First you will need a test target. One easy way to get one is to make an image on the computer with 10 lines pairs per inch. Create a new white, gray-scale canvas with large pixels, like 20 pixels per inch. Then use the pencil tool. Add vertical lines: one column of black and the next column of white. It helps to enlarge the image so the pixels are fairly large on the screen. Once about 20 black lines are drawn, send the image to the printer. You should have black and white lines with 10 line pairs per inch on the paper. Measure to be sure. You have made a quick and easy test target.
It is easier if you mount the test target onto some gray cardboard that is larger than the sheet of paper. Now affi x the test target to a wall so that it is vertical. The wall should be some neutral, medium color or the auto- matic exposure system in the camera will be fooled into making the white paper gray.
Check the manual that came with your camera. It will tell you how many pixels it has. Some give the total (e.g., 7.9 megapixels) and others will tell you how many there are across the frame height and width. If you don’t have this, you can compute the values for the rows and columns as follows.
C H A P T E R 6 : Resolution
100
The total number of pixels is Pt, the number per row is Pr, and the num- ber per column is Pc. The basic formula is:
Pt ⫽Pc * Pr
You also need to know the aspect ratio of the camera. This is the ratio of the frame width to the frame height. Some cameras are 3:4, some are 2:3, and others might be 9:16, or even some other number. If you don’t know your camera’s ratio, take a picture and print it such that it does not fi ll the full sheet of print paper. Then measure the height and width. Or simply look at the pixel dimensions in the Image Size window when the image is opened in your computer.
In this case, assume that the camera has a 3:4 aspect ratio. Also assume that the camera has 6 megapixels. (In reality, camera manufacturers round off the indicated number, and in this case, the true value is 5,993,240 pixels). These factors given, it is such that:
5 993 240, , ⫽Pc * Pr
and
Pr/Pc⫽ 4 3/
which is to say that,
Pr ⫽4* Pc/3 Substituting back, 5 993 240, , ⫽Pc * 4 * Pc/3 or 5 993 240, , ⫽ 4* Pc2/3 Doing the arithmetic,
Pc ⫽2 120,
and since,
Pr ⫽4* Pc/3
then
Pr ⫽2 827,
So, for the given camera, the P w is 2,827. A similar calculation will be needed for the camera you wish to test if the value is not given in the
manual. With this number you can make some calculations that will be used to set up the camera test. From the formula given earlier, solve for the equation,
D ⫽ W *o 3 /P ,w
for the frame width. That is,
Wo ⫽D * Pw/3
The value for D is known; it is 0.1 inch, the distance across a pair of lines on the test target just made. And P w is obtained from either the man- ual or the calculation just demonstrated. In the example, this comes out to:
Wo ⫽0 2 2 827 3. * , / ⫽ 94 23. inches or 7 feet 10 inches
On the wall where the test target is mounted, measure 3 feet 11 inches on either side of the center of the test target, and place some sort of mark, like blue masking tape at that location. For completeness, place marks at 2 feet, 6 inches on either side, and also at 8 feet either side.
Put the camera on a tripod and set it at the height of the test target and have the camera pointing at right angles to the wall. Use center-weighted exposure, some moderate f/stop, and be sure to focus very carefully. It might help to have a large fl at object on the wall in addition to the test target in order to focus more easily. Do not use JPEG image output. Set the three distances between the camera and the target such that your pairs of markers are just on the edge of the respective frames. You will be taking a picture of an object with a detail size of 10 line pairs per inch at 3 frame widths. One will have the line pairs well resolved, the next will have them just resolved, and in the third the lines will not be resolved at all. Examine your images on the computer screen to verify that you know the camera’s performance.
Since most people would rather check a table than run calculations, it is possible to take the process just described and produce a pair of tables. One table shows the detail size that is just resolvable for a variety of frame widths. The other shows the maximum frame width that can be used for a variety of detail sizes. These are shown in Table 6.1 . These are for the cam- era assumed in the example in the previous paragraphs. Some typical object sizes are shown in Table 6.2 .
Please note that it is frame width and not lens focal length that is impor- tant in determining the relationship between detail size and frame width. If a wide-angle lens is used, the pixels on the sensor chip will be spread over a wider frame, and if a telephoto lens is used, the frame will be narrower. But the same number of pixels will be spread across the frame. The photogra- pher will adjust the camera-to-object distance to get the same frame width. So the result is that the number of pixels per frame width is the same no matter the focal length of the lens.
C H A P T E R 6 : Resolution
102