Bellows Extension Factor: What it is and how to calculate it.

What Is Bellows Extension Factor?

When you’re using a camera with a bellows, like an analogue large format camera, you sometimes need to extend the bellows beyond the lens’s focal length to focus on something closer. This is where the bellows extension factor comes into play.

The bellows extension factor is a term used to describe the amount of light lost when the bellows are extended beyond the lens’s focal length. This happens because the increased distance between the lens and the film plane causes the light to spread out and become less intense.

Here’s a how to calculate Bellows Extension Factor:

1. Measure the focal length of your lens.
   

2. Measure how far the bellows are extended (this number should be greater than the focal length for the factor to be relevant).
   

3. Divide your extension by the focal length, and then take that number and square it.
   

This will give you the factor by which your bellows are drawn. Since light falls off exponentially, you apply your extension factor to the logarithm base 2 of your number and that’s how many stops of extra light you’ll need in your exposure. This means you would need to compensate for this by allowing four times the amount of light for your exposure.

Hang on, WHAT??? 🤯

Yeah, I know - confusing isn't it! Here's how I do it. You don't need any fancy tools or complicated maths skills for this (phew!):

Step One: Make a list of f-stop numbers. These numbers help control how much light enters the camera. You can find them on your light meter or make a list like this:

3.5, 4, 4.5, 5, 5.6, 6.3, 7.1, 8, 9, 10, 11, 13, 14, 16, 18, 20, 22, 25, 28, 32.

(those in bold, underlined and italicised indicate tranditional whole f-stops)

Step Two: Calculate the focal length of your lens in inches. Common large format lens focal lengths for 4x5 format, along with their equivalents in inches, are as follows:

1. 90mm (wide-angle)

• Equivalent in inches: 3.54"

1. 135mm (standard)

• Equivalent in inches: 5.31"

1. 150mm (standard)

• Equivalent in inches: 5.91"

1. 210mm (portrait/telephoto)

• Equivalent in inches: 8.27"

1. 240mm (telephoto)

• Equivalent in inches: 9.45"

1. 300mm (telephoto)

• Equivalent in inches: 11.81"

Don't worry too much about exact numbers; rounding is okay!

Step Three: Measure the distance from the ground glass to the lens board in inches. This is your bellows extension. Then, compare this distance to the focal length of your lens. The difference tells you how much you need to adjust your exposure.

For example, if you have a 210mm (8 inch) lens with 11 inches of bellows, you need to increase the exposure by one stop (the difference between f8 and f11).

8, 9, 10, 11, = 1 stop

If you have a 240mm (9 inch) lens with 14 inches of bellows, you need to increase the exposure by one and one-third stops (the difference between f9 and f14).

9, 10, 11, 13, 14 = 1 and 1/3 stops

And if you have a 90mm (3.5 inch) lens with 4.5 inches of bellows, you need to increase the exposure by two-thirds of a stop.

3.5, 4, 4.5 = 2/3 stops

Then meter your scene and add the appropriate amount of extra light and you're sorted!

Here's an example from my trip to Anglezarke at the end of March 2024:

1/15s became 1/4s with bellows extension factor

I racked my intrepid all of the way out to 300mm of bellows draw, but I was shooting with my Schneider-Kreuznach150mm F5.6 Symmar S, so I needed to convert 150mm and 300mm to inches.

150mm in inches is approx 6

300mm in inches is approx 12

Using my list of f-stops above you will notice that it doesn't actually fit, but not to worry I just calculated based off 5.6 and f11:

5.6, 6.3, 7.1, 8, 9, 10, 11 = 2 whole stops

I had metered my scene with my incident meter as 1/15s therefore I adjusted this 2 stops to give me 1/4s and took the exposure.

I hope this short tutorial helps!

Cheers

Olly!