The full title/description of this lens is a real mouthful.
Mongo hunted down a good one of these for Mrs Mongo recently. Its now had a reasonable trial at the hands of Mrs Mongo (and Mongo got his paws on it a few times too).
Used with a Pentax K5 it gives the equivalent of 450mm at f4 wide open. Mongo heard largely good about this lens (and some doubtfuls) before doing his own research which led him to to buy one.
Purchased used for $900 - still boxed and used only a few time at best. It looked new ! This is great value.
weighs only just over a kg and is physically small for a lens of these specs. To give some idea, it is about 30% smaller and about 20% lighter than Mongo's nikon 300 f4. Price second hand, is virtually the same.
It has an ultrasonic AF motor (which is deadly quiet and reasonably quick on the K5. THis may vary with other camera bodies), possibly 2 ED elements (although not even top reviewers seem to know as Pentax is not great about giving information or specs), it is weather sealed and has special protective coating on the front element.
Mrs Mongo rates its performance as very good and one of the easiest lens to carry and use for its specifications. Mongo also rates it as very good on the K5 body. Overall , both Mrs Mongo and Mongo were impressed with it and thought it was an excellent buy in a lens. On a straight comparison with the nikon 300mm f4 , in image quality terms, Mongo would rate the Pentax at about 94%. That is pretty darn good given the quality of results results from the Nikon. Also , like the nikon, it is great from wide open and peaks at only 1 stop down (f5.6) whereas the Nikon peaks at f8. It handles better than the Nikon mainly due to its more manageable size and weight. Like the Nikon , it does not have vibration reduction but this is provided by the K5 body. So, in this regard, it is better than the Nikon and better value for that reason.
The one very big annoyance with this lens (and Pentax generally) these days is The failure of Pentax to make any teleconverters for its own telephoto lenses. Kenko, Sigma and others are problematic as they can not be sure to work with this ultrasonic AF system OR not work in combination with this system and all high end Pentax DSLR bodies. You basically have to try each combination to ensure that they work together before buying.
Enough of the tech talk - a picture or two are worth a thousand words. So, here are a couple with this IMPORTANT DISCLOSURE - PHOTOS TAKEN BY MRS MONGO BUT PROCESSING PROVIDED BY MONGO.
These images and many more were taken last Monday when birding with Lance and Sar
K5, 1/2000th @ f8
K5, 1/320th @ f4