|
|
|
||||||||
Here's an easy way to measure the resolution of a camera-lens system. In the process you can use the resolution of the images on the matte to estimate the resolution you should expect on the negative and the print. The process requires that you use a test target.
Test target
You can use the test target below or you can make your own by following the instructions at the end of this text.
Most browsers will allow you to capture the test target alone by entering on of the commands shown below. (Make sure that you choose the format that gives equally spaced black and white rectangles at the bottom of the pattern. If all the patterns fail to deliver this clarity, follow the instructions at the end of the text to create your own test pattern. If you can suggest a file format that might allow for a better image, please contact me below.)
http://Members.aol.com/tjacmc/resolve/tja1999.jpg
http://Members.aol.com/tjacmc/resolve/tja1999.gif
http://Members.aol.com/tjacmc/resolve/tja1999.png
After you print the target, measure length of the bar under "L= mm" in millimeters and write the length in the space provided. Use the formulas on the left of each resolution pattern to calculate the line-pair densities (lp/mm). Write the densities next to each pattern with a felt marker so you can see it through a viewer.
Test target setup and line-pair density
Set the test target at a distance from the camera/lens so that you can clearly see about half the patterns on the matte surface in the viewfinder. Record the line-pair density of this pattern. It will be used to compute the resolution of the image on the matte. Later, you can use the line-pair density to find the resolution (lp/mm) of the image on the print and on the negative (with a magnifying glass or microscope).
To compute the resolution of any image you will need the line-pair density at the object, the total distance (TD) from the film to the object and the focal length (f).
Total distance.
Measure the total distance (TD) from the back of the camera body (image) to the test target (object) in feet or meters.
Convert the total distance (TD) to meters.
If the total distance (TD) was measured in inches, multiply it by .0254
If the total distance (TD) was measured in feet, multiply it by .3048
If the total distance (TD) was measured in passes, multiply it by 1.4
Focal length
Record the focal length (f) of the lens in millimeters.
Convert the focal length (f) to meters by multiplying it by 0.001
Object and image distances
Calculate the approximate or exact object (So) and image (Si) distances by using one of the two sets of formulae.
Approximate formulae:
Si = f
So = TD - f
Exact formulae:
Si = 0.5*TD - 0.5*SQRT(TD*(TD-4*f))
So = TD - Si
Resolution
Calculate the resolution of the image on the matte, the print or the negative as:
Image Resolution (lp/mm) = (So/Si)*Object line-pair density
Making your own resolution test targets
If you make your own target, it should consist of a series patterns. Each pattern consists of closely spaced horizontal and vertical black rectangles.
You can make the patterns in a graphics package by drawing a series of black filled rectangles with NO perimeter lines and the 'Snap-to-grid' option set ON.
Draw ten 1mm by 25 mm rectangles spaced 1mm apart. After you get a set of ten rectangles, select all the rectangles and group them together with the GROUP command.
Select the group and duplicate it several times. Then, starting from the second pattern, stretch each pattern one division longer than the pattern before it so that each pattern will have a lower line-pair density than the pattern before it. Finally, duplicate the patterns and rotate them 90-degrees so you will have a set of horizontal and vertical patterns .
Measure the individual patterns and label them with their line-pair density (number of lines per millimeter). For example: If 5 black and white areas (line pairs) are 50 mm wide, then they are 0.10 lp/mm.
If you have any questions or comments, don't hesitate to contact me via email. Ted Andresen