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06-02-2015, 09:53 AM   #16
dms
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If it wasn't for other aberrations the lens resolution doubles for every halving of fstop--as the diffraction effect is reduced. But to fix aberrations (which cannot be done perfectly) requires more optical elements. And given a couple of "very well" designed/made lenses that are designed to similar priorities, the faster one generally has the greater resolution. Actually since the priorities are usually very different for fast vs slow lenses this conclusion is not always true. As an example aspherical lens design often leads to higher resolution at/near wide open--but less improvement stopping way down and the non-aspherical (slower) lens is sharper at f/11 for example. But if cost is not a factor, the sharpest lens would generally be the one with the larger (wider) aperture.

An example of a lens type designed to be used wide open are very fast /specialized/very expensive macro lenses designed to be used over a very limited macro range. But these have no other practical priorities in their design.


Last edited by dms; 06-02-2015 at 10:04 AM.
06-15-2015, 02:56 PM   #17
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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?
Sigma AF 400mm f/5.6 HSM APO macro - Review / Test Report

But even then centre sharpness improves from f5.6 to f8.

QuoteOriginally posted by pathdoc Quote
"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.
As someone who used a superzoom as my only lens for 6 years I think there is a lot of truth in this. I got thousands of enjoyable photos with it. On a sunny day at f8 there isn't a massive difference between uncropped images taken with my Tamron 18-250 (RAW and PP'd a bit) and those with the Pentax DFA 100 macro (a lens with stellar sharpness) without pixel-peeping. But in any other conditions, you can see the difference immediately - even on a 6 x 4 print. I am a bit spoilt for the superzoom now, although the convenience is great.

QuoteOriginally posted by drypenn Quote
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.
I found I could squeeze a fair bit out of the Sigma 170-500 with increasing saturation and contrast in PP. But you start from behind. it's almost the mark of a really good lens that you don't have to. With the FA 77, I often feel that I can't improve on the shots that come from the camera.
06-15-2015, 09:47 PM   #18
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QuoteOriginally posted by PJ1 Quote
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.
The K50/4 Macro and the older Takumar version which has the same optics, were designed for document copying. The lens is/had to be sharp corner to corner which it is even on FF film.

Phil.
06-16-2015, 04:14 AM   #19
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QuoteOriginally posted by gofour3 Quote
The K50/4 Macro and the older Takumar version which has the same optics, were designed for document copying.
Making a prime lens with f/4 is almost like tying a brick around Usain Bolt's ankle. In the field, such a low speed would be a deal-breaker. Under the controlled conditions prevailing for its intended use, I guess it's a non-issue.

I guess that's why the DA-15 gets away with being so slow. Being primarily a landscape lens, I'm guessing the designers figured it would mostly be shot either in bright sunny weather or (if things got dark and stormy) from a tripod, so they could afford to lose a lot of speed. It's important to note in this context that the 14mm is a full stop faster, with size rising as appropriate. A phrase I've seen on YouTube comes to mind here: "Philosophy of Use." It's what I consider whenever I buy a lens, and I often wonder out of curiosity what was going through the designers' minds when they put a particular lens together.

06-16-2015, 04:18 AM   #20
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QuoteOriginally posted by trevorg Quote
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?
There are two issues here, one is comparing speeds, one is comparing lens types, zooms vs primes.

Almost any prime lens at 50mm developed over the last 50 years will be sharper than a zoom, simply because the prime lenses are very simple designs, optimized for only one focal length. Spoons are optimized over a whole range of different conditions, so sharpness is a compromise especially wide open.

Now take a 50/1.8 and compare it to a 50/1.4 or 50/1.2 and I'll bet you will find that the 1.8 is sharper wide open than the 1.4 or 1.2 lens. The reason here is that as the front element gets bigger, it is more difficult to account for the difference in element thickness, and the distortions that take place because of the front element. So what you find is that ultra fast primes are a little soft wide open, even when focused perfectly, and also sometimes display a shift in focus as you stop down

Nothing is perfect. You want something , you give up something.
06-16-2015, 03:04 PM - 1 Like   #21
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QuoteOriginally posted by Lowell Goudge Quote
Almost any prime lens at 50mm developed over the last 50 years will be sharper than a zoom, simply because the prime lenses are very simple designs, optimized for only one focal length. Spoons are optimized over a whole range of different conditions, so sharpness is a compromise especially wide open.
Spoons aren't supposed to be sharp - they're optimized to keep your mouth cut free while they deliver either ice cream or soup. But not both at the same time, there's always a compromise.
06-16-2015, 03:17 PM   #22
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QuoteOriginally posted by pathdoc Quote
Making a prime lens with f/4 is almost like tying a brick around Usain Bolt's ankle. In the field, such a low speed would be a deal-breaker.
It depends upon what you want, I suppose. f3.5 lenses were made in almost every focal length for years to save on cost and weight over faster lenses. If you were shooting landscapes, where you may likely be hiking into an area and would be using a tripod anyway, f3.5 wasn't that big of a deal.
06-16-2015, 05:23 PM   #23
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QuoteOriginally posted by pathdoc Quote
. It's important to note in this context that the 14mm is a full stop faster, with size rising as appropriate.
Yes, and we would struggle to find 14mm landscapes taken at f2.8. I have the Samyang and have never done so.

