are faster lenses inherently better than alternatives if shooting with same aperture?

Question

Let's suppose there is a faster prime and slower zoom lens which covers the same focal length and I want to shoot landscape scenes with certain fixed aperture to make sure the depth of field is big enough.

Is there any reason to prefer the faster prime for this task? Is there anything that makes such a lens inherently better performing than the slower zoom lens in terms sharpness, transmittance and other parameters? Or there is no such dependence and speed of a lens doesn't dictate any other of it's characteristics and thus every comparison of lenses should be done on individual case-by-case basis?

Answer 1

Ultimately, the preference is up to the individual photographer. And the speed of the lens is only one consideration.

The prime lens, whether faster or not, is generally a simpler design, which generally results in better performance with regard to sharpness, lack of aberrations, etc. It doesn't have to make the many compromises a zoom lens does in order to be "good enough" at both ends of the zoom range and everywhere in between, and there are usually fewer elements and other moving parts to reduce transmittance and introduce other errors. However, it does mean that you might have to "zoom with your feet" to get the framing you want.

For landscape photography, even on an overcast day, there is generally more than enough light available, except early morning or late evening, so the speed of the lens is not as critical as for indoor photography or other low light situations.

The zoom lens is for many more convenient because you can get a variety of framings of your scene without having to move much, but the additional design decisions mentioned above can result in more image issues to deal with. That said, though, lens design these days has made those differences generally far less noticeable than they used to be, so the convenience of not having to move around so much is often considered a benefit.

Answer 2

The one absolute benefit of the fast lens (even if "cheap") over a slow one (even well built) is that even if you shoot at the same aperture, during the measure/focus phase the fast lens is more open than the slow one (because this is done with the lens at its maximum aperture), and on a DSLR this puts the phase-detection autofocus in better working conditions, and, in some cameras, even allows a more accurate autofocus sensor to be enabled (usually if the lens can open at f/2.8 or better).

Answer 3

Is there anything that makes such a lens inherently better performing than the slower zoom lens in terms sharpness, transmittance and other parameters?

No. Sometimes the prime may be better at a specific aperture than the zoom, sometimes the zoom may be better at the same focal length and aperture. "Better" could be in terms of things such as how "sharp" it is, vignetting, geometric distortion, flare resistance, chromatic aberration, etc. Or maybe one makes using the kinds of filters popular with landscape photographers easier than the other which has a bulbous front element that makes using filters difficult or requires internal rear mount filters?

Or there is no such dependence and speed of a lens doesn't dictate any other of it's characteristics and thus every comparison of lenses should be done on individual case-by-case basis?

That is correct. Every lens comparison should ultimately be done on an individual lens to individual lens basis.

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I've used an extremely cheap 35mm f/2 lens that was awful blurry wide open. It had to be stopped down to around f/4 before it got reasonably decent. Even at f/5.6 it was still slightly worse than my 24-70mm f/2.8 zoom at 35mm and f/5.6, and the 24-70 is pretty sharp even at f/2.8. Of course the zoom was about 14X the cost of the cheap 35mm. After only a handful of uses the cheap 35mm refused to stop down no matter what aperture was set in-camera. So it is now essentially useless stuck wide open. It's not even heavy enough to make a proper paperweight.

On the other hand, I've got a nicer 35mm f/2 lens that cost between one-third and one-half what my 24-70/2.8 did, and it's very good wide open.

• It's probably a tad bit sharper than the zoom at f/2.8 and other wider apertures both share in common, though both are really good.
• It's useable at f/2.
• It has IS, something my 24-70/2.8 does not.
• It is much smaller and lighter than the f/2.8 zoom.

But by the time I've bought a 24/2.8, a 35/2, a decent 50/1.4, and an 85/1.8 I've spent well over what the 24-70/2.8 cost me. Sometimes the zoom works better for me, sometimes a couple of the primes do.

Cheaper primes, like the "nifty-fifty" lenses sold by most camera makers, tend to be soft wide open (though not nearly as bad as that Yongnuo 35/2). They need to be stopped down to f/2.2 or f/2.5 to get even close to the same optical performance as a 24-70 zoom at f/2.8. But in terms of bang-for-the-buck, it's hard to beat a "nifty-fifty'! High quality primes have moved into the same price range that good constant aperture f/2.8 zooms used to inhabit. But now the price of the zooms has, if you'll pardon the pun, zoomed plumb out of sight! So by the time you buy 2-3 mid to high end primes you're spending more than what you'd spend on one good f/2.8 zoom in the same focal length range.

In the end it comes down to a comparison of the two lenses in question, how much performance you need the lens to offer, personal preference, and budget.

Answer 4

When you compare lens with the same focal length and same aperture you will see the same exposure (when shutter speed and ISO are the same). Also you will have same depth of field, field of view, etc. If you use F9 for example it does not matter if your lens can open to F1.

To be precise some lens can have on some aperture slight difference in aberrations, which can change the quality of the image. But the change will be very small.

The other side of the question is the quality of the glass in lens and relation to the image quality but this (AFAIK) is not your question.

Answer 5

Or there is no such dependence and speed of a lens doesn't dictate any other of it's characteristics and thus every comparison of lenses should be done on individual case-by-case basis?

There are some generalities that one can expect, but there will also be exceptions. The determinations should be made on a case-by-case comparison; with actual testing if possible.

Your rephrased question (making it a zoom lens), adds some additional generalities; but again there may be exceptions.

And then there is the "certain fixed aperture" for adequate DOF in the rephrased question... that aperture, combined with the sensor resolution, could introduce an equalization factor of diffraction... e.g. no lens resolves more that 16MP (average/green) on a FF sensor at f/11; no matter how sharp it is, how much more the lens can resolve at wider apertures, or how much resolution the sensor has.