Hi, I have read multiple threads on thoughts from folks on the best way to use the DX mode on the D600, both on this site as well as many other forums on the net. The overall recommendation so far regarding this has been that you never need to use the DX mode as you can always shoot FX and just crop as needed. However, based on my recent experiences in 2 different situations, I think there may be some value in actually using the DX mode at the time of capturing the picture.
Scenario 1: Macro Photography I was shooting the close up of a cherry blossom using a 150mm macro lens, which was still falling a bit short in being able to reach the flower. The flower was higher up, further from the lens - in this case, my subject was just the flower and I was sure i did not require any of the surrounding leaves/branches around it. So in this case, i used the DX mode and was able to focus just on the flower - it made it easier to review on the LCD screen, as i had just the composition i wanted, didn't have to zoom in to check that; also, saved the file size of the RAW file by 50%. The FX RAW size was 30MB and the DX one was around 15MB. Why store more when you are SURE you don't need more?
Scenario 2: Bird Photography This was another case when I was using a 80-400 lens with a 1.4x tele converter, but was still not able to reach the small bird. If I used the FX mode, I would get all the surrounding branches and twigs that I didn't want in the frame - using DX helped again, as i was able to zoom in on just the bird.
Now, with either of these 2 scenarios, I am not sure if there's a difference in picture quality between cropping in the camera (DX mode) and post-crop (FX mode). However, I know that it was a much smaller file, easier to compose as well as review on the LCD screen with the DX mode. There's probably a similar case if you're shooting some small wild life from a distance and wanted more of the animal, than the environment. I have not encountered this situation yet btw.
So, based on this my overall recommendation would be that make use of the DX mode when you are SURE that you want to crop the frame. I'm curious to hear what others think.. Also, does anyone have other scenarios where this may be applicable?