The mysterious depth-of-field preview button; why is it still with us?

I’m sure that many photographers will agree that modern DSLR cameras have become burdened with too many buttons, switches and knobs and would like to see a number of controls culled in pursuit of a less cluttered design. A prime candidate for this purge has to be the one hidden away on the front of the camera body, the obscure little button that seldom feels the warmth of a human finger, the depth-of-field preview button. However, before we imagine a world with one less switch, maybe we should first explore what it actually does.

Centre of frame: the mysterious depth-of-field preview button.

The peculiar thing about the depth-of-field preview button is that, often, absolutely nothing happens when it’s pressed. In this instance, the button’s lack of participation is a consequence of the camera’s aperture being set to its largest possible value, for example, f/1.8 if you are using a 50 mm f/1.8 lens.

Depth of field, the matter of importance for the isolated button, is inextricably linked to the size of the aperture opening and to see the button at work requires something other than the largest possible value. For instance, pushing the depth-of-field preview button once the lens has been stopped down to an aperture value of f/16 will have an effect; it will cause the aperture diaphragm to close to its f/16 position for as long as the switch is depressed.

An illustration depicting the relationship between depth of field and aperture values.

Obviously, the fact that there is any effect at all at least shows that the depth-of-field preview button is operational, but it also opens up a whole new set of questions. First, How is the depth-of-field preview button useful to photographers? and second, Why, if the camera’s aperture is set to f/16, does the aperture diaphragm only assume its f/16 position once the depth-of-field preview button is pressed?

The easiest way to answer these questions is to look back in history. Not too long ago, setting a camera’s aperture required rotating an aperture ring found on the barrel of the lens. In much the same way as one might manually focus a camera today, adjusting the aperture required turning the aperture ring through the various aperture values. The system was entirely mechanical with no meddlesome electronics, which meant that once a value had been selected and the aperture diaphragm was in position, it remained that way until a sentient being decided that it needed to change.

A retro camera lens with a mechanical aperture ring. The f/2 to f/5.6 markings are visible, with the small black line on the silver ring indicating that the aperture is currently set to a value of f/2.

While some photographers, especially those involved with time-lapse photography*, may look back on the old aperture ring with fondness, the system was not without its flaws. Mechanically fixing the aperture meant that as users dialled in smaller and smaller aperture values, the brightness of the image seen through their viewfinder diminished accordingly. A smaller aperture opening means less light entering the system and the viewfinder is nothing more than an extension of that very same system.

To work around this problem, photographers who use these older lenses compose their photos with a wide aperture value, allowing for the best possible sighting, stopping down to the desired narrower aperture for the purpose of taking the picture. It’s an elegant solution to the difficulty of having too little light in the eyepiece and it is precisely the same approach that modern cameras seek to automate today.

An electronically controlled aperture diaphragm in its closed position.

The contemporary DSLR holds its aperture diaphragm at its widest possible setting, letting in as much light as it can, only shrinking down to the photographer's desired aperture value a moment before the shutter curtains open. Pressing the depth-of-field preview button simply overrides this neat feature, forcing the aperture to assume the set position, forfeiting the increased viewfinder brightness.

The fact that the depth-of-field preview button gets almost no attention at all and finds itself in an exceedingly awkward position on the camera body, is testament to just how well the automation scheme works. It also brings to the fore the question of why the button still manages to find its way onto camera bodies at all.

The answer is that by the camera holding the aperture in its wide-open position, the view through the viewfinder is bright but also shows the image with the shallowest depth of field. If, for example, you wish to see how far back the sharpness in your photo extends, the scene in the eyepiece is all but useless. Press the depth-of-field preview button, though, and the camera provides the correct aperture setting, leaving the viewfinder to show a potentially dark but entirely accurate representation of the actual depth of field.

An electronically controlled aperture diaphragm in an open position.

In practice, I personally don’t know anyone who uses the depth-of-field preview button as its designers intended. The modern DSLR affords photographers the opportunity to shoot, review and shoot again and, as such, it is significantly easier to simply guess at which aperture value will give you the desired depth of field and to adjust that value on subsequent shots should it all go wrong.  

Nevertheless, even if we don’t use the depth-of-field preview button an awful lot these days, it would be an unspeakable injustice if a scientific apparatus such as the modern DSLR left any one of its settings entirely up to trial and error. Adopting the lottery approach to depth of field certainly works but removing the preview button would be a lot like losing a baby toe; you’d be just fine without it but you sure wouldn’t be whole anymore!

Aside

When shooting in Live View, cameras do everything possible to ensure that the image displayed on the screen is an accurate representation of the final photo. This means that when a user is in Live View shooting mode and presses the depth-of-field preview button, the camera compensates for the loss of light in the system by artificially increasing the brightness of the image on the screen. Although I have never seen this method adopted in the field, it is, without question, the most accurate representation of a photo one can get without actually taking the picture.

Footnote

* Small variations in aperture that occur with today’s electronically controlled systems are responsible for an unwanted flicker in time-lapse sequences. Mechanically controlled aperture diaphragms do not suffer from the same problem.

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