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PostPosted: Sat Sep 08, 2007 2:47 pm 
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You can also download this little gem here (copyright D. Bethke) and print it (at least DIN A4 better even DIN A3) and make a close-up shot that covers the corners. There are Siemens-stars in each corner and other structures where you can easily judge sharpness like this from the Tamron 11-18mm @18mmf5.6 (clickable for larger view):
Image
With this sort of test it is ABSOULTELY CRITICAL to have the camera aligned COMPLETELY PARALLEL to the sheet of paper, because even small deviations make problems with the MINIMAL DEPTH OF FIELD :!: :idea: :shock:
At this magnification (1:10) and this aperture the dof is about 0.4mm for critical viewing :!: :idea: :shock:
And you CANNOT stop the lens down to make the dof larger, because then the IQ-problems with the lens will also be reduced :?
Fortunately you can control parallelism in the viewfinder quite well as even slight deviations from that easily show up in non-uniform geometry.
So you can give this method a try but again, test it first at home. If you cannot get consistent results, forget it. (I use a tripod for these tests, naturally)

Sorry for SHOUTING but my experience and trivial mathematics show that this test relies very heavily on absolute parallelism and is much more unforgiving than the test with objects at a distance of at least 100xfocal length or even better "virtual" infinity as infinity is always parallel to the camera :wink:
So use at your own risk...virtual

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PostPosted: Tue Sep 11, 2007 8:25 pm 
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Finally on to my test-results!
The Tokina made a good first impression, although it was not as "sexy" as the Sigma 12-24mm (FF/FX, extremely low distortions and light fall-off), or the Sigma 10-20mm (the widest of the gang). But it has for example much lower ghosting/flare than both Sigmas and it is certainly cheaper than the otherwise equivalent Sigma 12-24mm and it is the brightest: f4.0 throughout the zoom-range!
So here we go:
My favorite shot is trees against the sky as the harsh contrast easily shows color-fringing and the fine structures are a good measure for sharpness. They are also sufficiently far away for a wide-angle lens so as not to bother about focus. Following is a shot taken at 12mm f4 1/350sec with two red rectangles marking the places where the 100% crops were taken:
Image
Crops reproduced at 100% (259x387)
|---------------------centre---------------------||---------------------corner---------------------|
Image Image
As you can see, the centre crop is quite sharp and "fringe-free" but the corner-crop is also not bad, with respect to fringing as well as sharpness. Remember this shot was made wide open @f4! As we will see later there are other shots were fringing is more prominent...

Let's have a look now on a "lab"-shot featuring the famous "Siemens-stars" and taken at 12mm f4 1/350sec:
Image
Followed again by a 100% centre-crop and crop from the upper right corner (which is the same corner as in the previous pictures because we're are now shooting in landscape format!)
Image Image
You see much less color fringing at the corner-crop than with the Tamron 11-18mm and you can easily see that the "grey disk" in the corner-crop is quite uneven. So sagittal resolution is much better (disk-diameter=7mm) in the corners than the meridional res (14mm diameter).
The diameter of the grey disk in the center-crop is around 9mm. That means: corner-resolution of this lens at 12mm wide open is sagittally 50% better (!) then center-sharpness and meridionally 35% worse :?
DISCLAIMER: On your screen you may measure other absolute values, but the ratio should be similar!
That means less center sharpness than the Tamron, but the corners are better defined. You can also look at the third Siemens-star in the lower left corner of the pic. This is a finer one than the other two, so you cannot compare the grey disk between the different stars, but you can compare it to the same star in the Tamron pic. You can see a much better definition of this fine structure with the Tokina.
This measurement confirms a feeling that the Tokina had no real "bite" at wide open aperture, not even in the centre. This is in stark contrast to Klaus Schroiff's test here, that showed excellent central resolution-figures over the whole aperture range :? Another case of decentering defect?
At 12mm/f8 the grey disk in the centre-crop shrinks to almost 7mm and in the corner-crop to 7/10mm (see here). At 24mm the corner-sharpness becomes clearly less astigmatic (and thus overall quite good), while the centre-sharpness stayed at sub-par levels. Interesting...

Now onto the next part. A full close-up shot of the Siemens-star, to determine the IQ under these circumstances. I often wonder why nobody is showing lens-quality at magnification of near 1:10. So here is a world second (for the "first" see here) taken at 24mm/f4:
Image
If you measure the grey disk in the centre star at approx. 8-9mm again. So no big deal here: centre-sharpness for close objects is as good as sharpness at infinity. If you measure the little stars in the corners (well not really in the corners) you get around the same8-9mm grey disk with not much deviation from corner to corner (which should be proof that I had the camera adjusted well and the lens is not obviously decentered). So all in all the close-up performance is good.
You can also see some brownish fringing in this test-shot and (as promised) the following 100% crop from the upper right corner also shows some fringing of this lens (click through to full pic) although I think it is quite controlled:
Image

Last but not least: Let's do the "white-dwarf". You know this one already? Well, I just wanted to show you how sensitive this lens is to contra light under almost controlled conditions. The details of this methodology are over here. Following is a typical and a worst case example. The full gory details are in flickr, wibble over here. The results are quite positive, ghost and flare were no big deal with this lens, only when shooting straight in the "sun" :D
The first shot is @12f4, the second @20f8.
Image Image

Summary--------------------------------------------------------------

Would I buy this lens? Well, I did not, because for me the overlap with my 18-200mm zoom was too great (only the range from 12mm-18mm was new)! If you cover the mid-range with a 50mm prime or a 24mm-70/120mm zoom, the fit is much better! I also missed the "bite", but perhaps I had a copy of the lens that was not well adjusted (although decentering should have come out with my tests...).

Is there a photo I love, made with this lens? Sure! The one shot that I love most with this lens is here. Again I'm not implying this is a "great" photo, though...

Ah, before I forgot: You know already that geometric distortions and light fall-off for me are not decisive factors, as both can be pretty much corrected via postprocessing. Light fall-off can be viewed at my photos and for geometrically inclined there is one very special shot this time: @12mm f4. It also shows corner-sharpness (much better than the Tamron) and color-fringing quite beautifully. And when you compare this shot to the respective Tamron-shot, you see quite clearly that the Tokina is much better in the corners.

Ok, so that was it, my second "full" lens review...
Have fun reading! And feel free to ask :idea:

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Last edited by Thomas on Wed Feb 06, 2008 8:15 pm, edited 2 times in total.

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PostPosted: Wed Sep 12, 2007 3:50 am 
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Great work Thomas - really enjoying your reviews!

PS - were those your feet?


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PostPosted: Wed Sep 12, 2007 9:20 pm 
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Thanks, Gordon!
Looking at the following picture in large size, I'm now convinced that this lens has a decentering defect. I've painted a red line, below which sharpness looks ok. With a well-centred lens there should be a longer range of sharpness on the right border of this pic (clickable for larger view)!
Image

What is your opinion on this?

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PostPosted: Wed Sep 12, 2007 10:41 pm 
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Well the simple explanation is that no lens is perfect! There are some odd optical artefacts on all lenses when you look closely... the trick is to select the models with the least problems...!


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PostPosted: Thu Feb 13, 2014 5:39 am 
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The link with siemens test doesn't work any more: :cry:
http://images.digicamfotos.de/media/23/siemensstern.zip

Could you guys share a new link to siemens lens test, please...

Thanks! :wink:


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PostPosted: Thu Feb 13, 2014 12:25 pm 
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For those who are interested, i've discovered the original place where all the test targets are being kept.
http://de.cyclopaedia.net/wiki/Siemensstern


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