Sigma 17-50 f/2.8 EX DC HSM or ?

cieslikowski · 01-04-2016, 12:00 PM - **1 Like**

Hi everyone.

I have a problem with make decision, anyone can help ?
Now I have a DA 18-55mm F3.5-5.6 II but this lens is too bad for me. I know this is a dark budget kit lens but I have a a some K-Series lens, Biotar 80mm and 120mm ... I searching a light remedium for dark lenses with a sharp and colorfull image. In forum user opinions Sigma from thread is a a very good lens, but anyone have idea for alternative lens?

**jatrax** · 01-04-2016, 12:04 PM

Here is a review of the three 17-50ish f/2.8 lenses for Pentax: DA* 16-50mm vs. Sigma and Tamron 17-50mm F2.8 Comparison Review - Introduction | PentaxForums.com Reviews

**BigMackCam** · 01-04-2016, 12:20 PM

I considered the three lenses John has just mentioned, and previously owned a fourth possible contender - the Pentax 16-85 WR.

I had two copies of the 16-85, both of which had issues, so I reluctantly decided to switch to another lens. If you get a good one, it's a great lens (though it doesn't have constant aperture which sounds like it may not be right for you, and MF is a bit too jerky for my liking) - and it's WR too.

I ended up choosing the Sigma because (a) it has excellent and fairly consistent IQ throughout the range, and good wide open performance (at least in the centre - in fact, only the corners let the side down), (b) the build quality is *great*, (c) I'm not concerned about the extra 1mm at the wide end, (d) I've found no reports of motor failure (which, from what I've read, seems to be a reasonably frequent occurrence with the Pentax 16-50), and finally (d) the deal I got on it made it excellent value for money.

The Sigma isn't a perfect lens (if such a thing exists!) - it's quite heavy, the zoom operates in reverse compared to Pentax models, it is a bit more prone to flare (though I've not experienced this yet), the focus throw is short (though smooth) for MF, and of course it isn't WR. But I'm very impressed with it and don't regret my choice.

FantasticMrFox · 01-04-2016, 12:37 PM

I was torn between the Pentax HD 16-85 DC WR and the Sigma 17-50 f/2.8 as an upgrade from the same lens you have. I wanted the Pentax for the weather resistance (I'm an outdoor person in a wet country

), but finally went for the Sigma as it was about half the price. I picked it up from the store New Year's eve, tried it out in Switzerland for a couple of days and in this time gave fallen completely in love with it. It's amazingly sharp (better wide open than the Pentax at any aperture), the AF is faster and finally silent, it's built very well and has a nice reassuring weight to it. The only one thing I miss is the weather resistance, but oh well, you can't have everything

**BigMackCam** · 01-04-2016, 12:46 PM

Originally posted by FantasticMrFox

...the AF is faster and finally silent

Whilst this wasn't one of my criteria in choosing the Sigma, I agree - the AF is both quick and very quiet indeed! For me, that's a "nice to have" - but extremely welcome nonetheless

**eminker** · 01-04-2016, 01:08 PM

I've been quite happy with my Sigma 17-50 F2.8 but I will say my Pentax 16-45mm F4 is a winner as well if you don't mind the being a bit short at 45mm or being F4.
Color saturation and rendering definitely are better with the Pentax especially doing landscapes and bright outdoor shots. Indoors if you have adeqaute light, its not too bad as
well. Build quality definitely is awarded to Sigma as opposed to Pentax. AF on the long end of the Sigma 17-50 in questionable light almost always hunts ....

cieslikowski · 01-04-2016, 01:12 PM

Hmm. Sigma vs Pentax 16-85 price = 1500pln (c.a. 375 EUR) vs 2500 pln(c.a. 580 EUR) incl. vat. Silence of AF and WR isn't a criteria for me. But sharpness and colors ... hmm weight and filter diameter of Sigma is a little problem with my opinion.

I have a idea - I can a borrow Sigma from shop and test this. But Pentax ...

Adam · 01-04-2016, 01:21 PM

Originally posted by cieslikowski

Hi everyone.

I have a problem with make decision, anyone can help ?
Now I have a DA 18-55mm F3.5-5.6 II but this lens is too bad for me. I know this is a dark budget kit lens but I have a a some K-Series lens, Biotar 80mm and 120mm ... I searching a light remedium for dark lenses with a sharp and colorfull image. In forum user opinions Sigma from thread is a a very good lens, but anyone have idea for alternative lens?

