Megabytes Versus Megapixels

Megapixel Chart

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!

Understanding Focus

Canon USM Feature Explained

A friend recently asked me why his camera was so slow to focus.  The answer naturally started with (as do most answers in photography) my standard phrase, “It depends…”  I realized after my conversation with him that this would be an excellent topic for exploring here on the blog.  So, today, we’ll be taking a look at focusing concepts in your camera gear.  To start off, it probably makes the most sense to identify that there are focusing features both on your camera, and on your lens.  On cameras, it’s most often referred to as the “auto-focusing system” or AF.

AF Systems

Canon 7D AF System

The above AF system comes from the Canon 7D, but generally speaking, newer cameras will have better AF systems than older cameras.  What makes them better is the presence of more AF points.  The more points the sensor has, the easier it is to identify a point to focus on.  The AF system identifies these points based on contrast between light and dark, so reflected light does matter to a degree as well. The bottom line though is that with more focusing points, more focus detection, better contrast algorithms, etc. and better sensitivity to light will allow a camera to find a subject quicker than its older counterpart with fewer AF points.  Of course, the AF system is only as good as the lens that it connects to, and the lighting conditions you are in.  If you have a great AF system (camera body) and a slower lens (an aperture in the range of f4 or f5.6), you will still be limited by light.

The upshot:  More AF points = Better Camera (in general)

While there are always exceptions and nuances or unique scenarios for specialty gear, focusing speed is really more a function of the lens than the camera body.  What you will find in bodies though, is that you get better target acqusition and tracking on the higher end bodies than the entry-level models (say an Xsi versus a 1DMark II).   The camera basically does the job of saying “here’s the point to focus on”, and then transfers the job to the lens of actually making that point the sharpest one in the picture.

One question that I often hear though is “So, what defines how fast the focus locks – the AF system or the lens?”  The answer is that the AF system does the focus lock – the “lock” is really just defining the point to focus on…it’s just a fancier way of saying it.  If you have no point locked in, even the fastest lens will just search and search and not bring anything sharp because it doesn’t know what to make sharp.

I would be remiss though if I didn’t acknowledge that newer cameras do work better in lower light too though, because they do.  Here’s why:  you will get better low light performance on a higher end body, simply because the sensor is larger, and more sensitive to light.  But when we start talking about light, we have to actually give more of a nod to the lens.

Lens Focusing

Where camera bodies pull their weight in target acquisition and tracking, lenses pull their weight when it comes to operation in lower lighting conditions.  The larger your aperture (the smaller number actually means a wider open aperture, remember?), the better it will perform in low light because you are simply letting more light get to the sensor.  That lens that opens to f2.8 will let more light in regardless of the body it is connected to!  This is why expensive lenses that can open t f2.8, f2.0 or even wider are referred to as “fast glass” because they let such a large amount of light onto any sensor.

The other part of what really gives a nod to lenses over bodies is that lenses are what actually does the focusing.  Adjusting the focus to something that is “sharp” is done by the lens.  Where the camera said “Hey, cool, I’ve found the point to focus on”, here, the lens uses the AF points from the camera body and basically says “okay, I am going to focus in on this point that you’ve defined.”

The last point to bring up here is to mention the focusing motor of the lens.  Terms that are often bandied about include USM, HSM, and other similar terms.  This is where it can get really confusing because each vendor uses unique terms, and even within a specific vendor, the same letters can mean different things.  Here’s (since it’s the CanonBlogger site), I am going to stick to the Canon nomenclature for now, which is the USM.  In the Canon family, USM is seen in many lenses and has quite a history.  It started in the body before the digital camera revolution.  When Canon introduced the EF lens mount in 1987, the technology was available to put the USM on the lens, which allowed it to operate even faster.  They never looked back!

Nowadays, USM is bandied about within the Canon family, and you have to be careful now, because one is used to identify a lens feature, while the other is more marketing and hype (in my opinion anyway).  The one that really matters are the ones that refer to the Ring USM.  These lenses are more expensive, heavier than their counterparts, are much quieter, and also tend to bring the subject into focus faster.

Canon USM Feature Explained

The alternate motor or Micromotor Ultrasonic Drives are the ones used in cheaper lenses, which by comparison are lighter, noisier, and focus more slowly on the subject.  Most kit lenses are of these variety.


What did we learn here?  First off, hopefully you’ve come away with a better understanding of body AF systems versus lens focusing.  On the most simple of explanations – a camera AF system says where to focus, and the lens is what actually does the focusing.  It should come as no surprise though, that both tie in very tightly to light.  In the body, better sensitivity to light allows for better contrast in the AF points, while in the lens, the wider aperture can bring more light in under conditions where a narrower opening would not suffice.  So, higher sensitivity and wider openings can impact contrast and focusing speed in bodies and lenses respectively.

The bottom line:  neither does this task independently, but when  it comes to speed – most of this comes from your lens, not your body!


Got questions, comments, or additional thoughts to share?  Sound off in the comments!  Have a great weekend, and we’ll see you back here on Monday!

Three Reasons You Should Be Using a Card Reader

Card Reader from B&H Photo

Card Reader from B&H Photo

One of the most surprising things I learned about during our recent trip to Brainerd, MN was that many photographers are tethering their cameras to their computers for transferring photos over for processing.  While tethering via cable is always an option, there are many risks to doing so, and during our talk, we took a few minutes to share some reasons why it’s better to use a card reader than to import from your camera.  Since there were so many people that seemed to appreciate the insights, I thought I’d take some time to share some of those insights here:

  1. Speed – Card readers have nothing else to do other than transfer data onto and off of a card to a computer.  No camera firmware is required, no menus need to be loaded, and no power is needed.  It’s a simple plug-and-play process for practically any computer and you can increase your transfer speeds significantly by taking this route over camera direct transfers.  Seriously…with the super fast UDMA cards out there today, you can really see a decrease in transfer times, which gets you on with other things even that much quicker.
  2. Reliability – Because cameras are such advanced devices that have so many working parts and components, the process of connecting them to a computer does take a certain toll and if the cable is frayed, or a connection is lost, or even power is lost, you can risk losing and/or corrupting your images during the transfer process.  Eliminate the risk of losing those precious memories you’ve created and use a card reader!
  3. Conserving Battery drain – In Minnesota it was cold!  We were in negative temperatures for a good portion of our stay, and with lower temps comes decreased battery time.  If we had to deal with draining our camera batteries to transfer images to a laptop or desktop computer, they likely would not have lasted.  It’s important here to remember that plugging a camera into a USB port isn’t like plugging in a phone or other devices.  Plugging in doesn’t charge the batter, it discharges the battery!  Something to keep in mind when conserving your battery life.

So, what kind should you get?  The answer here (as always) starts with “It depends…”  What kind of camera and card are you using? Do you have multiple cameras and multiple card format types?  Cards range in size and format from SD, CF, to Memory Sticks, and other proprietary sizes and shares.  Formats also include the new UDMA which increases write/read transfer rates, and varying capacities from extended capacity (XC) and other older ones which may require specific types of readers.  Check these considerations before making a purchase.  My suggestion though is to get a reader that can read both standard capacity cards as well as the XC formatted ones.  These can be found for as little as $5 through sites like and Amazon, but the better quality ones are available through camera retailers like B&H Photo, Adorama, and Sammy’s.

There are, of course, other reasons to use a card reader over a camera for data – and I could go on with a quite extensive list here, but would really like to hear from others:  what are your reasons for using a particular transfer setup?  Do you have a reader preference?  Or are you using the camera to tether?  Sound off in the comments with your own ideas and suggestions!

Enter your email address: Delivered by FeedBurner