Have you actually tested this hypothesis, I have....
https://www.pentaxforums.com/forums/10-pentax-slr-lens-discussion/297821-50mm-what-use.html Quote: My 18-135 is often unsatisfactory in the range 110-135, you see - a bit blurry in the corners. Whereas the 100WR is sharp even for massive crops, possibly even approaching 200 mm equivalent.
SO to take an 18-135 to 200mm just crop off the blurry corners and you're just as good as the 100.
The FA 100 has lw/ph of 2194 ( with a max scale of 2350) as tested at Photozone, cut that in half and you have 1100 lw/ph. That's good for an 11 inch print.
A DA* 200 will achieve 2074 at ƒ 5.6 that will easily give you a 30x20 print.
The resolution of one is imperceptably close to the other. So if you can easily crop a 100 to 200, you can also easily crop a 200 to 400. It's far from the same functionality.
The DA 18-135 was measured at 2405 lw/ph in the centre (out of 2750) measured on a 16 MP sensor, as opposed to a 10 MP sensor of the other two.
Since when cropping, the 18-135 is only losing 1/3 of it's lw/ph, it could easily be argued that the 18-135 will give you a much better crop reduced to 200mm than an FA 100 does. At 2/3 size the 18-135 should give you 1600 lw/ph on a K-5. the FA 100 will probably give you about 1200-1300.
There's not much chance based on the math that you're right, but if you were to show me some images proving your point I'd be willing to reconsider.
But back to the original topic... the DA 50-200 rated at 10 MP is 1865 lw/ph at ƒ8 and 1906 at ƒ11... if you look at the above 1100 quoted for half an FA 100 image, you'll see using the FA 100 is about 1/3 less resolution than using the DA 50-200. So maybe using the FA 100 is good enough for web purposes, but lets not pretend it's the same.
Quote: However, the 18-135 is not as good as the 35 and the 50, and not a lot better than the 18-55 the OP already owns,
Not in my test. In my test the 18-135 is interchangeable with FA 50 ƒ1.8, unless you need speed, and the Tamron 17-50.
https://www.pentaxforums.com/forums/10-pentax-slr-lens-discussion/297821-50mm-what-use.html