[dave9t5]

Dear all,

I would like to get a wide/mid angle zoom for a full frame camra (nikon d750)

I'm looking to the following models:

- tamron AF 24-70 f/2.8 SP DI USD VC, or

-Sigma 24-105 f/4 DG OS HSM A
My dilemma is that the Tamron has a better image quality in term of sharpness, distortions, aberrations, speed etc

 

The differences between these two lenses in the PZ test results don't seem very significant.

 

The only thing I see is that the Tamron is significantly better at f/2.8 and the Sigma seems better at FL's over 70mm...   :lol:

[Apedra]

The differences between these two lenses in the PZ test results don't seem very significant.

 

The only thing I see is that the Tamron is significantly better at f/2.8 and the Sigma seems better at FL's over 70mm...   :lol:

Be careful comparing the diagrams since PZ uses different scales for MTF (resolution) for the two lens, the real difference can be appreciated if you compare numerical values or put into a single diagram the value of both lens

[dave9t5]

Be careful comparing the diagrams since PZ uses different scales for MTF (resolution) for the two lens, the real difference can be appreciated if you compare numerical values or put into a single diagram the value of both lens

 

The scales are exactly the same:

http://www.opticallimits.com/canon_eos_f...os?start=1

http://www.opticallimits.com/canon_eos_f...ff?start=1

[JJ_SO]

The scales are not the same, if one tries to compare a Canon test with a Nikon test  ^_^ But who would do that ?  :lol:

 

 

Maybe this helps, too: http://sigma-rumors.com/2015/01/more-sig...05mm-talk/

 

Sigma appears to be overrun by their own success   So, if you decide to go Sigma, I can only hope you're a patient person

[dave9t5]

The scales are not the same, if one tries to compare a Canon test with a Nikon test  ^_^ But who would do that ?  :lol:

 

Rhymes with Scooby ? 

[Scythels]

To avoid hiding it at the bottom, I'll address Apedra first.

 

 

if I well understood it is the same methodology adopted in the photozone tests

 Interferometry is not performed by photozone.  Interferometry is a remarkbly, painfully slow process when you evaluate multiple positions within the image field.  To give you an idea, in my research group we have done a single test-test to evaluate our procedure for testing a particular freeform optical system on an interferometer.  The procedure is fully automated, and it took two hours for that one position.  The test re-runs itself if it gets bad data, but even with good data it took two hours.  We will ultimately test 81 field points and the system must be precisely moved by hand between the points, so it will take north of 200, potentially more like 300, hours to evaluate the entire system.  This is a much more complex system than a photographic lens, but interferometric testing would still be much more time consuming than MTF50 testing, such as Imatest which is done by PZ. 

 

It is also very expensive, for non-flat surfaces you must have a reference lens or surface matched to the focal length you are testing.  It is possible to test non-flat systems with a 'stock' reference flat but you must know a great deal about the system being tested for this to be accurate.

 

 

how do you judge the three lenses (tamron 24-70 f2.8, sigma 24-105 f4 and nikon 24-120) in term of : sharpness, distortion, chromatics aberrations?

The sigma 24-105 is tested on the 5D2, which has 2 fewer megapixels.  If we compensate for that by adjusting (24/22)*line pairs per picture height we will get a very close approximation to what the sigma would do on the D3x, used by photozone.  This ignores differences in the resolving characteristics of the individual sensors, but none of these lenses are so good that the sensor provides the greatest contribution to the overall performance. 

 

With that in mind, the center of the frame of the sigma is consistently the best at all focal lengths.  The tamron would win the off-center competition.  The sigma does well and is competitive there, though.  I would mention that the very very edge of the picture - just maybe 50px*50px in the corners from the sigma is ruled by astronomical astigmatism and is, frankly, shit.  But, it is a very small area so it may not affect your usage.  The nikon does the worst in terms of resolution away from the center.

 

Regarding distortion, the tamron is the best corrected overall.  Sigma takes second, nikon is worst.

