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Bellows factor in digital photography



This page was written for better explaining a lesser known photographic issue (Bellows Factor) and it is indeed useful for a better understanding of how photography works.
Yet, no average digital photographer will really need to get involved in the matter: thanks to the way exposure is calculated today, most people can get past of the whole thing and just keep on shooting.


Orvieto - Pozzo di San Patrizio

Orvieto — Italy

Most of the digital cameras we use today do not resemble any more their distant ancestors: old huge bellow cameras of one century ago seem to have nothing to do with nowadays’ technology-packed digital devices that fit in a pocket and upload tons of color (even three-dimensional) pictures to the internet in seconds, ready to be spread across the whole world.
Right?
No.
The way photography works is just the same as before, and so is the majority of the issues involved in the making of a photograph.
Here we deal with one technical matter called Bellows Factor, which has to be taken into account when shooting macro.

Bellow? What bellow?

I have many friends asking for an advice about the best camera for really learning about photography. Most of them seem astonished as I tell them that the best device is a view camera, something like this one:

View camera extending its bellow for focusing

While there are much cheaper and more practical ways to learn photography, view cameras allow for a straightforward, in-depth understanding of photographic issues and process.

In this particular case, you will clearly notice that the lens, attached to a bellow, can be moved at different (focusing) distances from the film/sensor plane (i.e. the camera back).
Distances range from the nominal focal length of the lens to the maximum stretch of the bellow.

Examples

In practical terms, imagine that you have a 150 mm lens attached (which is considered a normal lens on a camera like the one you see just above):

  • set it a 150 mm (15 cm) for focusing objects an infinite distance off, or very very far anyway (moon, mountains, landscapes,…)
  • set it at greater distances to get closer subjects in focus; you will notice that as you move closer to your subject the bigger enlargement of it you will get.
  • set it at twice the focal length (30 cm) to have your subject image projected life-size onto the film/sensor (there is a rule of optics behind this regular behaviour, of course). This life-size (1:1 subject to image ratio) is a typical achievement in macro photography and you will also notice that the subject to lens distance will be the same as the lens to film/sensor distance (30 cm) at this point.
  • set it further off the film/sensor (i.e. >30 cm) in order to get progressively closer focusing distances and more than life-size reproduction ratios. For such kind of enlargments you will end up working with your subject placed between 1 and 2 focal lengths in front of your lens.

Two clear examples more:

1. Focusing distant subjects

Lens is close to the sensorMountain - a far away subject
We use here an old-style bellow camera to show you how things work: you can see the lens in front of the bellow and the sensor plane at the back side. Focusing is manually set by changing the sensor-to-lens distance.

General purpose lenses are designed to offer the best optical performance only when focused at medium-to-long distances. That is, they are good at photographing subjects like: the moon, distant landscapes, family holidays at the seaside, birthday cakes and party guests, corn fields, trees and any other general view.

When focusing on such distant subject, the lens is positioned very close to the sensor.

The further the subject from the lens, the nearer the lens to the sensor. Any subject focused at infinity, just like the moon or a mountain, will cause the lens to be placed at the minimum possible focusing distance from the sensor for that lens. Such distance is equal to the focal lenght of that lens.

2. Focusing close-by subjects

Lens is far from the sensorFlower - a very near subject
What we wrote above is a basic rule of optics and the interesting thing is that, predictably, the reverse applies when the subject is instead focused close to the lens: the sensor will have to be placed at a remarkable distance from the lens. That’s the way optics goes.

When focusing a nearby subject, the lens is positioned far from the sensor.

As written above, general purpose lenses are designed to offer the best optical performance only when focused at medium-to-long distances: that is, the best optical performance is attainable when there is much distance in front of the front lens and small distance in front of the rear lens. Such a behaviour leads to a simple thought: why not reversing the lens, a normal, plain, prime lens, in order to get the best out of your lens when shooting macro? This way your lens is optimized for greater-than-life size shots.

Now, it is fairly clear that bellow extension is a key factor in focusing your subjects.
But there’s a catch…

There’s a dim light at the bottom

Have you ever noticed how the amount of light rapidly decreases when entering a tunnel?
The further you go, the darker it gets.

Just a small amount of light travels across a long tube — photo by CatDancing

Light hardly reaches the inner part of the tunnel ©CatDancing


Be it a dark gallery, a cave, a deep well or a long corridor, available light amount falls not too far from its aperture.
And it’s just about the same with camera bellows!
With an added issue: photographic apertures are usually quite narrow.

Exposure compensation

Such a light drop, due to very extended bellows (itself due to very close focusing), usually results into underexposed photographs, contrasting which requires some exposure compensation.
The reason you “loose” light is that you are using less of the image circle than your lens f-numbers are adjusted for.

How much should one compensate?

Checking magnification and bellows factor on a view camera ground glass

As a rule of thumb, bellows factor is to be taken into account when bellow extension is more than 8 times the focal length. Beyond that threshold distance, the amount of´╗┐ light reaching the film/sensor decreases too much, the actual f-stops of your camera will not be accurate any more (in that they are calculated for long distances focusing), and you will need to compensate by exposing a little more (changing time, as opposed to aperture, preferably).

There are many methods for this, the easiest (if you are using a view camera, which you probably aren’t) being to measure the magnification on the ground glass and then make some calculation.
Or you can get your QuickDisc instead.

Bellows factor in digital cameras

But then, are these issues of any interest for the average digital photographer?
Not quite.
On one hand digital photography got rid of the ground glass, making it impossible to actually measure magnification while shooting.
On the other hand a couple of test shots and a quick look at the histogram should be enough for proper exposure settings. The in-camera histogram is indeed the best exposure meter one could dream of, and should be extensively used.

Histogram is one of the most practical ways to meter light in digital photography




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