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08-13-2014, 06:23 AM - 23 Likes   #1
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Pentax FA31mm f/1.8 Limited Vs Sigma 18-35mm f/1.8

Now that I have a perfect, optically aligned K mount copy of the Sigma 18-35mm f/1.8 I think it would be interesting to compare it to the SMCP-FA 31mm f/1.8 Limited Ė many people refer to this zoom as a stack of primes, well it certainly is big enough:



SMC-24mm f/2.8 - SMCP-FA31mm f/1.8 ASPH Limited - Sigma 15-30mm f/3.5-5.6 EX - Sigma ART 18-35mm f/1.8


The closest thing Pentax makes to this lens is the HD DA 20-40 mm f/2.8-4.0 ED Limited DC WR . To be honest I was never all that interested in the Pentax HD Limited lens. Physically the Limited Zoom Lens looks lovely, but the Limited zoom was completely overshadowed by the faster Sigma 18-35mm f/1.8 ďArtĒ

The prime Iím comparing it to is the SMCP-FA31mm f/1.8 Limited. This lens is (at the time of writing here in Australia) nearly twice as expensive as the sigma zoom. The FA31 has been one of my favorite pentax primes for a number of years, and in my experience it is a superb lens optically and Iím familiar with all its quirks.



Just to sum up the physical features of each lens






Flare is handled surprisingly well:






In my testing, the Sigma lens trailed behind the FA31in terms of sharpness across the frame when stopped down, The sigma zoom still did surprisingly well at f/1.8 compared to the FA31, it has higher contrast, but that advantage abruptly vanishes as soon as the FA31 is stopped down. At apertures f/4 and f/8 there is a visible difference in resolution between the two lenses, the FA31 overall being the sharper of the two:



Un-sharpened 100% center crop, Both lenses at f/8, No WB or lens profiles applied
.

The Sigma Zoom lens generally handles Chromatic aberration better than the FA31, however I feel this comes at the expense of Bokeh quality. I disagree with Sigmas ďartĒ designation with this lens, to me it is really a contemporary lens in ďartĒ drag. Donít get me wrong, this lens is extremely sharp for a zoom: but if that is all Sigma thinks there is to art, they are in for a surprise. I found that this lens is inconsistent in its rendering of out of focus areas: in some situations it can produce outstandingly good bokeh, but it can also produce atrociously bad bokeh. I took images on the FA31mm f/1.8 under exactly the same circumstances for comparison and the FA31 has higher amounts of LoCa, but the OOF rendering from the FA31 is simply smoother than the Sigma zoom.





Right: Sigma 18-35mm @ f/1.8 Left : Pentax FA31mm f/1.8 @ f/1.8
Both lenses at 31mm 100% center crop.






The good news is that at the 35mm setting the lens is quite consistent with its rendering of out of focus elements. However at short focal lengths and wide apertures things seem to fall apart, even stopping the lens down cannot completely eliminate the "frog egg" bokeh. Focusing accuracy on my Pentax K5IIs is also an area where the FA31 has a distinct advantage, AF was noisier and slightly slower on the FA31 Ė but it was accurate. So far my FA31 has needed no AF adjustments at all.

Presently I have the Sigma lens dock on order and I will report on its efficacy however, there were still issues where the sigma lens would sometimes focus on the wrong target or sometimes mis-focus completely. I was prepared to experience some issues with AF accuracy and I was pleased to see that the Full time manual focus capability of this lens coupled with my use of the O-ME53 viewfinder magnifier allowed me to tweak the precise point of focus with this lens, but I had to tweak the focus frequently, so Iím devoting a User setting mode specifically for it.

Even with these issues the Sigma 18-35mm f/1.8 Is an extremely good lens. The issues I have pointed out are things to be aware of and can be avoided to a certain extent. Personally I would have no issue with recommending the Sigma 18-35mm f/1.8 to another experienced photographer who would be able to make the most of its strengths, and be aware of its weaknesses and find their own way around them. However I wouldnít recommend this lens to someone starting out with photography as the weight and focusing issues would be discouraging to someone who isn't familiar with wide aperture lenses.

