Certified Professional Photographer (CPP) Practice Exam

How many stops does a 3x converter reduce effective f-stops?

1. 1 stop

2. 2 stops

3. 3 stops

4. 4 stops

The correct answer is: 3 stops

Using a 3x converter with a lens results in a reduction of the effective f-stops by three stops. This occurs because a teleconverter enlarges the image projected by the lens, effectively reducing the amount of light that reaches the sensor or film. As a general rule of thumb, each stop of light corresponds to either halving or doubling the amount of light entering the camera. When a 3x converter is used, it multiplies the effective focal length of the lens. However, this increase in focal length also has the effect of reducing the aperture's effectiveness at allowing light to hit the sensor. Therefore, if one assumes that each full stop is a halving of the light—one stop means half the light reaches the sensor—then with a 3x converter, the light is significantly reduced by three full stops. This means you would need to open the aperture wider by three stops to maintain the same exposure, resulting in a more pronounced effect on the depth of field and overall exposure of the image.