Dual card slots
Dual card slots let a camera write to two memory cards, either as an instant backup, as overflow when one fills, or with RAW on one card and JPEG on the other.
Many professional cameras have two card slots. They exist for safety and for convenience: a second card can hold a copy of every frame in case one card fails, take over when the first is full, or keep RAW and JPEG files apart so the JPEGs can be handed over without touching the RAWs.
Each brand names the options differently, but the choices are broadly the same.
How it works
The camera has a recording setting for the second slot: backup (write every frame to both), overflow (switch when the first fills), or separate (RAW to one, JPEG to the other). Slots may differ — for example one CFexpress and one SD — and the camera can only write as fast as the slower card in backup mode. Playback usually shows one slot at a time, and a menu setting chooses which.
An example: a wedding with a client handover
A wedding photographer records RAW to the CFexpress card and JPEG to the SD card. At the end of the evening the SD card holds every JPEG for a quick slideshow, while the RAW card goes home untouched for the edit.
Two cards and CloudTether
CloudTether works with RAW+JPEG on one card or RAW on one card and JPEG on the other. With JPEG only on, it copies just the JPEGs and leaves RAW on its card. With rated send on Canon and Nikon, rating either file of a RAW+JPEG pair counts for both — rate whichever playback shows. Tethering adds an off-camera copy, but with JPEG only the card still holds the only RAW, so a backup slot is worth keeping on.
Choosing a mode
For irreplaceable events, backup is the safe choice. For long days where card space runs short, overflow. When JPEGs are needed separately, separate recording — accepting that each card holds only half of each frame.
Tips and common mistakes
- Check which slot playback is showing before you rate or delete.
- Format both cards in the camera before a job.
- In backup mode, the slower card sets the burst speed — match cards where you can.
Put it into practice
CloudTether tethers your camera to your phone and delivers every shot to the cloud, live as you shoot — no laptop.