Below is the text of a review I posted in the lens review database over at pentaxforums.com, but since this lens can be used with most cameras due to the interchangeable nature of the Adaptall mount, I thought I'd repost it here, augmented with photos at the end...
The Review...
This represents my initial impressions after a couple weeks of use. The only lens I can compare it to with regard to focal length is the Sigma 50-500mm Bigma, so I'll make those comparisons when I can, but unfortunately I can't compare it with any of the Pentax or Sigma fast super-telephotos...
Sharpness:
Wide open, the lens is quite sharp in the center, probably comparable to the Bigma wide open (I didn't do any head to head comparisons though). The, the advantage of course is that you are 1.7 stops faster than the Bigma is at 400mm. Corner performance is weaker than in the center, but the application of sharpening in post processing makes even corner crops crisp.Colors, Contrast and Bokeh:
Stopping down, improves sharpness across the board, with the corners at f/5.6 matching the center at f/4, and sharpness appearing uniform (and still slightly improved) by f/8.
Even using the Adaptall-2 1.4x tele-converter (#140F) for the equivalent of 560mm f/5.6 the center sharpness holds up fairly well, and while may be slightly softer wide open than the Bigma @500mm, only 1:1 pixel peeping would tell it, and I think the difference would be slight. The application of sharpening (USM) in post processing makes even the corners (on APS-C) usably sharp wide open, even with the 1.4x TC.
Colors and contrast are very good at all apertures, though the slight softness at f/4 is visible as a slight loss of contrast on a small scale. It might be said that micro-contrast is what is hurt at wide aperture, more than resolution and that's why the sharpness benefits so much from sharpening (but I don't really know the validity of that, so perhaps it shouldn't be said ;-).
I find the bokeh produced by this lens to be very nice... very smooth, and of course with the shallow DOF generated by wide apertures at this long focal length, the bokeh is plentiful - it's easy to isolate your subject from the blur obliterated background with this lens.
Fringing and Chromatic Abberation:
Exposure:
Yes this lens does exhibit both CA and PF, but not to the extent that I'd even list it as a "Con".
CA seems very well controlled, and can be virtually eliminated using a "CA tool" like that found in Lightroom. When using the teleconverters, CA is more apparent when pixel peeping, but still the Lr CA tool does a great job minimizing it.
"Non-Destructive Purple Fringing" (to coin a phrase) is fairly prevalent in the dark sides of high contrast areas when using the TCs (it exists to a small extent when the lens is used alone, but it becomes more prevalent with the teleconverters). I'm calling it non-destructive because detail is still preserved, it's just purple rather than the dark color it ought to be... In 99% of my shooting with this lens, there is no purple in the shot, so removing this PF is as simple as desaturating the purple channel, and darkening it somewhat and calling it a day. I've created a Lightroom preset that does this, and I can apply it to troublesome photos as needed. If the shot contains a bunch of purple flowers as well as PF, then I have to go into Photoshop and use masking to selectively desaturate the trouble spots.
Auto Exposure with the PK/A adapter seems spot on when the lens is wide open, and needs -0.7 EV compensation when stopped down at all. Luckily, the -0.7 seems to hold true from f/5.6 through at least f/11 (I've rarely ventured above f/11 so I can't say for sure beyond that).Build Quality, Finish and Usage:
Overall I'm very impressed with this lens. Now to hire a Sherpa to carry it around for me....
This lens is built like a tank (right down to the color scheme). The metal body feels quite solid. The paint scratches fairly easily, when compared with something like Sigma's "EX Finish" or the finish of my Pentax DA*, and modern plastic Tamron lenses - luckily any copy you find of this lens is likely to be pre-scratched by the previous users, so it's not so painful when you add your own.
The action of the focus is silky smooth and very light (basically perfect in my opinion). Focus force can be increased by setting the focus preset detent ring in a fixed position. This ring allows you to preset a focus point, and the lens will "snap" into it when you move the focus to this point, while still allowing you to focus throughout the full range. I haven't used this feature yet as I don't like how stiff the focusing action becomes, but it would be supremely valuable to keep the focus fixed on a point (a birds nest for instance) and resistant to being bumped out of focus.
When my copy of this lens arrived, the portion of the lens behind the tripod collar (where the adaptall adapter mounts) was slightly loose - not good - I found that this was caused by looseness in the six screws which hold the spacing between this rear portion and the main body of the lens. These screws can be accessed through the 3/8" hole in the tripod mount (turning the mount allows you to access each screw in turn), and once snugged, the lens is solid again. I did have these screws work loose a couple times, but I recently bought a proper jewelers screwdriver, and torqued them down with a small drip of loctite each, and so far this has held. Incidentally, I found that positioning this ring at maximum extension made the lens focus accurately at the infinity stop rather than focusing past infinity as it did when the ring was more forward.
The weight of this lens is a lot to hand hold, but it's doable with practice (I've captured birds with it in flight - some are even in focus). It really doesn’t weigh much more than the Bigma, though of course it’s bulkier. The lens seems a bit nose heavy when mounted on a tripod even with the battery grip on my K20D. I heavy duty tripod/head combo is mandatory, and a gimbal style head would be ideal. I've had good success using this lens on a monopod as well.
I often find manually focusing is necessary when photographing wildlife, even with AF lenses, as often branches or other objects are present to confuse the AF system, so the MF only aspect of this lens doesn't bother me much, though of course AF does often allow you to get some "snap" shots you might miss having to manually focus - I find my speed and accuracy is improving the more I practice, and I'll probably be super fast by the time I can afford an AF super-tele like the APO EX 500/4 or an FA* 600/4 or similar.
Now Sample Photos:
These are taken from my use of the lens for general shooting rather than a special test series, so in general, I'm not shooting wide open as I'm shooting for "keepers" rather than lens testing. Often these are f/5.6 or f/8 (true aperture of the lens itself - not accounting for the light loss caused by any teleconverter used). Performance at f/4, even on the 2x TC is impressive IMHO, as you can see with the last few examples. (click for full size versions)...
No Teleconverter, f/8 -
you can see PF in the eye...
full frame...
crops...
full image...
crops...
Here's a hawk in my yard, shot with the Tamron 1.4x TC (Model 140F), f/6.3, hand held... again, PF is the noticable flaw, and it's handled well by just desaturating the purple channel...
full image...
crops...
Now a couple on the Tamron 2x TC (Model 01F) - f/7.1 on a monopod. PF is larger in these examples, but still admirably handled by desaturating and darkening...
somewhat cropped...
crops...
full picture...
crops...
Now, some really impressive shots from the lens wide open on the 2x TC on a monopod...No correction for PF or CA needed on the first...
full picture...
now the crop...
pretty impressive to make out these hairs on the leaf from nine feet away, wide open on a 2x Teleconverter!
full picture...
now the crop...
As you can see, performance stays high on the 1.4x teleconverter, and is still pretty darn usable on the 2x converter...
More sample shots with 100% pixel peeping are available here.
Photos taken with this lens on display on my web-gallery are here.
