The flash cards used in the digital cameras today have capacities mostly between 1 GB and 64 GB. It seems a lot, isn’t it? The answer requires a longer discussion; those not interested in details can skip this part, but I assure you that it will be an interesting exercise.

The Size of an Image

Let’s take an image with a resolution equal to HD (1920 × 1080) – most monitors and TV sets today are capable of displaying natively such an image. How much data is necessary to store this image?

Most will jump and say: approximately 2 MB. Why? Well… 1920 × 1080 = 2,073,600 i.e. 2 MB. Wrong! There are two mistakes in this assumption:

• One pixel is not represented by 1 byte; in most cases, a pixel requires 3 bytes (or 24 bits) – one byte for each fundamental color: red, green and blue. Which means that our image size becomes ≈ 6 MB.
• The image is not stored on the digital media as it comes from the sensor: because of the relatively high redundancy in the information, most image formats use compression to reduce the size.

Depending on the format and the algorithm used for compression, the size of our image can be reduced by a factor of 2 (very high quality but large size) to 50 (very low quality but small size). Obviously, most will prefer a compromise between quality and image size with compression factors between 6 to 10 (JPEG format). In our example, a factor of 8 will reduce the size of the image from 6 MB to ≈ 760 KB. Camera manufacturers can be more conservative sometimes but not too far from these compression factor values.

Now, let’s take a typical compact camera – our Canon PowerShot SX260 HS is a very good example. This camera has a sensor with a resolution of 12 megapixels (4000 × 3000). If we repeat our calculation we get:

• Uncompressed size: 4000 × 3000 × 3 = 36,000,000 bytes.
• Compressed size using a factor of 8: 36,000,000 / 8 = 4,500,000 bytes or ≈ 4.4 MB.

In reality, the compression factor varies with the amount of detail in the image: for images with a lot of details and contrast the factor can be 4, while for soft images with very little details this factor increases to 50! However, for storage estimate a factor of 8 is a good starting point; actually, I averaged the size of about 2500 images taken in various conditions and my result was amazingly close: 7.5!

Please note that our example assumes cameras that use the JPEG format – currently used by most digital cameras, especially the compact models. This is a lossy compression that discards some of the data (i.e. loss of quality) in order to reduce the size significantly. This is similar to audio compression used by popular formats like MP3 or WMA. When it comes to the default compression factor used for image quality, the camera manufacturers are conservative (factors between 6 and 12 are used, especially in the so called “creative modes” – a discussion about this later). Many cameras allow the user to select the quality of the image in three steps:

• high (large) – good for prints and editing,
• medium (average) – good for screen display,
• low (small) – good for web use.

High end cameras may use different formats, including RAW, which is usually a proprietary format of each camera manufacturer. The RAW format corresponds to an unprocessed image (somewhat equivalent to the negative in the film photography) but compressed as much as possible without altering the original information coming from the sensor. This is a lossless compression method similar to the data compression used in ZIP file archival on your computer. The compression factors for the RAW format can vary from 1 (uncompressed) to 4 (if the image is not rich in details); normally, you should probably consider a factor of 2 if the RAW format for your camera uses lossless compression. For example, a 12 Mpixel sensor (36 MB uncompressed) could produce RAW images with sizes of around 18 MB.

How Many Images Can Be Stored on a Memory Card?

Knowing approximately the size of an image, it is relatively easy to calculate how many images (photos for all practical purposes) a card can hold. Use the following factors as a starting point:

• High quality (large image): 8
• Medium quality (average image): 12
• Low quality (small image): 20
• RAW (if available): 2

For a 12 Mpixel camera the following table may serve as guidance:

The Speed Requirement

This is usually indicated by the manufacturer in the user manual of the camera. Knowing that the cost increases not only with the size but also with the speed, the best thing to do is to look for the recommended speed: no less, no more.

A low speed class card will slow you down when capturing photos in bursts: for example, in place of 3 captures (frames) per second you may see just one capture (frame) per second. Also, video recording (if the camera allows) will become problematic (skipping frames is the most common phenomenon).

A faster card is just a waste of money: it won’t bring you any advantage. The only time you would buy a faster card is when a card with your speed requirement is not available.

At the time of writing this post, most compact cameras can use SD cards class (6) with few exceptions. However, class (10) may be required by some cameras allowing full HD recording at 60 frames per second (progressive mode) or allowing full resolution bursts at high speed (e.g. more than 10 captures per second). More recently, large capacity cards (16 and 32 GB) are only available in class (4) and class (10) speeds; in such cases the choice is obvious: if the recommended speed class is † (not available), get a class (10) card.

