Why not another bump for an old thread? The question keeps getting asked.
The key questions for anyone buying a wildlife lens for using with a Pentax DSLR are:
1. budget?
2. zoom or prime? If a zoom, would you use it often at shorter than the maximum focal length? What else is in your kit that might affect the choice?
3. What about focus? Can you live with manual focus (and if so, can you also live with manual exposure)? If you need autofocus, does it need to be quiet and fast?
4. Is weather-resistance essential? (If the answer is yes, you will rule out many fine lenses. Maybe a protective sleeve will suffice?)
5. How much reach do you really need? 250mm? 300mm? 400mm? 500mm? (In other words, how big and how far away will your subjects be?)
6. How fast do you need the widest aperture to be? (Speed matters particularly for low-light shooting and use with a TC. Bumping up the ISO and/or dropping shutter speed is often going to compromise results.)
7. How important is absolute IQ? (This is tied to the previous question.)
8. What minimum focus distance (MFD) is acceptable?
9. How much weight and bulk can you cope with? (Will you mostly be using handheld? Do you plan to take it on walks? Do you plan to travel with it?)
For me, the last question is really important, and under-rated. If your experience is limited to <500g consumer zooms, you are in for a shock when you want more than 300mm. The 170-500 I had was about 1.4kg - light for a 500mm lens, but still quite a burden to carry around. Good luck to those people who happily cart around a 2kg lens like the Pentax DFA 150-450, or the Sigma 150-500 or 50-500 on a walk, especially if they are also carrying a 700g-1kg camera, spare batteries, a flash and extender, and another lens or two and some filters, not to mention a tripod. And don't forget your water bottle, food, jacket, phone, etc etc. I'm not that fit or dedicated, and I can't afford a Sherpa.
Like it or not, every lens is a compromise. (Otherwise we would all have 12-600 f2.4 superzooms that were WR, had fast and silent AF, weighed 400g and produced images like an FA Ltd.) Here are some observations on the various telephoto lenses I have had. I offer these as they are probably representative of particular classes of lens:
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Superzoom - Tamron 18-250 (same as Pentax DA 18-250) : The superzoom was good for what it was, and the versatility and portability were great. Short MFD made it good for close shots. But the reach was insufficient (not only because 250mm often wasn't enough, but also because focus breathing meant the real magnfication was considerably less). At f6.3 at the long end, it was slow - especially when it needed stopping down one or two stops for best results. Images benefit a lot from post-processing. Not a patch on the 55-300 at the long end. The latest Sigma 18-300 might be better, but don't expect the same performance as the 55-300 at 300. (If someone proves me wrong about this I'll buy one immediately.)
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Consumer tele zoom - Pentax DA-L 55-300 : Cheap, lightweight, versatile. Outstanding value. Excellent for travel and hikes. Images benefit from PP. Not an IF lens; really good magnification at 300mm regardless of distance to subject. MFD about 1.6m is ordinary. Faster (ie wider maximum aperture) than the 18-135 across their common range, but the 18-135 is much more pleasant to use. Surprisingly good at the long end, when stopped down. At a pinch, f5.8 at 300mm is useful, but aim for f8-f11 whenever possible. (A flash and extender are good for this.) Slow AF, hunts like crazy, sounds like a coffee grinder. (The new PLM is obviously much quieter.) Not a great option for moving subjects. Better on the K-3 than on the K-30 because of the faster AF motor, and better resolution.
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Consumer long tele zoom - Sigma 170-500 : Affordable, and the extra reach was great. Light for a 500mm lens; later iterations of the xx-400 or xx-500 are all heavier. Still something of a beast to carry. Usable handheld, but better on a tripod. Slow f6.3 at the long end; at it eats light at 500mm. Slow and noisy AF, like the 55-300 - maybe a little better. A frustrating 3m MFD. Good images within its limitations: subject not too far away, good light, stopped down. Better at the wide end; at 300mm (maximum aperture f5.6) performance would be roughly comparable with the 55-300. Of all lenses I have used, this produced the most vignetting, and the dullest colours and least contrast. Photos benefit from a lot of treatment in PP. From what I have seen, the IQ from later versions like the Bigma and little Bigma is significantly better, but at the price of much more weight.
