Lesson 8: Filter Factors

Kozak02Filter Factors
by Steve Kozak
M. Photog., CR.
CPP

 

 

If we go back and review the basics of f-stops, we know that an moving from one f-stop to the next larger lens opening (whole stop) allows twice the amount of light to reach the sensor.

For example, moving from F16 to F11 doubles the amount of light reaching the sensor.

Conversely, moving from one lens opening to the next smaller lens opening (whole stop) cuts down the amount of light reaching the sensor by half.

So, moving from F11 to F16 reduces the amount of light reaching the sensor by half.

It is important to recognize that the increase or decrease of exposure through the f-stops (and shutter speeds) is exponential – not linear. So lets take a look at how this is carried out over a number of f-stops:

Starting at F16, moving to F11 doubles (2X) the amount of light reaching the sensor.

F16 to F11 = 2X the light

Moving from F11 to F8 doubles the amount of light again!  So, F8 allows 4X the amount of light to reach the sensor than F16 allows.

F16 to F11 = 2X the light
F16 to F8 = 4X the light

(If you had $1 and I doubled it for you, you would have $2, If I doubled it again, you would
have $4.)

Moving from F8 to F5.6 doubles the amount of light again! So F5.6 allows 8X the amount of light to reach the sensor than F16 allows.  (If I doubled your previous $4, you would now have $8.)

F16 to F11 = 2X the light
F16 to F8 = 4X the light
F16 to F5.6 = 8X the light

Moving from F5.6 to F4 doubles the amount of light again! So F4 allows 16X the amount of light to reach the sensor than F16 allows. (If I doubled your $8, you would now have $16.)

F16 to F11 = 2X the light
F16 to F8 = 4X the light
F16 to F5.6 = 8X the light
F16 to F4 = 16X the light

By now, you probably see the trend and that moving from F16 to F 2.8 allows 32X the amount of light that F16 allows.

F16 to F11 = 2X the light
F16 to F8 = 4X the light
F16 to F5.6 = 8X the light
F16 to F4 = 16X the light
F16 to F2.8 = 32X the light

With whole stops, F1.4, F2, F2.8, F4, F5.6, F8, F11, F16, F22, F32 and beyond, whichever
f-stop you begin with, the following is true:

                                       Opening the lens by / Increases the light reaching the sensor
1 stop = 2X
2 stops = 4X
3 stops = 8X
4 stops = 16X
5 stops = 32X
6 stops = 64X
7 stops = 128X
8 stops = 256X

The converse of all of this is:

                                            Closing the lens by / Decreases the light reaching the sensor
1 stop = 1/2X
2 stops = 1/4X
3 stops = 1/8X
4 stops = 1/16X
5 stops = 1/32X
6 stops = 1/64X
7 stops = 1/128X
8 stops = 1/256X

Keep in mind, in the above examples, the amount of light given off by the light source is not changing—only the amount of light passing through the diaphragm of the lens and reaching the sensor is changing. Think of it as opening or closing the blinds in a window to allow more or less light into a room. The light outside is not changing, only the amount of light allowed into the room (sensor) is changing.

Filter Factors
When using filters on the front of the lens, some degree of exposure compensation will be required for the loss of light as it passes through the filter. (Clear filters such as a 1A Skylight filter do not require any measurable compensation.) The denser the filter, the more the exposure value will need to be increased over the measured exposure of the scene.

Most filters provide a “Filter Factor” right on the rim of the filter itself. You may, however, have to consult the filter’s instructions. Filter Factors are expressed as an increase in exposure values.

So, a filter with a “filter factor” of 2X means that you will need to increase the exposure
by twice the amount. Again, twice the amount of exposure is equal to one stop.
To achieve this one stop increase in exposure, you could:

  • open up the lens by one stop
  • or slow the shutter down by one stop
  • or increase the ISO by one stop. (100 ISO to 200 ISO, for example.)

Filter Factors
2X = 1 stop increase
3X = 1 1/3 stop increase
4X = 2 stop increase
8X = 3 stop increase
16X = 4 stop increase

Welcome to your Lesson 8: Filter Factors

1. Which line has the “full” f-stops in the correct order?
2. Which line has the “full” shutter speeds in the correct order?
3. If you move the f-stop from F11 to F8, what happens to the amount of light that reaches the sensor?
4. If you move the shutter speed from 1/125 to 1/250, what happens to the amount of light that reaches the sensor?
5. How much more or less light reaches the sensor if you move the shutter speed from 1/60 to 1/30?
6. How much more or less light reaches the sensor if you move the shutter speed from 1/500 to 1/125?
7. How much more or less light reaches the sensor if you move the shutter speed from F4 to F16?
8. A filter factor of 4X represents a required increase of how many stops?
9. A filter factor of 8X represents a required increase of how many stops?
10. A filter factor of 16X represents a required increase of how many stops?
11. Given an exposure of F8 @ 125 at 100 ISO, what would be the new exposure if using a filter with a 4X Filter Factor?
12. Given an exposure of F5.6 @ 500 at 100 ISO, what would be the new exposure if using a filter with a 4X Filter Factor?
13. Given an exposure of F16 @ 15 at 100 ISO, what would be the new exposure if using a filter with an 8X Filter Factor?
14. Given an exposure of F11 @ 60 at 400 ISO, what would be the new exposure if using a filter with an 16X Filter Factor?