Fast makes more sense at the long end for sports and wildlife.

06-16-2015, 08:58 PM   #24
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f2.8 14mm would be useful for astrophotography, no?
06-16-2015, 10:56 PM   #25
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QuoteOriginally posted by trevorg Quote
f2.8 14mm would be useful for astrophotography, no?
Yes, it would be, Trevor.

I think the star shooters like the Samyang because there's little 'coma' (thanks to aspherical elements).
06-17-2015, 03:44 AM   #26
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QuoteOriginally posted by pathdoc Quote
Making a prime lens with f/4 is almost like tying a brick around Usain Bolt's ankle. In the field, such a low speed would be a deal-breaker. Under the controlled conditions prevailing for its intended use, I guess it's a non-issue.

I guess that's why the DA-15 gets away with being so slow. Being primarily a landscape lens, I'm guessing the designers figured it would mostly be shot either in bright sunny weather or (if things got dark and stormy) from a tripod, so they could afford to lose a lot of speed. It's important to note in this context that the 14mm is a full stop faster, with size rising as appropriate. A phrase I've seen on YouTube comes to mind here: "Philosophy of Use." It's what I consider whenever I buy a lens, and I often wonder out of curiosity what was going through the designers' minds when they put a particular lens together.
You get prime lenses for a number of reasons -- fast aperture being only one of them. Small size, resistance to flare, and lack of distortion are others that are present in the DA 15 and many other primes, particularly those with a slower aperture. As you say, the DA 15 is consider a landscape lens and works best if shot from a tripod. Is that doesn't work with a particular photographer's style of shooting, than there are other wide and faster lenses out there, although none of them have the same flare resistance of the 15.

To the OP's question, most lenses are sharpest stopped down a stop or so. A lens like the DA *55 or the FA 77 are therefore really sharp at f2 and f2.8 -- places where slower lenses may not even have an aperture setting. It is possible to design a lens that is "sharp wide open," but few companies engineer their lenses that much.
06-17-2015, 04:37 AM   #27
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I'll throw this out as I don't think it's totally irrelevant to this thread.
For my bird shots I use a 560mm, f/7.1 (fixed - no diaphragm), three element APO triplet scope.
Under actual real world use it easily out-resolves any other glass that I have including the 50mm Zeiss Zk and the Pentax FA 35.

The following files are nothing special - originally shot in jpg, not RAW, on the old K20D with the usual PP that I would apply to any file.
Both at a distance of about 25 feet or so.
Full frame and crop.

Last edited by wildman; 06-23-2015 at 07:42 PM.
06-17-2015, 05:23 AM   #28
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Those photos are amazing, beautiful colours! That's a very good argument against lenses with narrow base apertures performing poorly.
06-17-2015, 11:05 AM   #29
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An f/7.1 lens/scope can perform very well as we've see but so can an f/1.4 lens if the designer has the time and money to do so. I would bet that the scope design uses expensive, exotic glass (maybe Barium combined with a super low dispersion glass) to help solve the longitudinal and lateral color. The 3 elements are probably air spaced to give the designer more degrees of freedom in correcting the 5 remaining aberrations. Physics tells us that the diameter of the optic does have an affect on performance. A 10 inch diameter lens will outperform a 2 inch diameter one if aberrations are corrected to the same degree in each. This is the main reason why telescopes are made with large diameters. The other reason being light gathering ability.
06-17-2015, 01:20 PM   #30
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QuoteOriginally posted by desertscape Quote
I would bet that the scope design uses expensive, exotic glass (maybe Barium combined with a super low dispersion glass)
Ohara FPL-53 fluorite and the other two elements are Schott.
Another thing to consider - simplicity. The Sigma 500 prime, for instance, has 14 elements. While all those elements correct aberrations they may also, in turn, introduce there own.

Same glass but with the tiny Q-S1 sensor hanging off the back end. This gives me approx 60x apparent magnification...

Last edited by wildman; 06-23-2015 at 07:42 PM.
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