Lens Comparison:
SMC Pentax-DA 18-135mm F3.5-5.6 ED AL [IF] DC WR and Tamron SP AF 24-135mm F/3.5-5.6 AD Aspherical [IF]
(and DA 35 f2.4, DA 50 f1.8, F35-70 f3.5-4.5 and Tamron 90 f2.8)

This comparison was motivated by a discussion on a thread in the forum highlighting the pictures taken by the DA 18-135.

Let me make some general comments at the outset.

• I bought it to replace the Tamron 24-135 which I had been using. It's a big and heavy lens (540g as compared to the 405 of the DA 18-135), isn't WR, and I did want something wider for my walkaround lens. Do note, however, that the Tamron is a full-frame capable lens. So, if Pentax ever delivers a full-frame camera...
• I love my DA 18-135. It's the lens that is on my K-30 most of the time. It's WR, wide enough, light and compact enough, and takes good enough pictures. This comparison is not intended to bash the DA 18-135. I have had much better results than the critical review on photozone.de.
• I am aware that there can be significant variations with any given lens. These are just my results with the lenses I have.

Some general observations:

• In making comparisons, I really noticed how sharpness changes across the frame depending on focal length and aperture. In most instances, but not all, the Tamron was better across the frame.
• There are lots of ways to evaluate a lens. Distortion, vignetting, aberrations, sharpness across the frame, aperture speed, contrast, handling, focusing speed and accuracy, etc. In this comparison, I'm mainly looking at the sharpness near the center of the frame. (If you have a specific question about some other aspect, ask. I still have all the original photos.)
• If you printed the pics out at 6"x4" resolution, I doubt you would see much difference. You would see differences showing up if viewed full screen on a monitor. My examples are at 100% to see the differences most clearly.
• In both tests, I kept the ISO consistent and used Av mode.
• For the indoor test, I used a tripod and 2 second shutter delay. I took shots with the lens wide open at any given focal length and also at f5.6 and f8. I then used the best shot as the reference picture. In all but a few cases, best results were at f8. In those few cases, f5.6 was better. (The only exception was the DA 35 f2.4 where all the shots were superb.)
• The outdoor shots, which were trying to duplicate actual usage of the lenses, were handheld. All were shot at f8.
• I took shots at (or at best approximation to): 18mm, 24mm, 35mm, 50mm, 70mm, 90mm, 135mm.
• I included some of my better lens in the comparison to see how the Tamron and Pentax compete with primes like the DA 35 f2.4, the DA 50 f1.8, the Tamron 90 f2.8, and the well-regarded F35-70.

General results

(primarily looking at sharpness but also taking into consideration color and contrast): listed in order starting with 1 as my preferred

• Not surprisingly, the primes did best. My highest quality lens is the DA 35 f2.4 by a significant margin.
• Somewhat surprisingly, the Tamron was a draw with the DA 50 f1.8 at f8. At larger apertures, the DA 50 was better.
• Also somewhat surprisingly, because I really love my F35-70, the Tamron beat it in every case.
• The differences between the DA 18-135 and the Tamron 24-135 lessened as it went out to the long end.
• The Tamron was usually better than the Pentax at the larger apertures, but the differences lessened as one stopped down.
• The Pentax consistently exposed 1/3 stop faster. (E.g., same focal length, some ISO, same aperture: Pentax would be at 1/160 and the Tamron at 1/125.) Subsequently, the Pentax pics were consistently darker. The Tamron pics thus appeared with more and better contrast. I adjusted levels (only) on some of the Pentax pics to try to minimize that aspect of the comparison, but the Tamron ones still had better contrast.

Pictures

I created a Flickr set. Be sure to read the captions on photos where I didn't think to add labels. Unfortunately, I wasn't entirely consistent in the way I ordered the pics as taken by each lens. HERE is the link.

Conclusion

As you can especially see in the outdoor crops, the Tamron really does deliver more detail, better contrast, and better exposure.

I'm not getting rid of my DA 18-135, but I'm even more impressed with the results of the Tamron 24-135 after this comparison test.