The Sigma 17-50mm is a very good choice, and probably the best value for the money if you can get it at US pricing ($400 ish).

Like jatrax mentioned, there are two other alternatives in the same focal length range. Other great lenses that offer a bit more versatility at the expense of some image quality include:

Sigma 17-70mm F2.8-4 Contemporary Review - Introduction | PentaxForums.com Reviews
HD Pentax-DA 16-85mm F3.5-5.6 Review - Introduction | PentaxForums.com Reviews

**macman24054** · 01-04-2016, 02:17 PM

If you can stand to loss some on the wide but pick up some in length the Tamron 28-75mm 2.8 is a jewel of a lens. It is by far the most used lens I have. It is a good all around focal length that has excellent IQ and the 2.8 throught the focal length take the veriable apeture out of the equation. You really can't go wrong with the DA* 16-50, Sigma 17-50 or Tamron 17-50. When you compare IQ and price the budget friendly Tamron 17-50 2.8 is the steal of the bunch.

mikeodial · 01-04-2016, 05:25 PM

Originally posted by cieslikowski

Hi everyone.

I have a problem with make decision, anyone can help ?
Now I have a DA 18-55mm F3.5-5.6 II but this lens is too bad for me. I know this is a dark budget kit lens but I have a a some K-Series lens, Biotar 80mm and 120mm ... I searching a light remedium for dark lenses with a sharp and colorfull image. In forum user opinions Sigma from thread is a a very good lens, but anyone have idea for alternative lens?

Here is the dxo mark review of comparative lenses. Pentax smc PENTAX DA Star 16-50mm F2.8 ED AL (IF) SDM on Pentax K-3 vs Tamron SP AF 17-50mm F/2.8 XR Di II LD Aspherical [IF] Canon on Canon EOS 760D vs Sigma 17-50mm F2.8 EX DC OS HSM Canon on Canon EOS 750D

I did own the 16-50 and liked the microcontrast, but didn't find it very sharp. If you can live without zooming the 31 FA Limited is a much better lens.

Dr. PhunkyPants · 01-04-2016, 05:31 PM

Tried the 17-50mm, briefly. Liked the image quality, but the size and weight...not so much!

Sent it back and went with an HD 21mm.

Alnjpn · 01-04-2016, 05:52 PM

My choice would be the Sigma 17-50 2.8 as it produces very good results for the money. The sharpness is excellent and pic quality far better than your kit lens. My only complaint is that it is big and heavy, so if that may be a factor depending on what you want it for. I chose mine for travel but the weight has become a factor so haven't used it as much as I thought. Tamron's 17-50 2.8 is lighter and cheaper and may be an alternative choice but make sure you get a good copy.

audiobomber · 01-04-2016, 09:03 PM

Originally posted by eminker

I've been quite happy with my Sigma 17-50 F2.8 but I will say my Pentax 16-45mm F4 is a winner as well if you don't mind the being a bit short at 45mm or being F4.
Color saturation and rendering definitely are better with the Pentax especially doing landscapes and bright outdoor shots. Indoors if you have adeqaute light, its not too bad as
well. Build quality definitely is awarded to Sigma as opposed to Pentax. AF on the long end of the Sigma 17-50 in questionable light almost always hunts ....

I upgraded from the 16-45mm to the Sigma 17-50. At first I shared your opinion that the 16-45mm had nicer colours. But I soon realized that it was because the 16-45mm was underexposing. When I equalized exposures using the histogram, colours were shockingly similar. In my experience, the 16-45mm underexposes by half a stop, which results in deeper more saturated colours. The 17-50mm had the more accurate exposure.

Originally posted by macman24054

If you can stand to loss some on the wide but pick up some in length the Tamron 28-75mm 2.8 is a jewel of a lens. It is by far the most used lens I have. It is a good all around focal length that has excellent IQ and the 2.8 throught the focal length take the veriable apeture out of the equation. You really can't go wrong with the DA* 16-50, Sigma 17-50 or Tamron 17-50. When you compare IQ and price the budget friendly Tamron 17-50 2.8 is the steal of the bunch.

The 28-75mm only goes from normal to mild telephoto. It has no wide angle capability.

The Tamron 17-50mm is a good lens but actually more pricey than the Sigma right now.
17-50MM PENTAX | B&H Photo Video

The Tamron is markedly inferior to the Sigma in build quality, less sharp at wide apertures, and famously variable in auto-focus across all brands, not just Pentax.

