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01-15-2012, 11:03 AM   #1
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Quirks of Lens Design

We mortals expect a Tele Zoom lens to be tack sharp at the tele end, and a UWA Lens to be tack sharp at the Wide end. And I am talking about 2x or 3x zoom lenses not the 7x to 15x super zoom varieties that obviously will have limitations. I have a Sigma 100 to 300 mm lens - bought lovingly mind you for birding - but it is soft at 300 mm, sharp at 200 mm and brutally sharp at 100 mm. So also a Sigma 10-20 zoom, which is tack sharp at 20 mm but not so sharp at 10 mm. This is frustrating and does not serve the purpose for which the lens was bought. I might as well have purchased 300 and 10 mm prime lenses losing some functionality.
Hello "Gods" of Lens Design are you listening. Have you lost your Samurai qualities?

01-15-2012, 11:21 AM   #2
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I don't know if any optic gods are listening here. But they might answer: This is why you should collect prime lenses! And some very very specific zooms. You want a zoom that's tack-sharp, even brutally sharp, throughout its range? I give you: Schneider Betavaron 50-125 enlarger zoom! So sharp, the subjects bleed and weep. So sharp, molecules are sliced in half as they approach it.

But we mere mortals must often be satisfied with primes for their quality and zooms for their flexibility. One member here told of his exalted DA*60-250 and his daughter's plebeian DA18-250. He said that despite the difference in image quality, the main difference between them was that she sold pictures and he didn't! The quality of the lens matters less than the quality of the eye behind it.
01-15-2012, 11:25 AM   #3
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Rico,

I THINK what he's saying is that you buy a longer zoom for its longest reach and that is where the emphasis on sharpness should be placed NOT in its lower range. Ditto for UWA lenses but in reverse - you buy them for their widest range and they're not sharpest there but at their narrowest range. Seems backwards to me too - optimize the zoom for its highest mm setting if its a telephoto and its lowest mm setting if its a wide angle lens.
01-15-2012, 11:40 AM   #4
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I think this carries from lend to lens. I have the first generation sigma 70-200/2.8 and it is incredibly sharp at 200mm and wide open. Later macro and HSM variants were softer. I think it is the compromises in design to add other features

For me, the 70-299/2.8 plus teleconverters was a much more useable option than the 100-300/4

01-15-2012, 01:00 PM   #5
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A thought: This *could* be about the specific copies of the mentioned lenses, in particular, it *could* be that AF accuracy at the long/short end is not at its best (which would be kind of fortunate as the remedy would be tuning the AF fine adjustment for the more important end of the FL?).
01-15-2012, 01:31 PM   #6
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QuoteOriginally posted by nanhi Quote
We mortals expect a Tele Zoom lens to be tack sharp at the tele end, and a UWA Lens to be tack sharp at the Wide end.
I doubt that's a realistic expectation with longer teles. Even with prime lenses, the best 200mm will likely be a bit sharper than the best 300mm, as it is difficult to maintain resolution as magnification is increased. UWAs used to be the same way in the sense of generally being less sharp at the wide end, but lately there are some that have reversed that trend, and are sharper at the wide end, like the Pentax DA 12-24 and even the DA 10-17.

QuoteOriginally posted by RioRico Quote
One member here told of his exalted DA*60-250 and his daughter's plebeian DA18-250. He said that despite the difference in image quality, the main difference between them was that she sold pictures and he didn't! The quality of the lens matters less than the quality of the eye behind it.
Of course, it could also be that this daughter's a better salesperson as well.
01-15-2012, 01:43 PM   #7
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While you might buy a tele zoom for its long end and a wide zoom for it,s short, and indeed, manufacturers should be expected to try to opimize those ends as best as they can, there is no getting around physics. Designing very long and very short lenses is just hard.
01-15-2012, 01:49 PM   #8
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Although I have a couple of zooms (80-200 2.8 Tamron 30A and a 35-105 Pentax A) I use them for convenience the majority of the time.
The Tamron is quite sharp at the shorter to mid end and the falls off slightly at the longer end but is quite good overall.
Bull work is done by primes.
The notion of using primes came a long time ago, not only by examination through the years, but also by some articles I read about lens design and the compromises made to create the zoom, especially in terms of sharpness.
Mr. Goudge makes an interesting observation that I am tending towards and that is the newer glass is compromised in quality for the sake of automation electronically. I do believe this compromise in design of the later glass is partly made up for by better sensor quality (as in the K5, for example).
In the end, I have found some marvelous glass of earlier vintage but at the cost of not having the bells and whistles of today (especially in two areas; flare resistance through superior multi-coatings and in auto-focusing). I can control flare problems to a great extent by avoiding bright direct lighting in the surrounding area of the FOV and in using a hood. Auto-focusing is great in time critical situations but when I have time, I always prefer to do the focus work myself with a self imposed "bracketing" routine.