 

When it comes to chromatic aberrations the tamron does better than the sigma - somewhat - but neither is offensive.  Nikon's 24-120 is very poorly corrected in that regard.

 

With that said, the nikon would be the most reliable lens of the three, mechanically.

 

Sigma has made great strides since the death of their CEO when it comes to producing better lens designs, but their use of premium materials in their new "A" and "S" lenses does not assure premium quality.  I would be very surprised were their lenses as well designed mechanically as canon and NIkon's.  It's possible, but I have enormous doubts.

 

That said as well, I must admit I have a bit of a personal bias or reservation against sigma due to years of scummy tactics in the way of design theft.  I haven't misrepresented the performance of their lens - but they stole their image stabilizer design from nikon, they stole their autofocus motor design from canon, and they stole their design method for the 35mm and 50mm "A" lenses from Zeiss. 

 

Out of the three I would get the Tamron 24-70, but an argument could easily be made for the sigma's greater zoom range.  Of course one can also be made for the max aperture of the tamron.

 

re: JoJu

 

One can compute an already computed lens, but what does that say about it's manufacturing quality

Interferometric testing can only be done on a real, physical lens.  Here is a poster on the subject created for presentation to freshmen optics students: https://dl.dropboxusercontent.com/u/3928...Poster.pdf

 

 

How does the analysis program gets it's formulas and coefficients?

Here is a page from slide notes on aberrations:

http://i.imgur.com/HQzqeU1.jpg

The equation/power series is for defocus in millimeters.  The merit function of a lens is a similar function but contains weighted terms for various aberrations and field postions (think center/mid/corner -- field = distance from center). 

Here is a page from slide notes on merit function terms:

http://i.imgur.com/9YrCrOy.jpg

 

Note that aberrations have different coefficients.  Third order spherical, which is the major aberration in the RIM plot I posted above, is w-sub-40, for example.  These terms sum to a merit function - an ideal lens has a merit function of 0, but this is an unattainable result. 

 

Back to rim plots for a moment...

 

An ideal rim plot would have a straight line through the x axis.  RIM plots graph defocus vs position in the pupil of the lens, essentially defocus vs distance from the center of the lens.  You generate a series of rim plots with different field positions (distances from the center of the image and analyze them to guide you through optimization when designing a lens.  The goal is to reduce aberrations, and different aberrations take different forms.  Here is a sample of the balancing of a few aberrations, again on slide notes:

http://i.imgur.com/nufW1oL.jpg

 

When you see that you have spherical aberration, there are measures that can be made to reduce it, same goes for coma, astigmatism, etc. 

 

For example, the length of the central airspace in double-gauss lenses primarily affects the astigmatism and vignetting without changing spherical, coma, field curvature, polychromatic aberrations, or distortion much. 

 

You could optimize from synthetic interferograms from the computer as well, in fact the interferograms I posted above (they are red...) are synthetic - they are the ideal result for the project my research group is working on right now.  When we complete the construction of the system we are building, we should get precisely that result. 

 

The trouble with interferograms is that while aberrations have unique forms, they can quickly become very convoluted to the human eye to the point where one has no idea what aberrations are present. 

 

So, back to the quote...

 

 

How does the analysis program gets it's formulas and coefficients?

A computer, luckily, is very skilled at deconvoluting complex mathematical situations.  The computer running the software for the interferometer will take the interference data, which is defocus in terms of waves (multiples of ~630nm).  That defocus can be used to produce a map of defocus across the field when several interferograms are taken, to which functions for all of the aberrations may be fit.  The coefficients will be determined by the best fit. 

 

You may wonder - "If it's only a best fit it must not be very accurate" - but historically interferometry has been remarkably accurate, and the experiments preceeding the one my research group is conducting right now had the result real interferogram be within 2% of the predicted theory.  And that is for a complex freeform situation, in a symmetrical system such as a photographic lens I would expect you could get within a tenth of a percent of the theoretical result. 

 

 

also can read the theory of an optical bench (also not to your taste, I guess  ) but I haven't seen one in the flesh nor tried to work with one, beware.

