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Handmade 1° spot meter for light metering

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.

Handmade 1 deg spot meter
Handmade 1° spot meter

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.

Functioning system of handmade spot meter
spot meter schematic

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.

Cds light meter cell diagram
CdS cell light meter circuit

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.

EV to Microamper diagram
Changes of light intensity according to EV, compared to Micro Amp in CdS cell

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.

1 degree Spot meter viewfinder
1° spot meter viewfinder

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.

Technical Specification

Angle Measurement: 1°

Photo cell: CdS

Light intensity measurable: EV=3-18

Based calibration zone: 1-8

battery: LR44x2

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

Weight: 200gr

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