Megabytes Versus Megapixels

One of the more common questions I get (I think it’s due to my technical background) is one where people are asking how many images they can store on their media cards.  The answer, as always, starts off with an “It depends…”.  For the quick feed readers curiosity, here’s the laundry list:

1.  File Format
2. Quantity of Light
3.  Varying Degrees of Color
4.  Bit Depth
5.  Megapixel Count (Resolution)
 

And for the more detail-oriented, here’s the extended version…

1.  File Format

Just one of the many considerations here is how you are saving your images.  There are also many facets in the “how you save your images” too.  For instance, RAW as a file format will always have more data in it than its more lightweight sRaw counterpart. Even further, jpg does a certain degree of compression in camera to help save on file storage space, so it will also decrease your file size usage.

2.  Quantity of Light

Another consideration to factor in is whether you are shooting in low light or bright light.  With digital photography, the more light you have in a scene, the more data there is to the image.  Conversely, darker images will have less data and take up less storage space on both your media card and your computer. Take, for example, this series of images I took of “Dino” outside on Sunday.  The exposure is set to under-exposed by 2 stops, neutral exposure, and over-exposure by two stops, as defined by my shutter speed.  The amount of storage space that was consumed on both the media card and my computer is indicated beneath each image.

Exposing Dino

3.  Varying degrees of color

The amount and types of color can also factor into how much storage space an image takes up.  I’ve actually addressed the issue of color in these exact terms before, so for a more thorough explanation of that, check out this article here.  Here, the summary is really the only relevant part, where green encompasses the largest amount of data, blue comes in second, and red encompasses the least.  Other color hues will fall somewhere between these three primary colors, so storage space will be a function of colors in your images as well.

4.  Bit Depth

Another factor that will enter into play (mostly in post production though) is that of bit depth, which is basically how you are saving your file out from processing.  Most cameras will capture in 16 bit depth, and will be imported in Photoshop or Lightroom at their native bit depth, unless you manually change it from 18 bit to 8 bit (which a lot of people do when using the full version of Photoshop, because that enables tools that are not available in 16 bit mode).  You can also output to24 bit or 32 bit mode, but these are mostly used for offset printing (think CMYK) and HDR imaging, which while popular does not speak to standard storage space for images captured natively in camera.  So, the bit depth is really beyond the scope here, but if you’d like to learn more about various bit depths and their usage, feel free to get started at the Wikipedia article here.

5.  Megapixel Count (Resolution)

The final element to consider in how much storage space an image will take up on either a card or a computer is the megapixel count.  Now, unfortunately there is no direct correlation from megapixel to megabyte as one is defined by the resolution of the image and the other is defined by a byte in computer terms.  While the former is mealleable depending on sensor type (CMOS vs CCD vs Foveon, etc.), the latter is pretty well delimited as a byte is a single unit of data.  So

Having said all of the above – raw versus jpg, light versus dark, one color versus another, bit depth, and megapixel count…there are some general rules of thumb we can draw based on significant research that has been done in this area (and by significant, I mean me hitting Google, Wikipedia, and various communities, asking if anyone knows of any authoritative resources I could check out).  The upshot is that images will largely be a function of their megapixel count.  Now, because of the variances in the other factors here, there is no hard and fast rule that is set in stone for image file size relative to MP count, but on a very rough scale, each megapixel of data will typically contain about a megabyte of data.  So the conversion is almost a 1:1 ratio.  Keep in mind of course that this is very rough, because I have seen with my 10MP camera that I have raw file sizes in excess of 17 MB!  It’s always better to work within an expected range, rather than using hard and fast rules anyway, so for that reason, here’s a chart:

Megapixels Resolution File Size
1.6 Megapixels 1536×1024 px 1.6-2.4 MB
2.8 Megapixels 2048×1365 px 2.8-4.2 MB
6.3 Megapixels 3072×2048 px 6.3-9.4 MB
10.1 Megapixels 3888×2592 px 10.1-15.1 MB
11.2 Megapixels 4096×2731 px 11.2-16.8 MB
17.5 Megapixels 5120×3413 px 17.5-26.2 MB
25.2 Megapixels 6144×4096 px 25.2-37.7 MB

Keep in mind that these formulas are very generic in nature as the methodology is not completely scientific, but can help you determine the expected capacity of your media cards for photos!

So, the natural extension of this takes us back to the original question:  How many images you can store on a media card given a certain pixel count?  Extrapolating things out is just a simple matter of math and Excel! :-)

Megapixel Chart

With smart phones exploding their own megapixel counts, many are also now looking to use the MP count as the benchmark for identifying image quality, thinking that more is always better, right?  Again, as a general rule of thumb, this is true, but there are laws of diminishing returns.

Think of it this way – a sensor in a camera is a finite size – it’s not going to change substantially within a given form factor.  So, an SLR will have a certain size of sensor, a point and shoot will have another sensor size, and a cell phone (smart or otherwise) will have yet another size of sensor.  If you had to choose between an SLR that has 10  megapixels in any given photo (of roughly 10-15 MB of data), is it safe to assume that this will produce the same quality image as a 10 MP camera on a cell phone?

The answer, of course, is no.  Sensor size is really what matters here because you can capture much more data in a megapixel (or a megabyte for that matter) if it’s a bigger size.  So again, bigger means better! :-)  What this hopefully tells you is that you can get some pretty big photos coming off a pretty tiny camera, and still get pretty lousy results.  In general, there are limits to what really matters on any given sensor size, because even though you can pack more megapixels on a sensor, the image quality really doesn’t return that much better a result after a certain threshold is reached.  What is that threshold?  Excellent question!

The answer:  It’s subjective, and open to interpretation, but here’s my take:

  • Smart Phone Cameras – The sensor is teensy tiny, so anything above 8-9 MP is just for fluff
  • Point & Shoot Cameras – A somewhat bigger sensor, and with the RAW capability, I’d say these can see benefits up to the 16-20 MP range…
  • SLR Cameras – With the biggest sensor in the category of portable cameras (I would not define a medium format or large format camera as “portable” in most scenarios_, these are seeing pixel counts in the area of 25-30 these days…a bit overzealous, and perhaps there is a difference, but certainly not for the purposes I use images for.  Even for stock images, I wouldn’t be using MP counts that high.  The logic is that if you start with a larger file, you have more capability to crop.  My response is, if you need to crop, you didn’t frame it right to begin with!

Until next time, keep on shooting!

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5 comments for “Megabytes Versus Megapixels

  1. Miguel Palaviccini
    at

    I didn’t know the file size would change so much by just changing the quantity of light. Pretty cool!

    I’m interested in reading the speed transfer article. I’m wondering if it’s worth buying the expensive cards, when USB 2.0 can’t keep up. Or is it just for in camera transfers that the cards really shine?

  2. Skylo@rear view camera
    at

    I also did not know that light plays such a big factor in file size, thanks.

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