Originally posted by derelict But an indicated 70mm on a K1 DFA lens when put on a K3 will not yield 70mm but a higher, narrower focal length. Hence, my looking for an APSC specific lens.
The APSC lenses on an APSC camera look just the same on that APSC camera as the Full Frame lenses of the same focal length.
Here's an example:
Take any focal length lens, let's use 15mm. Put the DA 15 on the K-3 - take a shot. Then the DFA 15-30 - take a shot. The scene will be rendered with the same basic magnification, height and width - there could be minor differences due to liberties taken with actual focal length vs. labeled focal length or the zoom design vs. a prime but substantially similar results will be had. Put the two lenses on a K-1 - set in full frame mode. The DA 15 will fail to fully cover the sensor and parts will be blacked out outside of the APSC area due to the lens lacking coverage there. The DFA 15 will show the same central portion as the DA 15, but will cover the entire sensor giving a wider result to the image. This only occurs because 1) the sensor is larger, and 2) the lens is able to cover the entire sensor optically.
So it is true that a FF lens on a FF camera gives a wider view at the same focal length, but it is NOT true that a FF lens on an APSC camera gives any different view than an APSC lens of the same focal length. You can get wider by going FF if the lens and sensor are both larger, but you don't get narrower because the lens is FF on APSC - you get narrower with both APSC and FF lenses on APSC because the sensor is smaller.
Imagine you built your house with 1 window per room and they faced towards a distant mountain chain. In one room you gave yourself a wide tall window in the other you gave yourself a narrow short window. If you took the storm windows off the big window and put them on the small window you don't see anything different than if you use the right size storm window. The lens on a camera is the storm window in this analogy. Put the small storm window up on the big window and you have to cover part of the window with wood to get it to fit. This blacks out a lot of the view. But the reverse is not noticed even if it may be less efficient in terms of weight and cost.