There are many, many many many many many many many many many many many many many different things that can be referred to as an "optical bench" - I suspect you mean LensRental's fancy tool from Optikos?  I've actually spoken quite a bit to the lead engineer for Optikos who was very involved in the design  of that machine, it's a cool thing. 

 

I have no qualms with the optical bench LR has, because it is a 'pure' form of testing.  In a nutshell a tiny glass plate or other test chart printed at an absurd dpi is placed behind the lens.  A very low divergence collimated light source illuminated the chart, which is imaged backwards by the lens.  A set of three diffraction limited telecentric lenses re-image a small portion of that at each of three field angles onto sensors and the result is used to compute MTF.  Alternatively, a tiny, tiny hole can be imaged to see the point spread function of the lens (which the machine likely can't compute MTF from - it is possible I suppose but I was offered a job at Optikos if I could figure out how to get the MTF out of the PSF so I do not think anyone has done it.) In essense, their machine is three of these in one: http://www.optikos.com/products/lenscheck-vis/

 

There are no confounding optics in this test which can play unfairly to specific weaknesses in lenses, only a physical limit to the resolution of the chart being imaged.  I'm okay with that, because it is impossible to adjust defocus specifically in this case, eventually you sill sample the dots used to print the chart, but that does not impact the performance of the lens other than hard-limiting resolving power. 

 

 

I guess (!) they use a big sensor and enlarge the picture given by the lens, sort of the way its' done by speed boosters on µ4/3 - and there the speed booster doesn't take away image quality.

The speed booster does still have aberrations - however because the field is being shrunk by a fairly well corrected lens, the defocus/aberrations are all reduced in magnitude.  The speed booster is also a superfast lens and is corrected for ideal resolution (untamed field curvature and distortion were traded to correct spherical, coma, and astigmatism) at the expense of less benchmark-visible image quality.  A perfomance evaluation done with a speed booster behind the lens still only provides a generalization of the performance of the lens.

 

 

I just ask what's the point of spending lots of money in it and go public with the results?

What's the point of many things?  It is a cool experiment, but it is still quasi-systematically flawed.  There is an error in the system for measurement, but because error can be positive or negative and positive+negative = 0 while positive+positive = more positive, the error is not truly systematic and cannot be computationally removed. 

 

I do not believe DxO tests every lens and every camera body together - what is the point of publishing the results?  But that is a rant for another day. 

[Apedra]

Scythel, thank you very much for your accurate replies and precious time spent to make such analyses and divulgate them

Andrea

[JJ_SO]

My my Scythels, what can I say if I see such a wall of words and knowledge?  B) I hope you don't expect me to understand/comprehend every bit of what you wrote nor could calculate some of those formulas. I don't want to defend LenScore, I just appreciate them as "another source" of collected data. If their process is actually that weak and full of flaws like it appears to be in your reply, how come they are more or less confirming your conclusion about those three lenses? Might be a coincidence...

 

I agree most of your conclusions about those lenses. However, I ended up with the Sigma because of 

• good experiences with two other Art-lenses (now three)
  
• at the time there was no Tamron available
  
• I get 24-105 instead of 24-70 at a smaller price about the same weight (more or less)
  
• the Sigma USB-dock does a nice job, though time-consuming and would be easier if I could wire the lens directly to a PC
  
• I already have fast primes, my zoom doesn't need to be super fast (and those are very tricky to correct and still weaker wide open)
  
• in Switzerland, the Sigma importer gives a free once a-year-service-check (without materials). Lifelong…
  
• Tamron's service is based in Germany, it's complicate, expensive (transport costs are huge) and takes a lot time to let them check a lens, but that's only my special situation. However, I'd include the service situation (and the experiences with) in my decision.
  