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Long prime - Sigma 400 f5.6 Tele macro (same as the one shown by @Ducatigaz above): A nice lens, but hard to find. About 1.4kg (about the same as the 170-500, but not as bulky). A step up in image quality from the 170-500. Resolves more detail than the 55-300 (I suspect mostly because of the extra magnification). Has a focus-limiter, which is useful. AF seems a little quicker than the other screw-driven lenses mentioned. It has a ring to switch to MF. I have used it a few times with my Kenko 1.5x TC (really 1.4x - it's the same as the Tamron 1.4 pz), for 560mm f8. For that it needs very good light and a tripod - and I'm not certain results were better than would have been obtainable just by cropping on the K-3.
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Premium 300mm prime - Pentax FA*300 f4.5: This is just a superb lens, optically. Sharpness, colours and so on are in the class of the DFA 100 WR Macro - which is saying something. Virtually no CA, pleasant bokeh. Images need little PP; OOC jpgs can be very good. It's compact (barely longer than the 55-300), but dense (it weighs about 900g, roughly double the 55-300). This is the Goldilocks lens. Not too heavy, not too bulky, fast enough and really sharp - so that images can take a lot of cropping. Screw-drive AF is faster and quieter than the other lenses mentioned (although noisy compared to a DC lens). MFD is about 2m, which can be frustrating. No focus limiter. No quick-shift, but it has a clutch to switch to MF. Doesn't come with a tripod foot (there are cheap options), which says to me that the makers expected it to be used handheld. It balances nicely on the K-3 and is easy to use handheld. Excellent from wide open - f4.5 is handy in low light, and lets you use a faster shutter and/or lower ISO. This has become my most used lens. I have only used it with the TC a few times (420mm, f6.3), and haven't properly compared it to the 400 f5.6, but I suspect that it is comparable if not better. I expect that the more recent DA*300 f4 would have many of the same virtues, with the bonus of quieter AF, WR, 1/3 stop faster, and shorter MFD. But the downsides are that it is bulkier, heavier, more expensive, and the SDM AF has been known to fail.
Other categories of lens not represented here include
MF lenses (e.g. K 300 f4, A 400 f5.6, Tair 300 f4, the Tokina 100-300 f4 and various mirror lenses),
premium tele zooms (e.g. Sigma 100-300 f4, DA*60-250 f4),
very long primes (e.g. Sigma 500 f4.5, DA 560) and
very long premium tele zooms (e.g. DFA 150-450, FA*250-600 f5.6).
Had I known at the start what I know now, I would have just got the F/FA*300 f4.5 and a good TC, and the best of the 55-300 lenses for walks and travel, and left it at that. The FA*300 cost about $A850 (say $US600), which seemed like a lot at the time; but it was definitely worth it. The DA*300 f4 would be very a nice option (especially with the DA TC), but I would worry about SDM failure.
If you are photographing birds within say 15m or so, with the 24mp sensor, 300mm is long enough, IMO, if the lens is really sharp. Sure 400 or 500 is better, particularly in a zoom, and particularly if the MFD is <2m, but for me the FA*300 f4.5 is a better package. For birds further away, more reach is invaluable: 300 + TC is good, but 150-450 or xx-500 would be better. So it depends a lot on what you are shooting.
One more thing to throw in. If you can use a flash, it can really help. I love this with the K-3:
https://www.pentaxforums.com/forums/22-pentax-camera-field-accessories/256288...-extender.html
Some samples from each lens I have had.
Tamron 18-250
Pentax DA-L 55-300:
Sigma 170-500:
Sigma 400 f5.6 tele macro:
Pentax FA*300 f4.5: