Originally posted by iheiramo: ...As this lens would be my choice to shoot against sun and for night time cityscapes ... This makes me wonder if found use for 24mm on FF? On the other hand, while on the APS-C, from time to time I have found 28mm too narrow and would like to have a wider lens, but is 24mm wide enough? ... I have seriously considered both DA 15 and 21, but not willing to spend on them, as they are not FF lenses. Otherwise very tempting choices with small size, flare resistance and night shooting capabilities. ...[heavily snipped for brevity]
There are many varieties of cityscapes. For shooting within a city with large buildings, I find ultra wide angle essential because a crowded city often doesn't allow zooming back with my feet. Since you already have 28mm covered, I suggest you go with 20mm or wider.
I think the DA 15 remains an option with the K-1. In APS-C crop mode you still get 16 megapixels at an equivalent 21mm. Square crop gives 24mp. You'll get amazing flare resistance, starbursts, and a compact ultrawide lens.
I wish Pentax sold a compact 20mm f4 prime to go with the K-1. They unfortunately do not. The 15-30 is a good lens but it's too large and heavy for some situations.