Re Dan's (audiobomber) observation:
Quote: The 18-250 has serious distortion at 18mm
.....not forgetting to mention some noticeable pincushion distortion at the opposite end of the spectrum.....lol !
Despite all of the above, I still regard Tamron's accomplishment in bringing the 18-250mm super-zoom to the market-place as a remarkable feat and believe Pentax's subsequent licensing of this particular design to be a ringing endorsement to this effect….something along the lines of that old adage about "Imitation being the sincerest form of flattery" etc.
Considering the cost constraints which many of today's engineers probably find themselves operating under, I suspect it is quite likely that they have reached the limit of what is presently achievable, regarding the minimising of optical distortion in super-zooms. This thorny subject is usually referred to as
compromise, but fortunately I did plenty of research prior to purchasing this lens and was thus fully aware of the inevitable trade-offs involved where such optical distortions might be concerned. Thankfully these defects can easily be dealt with nowadays when post-processing images and therefore I honestly fail to see the point of this argument, unless you don't have access to a computer.
I don't think anybody here is claiming that Tamron's 18-250mm is perfect in every respect because it clearly isn't, but nonetheless there are evidently many satisfied users out there taking excellent photographs with it any enjoying the all-in-one convenience factor !
The main point of all this is that if you feel a 2-lens solution might offer you a perceivable benefit, simply try it and see.
Best regards
Richard