How to Choose a Memory Card

Understanding Performance vs. Cost and What it Means to You
By Brent Durand

Digital memory card shopping is one of the last tasks in putting together a new camera and underwater housing rig. Most photographers do not put a huge amount of thought into the process, however there is much to be gained by doing a little research before placing the order. What is SDHC? Why are there two speed classifications? Do I really need the fastest card on the market? How many gigs do I need?

This reference guide summarizes (and simplifies) the most important specs to consider when shopping for your next memory card.


Having the right memory card is very important, especially when shooting behavior. Finespotted Jawfish, La Paz, Mexico.


Types of Memory Cards

SD Cards

Secure Digital cards are used with all compact, mirrorless and most crop (DX) sensor DSLR cameras. They are smaller and thinner than CF cards. Most photographers currently look towards SDHC, which is a newer generation with higher storage capacities and faster processing speeds. SDXC cards are new in the market and cost-prohibitive for most, but look for these to become popular in coming years.


CF Cards

Compact Flash cards are significantly larger and stronger than SD cards and generally used with full frame (FX) sensor DSLRs and HD video cameras. These cards are tough, provide more data recovery options and perform well in less-than-ideal shooting conditions. They also cost significantly more than SD cards.


Micro SD Cards

These tiny memory cards have great capacity for their size and are used in GoPro cameras and often in cell phones.


Card Speeds Revealed

Aside from the storage capacity, speed (or transfer rate) is the most important factor in choosing a memory card.


Write Speed

Write speed is the speed at which memory card can write data from the camera buffer. For beginner still photographers it’s not much of a worry, however it becomes important for photographers who shoot many continuous frames per second or record HD video. If the card is too slow, the camera’s buffer memory will fill and stop recording. If the card is as fast or faster than the camera’s internal buffer, then the only limitation is the camera.


Read Speed

Read speed is the speed at which you can transfer images from the memory card to the computer. And unless you are shooting many images with critical deadlines, this is less important in selecting a card than the write speed.



Speed Ratings

This is where things get confusing and make it difficult to compare various memory cards. Speed classifications are constantly changing and transfer rates are getting faster, but it’s important to understand the differences in order to save money while maintaining the performance you need.


"X" rating

The “x” rating (i.e. 400x Speed) compares processing times to original recordable CDs (1x = 150 KB/s). This means that a card labeled 40x has a maximum speed of 6 MB/s and a card labeled 400x has a maximum speed of 60 MB/s. Lexar SD cards use the “x” system, SanDisk uses the MB/s system and Delkin uses both. Transcend, on the other hand, only includes the card’s Class rating on their labels (note: Lexar, SanDisk & Delkin also include Class).



The Class system was developed to help simplify card speed ratings. The memory card must meet a minimum processing speed in order to qualify for a particular class. Most new cards on the market are Class 10, which is designated as a 10 with a C around it. That said, a high-quality Class 6 card can also record HD video.



UDMA is another maximum speed classification labeled on many CF cards. UDMA 6 cards feature transfer rates up to 133 MB/s, while UDMA 7 cards feature transfer rates up to 167 MB/s.



UHS speed class can also be found on many SD cards. UHS 1 denotes a 10 MB/s minimum write speed while UHS 3 denotes a 30 MB/s minimum write speed. These are indicated by either a 1 or 3 inside a U.


To summarize, Class and UHS indicate minimum speeds whereas the “x” and UDMA ratings are maximum write speeds.



What Card is Right for Me?


Choosing a memory card does not need to be difficult. Here’s the best way to figure it out:

  • Look in your camera user manual and see what transfer rates are recommended for maximum speed and performance.

  • Determine what file size you need. Underwater photographers will benefit from larger capacities so that you do not need to open your housing to swap cards. It’s better to wait for a lens or battery change to do this. Different cameras record different amounts of data per photo, so it’s worthwhile to call the folks at Bluewater Photo to see what memory card size is best for you.

  • Once you know the minimum speed and capacity you’re looking for you can begin shopping based on price. Keep in mind that if you plan to upgrade cameras in the next year or two, you may want to plan ahead and get faster cards.



Schoolmaster Snapper tucked away from the current in Cozumel, Mexico.


Memory Card Tips


  • As you use your card it becomes fragmented, which reduces speed and capacity. Make sure to reformat your card frequently in order to keep performance optimal.

  • Never fill your card to maximum capacity. Always leave a little room in order to minimize risk of corruption.

  • Get a Fast Card Reader. There is no point to fast read speeds on a card if your card reader can’t keep up.

  • Buy from name brand and reputable dealer. These cards perform the fastest, have the least chance of corruption and more options for recovering data if the card does corrupt.


   Further Reading


Brent Durand is a weekend wanderer and story teller from California.

Brent is an avid diver and adventure photographer, and shoots underwater any time he can get hands on a camera system. He can be reached at

Follow Underwater Photography Guide on Facebook or Instagram.


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