Originally posted by mikeodial

Here is the dxo mark review of comparative lenses.

DXO lens tests rely on the combination of lens and camera. The comparison below is a more level playing field, 24mp sensors, no blur filter:
Pentax smc PENTAX DA Star 16-50mm F2.8 ED AL (IF) SDM on Pentax K-3 vs Sigma 17-50mm F2.8 EX DC HSM Pentax on Pentax K-3 vs Tamron SP AF 17-50mm F/2.8 XR Di II LD Aspherical [IF] Nikon on Nikon D7100

More interesting than the overall scores are comparisons of sharpness at each aperture and focal length, which is available by clicking on MEASUREMENTS > SHARPNESS > FIELD MAP.

This shows the superior sharpness of the Sigma at wide apertures. Sharpness at wide apertures is why I wanted an f2.8 zoom.

The Sigma focus and zoom rings turn the wrong way which is annoying. The Pentax has superior features; quick-shift and WR.

**macman24054** · 01-04-2016, 10:04 PM

Originally posted by audiobomber

The 28-75mm only goes from normal to mild telephoto. It has no wide angle capability.

No where did I say the 28mm side of the lens is wide. In comparison to all the other lenses mentioned, and 16's and 17's the 28-75 would loss some on the wide end of the focal length and with the exception of the 16-85 it gives you a gain in the long end of the focal length. I will admit the Tamron 17-50 lacks in build quality. It does hold it's own quit nicely when compared to the Pentax 16-50 and Sigma 17-50 in IQ.

Nicolas06 · 01-05-2016, 12:54 AM

Originally posted by audiobomber

The Tamron 17-50mm is a good lens but actually more pricey than the Sigma right now.
17-50MM PENTAX | B&H Photo Video

The Tamron is markedly inferior to the Sigma in build quality, less sharp at wide apertures, and famously variable in auto-focus across all brands, not just Pentax.

DXO lens tests rely on the combination of lens and camera. The comparison below is a more level playing field, 24mp sensors, no blur filter:
Pentax smc PENTAX DA Star 16-50mm F2.8 ED AL (IF) SDM on Pentax K-3 vs Sigma 17-50mm F2.8 EX DC HSM Pentax on Pentax K-3 vs Tamron SP AF 17-50mm F/2.8 XR Di II LD Aspherical [IF] Nikon on Nikon D7100

More interesting than the overall scores are comparisons of sharpness at each aperture and focal length, which is available by clicking on MEASUREMENTS > SHARPNESS > FIELD MAP.

This shows the superior sharpness of the Sigma at wide apertures. Sharpness at wide apertures is why I wanted an f2.8 zoom.

Beware we speak of innexpensive third party lenses and so this will be very different from samples to samples. If you look on photozone for example on Nikon APSC were both lenses have been tested, the Tamron is significantly better than the sigma. That last one has terrible borders wide open whatever the focal length and the borders never match the performance of the tamron.

On DxO this is the opposite ! I think sample variation play a bigger role than we might want to admit here.

In term of size/weight the tamron is significantly smaller/lighter than the sigma. To me this is also an important factor.
Tamron 17-50: 435g 67mm filters, 7.2cmx8.2cm
Sigma 17-50: 565g, 77mm filters, 8.3cmx9.2cm

Last thing, the tamron is said to have field curvature on the photozone. That may explain the weak borders wide open on DxO ?

01-04-2016, 12:00 PM - 1 Like	#1
cieslikowski Junior Member Join Date: Jan 2016 Location: Warsaw Photos: Gallery \| Albums Posts: 32	Sigma 17-50 f/2.8 EX DC HSM or ? Hi everyone. I have a problem with make decision, anyone can help ? Now I have a DA 18-55mm F3.5-5.6 II but this lens is too bad for me. I know this is a dark budget kit lens but I have a a some K-Series lens, Biotar 80mm and 120mm ... I searching a light remedium for dark lenses with a sharp and colorfull image. In forum user opinions Sigma from thread is a a very good lens, but anyone have idea for alternative lens?