I don't want to talk your doubts about mechanical design away. I'm just more confident since none of the 4 Sigmas (Art series) and none of the other 4 I had with Pentax showed mechanical flaws or was a repair case. While the pretty expensive Nikkor 85/1.4G needed a new focus drive after only two years, no rain or other weather conditions involved. Of course, there's no Sigma 85/1.4 Art to compare for me. Yet. But here comes the next list where Sigma does a little bit better than Nikon

• both caps are better designed and don't slip out of my fingers
  
• the lens hood (okay, that's nothing for a zoom, because there it's useless in tele position) of the 50/1.4 Art makes the lens less sensitive to certain front light situations than even Zeiss' Otus round shaped hood is.
  
• they are better damped when manual focussing than the Nikons and there's no gap in the focus rings 
  

My conclusion is, Sigma cares a lot about details. Question is "also inside the lens?" Can't say, haven't open one yet.

 

Thanks a lot for your text about Interferometry. Like you said, a time consuming process and maybe not the best suitable for testing photographic lenses. I suspect, the interferometry test of those three lenses wouldn't come to surprisingly different results as the less precise/reliable MTF 50 test.

 

And right, DxO mark doesn't test all body-lens combinations. But they do more than anyone else out there and it's a good chance to find popular contemporary combinations tested - while PZ for each system (APS-C, FF, manufacturer) only tests on one body.

 

A last bit about your simulation of how Canon and Nikon tests could be interpolated by MP count: Canon uses AA filter, Nikon D750 doesn't - that IS increasing the resolution, but less than 10% (D800 / D800E being the only camera types to be comparable in this aspect)

[JJ_SO]

I forgot that one:

 

That said as well, I must admit I have a bit of a personal bias or reservation against sigma due to years of scummy tactics in the way of design theft.  I haven't misrepresented the performance of their lens - but they stole their image stabilizer design from nikon, they stole their autofocus motor design from canon, and they stole their design method for the 35mm and 50mm "A" lenses from Zeiss.  

 

Well, talking about "stealing" is talking about Japanese (and lately Chinese and Korean) manufacturers who stole a lot knowledge from German manufacturers and after that protected and improved their copy work.

 

In Sigma's case: They improved Nikon's VR quite a bit. I can't judge if they improved also the focus drives but I know they do reverse design - how else could they come to equal AF qualities, at least in terms of accuracy? Sigma's lenses are not worse than Nikon's (I'm talking about the Art series, can't say much about Contemporary or Sports). And the end-result is better than what Nikon has to offer at a remarkably lower price. Of course, development and design does cost a lot of money and just copying it appears to be unfair.

 

I'm just saying Sigma's not only copying, I see improvements in lots of aspects and after all, coming so close to an Otus at ¼ of the price plus implementing an (as Zeiss tries to make it look impossible because of the lack of accuray) AF is a performance of it's own.

 

The adjustment via dock is unique - Sigma didn't copy that bit!

The mount changing service is unique - every other lenses you have to buy again when changing your system

The lens testing with Foveon sensors is unique and each lens is tested (however, I read quite a bit about lenses to be changed and have no good explanation for that)

The lenses are made in Japan - Nikon goes to China and Thailand and is still more expensive than Sigma.

 

I wouldn't have become a Sigma owner, if Nikon did as good as Sigma on their designs of the 35 and 50 mm. Also, Nikon has nothing to contribute in fast wide angle zoom (18-35/1.8). But optically, the Nikkors 35 and 58 are weaker wide open and still more expensive. And since Nikon does neither guarantee weather resistance nor defines against which kind of weather their sealing should resist - where's the point in paying so much more? Same goes for the 24-120/4 vs. 24-105/4 from Sigma: It's worse and more expensive. So I welcome Sigma giving Nikon a good reason to improve. Unfortunately this takes time and I see Nikon still behaving a bit too arrogant...

[Scythels]

@JoJu

 

 

I don't want to defend LenScore, I just appreciate them as "another source" of collected data.

Certainly, you've been here for quite a long time, it is clear you have no affiliation with them. 

 

 

If their process is actually that weak and full of flaws like it appears to be in your reply

My criticisms of their process are perhaps strongly worded, but they are marketing themselves as "the ultimate definitive best-ever lens test which places all lenses on equal footing" and so on.  The trouble is, their test is not putting everything on precisely equal footing.

