Have you ever wondered what the button next to the lens mount on your DSLR is for? Not the one that you use to detach a lens, but that other one? That’s the depth of field preview button. Let’s dig in.

Depth of Field Redux

The depth of field of an image is how much of it is in focus. An image with a shallow depth of field, like this one, has very little except the subject in focus.

An image like this one with a deep depth of field has pretty much everything in focus.

You control the depth of field by setting the aperture on your lens. A wide aperture gives you a shallow depth of field, while a narrow aperture gives you a deep depth of field. A wide aperture also lets in more light which means you need to use a faster shutter speed.

RELATED: What Is Aperture?

What You See Through the Viewfinder and the Depth of Field Preview Button

When you look through the optical viewfinder on your camera, you don’t actually see the scene exactly as it will look in your photographs. Since you’re viewing things live, you don’t see the affect of shutter speed in your images. You also don’t see the affect of aperture.

When you use an optical viewfinder, the aperture is set to the widest aperture of the lens. Only when you take the photo, does it close down to the aperture you’ve set. This is so you get as much light as possible through the viewfinder to make it easier for you to see what you’re photographing. But that comes at the expense of getting an accurate idea of the depth of field.

If you press the depth of field preview button while looking through the viewfinder, the aperture will close down to the value you’ve set. This gives you an idea of what the depth of field for the scene will be, but everything will appear much darker. I’ve simulated the effect in the image below.

Notice how without the depth of field preview, the image is bright but the foreground and background are blurry while, with the depth of field preview, the image is darker and harder to see but the foreground and background are sharp.

Electronic Viewfinders and Live View Screens

The depth of field preview button is a throwback to the days when the only way you could preview what you were photographing was through the viewfinder. They were very important for landscape photographers and portrait photographers working with groups who wanted to check they’d manually focused correctly. That’s why it still has such a prized place on DSLRs.

Now, however, you really don’t need the depth of field preview button. Both electronic viewfinders (which you find on high end mirrorless cameras) and live view screens (which are on almost all cameras) show an accurate preview of the depth of field without the darkening you get with an optical viewfinder.

Although I mainly shoot landscapes at the moment, I’ve never used the depth of field preview button in anger. Instead, I use the live view screen as it makes everything much easier.

RELATED: How to Take Good Landscape Photos

While the depth of field preview button is pretty obsolete, it’s hard to figure out what it does on your own since you can only see its effect when you’re looking through the viewfinder with your aperture set to a low value. Now you know.

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Harry Guinness is a photography expert and writer with nearly a decade of experience. His work has been published in newspapers like The New York Times and on a variety of other websites, from Lifehacker to Popular Science and Medium's OneZero.
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