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06-02-2015, 12:32 AM   #1
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Why are fast lenses sharper?

Hey guys,


Why do lenses that have wide apertures, zooms or primes, generally have better sharpness than small aperture lenses (like the kit lenses)? I'm talking about wide open, ie. the 50mm prime is sharper wide open (f1.8 for example) than the kit lens at 50mm wide open (f5)?


Is it just because lenses with small base apertures are generally cheaper, so the optics are "cheaper" too?

06-02-2015, 01:02 AM   #2
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As a general rule, more expensive = better optics. There will always be exceptions. As a general rule, also, prime lenses will be sharper than zoom lenses. It is inevitable that there will be some trade-offs in zoom lenses, and a decrease in sharpness is usually what we get (and often expect). But the sharpest lens is not always the best for the job.
06-02-2015, 01:42 AM   #3
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But is there a "physics" explanation for a lens with wide base aperture to inherently have better sharpness, or is it just a manufacturing decision?


Does there exist a lens that starts at f5.6 for example and is really sharp at this aperture?
06-02-2015, 02:25 AM   #4
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QuoteOriginally posted by trevorg Quote
But is there a "physics" explanation for a lens with wide base aperture to inherently have better sharpness, or is it just a manufacturing decision?


Does there exist a lens that starts at f5.6 for example and is really sharp at this aperture?
Fast doesn't mean sharper in any case. To achieve a large relative aperture simultaneously with larger angle of view, the designer has to apply a more complicated optical formula.
That means more border surfaces (glass-air and air-glass), more thicknesses, more indices. All in all, more parameters to vary with in the process of optimization search.
This is the reason why such a systems (usually) prove to be better corrected when stopped a bit. But there are a lot of examples when the situation is just the opposite.

06-02-2015, 03:22 AM - 3 Likes   #5
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QuoteOriginally posted by trevorg Quote
Does there exist a lens that starts at f5.6 for example and is really sharp at this aperture?

How about sharp starting at f4? S-M-C/Super Macro-Takumar 50mm F4 It will probably out resolve a lot of lenses at f4 but it still gets sharper as it is stopped down.


A good macro is designed to be sharp across the frame but nearly all lenses (even macros) will be sharper in the centre than at the edges. After all, it is mostly subjects towards the centre of the frame that we want to capture. Without getting too much into optics, if a lens is sharper at the centre, it will get sharper as it is stopped down because we are using more of the "better" part of the lens (That is putting it a bit crudely but one of the factors to consider is that a lens has a curved surface and it is projecting onto a flat surface. Sony are developing a curved sensor which will supposedly to address that. But how finicky do we need to get?). If we stop down further - usually past f11 but it depends on sensor size - diffraction effects start to degrade the perceived sharpness of the lens. That is why some lenses may be described as "diffraction limited". It means that the decline in sharpness past a certain point is due to diffraction and not the optical qualities of the lens. If you look at graphs of lens tests you will see that almost all lenses get sharper as the are stopped down and then start to decline after f8 or f11 (some even sooner). Diffraction is there at wider apertures too but it is not so noticeable.


The dpreview site allows different lenses to be compared - with caution, because there are only a couple of Pentax lenses there and other comparisons must be made on the same sensor to be meaningful. It is here: Lens review data: Digital Photography Review


The Zeiss Otus 55/f1.4 is about the sharpest lens around at the moment. It is at the bottom of the list. It is very sharp at f1.4, peaks at f2.8 and then starts to decline (while still being very sharp). It has 12 elements in 10 groups. They must have gone to all that trouble for a reason. It is undoubtedly a good lens, but costs about $5000. The SMC Pentax-DA* 55mm F1.4 SDM can be had for about one eighth of that price. It has nine elements in eight groups - less glass, less weight, but also a reputation for being very sharp, and getting sharper as it is stopped down. It is not so much a question of why manufacturers don't make all lenses as sharp as possible. They all work to a price and even an ordinary kit lens these days is produced to fine tolerances and can deliver a great image in the right hands.


If you want to get right into the optics side though, I propose this: If we had an optically perfect lens projecting onto a matched, curved sensor, It would exhibit maximum sharpness at maximum aperture because diffraction would be at the minimum. The moment we began to stop down, sharpness would decrease as diffraction increased. In our imperfect world, it doesn't quite work that way, but the Zeiss Otus optics are pushing towards that limit.


That should open a can of worms! ,
06-02-2015, 04:11 AM   #6
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Well that all makes sense! Thanks for the detailed reply. Are there any kit lenses from other manufacturers that are sharp at the widest aperture and shortest focal length (18mm), or are they all basically the same design therefore soft at this point? Kit lenses from Canon, Nikon, Sony, Fuji, Samsung, Olympus, etc.
06-02-2015, 04:25 AM   #7
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QuoteOriginally posted by trevorg Quote
Are there any kit lenses from other manufacturers that are sharp at the widest aperture and shortest focal length (18mm)

The Sigma 18-200/f3.5-6.3 II DC OS HSM is at about its best wide open - f3.5 - at 18 mm. See Sigma 18-200mm f/3.5-6.3 II DC OS HSM Review


Some reviewers considered it "best in class".
06-02-2015, 04:30 AM   #8
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QuoteOriginally posted by PJ1 Quote
The Sigma 18-200/f3.5-6.3 II DC OS HSM is at about its best wide open - f3.5 - at 18 mm. See Sigma 18-200mm f/3.5-6.3 II DC OS HSM Review


Some reviewers considered it "best in class".