01-04-2016, 12:20 PM	#3
BigMackCam Digitiser of Film Join Date: Mar 2010 Location: North East of England Posts: 20,674	I considered the three lenses John has just mentioned, and previously owned a fourth possible contender - the Pentax 16-85 WR. I had two copies of the 16-85, both of which had issues, so I reluctantly decided to switch to another lens. If you get a good one, it's a great lens (though it doesn't have constant aperture which sounds like it may not be right for you, and MF is a bit too jerky for my liking) - and it's WR too. I ended up choosing the Sigma because (a) it has excellent and fairly consistent IQ throughout the range, and good wide open performance (at least in the centre - in fact, only the corners let the side down), (b) the build quality is great, (c) I'm not concerned about the extra 1mm at the wide end, (d) I've found no reports of motor failure (which, from what I've read, seems to be a reasonably frequent occurrence with the Pentax 16-50), and finally (d) the deal I got on it made it excellent value for money. The Sigma isn't a perfect lens (if such a thing exists!) - it's quite heavy, the zoom operates in reverse compared to Pentax models, it is a bit more prone to flare (though I've not experienced this yet), the focus throw is short (though smooth) for MF, and of course it isn't WR. But I'm very impressed with it and don't regret my choice. Last edited by BigMackCam; 01-04-2016 at 12:43 PM.

01-04-2016, 01:21 PM	#8
Adam Administrator Site Webmaster Join Date: Sep 2006 Location: Arizona Photos: Gallery \| Albums Posts: 51,595	Originally posted by cieslikowski Hi everyone. I have a problem with make decision, anyone can help ? Now I have a DA 18-55mm F3.5-5.6 II but this lens is too bad for me. I know this is a dark budget kit lens but I have a a some K-Series lens, Biotar 80mm and 120mm ... I searching a light remedium for dark lenses with a sharp and colorfull image. In forum user opinions Sigma from thread is a a very good lens, but anyone have idea for alternative lens? The Sigma 17-50mm is a very good choice, and probably the best value for the money if you can get it at US pricing ($400 ish). Like jatrax mentioned, there are two other alternatives in the same focal length range. Other great lenses that offer a bit more versatility at the expense of some image quality include: Sigma 17-70mm F2.8-4 Contemporary Review - Introduction \| PentaxForums.com Reviews HD Pentax-DA 16-85mm F3.5-5.6 Review - Introduction \| PentaxForums.com Reviews Adam PentaxForums.com Webmaster (Site Usage Guide \| Site Help \| My Photography) PentaxForums.com server and development costs are user-supported. You can help cover these costs by donating or purchasing one of our Pentax eBooks. Or, buy your photo gear from our affiliates, Adorama, B&H Photo, KEH, or Topaz Labs, and get FREE Marketplace access - click here to see how! Trusted Pentax retailers:

01-04-2016, 09:03 PM	#13
audiobomber Veteran Member Join Date: May 2008 Location: Sudbury, Ontario Photos: Albums Posts: 6,806	Originally posted by eminker I've been quite happy with my Sigma 17-50 F2.8 but I will say my Pentax 16-45mm F4 is a winner as well if you don't mind the being a bit short at 45mm or being F4. Color saturation and rendering definitely are better with the Pentax especially doing landscapes and bright outdoor shots. Indoors if you have adeqaute light, its not too bad as well. Build quality definitely is awarded to Sigma as opposed to Pentax. AF on the long end of the Sigma 17-50 in questionable light almost always hunts .... I upgraded from the 16-45mm to the Sigma 17-50. At first I shared your opinion that the 16-45mm had nicer colours. But I soon realized that it was because the 16-45mm was underexposing. When I equalized exposures using the histogram, colours were shockingly similar. In my experience, the 16-45mm underexposes by half a stop, which results in deeper more saturated colours. The 17-50mm had the more accurate exposure. Originally posted by macman24054 If you can stand to loss some on the wide but pick up some in length the Tamron 28-75mm 2.8 is a jewel of a lens. It is by far the most used lens I have. It is a good all around focal length that has excellent IQ and the 2.8 throught the focal length take the veriable apeture out of the equation. You really can't go wrong with the DA* 16-50, Sigma 17-50 or Tamron 17-50. When you compare IQ and price the budget friendly Tamron 17-50 2.8 is the steal of the bunch. The 28-75mm only goes from normal to mild telephoto. It has no wide angle capability. The Tamron 17-50mm is a good lens but actually more pricey than the Sigma right now. 17-50MM PENTAX \| B&H Photo Video The Tamron is markedly inferior to the Sigma in build quality, less sharp at wide apertures, and famously variable in auto-focus across all brands, not just Pentax. Originally posted by mikeodial Here is the dxo mark review of comparative lenses. DXO lens tests rely on the combination of lens and camera. The comparison below is a more level playing field, 24mp sensors, no blur filter: Pentax smc PENTAX DA Star 16-50mm F2.8 ED AL (IF) SDM on Pentax K-3 vs Sigma 17-50mm F2.8 EX DC HSM Pentax on Pentax K-3 vs Tamron SP AF 17-50mm F/2.8 XR Di II LD Aspherical [IF] Nikon on Nikon D7100 More interesting than the overall scores are comparisons of sharpness at each aperture and focal length, which is available by clicking on MEASUREMENTS > SHARPNESS > FIELD MAP. This shows the superior sharpness of the Sigma at wide apertures. Sharpness at wide apertures is why I wanted an f2.8 zoom. The Sigma focus and zoom rings turn the wrong way which is annoying. The Pentax has superior features; quick-shift and WR. Last edited by audiobomber; 01-04-2016 at 09:18 PM.

