p.2

To make a working chart is the largest hurdle of all here. It may take a couple of hours. You will need a 20x30" cardboard or a foam support (look for those in an office supply store). If you use the large chart only, you may test cameras with up to 5M pixels (6.3 M of 10D is a stretch). For testing cameras with resolution over 6M pixels, please download and print the small circle (below) separately. If you don't have a decent printer or you'd like to have the other key elements of the chart at very high resolution (100 dpi/mm), you can purchase a CD with the appropriate files in a TIFF format or even the printed images on a 2880 dpi resolution printer, for a nominal fee.

This is the chart that will allow you to check the resolution of your camera. The chart contains various circles and straight lines resolution guides, and all 6 basic color gradients. I've also added a 1.4x0.9" portrait of my daughter that will be analysed in this tutorial. To download the file (zipped jpg format made with Photoshop) click on the picture on left. It is an 848 KB jpg file. It is a compressed image file that should be printed in a 48x72 cm format (could be on several sheets of paper). For a fast test, print just the center of it on a letter size format (do not scale it !), and mount the printout on a 20x30 inch board. Do not use glossy paper. Using a marker draw the frame edges, a 480x720 mm (18.9x28.3 inch) rectangle. Electric black tape is also a good choice to mark the frame edges. You may even just mark the edges on a wall with a dark tape or so, and affix the printout in the center. This will be your photo frame enlarged 20x when compared to a 24x36 mm standard negative format. You may also hand-draw horizontal lines, as in the original chart to test for lens distortions.

This is the key element of the chart. If you have a good printer, this entire chart above will allow you to test cameras up to 5M pixels. If you click on this photo, however, you will download a much higer resolution circle chart that will let you test cameras making photos up to 15M pix. This is the only one of the three circle guides of the chart but it's the most important one. The high resolution file (2540 dpi or 1000 pix/cm) is 1Mb large, zipped gif format. Click on the circle to download it. You need to print 2 of them. The numbers on the right indicate resolution in the number of lines per picture height times 100. This means that a 1800x1200 pix photo may have its theoretical maximum resolution equal to 12 (twelve times 100 is 1200, which is the picture height in landscape format).

The Test.

Make a photo of the chart such that the entire chart fills the entire frame (not the viewfinder), preferably at 2 extreme zoom settings. Most optimal photo setting is with the sun light or at least outside, with a tripod, and no flash used. If you can't do that, make sure the light is even or use a flash if you have to expose it hand-held and for longer than 1/200s. Make a few photos from slightly different distances because viewfinders usually don't show the entire frame. The lens of the camera should be right at the center of the chart, otherwise you will produce strange distortions. If you use white background, you may want to overexpose by 1 f stop. You may 'personalize' your chart by adding some photos or other objects to it - this may even cause correct automatic exposure. Once you made the photos, download all of them to your computer and choose a sharp one that shows the upper and lower edges of your chart closest to the frame edges.

 Analyze the photo. Find the thinnest line that you can see on the circles or on the line chart. Read what number is displayed by that line. Circles do not show the numbers, and you need to download the chart guide (see the picture above). The number indicates how many hundreds of lines (black and white) can be resolved vertically on a landscape formatted frame. Check what is the vertical resolution of the camera. If it is 1500 pixels across, you should be able to see line #15 or even higher (sample on the next page). If you see only thicker lines, say up to #11, the sensor uses only 11/15=73% of its resolving power due to poor lens sharpness. Therefore, the lens is not a good match with the sensor (I consider 75% to be the lowest acceptable number), and I suggest returning the camera to the store and buy a cheaper one with a lower number of pixels, if available.

If you need to be more accurate just because your number is very close to 75%, or you'd like to see a few examples, turn to the next page.

If you think that making those charts is too complicated click here to learn how to make this test as simple as possible (only slightly less acurate).

 

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