# ASTM Method for Light Loss Testing - Enclosures for the CCTV industry

M

#### mshell

Does any one know the document number for an ASTM for Light Loss Testing (if there is one)?

#### Al Rosen

Super Moderator
mshell said:
Does any one know the document number for an ASTM for Light Loss Testing (if there is one)?
Can you provide some details or context? Describe the conditions.

M

#### mshell

Sorry for the delay in responding.

We manufacture enclosures for the CCTV industry. One of the requirements is a specific f stop or light loss. We currently use a black box and light loss controller to calculate the amount of light loss.

Also, how is f stop converted to a light loss %?

Thanks in advance for any help.

#### Al Rosen

Super Moderator
mshell said:
Sorry for the delay in responding.

We manufacture enclosures for the CCTV industry. One of the requirements is a specific f stop or light loss. We currently use a black box and light loss controller to calculate the amount of light loss.

Also, how is f stop converted to a light loss %?

Thanks in advance for any help.
Mshell,

If I understand, you're asking what is the attenuation of the light for different f-stops?

If that is the case, the F stop is equal to the ratio of the Diameter of the aperture to the focal length of the lens (focal length/diameter). There are standard f-stops like f1.0 f1.4, f2.0, f2.8......f22 ad infinitum. The amount of light is double when you go down from one standard f stop to the next lower or 1/2 when you go up a standard f-stop. The reason it is inversly proportional, is because the area of the aperture (a circle) increases as the f stop decreases.

Look at A Tedious Explanation of the f/stop for a more detailed (and probably clearer) explanation. You will also be able to calculate the percent difference in light for non standard f-stops after reading this.

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