Performance with the Pentax O-GPS1:



After a Precise calibration and careful manual focusing with Live view, I was able to get this image taken from my back yard:




Pentax K5IIs - Sigma 18-35mm f/1.8 @ 18mm f/1.8 ISO800 20s Astrotracer Mode (cropped image)


Unfortunately some cloud cover crashed the party before I was able to get more images, however I will point out this lens isnít parfocal, you need to re-focus each time you change focal length. Coma is remarkably well controlled with this lens at f/1.8, much better than most primes I have worked with. Though vignetting at the f/1.8 setting at all focal lengths is something to be careful of.


Last edited by Digitalis; 11-28-2015 at 05:13 PM.
08-13-2014, 07:02 AM   #2
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Cheers for the great review, excellent work Digitalis!
08-13-2014, 08:07 AM   #3
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Thanks for the comparative review! Very helpful!
08-13-2014, 08:14 AM   #4
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Sigma is now making top notch lenses. I think the 18-35 is for photographers who value flexibility a lot. Undoubtedly the best zoom right now considering that the HD 20-40 is significantly slower.
That being said the only prime lens that can beat the FA31 would be a new FA31 with less CA and, let's be crazy, a weather sealing. NOW THAT would be the perfect normalish lens.

08-13-2014, 08:30 AM   #5
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QuoteOriginally posted by nicoprod Quote
the only prime lens that can beat the FA31 would be a new FA31 with less CA and, let's be crazy, a weather sealing.
Even then I probably wouldn't buy it, even in the unlikely event pentax made the FA31 fully apochromatic. I have used my FA31 in rain without any crippling issues. The important thing when shooting in inclement weather is not to do anything stupid, protect your lens and camera. After all, you are working with precision mechanical and optical equipment here.


un-sharpened center 100% crop from the Sigma 18-35mm f/1.8 @18mm f/1.8 ISO 800 20s Astrotracer mode on.

Truly Remarkable performance for a zoom lens.

QuoteOriginally posted by jeffshaddix Quote
Cheers for the great review, excellent work Digitalis!
QuoteOriginally posted by seventysixersfan Quote
Thanks for the comparative review! Very helpful!
I'm glad you appreciate it - I have been changing lenses back and forth for most of today to make this comparison. Comparing these two excellent lenses and pouring over the results is really, really tedious work, I need some aspirin before I go to bed.

Last edited by Digitalis; 08-13-2014 at 03:45 PM.
08-13-2014, 09:10 AM   #6
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Amazing review digitalis, but what I've come to expect in your posts, thanks a heap.
08-13-2014, 10:04 AM - 2 Likes   #7
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QuoteOriginally posted by nicoprod Quote
Undoubtedly the best zoom right now considering that the HD 20-40 is significantly slower.
The best lens for real-world photography is the lens you have with you.
And while I have no problem taking the DA 20-40 everywhere,
I wouldn't want to be dragging that Sigma around for long!
08-13-2014, 10:35 AM   #8
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Thanks a lot for the effort.

08-13-2014, 10:46 AM   #9
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Thank you so much for your efforts here, Digitalis. This is very valuable information. Newton simply quit in the middle of doing the iterations necessary for calculating gravity influenced motions in the "three body problem" when he claimed it was giving him a headache. Let the grad students do it...

What I'd like a bit more clarity on is the manual focusing process here. Are you just using the standard focusing screen in your K5iis? I plan to pick up the O-ME53 for my new K3 based on numerous user comments here, and I note that some have suggested that the K3 offers a slightly improved manual focusing experience compared to previous flagship models. Given that I'm supposing my loss of eyesight contrast and presbyopia 'issues' fall within a range of what I'll call "typical" for early sixties males, how would you conjecture I'd get on with critical focusing following your mode of operation with the Sigma? My eyesight resolution is still pretty good with a 1 diopter correction.