01-16-2012, 09:04 AM   #9
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Thank you so much Docrwm and Marc for a much clearer explanation than mine. A lens designer should aim to ensure a 100-300 zoom is at its sharpest best at the 300 mm tele end. Just as a SWA lens like the 10-20 should be designed to be the sharpest at its 10 mm end. I believe it should not be too difficult with today's hi tech glass, lens coating, computer aided design and quality control.
And Jolepp has an excellent suggestion which I will definitely try - test a tele zoom at its tele end for back/front focus. Similarly test the Sigma 10-20 at its 10 mm end and input the adjustments into the camera body.
You will note I am not talking about the super zooms like the 18-275 ... - 15x - which is bound to have compromises.
I do hope all the magnificent lens companies - Pentax, Sigma, Tamron, Tokina, Samyang ... and all the rest are listening.
01-16-2012, 09:14 AM   #10
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From my own results with using a Sigma 70-300 for a few years, the 200-300mm range is softer although acceptable if stopped down. It's much sharper in macro mode though and I think that particular lens is optimized for close focusing on the long end of the zoom. Still, I have had a lot of very good tele shots from it. Good light and f/8-f/11, it does fine and that's 90% of what I use it for.
01-16-2012, 09:30 AM   #11
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Yep Reeftool Sir, I still have a great opportunity to buy a like new Sigma 70-300 APO zoom with (2) filters just 2 Km away from my home for $165. I am resisting the LBA for the same reason you mention here - soft at the 300 mm end. Thanks for the input.
01-16-2012, 12:47 PM   #12
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QuoteOriginally posted by nanhi Quote
Yep Reeftool Sir, I still have a great opportunity to buy a like new Sigma 70-300 APO zoom with (2) filters just 2 Km away from my home for $165. I am resisting the LBA for the same reason you mention here - soft at the 300 mm end. Thanks for the input.
you need to look at each lens differently. I know for certain, that the sigma 70-200F2.8, has changed in optical design significantly between the first version , the APO 70-200F2.8 EX
and subsequent versions
APO 70-200F2.8 EX DG
APO 70-200F2.8 EX DG Macro
APO 70-200F2.8 EX DG Macro II
APO 70-200F2.8 EX DG HSM Macro
APO 70-200F2.8 EX DG HSM Macro II

I believe the progression is as follows, the first DG version was the same as the origonal, with rear element coatings, and was also very sharp at 200 mm and all apertures, when they added macro, the optical design and formula changed, and not for the better (IMO). it is only the HSM Macro II that has come back with a new optical formula and improved sharpness at 200mm. So yes sure newer designs can, with computers and materials get the image quality better , but there are still trade offs, and each design carries some. I couild care less about macro, so I never changed out my origonal lens, and am glad I did not
01-16-2012, 12:53 PM   #13
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Tamron sure got it right on their 70-200 2.8.... sharp on the long end. Also sharp on the short end. Humorously less sharp at 135mm.

It doesn't sound so bad, but when a person has a lens, they notice the unsharp end more often even if they think they won't.
01-16-2012, 11:25 PM   #14
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QuoteOriginally posted by nanhi Quote
Yep Reeftool Sir, I still have a great opportunity to buy a like new Sigma 70-300 APO zoom with (2) filters just 2 Km away from my home for $165. I am resisting the LBA for the same reason you mention here - soft at the 300 mm end. Thanks for the input.
It is still pretty good at 300 in good light and stopped down and for the price, you really can't complain. It's going to get a good workout this week at Killington for the Dew Tour as I'll be catching my daughter competing in the Superpipe. It's been a couple of years. This is one of my 70-300 shots from the VT. Open in 09. It's not that bad at 300.
01-17-2012, 06:03 AM   #15
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QuoteOriginally posted by RioRico Quote
I don't know if any optic gods are listening here.
'm far from being a God, but I'm an optical designer. Will that serve?

QuoteOriginally posted by nanhi Quote
We mortals expect a Tele Zoom lens to be tack sharp at the tele end, and a UWA Lens to be tack sharp at the Wide end.
It would make sense but that's rarely how these things work.

A zoom must be optimized so it will perform as evenly as possible across its range. As you move away from the optimum IQ starts to degrade but some focal lengths are better than others for this starting point (and this varies with the design of each lens). Most zooms are optimized near the center of their range, but that's not always the case.

QuoteOriginally posted by nanhi Quote
I have a Sigma 100 to 300 mm lens - bought lovingly mind you for birding - but it is soft at 300 mm, sharp at 200 mm and brutally sharp at 100 mm
In the case of your lens, I'm guessing the designers did not find a satisfying design that allowed good performances across the range if they tuned it to be perfect at the tele end. That would be normal : at the tele end, you risk getting more vignetting, coma, and all sorts of aberrations. In addition, the tele end limits the maximum aperture. Let's say you have a f4 lens across the range. At the tele end it will use the whole area of the glass to give you f4. at the wide end, it's quite probable that it will NOT : in fact your lens might be a bit faster than f4, but limiting this gives you better performances, so that's what designers do sometimes.

QuoteOriginally posted by jolepp Quote
A thought: This *could* be about the specific copies of the mentioned lenses, in particular, it *could* be that AF accuracy at the long/short end is not at its best
That can very well be the case. Getting a sharp focus at 300mm is NOT always easy, it could simply be camera shake or FOVs that are too narrow.
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