First, the working principle

 

Implementation process of semi-automatic exposure : metering devices ( eg photoresistors and photodiodes ) detects the brightness of the scene being shot with a corresponding optical signals. Comparison with the reference signal , using light-emitting diode LED shows the current actual deviations exposure parameters ( aperture , shutter speed and film speed ) Composite Index and the benchmark Composite Index . The most common form is the so-called " three light" indication - with "+ , 0, - " three symbols represent overexposed ( red ) , the right ( green ) and underexposed ( red ) . According to this directive , the photographer to manually adjust the aperture or shutter speed exposure parameters ( film speed for film into the camera , its value is specified ) to display "0" green light so far. At this point take the good times , adjusted the focus , you can press the shutter button to take a picture right exposure .

 

The two most representative SLR camera is Haiou DF-1ETM type ( domestic ) and really good energy Cosina CT-1 type ( Japan) , its photometric circuit diagram shown in Figure 1 and Figure 2 respectively . Their common characteristic is measured using both light metering mode , and both with three light display ; biggest difference lies : the former uses a metering element silicon photodiode , which is used CdS photoresistor .

 

Haiou DF-1ETM type camera's metering circuit using IC F3S05, metering element silicon LED SPD, aperture , shutter speed and sensitivity potentiometer function introduced by RF and RT. RW3 adjustable resistance for semi- linear adjustment , resistive and capacitive components R, C for the delay element . S1 is connected to the shutter button photometric switch , while S2 is the B gate power switch. Light-emitting diodes H1, ​​H2 and H3 , respectively overexposed "+" ( red ) , the right to "0" ( green ) , and less exposed " - " ( red ) .

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