 

Take this rim plot:

 

[sharedmedia=gallery:images:1315]

 

It is almost exclusively spherical aberration.  There are some minor polychromatic aberrations, but what is driving the 500-micron spot size is spherical.  If you introduce a componenet behind this that has the same spherical, but with a negative sign instead of a positive sign, you get a perfect system. 


Unless Lenscore's optic before the sensor is diffraction limited at a bigger aperture than the test lens, the aberrations it contributes will be significant.  The sign and magnitude of aberrations in that optic, then, becomes very important.  If you have a lens such as the fuji 56/1.2 or the panaleica 15mm f/1.7 which undercorrect the spherical aberration (most lenses do, these are mere examples) and the optic inside their camera overcorrects spherical, you have a net reduction in the spherical which will boost the contrast (and to some extent resolution) of the result image.  Similarly, if you take a lens such as the zeiss 100/2 which overcorrects the spherical aberration and have that optic in the camera also overcorrect the spherical aberration, the result is worse contrast and worse resolution. 

 

It is in this way that their test is flawed, but chances are the optic inside the camera doesn't contribute too too significantly to the overall result.  It would only be when comparing diffraction limited or nearly diffraction limited systems that the aberrations of that optic would contribute in an overly significant manner.  Of course, those lenses (such as canon's 300/2.8) are very rare, but then perhaps fans of one manufacture or another would use this particular cases to show the merit of their favorite manufacture over another, using such "ideal" data as Lenscore's data to split the hairs. 

 

Interferometric testing would be a very expensive and "wasted" effort on photographic lenses.  One must have an expensive reference sphere matched to the focal length being tested - to test the full range of photographic lenses one would need dozens of reference spheres, each costing between 5 and 15 thousand USD - not to mention the often months-long wait time for one to arrive after ordering.  This is on top of the purchase of a $100,000-$500,000 interferometer, as well as a couple thousand for the vibration isolation table things should sit on, a few more thousand for the staging equipment to hold everything...  You see where this is going vs perhaps $10,000 for a full imatest setup. 

 

A very real argument can be made for time as well - but alas if you want the "ultimate" lens test and to test the lens without influence of any other anything... interferometry would be that test.  In a nutshell, all of the optics behind the test optic are removed computationally.  What an interferometer does is take the reflection (about 2%) off of the final reference surface and interfere it with the beam from the test surface.  Because the difference is what is measured, you isolate the contribution from the test surface entirely. 

 

 

And right, DxO mark doesn't test all body-lens combinations.

Oh no - I meant everything they publish.  They claim it takes about 2 hours to perform each of their tests yet when new lenses or bodies are released within a couple of days results for most compatible lenses/bodies magically appear.  Ignoring the time constraints (this would require something like 80-150% of the time available since the announcement of the body/lens...) there is also a question of inventory - does DxO have an inventory of hundreds of thousands of dollars of photographic equipment lying around?  Possible, but I still doubt it.  Often very old and unpopular lenses appear for brand new bodies, one must doubt that the test was truly done rather than just mathematically simulated...

 

 

A last bit about your simulation of how Canon and Nikon tests could be interpolated by MP count: Canon uses AA filter, Nikon D750 doesn't - that IS increasing the resolution, but less than 10% (D800 / D800E being the only camera types to be comparable in this aspect)

Of course, but the D3x test camera features an AA filter, as does the 5D2.  There will be some difference in the strength of the filter but this won't account for more than a 5-10% error at most.  By shifting the 5D2 data so the "mean value" matches you in essence "align" the results so that the contribution of the AA filter is more likely to at most affect things that 5-10%, rather than a 5D2 with a stronger AA filter (-10% result resolution) suddenly making the 5D2 result 19% worse than the same lens on the D3x. 

 

 

Well, talking about "stealing" is talking about Japanese (and lately Chinese and Korean) manufacturers who stole a lot knowledge from German manufacturers and after that protected and improved their copy work.

