Memory cards for burst shooting: CFexpress, SD, speed and buffer

The A350 lifts off, you hold the shutter button down – and after a few seconds the rattle turns into a stutter. The camera is still writing while the aircraft is long since in the best light above the end of the runway. The culprit is rarely the camera body alone but the interplay of internal buffer and memory card. This guide explains what the abbreviations on SD and CFexpress cards mean, which values really count for burst shooting, and how to choose the right card for your camera and your spotting day.

The short answer: which card for burst shooting?

Buy the fastest card your camera’s slot can actually use, and pay attention to the guaranteed write rate rather than the peak read speed. On SD cards you recognize the write rate by class symbols such as U3, V60 or V90, on CFexpress cards by the VPG certification. If your camera has two different slots, the faster card belongs in the slot the camera primarily writes to during burst shooting.

Everything else – capacity, brand, price – comes after that. An expensive card that cannot play out its speed in the wrong slot is money thrown away. A card that is too slow in a fast camera, on the other hand, costs you exactly the pictures you drove to the fence for.

Buffer and card: what happens during a burst

Every camera with a burst function has a fast internal temporary memory, the buffer. When you press the shutter button all the way, the pictures land there first. In parallel, the camera writes the buffer to the memory card. As long as the camera produces images faster than the card can write them away, the buffer fills up. Once it is full, the frame rate collapses: The camera fires only as often as space becomes free again.

For plane spotters this is a very concrete question. A takeoff or landing lasts only a few seconds in the best part of the frame. If the next aircraft or a taxiing movement on the taxiway follows right after, you need an empty buffer again. Two values are decisive here:

• Buffer capacity: How many pictures the camera can capture in a row before it throttles. The manufacturer usually states this value per file format. For the Sony Alpha 1, around 155 compressed RAW files are stated at 30 frames per second.
• Clearing time: How long it takes until the full buffer is written to the card. This is where the memory card comes in – and this is where the difference lies between a camera that is ready again after a few seconds and one that is still writing at the next approach.

A full buffer also slows you down when switching to the playback menu, zooming into the image view or changing settings. Many cameras lock certain functions while writing is still in progress. How to sensibly combine burst mode, frame rate and shutter speed is explained in the guide to camera settings for plane spotting.

The abbreviations on SD cards decoded

SD cards carry several markings at once, which are easily mixed up. They can be sorted into three groups: capacity class, bus interface and speed class.

Capacity class: SDHC, SDXC, SDUC

The SD Association distinguishes by storage size. SDHC goes up to 32 GB, SDXC up to 2 TB, SDUC theoretically up to 128 TB. SDXC and SDUC cards run with the exFAT file system. Practically every current mirrorless camera supports SDXC; still, with older bodies a look at the manual is worthwhile.

Bus interface: UHS-I, UHS-II, UHS-III, SD Express

The Roman numeral on the card indicates which interface it uses to talk to the device – that is, the maximum transfer rate of the connection, not that of the card itself.

Interface	Max. bus speed according to the SD Association	Identifying feature	Meaning for spotting
UHS-I	up to 104 MB/s	one row of contacts	Standard in many entry-level and mid-range cameras
UHS-II	up to 156 MB/s (full duplex) or 312 MB/s (half duplex)	second row of contacts	noticeably faster buffer clearing in UHS-II slots
UHS-III	up to 624 MB/s	second row of contacts	hardly found in cameras
SD Express	up to 985, 1,970 or 3,940 MB/s depending on the PCIe variant	PCIe connection	not very common in cameras so far

Important: The interfaces are backward compatible. A UHS-II card also works in a UHS-I slot but then uses only the first row of contacts and runs correspondingly slower. In that case the fast card mainly pays off when reading it out on a computer with a UHS-II reader – not when shooting.

Speed classes: C, U and V

These symbols are the most important for burst shooting because they guarantee a minimum write rate during sustained writing:

• Speed Class: C2, C4, C6, C10 – at least 2, 4, 6 and 10 MB/s. Too slow for bursts with modern cameras.
• UHS Speed Class: U1 with at least 10 MB/s, U3 with at least 30 MB/s.
• Video Speed Class: V6, V10, V30, V60 and V90 – the number corresponds to the minimum write rate in MB/s.

For RAW bursts, V60 or V90 in a UHS-II slot is a sensible starting point. The big numbers on the front of many cards, such as “300 MB/s,” on the other hand, usually describe the maximum read speed under ideal conditions. The actual write rate is often well below that and is stated, if at all, in the fine print or the data sheet.

