Instruction: Extract information from the supporting document to answer the given question.

Input:
Question: Is it fully compatible with the Pentax k-3?
Document: I like my K5 but I have not bought any new lenses other than the 18-55WR that came with the Camera. I have some Film Era zoom lenses, a FA50 f1.7 prime, and some manual focus primes as well as 3 other Pentax Film bodies. The FA50 is good glass, the other primes are OK glass, and the zooms are a mixed bag.Now this lens, 100mm f/2.8 WR, well it is Terrific Glass (I need a stronger synonym but I am too lazy to find one right now).
This lens was designed using top-notch technology with photographers in mind. Minimal weight, nothing sticks out, the focusing ring is at the end of the lens, an additional central groove makes it very tactile - easy to find w/o looking and easy to turn w/o slipping - it just "sticks" to your fingers.The lens is really compact and sits well on K-5, easy focusing. Sample pics were taken handheld 1/500 around 6pm under partially cloudy sky with AWB.I don't miss the aperture ring but I would appreciate a focus limiter (was focus hunting 3-4 times out of 50) and slit in the hood for CPL.
Plus, if Pentax ever releases their FF camera - you are good to go. Highly recommended.
I've got this lens with Pentax K5 for my 50th birthday. I did not use much the lens, but for portrait photography is good. The subject is in focus and the background is blurred even with aperture f9. It searching the subject for focusing, but if you help - it doing fast and quiet. That is the reason for 4 stars. Just to mention - you can focus with this lens by hand wit-out switching manual mode.Filter size is 49mm - the cheapest in the market. I hoop it will last long.
For those familiar with the Pentax brand, for years, Pentax has specialized in small, tightly built primes. Those with all metal builds are called the limiteds. For whatever reason, the D FA 100mm WR does not receive this designation, but it certainly fits with the overall theme of both the DA and FA limited lenses.Build is metal and very sturdy. Hood snaps into place and is fairly good sized. Lens does not have an aperture ring or a focus limiter. It does have quick shift capability which is very handy and can limit hunting.
Stopped down to f4, they are pretty even.Out of focus areas are a step above the olderPentax D FA 100mm f/2.8 Macro Lens for Pentax and Samsung Digital SLR Cameras. Pentax basically updated this design and added rounded aperture blades, so that out of focus areas tend to be smoother with more rounded bokeh.All in all, this lens is a joy to use. Having an aperture ring and focus limiter would be handy, but the lack of these is not enough for me to drop the lens from a 5 star rating.
Focus is throw is quite long (it is a macro lens after all) and if lens misses focus, it takes awhile to cycle from one end to the other.Optics are very nice and quite sharp from f2.8. Distortion is completely absent as far as I can tell. This lens is definitely a notch above thePentax SMC DA* Series 50-135mm f/2.8 ED IF SDM Telephoto Zoom Lens for Pentax and Digital SLR Camerasoptically wide open -- less vignetting, less distortion, just sharper.
It was a tough decision between the famous Tamron 90mm f2.8 macro and the new Pentax 100mm f2.8 macro. Decided on the Pentax  despite ~50% higher price: WR, 49mm filter, lighter and more compact, all metal outside, excellent sharpness across the field and one stop down from 2.8 (still very-very good at 2.8, compare the graphs for the Tamron and Pentax on LensTips). The fact that protruding front lens is completely hidden inside the hood makes it very handy for street-candids-portraits.
If your running Pentax - this is a great lens. Note - prices are going down here in June 2012. Excellent macro capability, fantastic color rendering, tack sharp and metal construction makes it an all around great package. Pentax makes some fantastic lenses - and if you are using their system - this is a great lens - especially with a weather resistant camera like the K7, K5 and the upcoming K30 - this is a great compliment for a good weather tough package. 100m focal length is a bit strong tele on crop sensor - but this lens works well for a prime.
Top notch quality lens, small, compact, elegant and very well made. The metal build and high tolerances are very close to the standards one has come to expect from the Pentax series of limited lenses. Optics and performance are excellent, and the weather resistant design certainly adds to the value of this lens. Unfortunately, the autofocus is less than perfect --the absence of a focus limiter for me is a perceptible drawback, since hunting happens frequently and takes an annoyingly long time. This of course means that for moving subjects manual focusing becomes necessary, something that might not be a deal breaker for some but that for me takes away from what could have been an almost perfect macro lens.
Answer: Yes it is I love it on my K-3.