Why one should use DX mode on an FX camera

One shouldn't. Not always anyhow. But there are some moments.

As a reminder, a camera with a big sensor to capture the photo is called FX in the Nikon world. Most FX cameras can take photos using only part of the sensor in "DX mode". There are plenty of good reasons to use DX mode.

1. A photo taken in DX mode is smaller, so it takes up less space on your memory card, so you can shoot longer without changing card, and you can get away with smaller cheaper cards.
2. You can fit more DX photos on your harddisk, so when you travel you have more photos to show your grandma or prospective employer.
3. The smaller size of DX photos means they take less time to transfer to your harddisk or tablet.
4. The smaller size of DX photos means that synchronisation with any cloud site you use to store photos is quicker and takes less bandwidth from other people on the same network - like your grandmother if she uses your WiFi.
5. DX photos are quicker to edit and display in post-processing software. You will be able to edit more DX photos in one day than FX photos.
6. With most cameras, shooting DX allows more fps (frames per second) than FX.
7. Shooting DX does not fill up the buffer and allows you to shoot for a longer time with your camera, when you hold down the shutter button in continuous shooting mode.
8. If you "crop" a photo shooting in DX mode, you still get a full RAW file, but smaller. If you crop an FX photo in post processing, you either simply hide part of the photo, which means the file is as big as it was to start with, or you have to save it as jpg or some other format, and you lose the advantages of having shot RAW.

Then there are of course moments when FX mode is better. Why would you otherwise have paid for it?

1. You can get a more narrow depth of field.
2. You get more detail.
3. You can get a wider angle of view with the same lens.

Sometimes the FX advantages matter. Sometimes they do not matter at all. Sometimes they matter a lot.

If I one day take 99 photos in DX mode with my camera and just one in FX mode, and the subject in FX mode absolutely deserved it, I will be happy that I brought an FX camera instead of a DX camera.