[i83N]

Link to PF Post for Hands-on opinion of the 20~40 at DFW PhotoExpo HERE

"The lens lineup included the 20-40 Limited; that lens on the K3 is just....solid. And fast. And solid. The combo reminds me of my classic Pentax stuff; the 135 1.8 ED on an LX, for example."

Link not working.

[Winder]

See my follow-up question above and please answer it if you can. I really want to understand all these things better, understand why my thinking is backwards. It doesn't seem backwards to me so I need someone to help me understand how and why it's backwards.

When I get back I will try to explain if someone hasn't already done it. There are a lot of points you can address with your question.

The simple answer is that APS-C lenses need to be sharper at wider apertures and faster in general to match the image quality of FF. To match FF you have to shoot at a wider aperture which means the lens needs to be designed for peak sharpness at a wider aperture. The Sigma 18-35 F/1.8 is the FF equivalent of a 27-53mm F/2.8. So the 18-35 has to be as sharp at F/1.8 as a 24-70 does at F/2.8.

There is a lot more too it, but that is one key reason.

[monochrome]

Link not working.

Fixed now - sorry.

[snake]

For those that don't want to click back to find that post, here's the link: https://www.pentaxforums.com/forums/16-pentax-news-rumors/242898-pentax-dfw-photo-expo.html

Fixed now - sorry.

[Alizarine]

Which is why I'm keen on getting the Sigma 24-70mm... a full frame lens with no-so-great edge performance and vignetting but looks fantastic on APS-C cameras haha

I considered that lens before, with with a front element and filter thread of 82mm and weighs like a couple of bricks in the bag, I decided to go with the Tamron 17-50 instead... (and foot zoom all the way, in case I have no time to switch to my 50-135!)