Application of spot meter
One of the significant tools in photography is spot meter. spot meter is a lightmeter, which has an viewing angle between 1 to 3 degrees. Always, the method for measuring of light intensity is very important for me. Especially, methods that are very simple. My reason for making such an instrument, has been used Zone system technique, which was developed by Ansel Adams.
The Zone system procedure, all stages of exposure, developing of film and photo printing, will be standardized and repeatable. Thus always print results will be the same in terms of tonality.
1° spot meter structure
I used in this instrument, from a 50mm lens of the 35 mm camera's. I removed all the mechanical parts. Then, I put it inside an aluminum tube that was made for this purpose. To get the true picture (the reverse and left – right) and visible, i put the mirror box and ground glass, the camera's 35 mm, inside body. Thus, the image composed of the lens and passes through the mirror, ground glass and eyepiece is seen correctly. But for measuring light in the center of the image, you can create a hole in the center of the mirror. Therefore, by special technique, I created a hole in the center of the mirror with a diameter of 1 mm.
I used a variety of cell to measure the light. Finally, after testing many different cells, I used a special type CdS cells (Cadmium sulphide). This cell must be able to measure the amount of EV = 3-18. Changing light on the optical cell, causing the changes in the amount of electrical current passing through the micro-ammeter. Thus, by changing the amount of electrical current passing, can be measured different amounts of light.
But the main problem is the CdS cell response, ie the light intensity variations depending on the Micro Amp. This variations is a logarithmic graph.
According to the above chart, current strength measuring, not accurate when the light is low or high. To avoid this problem, I have tried changing the electronic circuit, and holding the electrical current passing of micro-ammeter in zone linear graph.
spot meter calibration procedure
Because of this photometer measures the intensity of light by reflection method, so using a large sheet of white, and another reflective light meter, by comparative method, I calibrated this spot meter. Then, I printed the EV values on transpart sheet and put on ground glass. Thus when you see the viewfinder, except the image of a scene and needle, will be visible a small circle in the center (circle 1 degree light meter) and EV values from 3 to 18.
But to convert EV values in the light meter viewfinder, to sensitivity values, shutter speed and aperture, I installed a separate three-ring on the lens. In addition, the engraved numbers, i used 8 gray scale, which is indicative of zone = 1-8.
Angle Measurement: 1°
Photo cell: CdS
Light intensity measurable: EV=3-18
Based calibration zone: 1-8
Battery check: yes
Material: Lens holder aluminum alloy 7075 , the body is made of black plastic
Lens holder coating: Black anodizing
External dimensions: W:100, H:150, D:40 mm