Buy several medium-sized cards from a brand you recognise, buy them from a retailer you trust, and replace them every couple of years. That is most of the advice, and the rest of this explains why each part of it matters.
Memory cards are the cheapest component in your bag and the only one whose failure loses a client's photographs permanently.
Reading the numbers on the card
There are four different speed markings on a modern SD card and they measure different things.
The big number, like 200MB/s. Maximum read speed, under laboratory conditions. It matters for offloading to your computer, and it is the number manufacturers advertise because it is the largest one available.
Bus type: UHS-I or UHS-II. UHS-I tops out around 100MB/s of real write speed. UHS-II has a second row of pins and goes several times faster, but only if your camera supports it. A UHS-II card in a UHS-I camera runs at UHS-I speed, so check before paying the premium.
Speed class: U3 or V30, V60, V90. This is the number that actually matters, because it guarantees a minimum sustained write speed. V30 means 30MB/s minimum, V60 means 60, V90 means 90. Bursts of raw files and high bitrate video need sustained speed, not peak speed.
Capacity class: SDHC, SDXC, SDUC. Just tells you the size range.
For stills photography, a U3 or V30 card is enough for almost any camera. V60 and V90 are for high frame rate bursts and serious video work, and they cost considerably more.
The size question: several medium cards, not one huge one
A 64GB or 128GB card is the right size for most photographers. Two reasons, and the second is the important one.
A single 512GB card holds an entire wedding, which means one component failure loses the entire wedding. Splitting the day across three cards means a failure costs you a third of it, and you will have changed cards at a natural break.
The second reason is habit. Cards that never fill up never get formatted, and cards that never get formatted accumulate half-remembered files and the vague uncertainty about whether the last job was backed up.
Buy four 128GB cards rather than one 512GB card. It costs marginally more and it is genuinely better practice.
Card slots, and the rule for unrepeatable work
If your camera has two slots, set them to write the same files to both cards simultaneously rather than using the second as overflow. That is a real-time backup at the moment of capture, and it is the only protection that exists between the shutter and your computer.
For weddings, this is not optional. For portrait sessions you can rebook, it is a judgement call. The distinction is whether the event can happen again.
Everything after that point is the 3-2-1 backup routine, and the card is only the first link in it.
Counterfeits are common, and they fail
Fake memory cards are one of the most widely counterfeited products online. They are typically small cards reprogrammed to report a larger capacity, so they work fine until they pass the real limit and then corrupt everything.
Buy from a manufacturer's own store or a retailer with a real returns department, and be suspicious of a 512GB V90 card priced like a 64GB one. If you have any doubt about a card, run a full write-and-verify test on it before putting it in a camera you will use professionally.
Habits that prevent lost photographs
- Format in the camera, never on the computer. Formatting in-camera builds the file structure that camera expects.
- Format only after the files are in two places. Not after import. After import and backup.
- Never delete individual images in the camera. It fragments the card for no benefit, and you will eventually delete the wrong one.
- Turn the camera off before ejecting, and give it a second after the write light goes out.
- Carry cards in a hard case, and use a simple convention for which side faces out to indicate shot versus empty. Any convention works as long as it is always the same one.
- Do not fill a card completely. Leave some headroom.
When to retire a card
Cards wear out. Flash memory has a finite number of write cycles, and a card being used weekly for professional work is doing real mileage.
Replace working cards every two to three years, or sooner if you see any of the warning signs: slower write speeds, an occasional file that will not open, the camera reporting an error that a reformat fixes, or a card that has ever needed recovery software. A card that has failed once will fail again, and the second time will be during something that matters.
Retired cards are fine for personal use, a dashcam, or a spare in the car. They are not fine for a wedding.
If a card does fail
Stop using it immediately. Do not reformat it, do not shoot more onto it, and do not run repair tools that write to it.
Recovery software will get files back from a card that has been formatted or partially corrupted surprisingly often, provided nothing has overwritten the data. Professional recovery services exist for the cases where software does not work, and they are expensive but frequently successful.
Then tell the client early if it looks bad. The conversation is survivable when it is honest and immediate, and how you handle the bad news is most of what determines the outcome.
Cards are the least interesting purchase in photography and the one with the worst downside. Spend the extra twenty on the brand you recognise.
