I wanted to upgrade from my K10D to a K1, the main reason being, I did not get the same pictures out of my APS-C, as I did out of my 24x36 cameras.
In film times I shoot 64 ASA which meant most time wide open and a shallow DOF. Even the pictures of a 2.8 / 35 mm M lens in FF have a much better look then from a 2.8 / 24 mm on APS-C in terms of DOF. Thus when trying the K-1, I tried it with my smc A 1.7/50 mm (among others) and the pictures where more satisfying then I ever thought, despite beeing ASA 1.600. Unluckily I found the K-1 way too bulky. Instead I bought a K-70 and a Sigma 1.4/35 mm. Comes near to the K-1 with the 1.7/50 mm, in price and weight also
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Brief summary, why I will sometime buy a FF :
- Better DOF and bokeh, escpecially in short focal length.
- General ergonomics (I think a K3 would do the job as well).
- A bulk of vintage lenses (the nice word for waste glass) to test on the K-1.
I still prefer ASA 100, so high ISO is not on my list.