This is three, three, three posts in one, which is just the thing for the beginning of a holiday weekend (it's Memorial Day weekend in the United States. We display the flag, hold parades featuring aged veterans and the occasional tank, and remember all those who served, some of whom also died).First,
an official, certified, sealed T.O.P. endorsement, unbidden and unbeholden as usual, for one sah-weet digital SLR lens.Second,
an announcement: SLRGear.com has just posted a review
of this lens.Third,
this seems as good a time as any to talk about SLRGear.com's outstanding "Blur Index" dynamic interactive graphic feature. This is really one swell application of web technology.
This is not immediately apparent to many people when they first visit the SLRGear site, but you have to click on
the static Blur Index illustration in order to get to the feature!
Once you do, a pop-up will open, and you'll find a great deal of very useful information packed into a very easy-to-understand visual. The Index gives the sharpness of the lens across the entire optical field at every focal length and aperture, which you control using sliders.
If you'll refer to the illustration above, I'll tell you briefly what you're looking for. First of all, the rectangle within the 3-D "box" is the image. The lower in the box it is—the closer to violet in color—the higher the sharpness. The corners of the rectangle are, obviously enough, the corners of the image. So the flatter the colored rectangle, the more uniform the lens's performance is across its field; and the lower it sits in the box, the sharper it is.
Like Phil Davis's masterful Plotter/Matcher program does for black-and-white film, developer, and paper combinations, the SLRGear Blur Index can show you almost at a glance what used to take weeks of use and patient observation to determine. Namely, it tells you where a lens is strong and where it is weak, and in what way. For instance, the above illustration is the Tamron 17-35mm wide open at 17mm (the worst case for this lens, as it is for most WA zooms). You see that the corners are very soft (they're much higher than the center); but you also see that the center is still extremely sharp. As most photographers have discovered, in siituations where you're using a lens wide open, very often you're concentrating on some central object and the corners don't matter much at all (consider my [in]famous apple
again, for instance). In such cases, the graph tells you the Tamron can be used safely wide open.
Move the focal-length and aperture sliders in the Blur Index pop-up back and forth to see that the lens gets very well behaved very quickly, and stays that way.
Using the SLRGear.com Blur Index, you can even see diffraction degradation setting in—that's when the entire colored rectangle, representing the image at the sensor plane, moves upwards as you close the aperture more and more. The graph reveals that there's not much diffraction penalty with this Tamron, but with other lenses it can very usefully show you what small apertures to avoid.
It's a quick and effective way to compare two lenses, too. For instance, a close comparison of the Tamron 17-35mm and the Canon 17-40mm shows that the Canon is slightly more consistent across the frame, but that the Tamron, at wider focal lengths and wider apertures, is a touch sharper in the center (remember that the Tamron is faster, too; the Canon at first looks much better wide open, but to be fair you have to "virtually" stop down the Tamron to ƒ/4. When you do that, the less expensive Tamron compares very well).
All in all, the Blur Index on SLRGear.com is one of the most innovative graphic representations of lens test information I've seen in ages. It will of course become more useful as more lenses are added to the database. But it's a fun and valuable feature that should be a regular surfing stop for anyone who likes to learn about how lenses perform.Posted by: MIKE JOHNSTON