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01-04-2016, 12:04 PM	#2
jatrax Site Supporter Join Date: May 2010 Location: Washington Cascades Photos: Gallery \| Albums Posts: 12,991	Here is a review of the three 17-50ish f/2.8 lenses for Pentax: DA* 16-50mm vs. Sigma and Tamron 17-50mm F2.8 Comparison Review - Introduction \| PentaxForums.com Reviews

01-04-2016, 12:37 PM	#4
FantasticMrFox Veteran Member Join Date: Oct 2014 Location: Munich Posts: 2,339	I was torn between the Pentax HD 16-85 DC WR and the Sigma 17-50 f/2.8 as an upgrade from the same lens you have. I wanted the Pentax for the weather resistance (I'm an outdoor person in a wet country ), but finally went for the Sigma as it was about half the price. I picked it up from the store New Year's eve, tried it out in Switzerland for a couple of days and in this time gave fallen completely in love with it. It's amazingly sharp (better wide open than the Pentax at any aperture), the AF is faster and finally silent, it's built very well and has a nice reassuring weight to it. The only one thing I miss is the weather resistance, but oh well, you can't have everything

01-04-2016, 12:46 PM	#5
BigMackCam Digitiser of Film Join Date: Mar 2010 Location: North East of England Posts: 20,674	Originally posted by FantasticMrFox ...the AF is faster and finally silent Whilst this wasn't one of my criteria in choosing the Sigma, I agree - the AF is both quick and very quiet indeed! For me, that's a "nice to have" - but extremely welcome nonetheless

01-04-2016, 01:08 PM	#6
eminker Site Supporter Join Date: Oct 2012 Location: Brooklyn, New York Posts: 150	I've been quite happy with my Sigma 17-50 F2.8 but I will say my Pentax 16-45mm F4 is a winner as well if you don't mind the being a bit short at 45mm or being F4. Color saturation and rendering definitely are better with the Pentax especially doing landscapes and bright outdoor shots. Indoors if you have adeqaute light, its not too bad as well. Build quality definitely is awarded to Sigma as opposed to Pentax. AF on the long end of the Sigma 17-50 in questionable light almost always hunts ....

01-04-2016, 01:12 PM	#7
cieslikowski Junior Member Join Date: Jan 2016 Location: Warsaw Photos: Gallery \| Albums Posts: 32 Original Poster	Hmm. Sigma vs Pentax 16-85 price = 1500pln (c.a. 375 EUR) vs 2500 pln(c.a. 580 EUR) incl. vat. Silence of AF and WR isn't a criteria for me. But sharpness and colors ... hmm weight and filter diameter of Sigma is a little problem with my opinion. I have a idea - I can a borrow Sigma from shop and test this. But Pentax ...

01-04-2016, 02:17 PM	#9
macman24054 Loyal Site Supporter Join Date: Aug 2013 Location: Axton, VA Photos: Gallery \| Albums Posts: 461	If you can stand to loss some on the wide but pick up some in length the Tamron 28-75mm 2.8 is a jewel of a lens. It is by far the most used lens I have. It is a good all around focal length that has excellent IQ and the 2.8 throught the focal length take the veriable apeture out of the equation. You really can't go wrong with the DA* 16-50, Sigma 17-50 or Tamron 17-50. When you compare IQ and price the budget friendly Tamron 17-50 2.8 is the steal of the bunch.

