If you shoot with a 4×5 large-format or any camera with expandable bellows, understanding bellows extension compensation is crucial—knowing when it applies and how to adjust for it. While there are various ways to calculate the necessary adjustment factor, all leading to the same mathematical outcome, I prefer a simpler approach. In this article, I’ll explain what a bellows compensation factor is, when it is needed, and how I handle it quickly and efficiently.
Using my 4×5 Sinar Norma monorail camera as an example, we first need to understand how bellows function before delving into bellows compensation factors. Let’s begin by looking at situations where no compensation is necessary.
Bellows Extension: Infinity Focus
My go-to lens for 4×5 cameras is the Nikkor W 150mm f/5.6. When focused at infinity, the bellows must extend to 150mm, precisely matching the lens’s focal length. This measurement—taken from the lens plane to the film plane—confirms the designed focal length necessary for achieving proper focus at infinity. In the image above, the Sinar Norma is shown with its bellows extended to 150mm.
This principle applies to all non-telephoto lenses used on 4×5 and similar large-format cameras. Whether it’s my Nikkor SW 75mm f/4.5, Schneider Digitar 120 f/5.6 Macro, Nikkor W 210mm f/5.6, or Nikkor M 300mm f/9, the bellows must be extended to match the lens’s focal length for proper infinity focus.
Why Telephoto Lenses Are Different
Telephoto lenses follow a different optical design, which allows them to achieve focus with a shorter bellows extension than their stated focal length. For instance, a 300mm telephoto lens may only require 200mm of bellows draw to focus at infinity. This design reduces the physical extension needed, making telephoto lenses ideal for cameras with limited bellows range.
Because of this difference, telephoto lenses do not follow the same bellows compensation principles as non-telephoto large-format lenses. Their reduced extension means they require a different method for calculating exposure adjustments, which is why they are not included in this discussion of bellows compensation factoring.
Convex Lens Boards & Top Hats
The Nikkor M 300mm f/9 shown here is mounted on a 21mm convex lens board, often called a “top hat.” These specialized boards extend the lens forward when the bellows alone cannot provide enough extension. Available in various sizes, they accommodate different needs. While my Sinar system offers ample extension for the 300mm lens and beyond, achieving this would require adding longer rails or additional bellows. However, that’s unnecessary in this case, as I primarily use the 300mm lens in the field with my Ebony RW45 camera, which requires just a bit more reach. Keeping the lens mounted on a convex board provides that extra extension while remaining compatible with the Sinar. When measuring for infinity focus with the 300mm on this setup, the reference point is the front of the convex lens board behind the lens rather than the front standard.
Bellows Extension: When to Apply Compensation
The bellows extension factor comes into play when the bellows extend beyond the lens’s focal length, which happens when focusing closer than infinity, like when doing close-up or macro work. As the bellows expand, the light reaching the film spreads out, reducing brightness. This means exposure compensation is necessary to maintain proper image brightness. The key is determining how much additional exposure is needed to correct the light loss.
Always ask yourself this before loading your film holder into the camera:
“Are the bellows extended beyond the focal length of my lens?”
A Quick Example
Below is an example of focusing beyond infinity using the Sinar Norma 4×5. For this demonstration, I’ve mounted a Schneider 120mm f/5.6 Apo Digitar Macro lens along with a Hasselblad CFV II 50c digital back. At infinity focus, the 120mm f/5.6 macro lens requires 120mm of bellows extension. However, for the close-up example shown below, I extended the bellows to 170mm—an additional 50mm beyond infinity. Based on my calculations, this extra extension introduces a bellows factor requiring one additional stop of light for correct exposure.
How could I determine that an extra stop of exposure was needed? There are three ways:
1. Mathematical Calculation – A straightforward approach based on formulae.
2. Manual Measurement – Use a ruler & chart to determine the bellows extension.
3. Phone App – My preferred method, a quick and convenient solution.
While exact calculations aren’t always needed in the field, these simplified methods make exposure adjustments easy. I’ll walk through each approach, all leading to the same result.
Method #1: Mathematical Calculation
Running the Equations
If you enjoy math (🧮), you might appreciate the two equations at the top of the graphic above. These formulas are the foundation behind the Manual Measurement and Phone App methods discussed next.
Method #2: Manual Measurement
The Quick Stick
The Quick Stick shown above is a 3-inch by 20.25-inch ruler—far from portable, but it has been a fixture in my studio toolbox for over twenty-five years! I’ve used it occasionally for close-ups and macro work with a 4×5 camera. With the advent of digital backs on my studio monorail cameras, I no longer rely on it, as Live View makes recognizing exposure adjustments much easier. However, it was an invaluable tool when needed, especially when teaching 4×5 photography years ago. It’s no longer listed on the distributor’s website, which doesn’t come as a surprise. Simple to use: place it at the film plane, follow your lens, and read the necessary exposure increase in stops at the lens’s plane.
Tape Measure & Chart
Many photographers create charts for the lenses they use in close-up and macro work. As an example, I put together a quick chart for my 150mm, 210mm, and 300mm lenses, listing only the necessary stop increases and omitting the bellows factor. This makes fieldwork easier—carry a small cloth metric tape measure, measure the bellows extension, and reference the chart. It’s simple: You only need to do the math once, create a chart, and keep a metric tape measure handy.
I eventually designed a chart with half-stop increments, which made field adjustments more efficient. When a bellows extension fell between the displayed half stops, estimating the correct compensation was simple—and it always worked.
Method #3: Phone App
NOTE: The apps I mention in this article are Apple-based, and I apologize for not including Android alternatives, but I have never used an Android device.
PhotoBuddy
The PhotoBuddy app offers a comprehensive set of photography tools—far more than I’ll cover here, as this article focuses on Bellows Extension Compensation (BEC). For a complete list of features, visit their website or check out their manual. I’ve used this app for years and highly recommend it for BEC.
Reciprocity Timer
The Reciprocity Timer app is a helpful tool for shooting. It allows you to take exposure notes for each frame or sheet of film. It’s highly customizable and includes a built-in Bellows Extension Exposure Compensation feature—just input the bellows length, and it does the rest. I’ve used this app for a few years without any issues.
Their website is currently under development, so for more details, visit the Apple App Store. As of March 2025, the app is actively supported and regularly updated.
Closing Statement
Understanding bellows extension compensation is essential for achieving accurate exposures in close-up and macro photography with large-format or bellows-focusing cameras. Whether you prefer manual calculations, a quick reference chart, or a mobile app, the key is finding a method that integrates seamlessly into your workflow. The techniques and tools outlined in this article—ranging from classic math & rulers to modern apps—offer practical solutions to simplify the process. By setting up a system that works for you, compensating for bellows extension becomes second nature, ensuring properly exposed images without guesswork.