Originally posted by DriftlessPhotographe I would love to get the Nikon D500 and pair it with the Nikon 200-500mm lens, but that's alot to spend currently. I currently use the kit 18-135mm as mentioned, which works well, but I have to crop in due to its short range, which then degrades image quality. The 300mm appealed to me being the longer reach and if I have to crop in the images would still retain good quality. That's my thinking at least haha.
Good thinking I'd say. I've tried a range of telephoto options and for me a 300 prime (with teleconverter where necessary) gives the best balance of image quality, portability/size, and value for money.
I have the FA*300 f4.5, one of the ancestors of the DA*300, and it's a superb lens. One of Pentax's best. The DA*300 is 1/3 stop faster, has newer coatings, quiet AF, QuickShift and WR. The only knocks on it are that the AF isn't lightning fast and the SDM focus motor fails occasionally. But most people manage OK with the AF, and if the SDM fails out of warranty it can be readily converted to screw-drive AF which apparently works well.
As @Azergoth points out, you would probably also want to get the Pentax 1.4x TC at some stage - it's expensive, but retains the WR, and seems to be a high end TC. Cropping with a 24mp sensor works quite well as @Chickentender shows, but there comes a point where more reach is better. Theoretically you could use one of the Kenko or Tamron TCs that has the pz contacts (I have a Kenko, and it's very good) to drive the SDM (and save some money that way), but user reports suggest the SDM AF doesn't work so well with those TCs as with the Pentax TC.
The alternative is one of the long zooms, like the Sigma 150-500 or 50-500, or (if the budget runs to it) the Pentax DFA 150-450. But each of those lenses is much bulkier and heavier than the DA*300 (even if you add the TC to it). Each of those zooms weighs 2kg or more, and believe me, that really limits its usability. I had a Sigma 170-500 for a while and even at 1400g I found it a beast to carry and use. The 300 prime is much more user-friendly.
I think the DA*300 should work well on the K-70, and even better if you later get one of the flagship models. I used long lenses on my old K-30 and got many worthwhile images. With the extra pixels and probably better AF the K-70 should be fine.
For those times when you want the flexibility of a zoom and coverage between 135 and 300mm add one of the 55-300 lenses. But while I'm a big fan of those lenses (I've got both a DA-L 55-300 f4-5.8 and a DA 55-300 f4.5-6.3 PLM) neither can hold a candle to the FA*300 at 300. And it's not just that the prime is about a stop faster - it just has much more resolution. That shows particularly when you are at a distance and need to crop.
If you get the prime, you won't regret it.
Incidentally, if you find the DA*300 too expensive, consider an F*/FA*300 f4.5. You may find one for around $US650-750. Add one of the better TCs (say $150) and you have something like a pro-level kit on a hobbyist's budget. If you look at the reviews of these lenses, you will see how much people love them. Here are some samples of my photos with this lens:
Flickr - Des(Australia) - Pentax FA*300mm f4.5 samples