The other thing that concerns me is the problem of securing a "plus" copy of this zoom: Did you get there just by brute force trial & error with multiple successive samples, via normal retail channels... or did you have a better option? Thanks for your advice, from the very, very curious.
08-13-2014, 10:47 AM   #10
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Reviews like this are what make this site great. Thanks for taking the time to do that for us.
08-13-2014, 11:01 AM   #11
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And I have always thought the 31 to be too large for my taste... Thanks for a very interesting comparison!
08-13-2014, 03:41 PM   #12
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QuoteOriginally posted by Kayaker-J Quote
What I'd like a bit more clarity on is the manual focusing process here. Are you just using the standard focusing screen in your K5iis?
Yes I am using the standard Focusing screen in the K5IIs, thankfully my eyesight is excellent so I can focus the SMCP-K 50mm f/1.2 manually without difficulty. I set my camera up so that it was in capture priority - I usually have my camera set to focus priority. Capture priority allows you to alter the focus without the camera disabling the shutter if the AF system isn't happy with the result. Even in AF mode on a pentax DSLR the Sigma 18-35mm f/1.8 allows you to change the focus by using the focusing ring (you don't have to change a switch or anything on the camera body or lens to do this) Which I think is an excellent design Idea.

QuoteOriginally posted by Kayaker-J Quote
The other thing that concerns me is the problem of securing a "plus" copy of this zoom: Did you get there just by brute force trial & error with multiple successive samples, via normal retail channels... or did you have a better option?
Initially the Sigma 18-35mm lens I got was faulty - so I tested it on my optics bench, found out there was some de-centering. I know my camera stores repairs and returns protocol: So I sent it to the supplier for repairs or replacement, they had it for a month and did nothing, saying the lens was performing optimally*. So I sent it to my usual repairer in Sydney, these people have a state-of-the-art optics testing bench and they spotted the problem immediately. The focusing group in this lens was rotationally de-centered by 0.3mm, this caused the DOF to be skewed laterally which was very noticeable at the 18mm setting. My repairer pulled the lens apart and re-centered the elements and returned it to me in less than a few days. I'm sending copies of the repair report and my test data to the supplier who did nothing about this lens - and see if they are prepared to accept that I was right and the lens was defective...I know I'm entitled to take legal action over this and I will if they refuse to admit that I had a faulty copy of the lens.

*they tested the lens on a Pentax K7 ISO400 at f/8 handheld at 35mm....really?

Last edited by Digitalis; 08-14-2014 at 06:01 PM.
08-13-2014, 03:53 PM   #13
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Your meticulous testing is appreciated--albeit out of my league. Stunning astrophotos, too!
08-13-2014, 09:21 PM - 2 Likes   #14
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I had a thought today - What would happen if you cropped an image from the Sigma 8-16mm f/4.5-5.6 at 16mm to match the 18mm end of the Sigma 18-35mm f/1.8?

There is only 1mm of focal length that separates them, but there is one small problem: At 16mm the widest aperture of the Sigma 8-16mm is f/5.6 at 16mm, so I would stop them both down to f/8 theoretically this should give a massive advantage to the faster Sigma 18-35mm f/1.8



Theorey is nice, but the reality looks more like this:



Pentax K5IIs Sigma 8-16mm f/4.5-5.6 - 16mm Vs Sigma 18-35mm f/1.8 - 100% center crops - no WB, sharpening or lens profiles applied.



Pentax K5IIs Sigma 8-16mm f/4.5-5.6 - 16mm Vs Sigma 18-35mm f/1.8 - 100% corner crops - no WB, sharpening or lens profiles applied.

Both lenses cost me roughly the same amount of money, both lenses are rather specialized. The fact that the results are so close is really surprising. I matched the framing as well as I could, but from what I'm seeing the 8-16mm has a different overall color balance than the sigma 18-35mm. The 8-16mm also has better control over CA than the sigma 18-35mm. The Sigma super-wide also has better control over distortion at this focal length. In terms of overall resolution it is a complete wash - they are basically identical in terms of performance. You could print these images at 13X19 and no one would notice any difference between them.

Last edited by Digitalis; 08-18-2014 at 03:01 AM.
08-15-2014, 03:52 AM - 2 Likes   #15
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I took another opportunity to compare the 18-35mm f/1.8 to the FA31 With both lenses at close focusing:



Fringing is more evident on the Sigma lens but the lens has higher contrast, also the 35mm focal length at the long end does give a slight boost to magnification. The FA31 has lower contrast, but the aberrations aren't any worse than they are at any other focus distance. In terms of sharpness both lenses are more or less the same.

Last edited by Digitalis; 08-15-2014 at 07:07 AM.
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