I could argue for days about different designs and who 'stole' from who, but the historical designs - such as the sonnar, tessar, double-gausse, ernostar, biotar, heliar, Landscape Lens, bertele's symmetrical wide-angle lenses, Angenieux's retrofocus, etc, are very very old - and they are "requirements" - truly certain combinations of f number and angular coverage cannot be achieved without some of these designs.  Most modern lenses are "original designs" - take for example the 400mm f/2.8 lenses from canon and nikon, nikon's newest looks like this:

[sharedmedia=gallery:images:1320]
While the canon equivalent design looks like this:

[sharedmedia=gallery:images:1319]
 

If we begin with first order analysis (the arrangement of lenses which focus and those which diverge) both are the same - they are telephoto lenses in a P-N arrangement wher P stands for positive and N stands for negative. 

 

But, the construction of the that "P" is different.  Canon has opted for a positive lens followed by a cemented achromatic doublet the formation of I don't know of the top of my head, followed by a gauss doublet, followed by another likely achromatic doublet of the same form as the one in front of the gauss doublet.  Nikon has opted for a flat protective front element followed by a large positive lens, then an achromatic doublet, then much further down the barrel an extreme steinheil doublet to form the "P" group.  

 

The designs are very different!

 

 

In Sigma's case: They improved Nikon's VR quite a bit. I can't judge if they improved also the focus drives but I know they do reverse design - how else could they come to equal AF qualities, at least in terms of accuracy?

I don't mean the protocol - the ring USM motor design, mechanically, was copied to produce "HSM." 

 

Of course one may improve it after stealing it, but they were not good enough to come up with it on their own to begin with...

 

 

I'm just saying Sigma's not only copying, I see improvements in lots of aspects and after all, coming so close to an Otus at ¼ of the price plus implementing an (as Zeiss tries to make it look impossible because of the lack of accuray) AF is a performance of it's own.

Retrofocus is a 'nice' design form to work with, sort of.  There are staunch limits on the length of the system, but it effectively works by having a very short/quick focus of light in the front of the lens, then that is spread out so it moves straight backwards in a collimated fashion, and finally the light is focused to the sensor.  The entire space between the front focus and the back focus groups is free for correcting aberrations, creating a focusing group, image stabilization group...

 

Before the 55mm otus, not a single manufacture had ever released such a long focal length retrofocus lens for the full-frame sensor size.  The otus was being shown cut in half for 1.5-2 years before its release which is more than long enough to produce an "original" copy of that design methodology for that focal length / aperture / angle of view combination.  I do not believe for a moment that of all the camera lens designers in the world, the ones at sigma were the first in history to come up with a design methodology implementation in parallel to the engineers at zeiss...

 

 

The lens testing with Foveon sensors is unique and each lens is tested

It is unique, but it is also a bit of a wash.  There is no point testing the lenses on a layered sensor design as it is no more sensitive than "flat" sensors.  You also introduce a great deal of complication regarding chromatic aberrations by taking that approach.  It's also an APS-C sensor to test full-frame optics.

 

Not defining the limits of your weather sealing is a method of protecting yourself from lawsuit.  It is not ideal, but these are not cheap devices to manufacture and having to replace them when people use them outside spec but claim they used them within spec and fried them could be quite expensive. 

 

Regarding "giving nikon a reason to improve" - R&D is very expensive.  Sigma, as well as the mirrorless manufactures, have chosen to burn through their bank accounts with massive overspending in R&D while nikon and canon have been more conservative.  Arguments can be made for the merits of both business strategies, neither is particularly superior.  Nikon has been expanding into alternative technologies, namely the use of fresnel lenses, electronic diaphragms, and fluorine coatings lately.  All very worthwhile, but I do question whether they should also be looking into MRF polishing for 5" class optics to further lighten their telephoto lenses, and of course MRF could be used to make more and better aspheres for smaller lenses as well.  If they could polish one of the ED elements in the "P" section of the 400/2.8 into an asphere, for example, they could take out one of the large front elements which drive weight, bringing the lens weight down perhaps 500g+ instantly.