Yes I have heard that is a good lens, especially considering it's range. Not exactly what I meant by kit lens though and a lot more expensive than one. By the way I'm just asking these questions out of curiosity, not because I am looking to buy a new lens or camera.

06-02-2015, 04:31 AM   #9
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"Softness" is relative. If you take pictures of calibration charts and print them out big enough to cover the back of your door, you will probably find softness in anything. If you're printing out an 8" glossy to sit proudly on your shelf to remember your spouse, kids, pets etc. by, a lens that fails to deliver in a harsh calibration test could well be good enough to show no flaw to the naked eye. This is why I tend to despise pixel peepers - they will find a way to make any near-affordable lens seem cheap, shoddy, and not worth having. It all depends on what you want to do with it. If you want that 8" glossy, any half-decent DSLR with any half-decent lens from the OEMs or one of the big-name third-party players (Tamron, Sigma, arguably Samyang if MF is not a deal-breaker) is good enough. If you want that massive back-of-the-door poster print to be sharp everywhere on eyeball criteria, you need to think about medium format.
06-02-2015, 04:42 AM   #10
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Yes, "softness" is relative. All photo lenses are some compromise. Diffraction limited and even better lenses are the special ones. Like the photolithographic for IC production.
06-02-2015, 05:00 AM   #11
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QuoteOriginally posted by trevorg Quote
...is it just a manufacturing decision?
I think it's mostly a manufacturing decision. What's the use of having an extra stop if it's not usable? To achieve that, a lens has to be more highly corrected. Generally, faster lenses will be bigger (to allow for the wider f-stop), heavier (due to the number of corrective elements), and far more expensive (because they needed more research & development and will sell in fewer numbers than a less expensive alternative.).

FYI, when talking about apertures, most photographers refer to the physical size of the aperture. The "smaller" apertures are not the ones with smaller numbers. Those are the widest ones. The physically smaller apertures have the bigger numbers...f16, f22, etc...
06-02-2015, 05:24 AM   #12
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QuoteOriginally posted by trevorg Quote
But is there a "physics" explanation for a lens with wide base aperture to inherently have better sharpness, or is it just a manufacturing decision?
There are a few reasons.

First, prime lenses are easier to make "fast" because they don't have to preserve their aperture across a range. And at the same time, primes are easier to optimize. As others have pointed out, a zoom is made of compromises, a prime can be tweaked just the way you want.

Second, when you work to improve a lens wide open, you more or less always also improve its performances closed down. Spherical aberrations, coma, chromatic aberrations and vignetting almost always improve when you stop using the borders of the glass elements (when the light rays get closer to the center of the optical axis). So if things are already quite optimized wide open, they'll get pretty darn good closed down.

Third, a prime normally commands a higher price 'per focal length" when compared to a zoom, but that gives designers more leeway to work and tune the design. Expectations are higher for a prime, in other words.

QuoteOriginally posted by trevorg Quote
Does there exist a lens that starts at f5.6 for example and is really sharp at this aperture?
Look at the new 16-85
06-02-2015, 05:58 AM   #13
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Faster lenses tend to be sharper simply because it requires some pretty good optics to get an acceptably sharp picture at a wide aperture. I said "tend" because back in the film days, there were a lot of relatively fast lenses that sucked wide open. They were "fast" mostly to have a brighter viewfinder and that was helpful with manual focusing. I own a few. Stopped down a little, even those old dogs do pretty good.
06-02-2015, 06:04 AM   #14
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Sometimes a "NOT" sharp lens just lacks a little contrast or saturation. A few nudges here and there at LR "makes" them sharp, which proves the point, that indeed they are sharp, but just a bit lacking in contrast.
06-02-2015, 06:18 AM   #15
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Closing down the aperture does two things as I understand it. First, it blocks out errant light that may be coming into the lens from a non-perpendicular angle. Imagine a light beam striking the front element slightly less than parallel to the surface. That light will enter the lens through refraction and start bouncing around between the surfaces of the glass. It will take a longer path than the focused light will so it will be out of focus. Closing down the aperture may help prevent that light from passing to far. If this out of focus light passes through and strikes the sensor it will blur out details. You lose sharpness and contrast. So, aperture blades act like a filter. I think it helps make the light more tele-centric. Second, the aperture blades refract light. They bend them. The smaller the aperture then the greater the effect. Think of an extreme case like pin hole cameras. With a small enough hole you don't even need a lens. Glass refracts light too. I think you could say that the two refracting systems, the aperture and the glass, can work together to help each other achieve better focus and sharper images. This is why smaller apertures have more depth of field. The aperture refracted more of the light from in front of and behind the subject than the glass alone.

Larger aperture lenses needs to have a more complicated design, element profile, and use more exotic materials which typically gives the lens a better starting point that a stopped down aperture can further increase. However, don't take this a universal truth. There are plenty of large aperture lenses that aren't sharp to begin with. A lot more depends on the design than the aperture by itself. The DA 35mm and DA 50mm are f/2.4 and f/1.8. Are there faster lenses? Sure ... but these "not as fast as possible" lenses are also wickedly sharp!
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