01-04-2016, 05:25 PM	#10
mikeodial Veteran Member Join Date: Jan 2014 Photos: Gallery \| Albums Posts: 370	Originally posted by cieslikowski Hi everyone. I have a problem with make decision, anyone can help ? Now I have a DA 18-55mm F3.5-5.6 II but this lens is too bad for me. I know this is a dark budget kit lens but I have a a some K-Series lens, Biotar 80mm and 120mm ... I searching a light remedium for dark lenses with a sharp and colorfull image. In forum user opinions Sigma from thread is a a very good lens, but anyone have idea for alternative lens? Here is the dxo mark review of comparative lenses. Pentax smc PENTAX DA Star 16-50mm F2.8 ED AL (IF) SDM on Pentax K-3 vs Tamron SP AF 17-50mm F/2.8 XR Di II LD Aspherical [IF] Canon on Canon EOS 760D vs Sigma 17-50mm F2.8 EX DC OS HSM Canon on Canon EOS 750D I did own the 16-50 and liked the microcontrast, but didn't find it very sharp. If you can live without zooming the 31 FA Limited is a much better lens.

01-04-2016, 05:31 PM	#11
Dr. PhunkyPants New Member Join Date: Feb 2014 Posts: 18	Tried the 17-50mm, briefly. Liked the image quality, but the size and weight...not so much! Sent it back and went with an HD 21mm.

01-04-2016, 05:52 PM	#12
Alnjpn Senior Member Join Date: May 2015 Location: Osaka Posts: 158	My choice would be the Sigma 17-50 2.8 as it produces very good results for the money. The sharpness is excellent and pic quality far better than your kit lens. My only complaint is that it is big and heavy, so if that may be a factor depending on what you want it for. I chose mine for travel but the weight has become a factor so haven't used it as much as I thought. Tamron's 17-50 2.8 is lighter and cheaper and may be an alternative choice but make sure you get a good copy.

01-04-2016, 10:04 PM	#14
macman24054 Loyal Site Supporter Join Date: Aug 2013 Location: Axton, VA Photos: Gallery \| Albums Posts: 461	Originally posted by audiobomber The 28-75mm only goes from normal to mild telephoto. It has no wide angle capability. No where did I say the 28mm side of the lens is wide. In comparison to all the other lenses mentioned, and 16's and 17's the 28-75 would loss some on the wide end of the focal length and with the exception of the 16-85 it gives you a gain in the long end of the focal length. I will admit the Tamron 17-50 lacks in build quality. It does hold it's own quit nicely when compared to the Pentax 16-50 and Sigma 17-50 in IQ.

01-05-2016, 12:54 AM	#15
Nicolas06 Veteran Member Join Date: Nov 2013 Posts: 4,854	Originally posted by audiobomber The Tamron 17-50mm is a good lens but actually more pricey than the Sigma right now. 17-50MM PENTAX \| B&H Photo Video The Tamron is markedly inferior to the Sigma in build quality, less sharp at wide apertures, and famously variable in auto-focus across all brands, not just Pentax. DXO lens tests rely on the combination of lens and camera. The comparison below is a more level playing field, 24mp sensors, no blur filter: Pentax smc PENTAX DA Star 16-50mm F2.8 ED AL (IF) SDM on Pentax K-3 vs Sigma 17-50mm F2.8 EX DC HSM Pentax on Pentax K-3 vs Tamron SP AF 17-50mm F/2.8 XR Di II LD Aspherical [IF] Nikon on Nikon D7100 More interesting than the overall scores are comparisons of sharpness at each aperture and focal length, which is available by clicking on MEASUREMENTS > SHARPNESS > FIELD MAP. This shows the superior sharpness of the Sigma at wide apertures. Sharpness at wide apertures is why I wanted an f2.8 zoom. Beware we speak of innexpensive third party lenses and so this will be very different from samples to samples. If you look on photozone for example on Nikon APSC were both lenses have been tested, the Tamron is significantly better than the sigma. That last one has terrible borders wide open whatever the focal length and the borders never match the performance of the tamron. On DxO this is the opposite ! I think sample variation play a bigger role than we might want to admit here. In term of size/weight the tamron is significantly smaller/lighter than the sigma. To me this is also an important factor. Tamron 17-50: 435g 67mm filters, 7.2cmx8.2cm Sigma 17-50: 565g, 77mm filters, 8.3cmx9.2cm Last thing, the tamron is said to have field curvature on the photozone. That may explain the weak borders wide open on DxO ?