Sony a7 III Memory Cards: Getting the Dual Slots Right

The Sony a7 III came out in 2018 and has since been replaced in Sony’s lineup by the a7 IV, but it’s still one of the most widely used full-frame mirrorless cameras around — and a common used buy. And for good reason — I loved shooting with it.

But when it comes to its memory cards, it has a hardware quirk in its dual-slot design that can trip you up if you’re not aware of it:

• Slot 1 supports high-speed UHS-II cards, while Slot 2 is limited to slower UHS-I cards.

If you put the wrong cards in the wrong slots — or configure them incorrectly in the menu — you’ll severely bottleneck the camera’s performance.

So here is exactly how to outfit your a7 III for maximum speed without wasting money on tech the camera cannot use.

The best SD cards for the Sony a7 III: Quick recommendations

Because the slots use different technologies, I recommend buying two different classes of SD cards to match the camera’s internal hardware perfectly.

Slot 1 (bottom slot): UHS-II SD cards

Slot 2 (top slot): UHS-I SD cards

SD card compatibility & technical requirements

Sony’s official documentation states the camera supports SD, SDHC, and SDXC cards.¹ It also notes that Slot 1 is UHS-II compatible, while Slot 2 only supports UHS-I (and legacy Memory Stick PRO Duo cards, which you should ignore entirely).

To understand what cards you actually need, we have to look at the data the camera generates. The a7 III shoots 24MP stills and 4K/30p video. Its absolute maximum video bitrate is 100 Mbps (which translates to just 12.5 MB/s of sustained data). Which is actually pretty low by modern standards.

This means video is actually the least demanding thing this camera does. The real challenge is clearing the buffer when shooting a 10 fps RAW burst. Here is how the features map to card speeds:

Sony a7 III Feature	Data Rate / Requirement	Minimum Required Card
4K/30p Video (100Mbps)	~12.5 MB/s Sustained	SDXC V30 (or U3)
1080p/120p Slow Motion	~12.5 MB/s Sustained	SDXC V30 (or U3)
10 fps RAW Burst	~240 MB/s to ~470 MB/s	SDXC UHS-II (V60 or V90) in Slot 1

Slot 2: The money-saving caveat

Because Slot 2 is physically incapable of reading the second row of pins on a UHS-II card, the camera cannot utilize the fastest cards money can buy in this slot. Overpaying for peak V90 or V60 speed for Slot 2 is a complete waste of money. If you insert a premium UHS-II card here, the camera will simply roll it back to standard UHS-I speeds.

While a faster card might speed up the process of offloading images to a computer with a fast reader, it offers absolutely no in-camera performance benefit. For Slot 2, you only need a reliable, high-capacity V30 UHS-I card. You can verify how these classes perform across different brands in my independent SD card speed tests.

It’s worth knowing that a faster card in Slot 1 doesn’t make the buffer wait go away either. Imaging Resource measured buffer clearing on the a7 III as slow even with one of the fastest UHS-II cards available in the UHS-II slot. Your RAW setting has more say in it than your card does: Sony rates the buffer at up to 89 compressed RAW frames but only around 40 uncompressed ones, because uncompressed files are roughly twice the size to write.

Formats to avoid

Don’t buy SD Express cards. I know they sound like they should be faster — and they are, but only in the right hardware. Which, right now, is hardly any hardware at all. Crucially, the a7 III does not have the PCIe interface required to use the SD Express standard, meaning it will default to sluggish speeds while costing significantly more.

The dual-slot bottleneck trap

The discrepancy between Slot 1 and Slot 2 creates a major bottleneck if you configure your camera to shoot redundant backups.

If you go into the menu and set Rec. Media Settings > Recording Mode to one of the Simult. options — meaning the camera writes the exact same 24MP file to both cards at the same time — the camera is forced to slow down to match the speed of the UHS-I card in Slot 2. If you shoot a 10 fps burst in this mode, the camera’s buffer will take significantly longer to clear, locking up the camera while you wait.

To maximize the a7 III’s performance, I recommend setting the camera to Sort (RAW/JPEG). Send your heavy RAW files to the blazing-fast UHS-II card in Slot 1, and send the lightweight, compressed JPEGs to the slower UHS-I card in Slot 2 as your backup.

Two things to know before you rely on that setup. Sort(RAW/JPEG) only splits the files if File Format is set to RAW & JPEG — with any other file format setting the camera ignores the split and writes to the prioritized slot instead. And in this mode the camera stops shooting as soon as either card fills up, not when both do, so a small JPEG card in Slot 2 will bring your RAW shooting to a halt while Slot 1 still has room.

What size SD card to get?

The a7 III is fully compatible with SDXC cards, meaning it supports capacities up to 2TB. 128GB to 256GB cards probably represent the perfect sweet spot for this 24MP sensor. A 128GB card will hold roughly 2,600 uncompressed RAW images or nearly 3 hours of 4K/30p video at its maximum 100Mbps bitrate.

Formatting cards in the a7 III

Always format your memory cards directly in the a7 III before a shoot to ensure the database files and folder hierarchies are built exactly as Sony’s firmware expects. Formatting wipes all data, so ensure your files are backed up first.

MENU > (Setup) > [Format] > Select Slot 1 or Slot 2

If your camera is failing to recognize the card or throwing read/write errors, formatting in-camera might not be enough. In those instances, you’ll need to decide between a quick format or a full format using the official SD Card Formatter tool on your computer to completely overwrite the drive sectors before trying again.

Notes & References:

1. Sony a7 III Help Guide, “Memory cards that can be used”. [↩︎]