How Large the Card Should Be?

The question may be formulated in a different way: one large card or many smaller cards? Please remember that memory cards should not be used for archival.

I am against the idea of buying the largest card and start shooting until the end of time… My reasons are simple:

• Large cards are expensive. I’m not talking about the cost of the whole card but the cost per GB of storage ($/GB). When you look for cards, divide the price by the capacity for each card (preferably the same manufacturer and the same speed class); the card with the lowest $/GB is probably a good candidate.
• Owning only one card is a recipe for disaster. The card may not be reliable and die in the middle of your trip; if the trip happens to be in the wilderness, then you are out of luck. The same is true if you lose the card somewhere. Not to mention that you may lose everything you shot since the last backup.
• Large cards may slow down your camera if you have too many images (files) on the card. The phenomenon is somewhat similar to your computer: if there are too many files on your hard drive the system becomes sluggish and the overall performance decreases. The phenomenon is not necessarily visible in the case of modern cameras that are equipped with fast image processors.

Those interested in video recording may take my advice with a grain of salt: they have different storage requirements and, for them, buying large cards may be a necessity.

I don’t suggest buying the smallest cards either: it is not cost efficient and the number of cards may increase above your limit of comfort. The only advantage of carrying a lot of small cards is the fact that a disaster happening to one card may have more bearable consequences (you could lose few shots but not all of them).

My Recommendations

My approach is somewhere between the two extremes: too large and too small. Just look for a card that can hold at least a day of shooting using the highest quality setting your camera is capable of; or use 1000 images if you have trouble estimating. Then refer to the previous table as guidance and be realistic on how productive you are.

Here is what I do:

1. I start with a reputable brand like SanDisk (the “king” of memory cards by far), Lexar or Kingston (in this order). Other brands are not necessarily excluded, especially if I need a card quickly to save the day (Patriot, Transcend and PNY are usable but stay away from obscure brands that you never heard of). It is all about reliability in the first place and not too much about performance.
2. I calculate the cost per GB of storage and select the best candidates; usually, this means a medium size like 8 or 16 GB (I tend to be very productive). If my camera has a high resolution sensor and produces large files (high quality), I select the larger card (16 GB); if my camera produces smaller files, I select the smaller card (8 GB). When using RAW format the capacity will be higher (32 GB is probably needed for sensors larger than 10 MPixel).
3. I always consider only 50…75% of the capacity of the card for use when shooting (over-provisioning). This has several reasons:

• if a card is only partially used, the overall performance of the camera is better;
• this extends the life of the card in some degree reducing the number of internal erase operations that are limited (probably not a big concern for the average shooter that is not very productive anyways);
• a smaller number of photos on one card means a better protection against the danger of losing photos in the case of a disaster (one card dying);
• it is easier to review a smaller number of photos with low cost picture viewers like media players, phones or tablets.

4. I always have at least three cards for one camera: one in the camera and two spares in the pouch (camera case).
5. I backup my images as soon as I have a computer available. Actually, I also own a battery operated portable hard drive designed for memory card backups. It may seem like an overkill, but I care about my photos. More about backing up your images in a future post.

Unlike the photographic film, the flash memory cards are not sensitive to electromagnetic field and X-ray: there is no danger to lose your photos in the airport. Static electricity is probably the only enemy of your cards – most cards have some protection but this protection can be ineffective against high energy static discharges. The best protection is to handle the card only from the plastic parts and keep them in the original case when not used.

The second type of protection is data corruption prevention in cases of misuse. Like other recordable media, SD cards can be locked so no further write or formatting can occur. In the picture below I highlighted the lock switch: if you slide it in the suggested direction (see the arrow or triangle before the word “Lock”), you will not be able to record more photos or to erase/format the card. Use this switch with care only if you suspect that there is a danger to misuse the card after you filled it with your precious images.

SD Card – the Write Protection Switch

I personally ignore the existence of this switch, especially if I’m using microSD cards with adapters. Anyways, don’t be surprised when you will find cards without this switch – some manufacturers (of cheap cards) decided not to provide this feature. This fact alone should probably say something about the quality of the cards because the lock switch is part of the Secure Digital (SD) standard and must be present on the card.

Finally, you have all the basics here. I will return to digital memory in different contexts in my future posts insisting more on usage. I will probably look at other type of cards if necessary. In the mean time, you can re-evaluate the storage needs for your camera and start looking around for some good cards as necessary. Good luck!