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Z-Cage

Review of the Canon EOS 5D Mark II with EF 24-105mm F/4.0 IS Lens

Written by Ron Risman
December, 2008

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21.1-Megapixel Full-Frame Sensor

Who needs an image sensor with this many pixels? While it is true that you really only need 5 or 6-megapixels to make beautiful 8x10" and 11x14" prints, manufacturers are not showing signs of stopping the megapixel race anytime soon. With that in mind lets take a look at some of the advantages and disadvantages of more megapixels.

Something to keep in mind is that more pixels doesn't necessarily mean sharper pictures. Yes, they can go hand in hand, but not always. I know that sounds counter intuitive, but in reality the resolution or pixel count of an image sensor has more to do with the image size (height & width) and maximum print size than it does actual detail - or what is often refer to as "resolution." A typical image captured with a 7-megapixel digital camera has an image size of 2,304 pixels high and 3,072 pixels across (2,304 x 3,072 = 7,077,888 pixels). In comparison a 21-megapixel digital camera will capture the same scene with an image size of 3744 x 5616 = 21,026,304. So what does this all mean? Well, in order to answer this question we need to consider print size.

Resolution and Print Size Comparison


When it comes to printing, you typically want to send the printer an image with around 300 pixels per inch (ppi) for any given print size to create what would be considered a photo lab quality print. So now let's look again at the image size of a 7-megapixel image. If the image captured with a 7-megapixel camera has 2,304 x 3,072 pixels, and the print is set to 300ppi (pixels per inch), then the maximum print size would be 7.68" x 10.24" (7.68" x 300ppi = 2,304 and 10.24" x 300ppi = 3,072). If you did this same calculation with a 21-megapixel camera you would discover the maximum print size to be about 13x19" (3744/300 = 12.48" x 5616/300 = 18.72"). But what if you print mostly 4x6" prints, don't you benefit from the extra resolution? The answer is probably not since the image being sent would have more pixels per inch (ppi) than the printer could handle. However, if you decide at some point to make a 16x20" print from a 7-megapixel image, then you WILL notice a lack of detail and resolution when compared to a higher resolution image, since the 7MP (2304 x 3072) image doesn't have enough pixels to fill a 16x20" page without lowering the pixels per inch to about 144-150 (instead of 300). Because of the lower ppi, you will notice that the image will appear softer and less sharp especially as you get in closer to the print. So what we have learned is that having more pixels provides larger print options, but not necessarily more detail in smaller prints.

The above paragraph only discusses image size, but pixels alone do not determine overall image quality. There are many other factors including lens quality, noise, exposure and focus accuracy, dynamic range, and tonality differences, just to name a few, that help determine the overall quality and detail of a photo. There are also times when a higher resolution image sensor can actually provide less quality due to increased noise in an image. Without dragging this subject on too much longer, it's worth nothing that an image sensor with higher pixel density will often exhibit higher noise levels when compared to a similarly sized image sensor with less pixels. This higher noise is due to smaller pixel sizes, a side effect of cramming more of them onto a chip. As pixels become smaller they become less light sensitive, which increases noise levels especially in lower light situations. To help combat this problem manufacturers have started putting micro lenses on their image sensors to help focus more light onto each pixel to give them increased light-gathering ability. This technology, along with Canon's new DIGIC 4 image processor, is what gives the Canon EOS 5D Mark II such incredible light sensitivity despite the resolution increase to 21-megapixels.

Another advantage to capturing images with 21-megapixel is the ability to crop the photo while still retaining enough resolution or pixels to make a decent size print. The following two photographs illustrate this cropping capability. The first image is the full capture of the swan near the edge of the marsh (reduced for web display). The second photograph was cropped to emphasize the face and neck, while still keeping the marsh in the background to show the surrounding. This cropped photograph is just 15% of the original size (3.2MP) but still large enough to make a quality 8x10 print.

In summary, 21-megapixel resolution won't necessarily do anything for your 4x6 or 8x10" prints, but it will benefit those who make larger big prints (12x16", 16x20", 20x30", etc.) on a regular basis. Using third party programs like Genuine Fractals you should easily be able to create enlargements that go well beyond the 20x30" print size. The extra megapixels also provide the ability to crop a photo in order to get in tighter on a shot. Since a full-frame sensor provides true 35mm equivalent zoom ratios, a 200mm telephoto is now "just" a 200mm photo, compared to a 320mm equivalent as it would be on a camera with a smaller APS-C sized sensor. With cropping in post-capture, a 200mm zoom can effectively act like a 320mm or 400mm, and with the 21-megapixel resolution you'll have plenty of pixels left over to make a decent size print.


Page 1 (Intro) Page 2 (High ISO) Page 3 (ISO Test Images) Page 4 (Sensor)
Page 5 (Features) Page 6 (HD Video) Page 7 (Conclusion) Page 8 (Sample Gallery)
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