Originally posted by Adam Focal length does not change based on the format being used, hence the aperture value does not change either. If you shoot with the 60-250mm @200mm on the K-3 and the 70-200mm @200mm on the D610 at F4 and the same ISO, you will need to use the same shutter speed on both cameras.
A 200mm lens on FF is not equivalent to a 200mm lens on APS-C, especially not at the same aperture. ISO is irrelevant as a comparator, because it varies wildly depending on camera design (e.g. ISO 400 on a Q, vs Sigma DP-1 vs Leica M9). Signal to Noise Ratio is what concerns photographers, not ISO.
For true equivalence, all of these parameters must be the same; Shutter speed, FOV, DOF and SNR.
It's easiest to demonstrate with a 4/3 vs. FF because of the convenient 2X crop:
- Shutter speed is a constant
- FOV: 200mm on FF = 100mm on 4/3
- DOF: f5.6 on FF = f2.8 on 4/3
With sensors from same generation, same manufacturer, the noise will scale similarly, therefore 2 stops:
- SNR: ISO 400 on FF = ISO 1600 on 4/3
Full-frame 200mm f/5.6 & ISO 1600 vs.
Four-Thirds 100mm f2.8 & ISO 400, and the same shutter speed, will provide true equivalence. Same exposure, FOV, DOF and SNR.