CFexpress: Type A, Type B and the VPG classes

CFexpress cards use the PCIe interface you know from SSDs in your computer. That makes them generally much faster than SD cards, but also more expensive. The CompactFlash Association defines three form factors:

Feature	Type A	Type B	Type C
Dimensions	approx. 20 × 28 mm	approx. 38.5 × 29.8 mm	approx. 54 × 74 mm
PCIe lanes	1	2	4
Theoretical maximum (version 2.0)	approx. 1 GB/s	approx. 2 GB/s	approx. 4 GB/s
Theoretical maximum (version 4.0)	approx. 2 GB/s	approx. 4 GB/s	approx. 8 GB/s
Use in cameras	so far practically only at Sony	widespread at Canon, Nikon and others	hardly relevant in cameras

Type A and Type B are not interchangeable. On models like the Alpha 1, Sony has built in combo slots that accept both CFexpress Type A and SD cards. The Nikon Z9 works with two slots for CFexpress Type B and XQD, and the Canon EOS R5 combines a CFexpress slot with a UHS-II SD slot.

A special feature is the relationship with XQD: CFexpress Type B has the same form factor and the same connector. Some older cameras with an XQD slot have learned via firmware update to read Type B cards as well. Whether your camera is one of them is stated in the manufacturer’s notes.

VPG: the write guarantee for CFexpress

What the V classes are for SD, the Video Performance Guarantee (VPG) of the CompactFlash Association is for CFexpress. It specifies a tested minimum write rate during sustained writing, such as VPG200 for 200 MB/s or VPG400 for 400 MB/s. With the newer profile VPG-5, VPG800 and VPG1600 were added. Originally intended for video, the certification is also useful for photographers: It shows that a card holds its performance not just in short peaks but over a longer time.

How long does your buffer take?

The clearing time can be roughly estimated: number of pictures in the buffer times file size, divided by the actual write rate. The camera itself additionally limits the write rate, so the result is a guideline – but it shows well how much a faster card matters.

The easiest way to find out the file size is to copy a few test pictures to your computer. It varies with sensor resolution, compression and subject. Whether you should shoot RAW at all and which compression makes sense is discussed in the article RAW or JPEG for plane spotting?.

How to choose the right card

The selection follows a fixed order. If you stick to it, you avoid the typical wrong purchases.

If you are still before the camera purchase, think about the slots right away: CFexpress cards are more expensive, so a camera with this slot also causes ongoing extra costs. An overview of bodies for every budget is in The best camera for plane spotting, in the comparison of camera systems and in the beginner gear under €1,000.

SD or CFexpress: what suits you?

Especially at airshows with fast aerobatic displays, the buffer is often the bottleneck because a display lasts for minutes and subjects constantly change. You can find tips on gear for such days in the airshow checklist and in the guide Photographing airshows.

Shoot bursts smartly instead of filling the buffer

The fastest card does not replace technique at the shutter button. With a few habits you get more out of every card:

• Short bursts instead of continuous fire: Fire deliberately in the decisive phases – at liftoff, at touchdown, when the aircraft reaches the best angle. In between you let the buffer work.
• Choose the frame rate to fit: For a taxiing aircraft you don’t need 30 frames per second. A medium burst setting is often enough and spares buffer and card. With propeller aircraft, shutter speed matters more anyway, as the article Photographing propellers shows.
• Use compressed RAW: Many cameras offer lossless or lossy RAW compression. Smaller files mean more pictures in the buffer and faster clearing.
• Use dual saving deliberately: Mirroring to both slots is a sensible backup but costs speed if one card is slower.
• Set up autofocus properly: A burst with locked focus needs fewer pictures. How to choose tracking and focus area correctly is in the guide Autofocus for aircraft.

At spots with dense traffic such as the Munich Airport visitors’ hill (Besucherhügel) or the Polderbaan at Schiphol, movements often follow each other closely. There you notice especially clearly whether your buffer is free again between two aircraft.

Care, safety and fakes

Memory cards are robust but not invulnerable. A few rules protect your pictures:

• Format in the camera: After backing up the pictures, format cards in the camera you use them in, instead of deleting individual files on the computer. That gives the card the file system the camera expects.
• Don’t remove during writing: As long as the access light is blinking, the card stays in and the battery stays inserted.
• Avoid fakes: Fake cards often report a larger capacity than is actually present or write slower than stated. Buy from reputable dealers and test new cards with a complete write and read test.
• Know the write protection: Many SD cards have a small slider on the side. When it is set to “Lock,” many cameras refuse to write – a classic when “Card protected” suddenly appears on the display at the spot in the morning.
• Use a card case: A case protects contacts from dust and moisture and helps you separate full from empty cards.

By the evening at the latest, your haul belongs on at least two storage media. How to build a reliable system is described in the backup strategy for spotting photos. For the subsequent review of large bursts, the tips on quick culling and batch editing burst photos help.

Conclusion

For burst shooting, the memory card is no bit player. It determines how quickly your camera is fully ready for action again after a takeoff or landing. The path to the right card is clear: first check the camera’s slot, then look at the guaranteed write rate – V60 or V90 for SD, a VPG class for CFexpress – and only then think about capacity and price. With several tested cards, short bursts and a fixed backup ritual, you won’t lose a decisive picture at the fence anymore. What else belongs in your bag is summarized in the gear checklist.