Depth of Field Preview: What the Button Does

What the depth of field preview button does

When you look through an optical viewfinder, the lens aperture stays wide open, regardless of the aperture you selected. This design keeps the view bright and makes manual focusing easier, but it also hides what the final photo will look like. The lens only stops down to the chosen f-stop at the moment of exposure. Press the depth of field preview button and the camera closes the aperture blades to the set value, so the image in the viewfinder darkens and the region of sharpness shifts to match the photo you will capture.

The button is most useful on DSLR cameras and other optical-viewfinder models, because the optical path shows the true lens opening. On mirrorless cameras, the electronic viewfinder and rear screen can display a live preview of the exposure and depth of field without moving the physical aperture, though many models still include the button as a physical control. You may also see the function assigned to a custom button on cameras that lack a dedicated one.

The core relationship between the aperture and the zone of sharpness is explained in the aperture guide. The preview button simply makes that relationship visible before you press the shutter.

Why the depth of field changes with aperture

Depth of field, the distance between the nearest and farthest points that appear acceptably sharp, expands as the aperture narrows. A landscape photo at a small aperture can keep the foreground and the distant ridge in focus, while a portrait at a wide aperture isolates the subject from the background. The preview button shows you the extent of that zone before you shoot, which is especially valuable when the difference between sharp and soft is critical to the image.

For a quick refresher, the lens aperture and depth of field article walks through the math and the creative uses. The button does not change the exposure; it only shows you what the aperture is doing. It also affects the bokeh, the quality of the out-of-focus areas, which is discussed in the guide to aperture blades and bokeh. Many photographers use the preview to check whether the blurred background is as smooth as they want it.

How to use the depth of field preview button

Find the button near the lens mount on most DSLR and mirrorless bodies, often labeled with the letters DOF or a small aperture icon. Press and hold it while you look through the viewfinder. The image dims at small apertures, but you will see the focus range expand. Release the button to return to the bright viewing state.

It can take a moment to read the scene with the darker viewfinder, particularly at f/11 or f/16. Practice by focusing on a near object with a background at a distance, then press the button and watch the background become clearer as you stop down. This is the same effect that the hyperfocal distance technique relies on, and landscape photographers often use the preview to confirm that everything from the closest rock to the far peak is within the sharp zone.

The preview is also useful in macro work, where the depth of field can be only a few millimeters. For portraits, press the button to see if the eyes are sharp while the ears fall off, a decision that matters more with the wider apertures common in full-frame cameras.

• Set the aperture to your intended f-stop, for example f/8 or f/2.8 depending on the scene.
• Frame and focus the shot as you normally would.
• Press and hold the depth of field preview button while you inspect the viewfinder.
• Note how much of the scene is sharp, then release and adjust the aperture if needed.

When the preview makes the viewfinder too dark

At small apertures, the viewfinder image can become quite dark, especially indoors or in low light. The preview shows a true representation, but it can be harder to judge composition when the image is dim. Some viewfinders, particularly on modern DSLRs, compensate by boosting gain in live view, and mirrorless cameras simulate the depth of field without the same loss of brightness. If the scene is too dark to evaluate, widen the aperture or move to brighter light before relying on the preview.

Cameras with electronic viewfinders or rear-screen live view offer a simulated depth of field preview, often automatically as part of the exposure simulation. The optical vs electronic viewfinder guide compares the two approaches. On many mirrorless models the simulation is not perfectly accurate at the extremes, so checking the final image after the shot is still a good habit.

Which cameras make the preview easiest

The depth of field preview is a standard feature on most interchangeable-lens cameras from DSLRs to mirrorless bodies. The usefulness depends less on the brand and more on the viewfinder type. A bright optical viewfinder makes the stopped-down image easier to see, while a high-resolution electronic viewfinder simulates the effect without dimming. If you use the preview regularly, look for a camera with a dedicated button or a customizable control.

The best DSLR cameras list compares optical-viewfinder models, and the best mirrorless cameras list covers electronic-viewfinder options. For specific needs, the best APS-C cameras and best full-frame mirrorless cameras guides include models with strong viewfinder systems.

What to pick for your use

Questions

Is the depth of field preview button the same as the aperture button?

Effectively yes. The button physically stops the lens down to the aperture you have set. It does not change any exposure settings; it only lets you see the depth of field at that aperture.

Does pressing the depth of field preview button hurt the camera?

No. The aperture mechanism is designed to open and close many times during normal shooting. Pressing the button just holds the aperture in the stopped-down position for as long as you keep it pressed.

Why does the viewfinder get darker when I press the button?

Because the lens stops down to a smaller opening, less light reaches the viewfinder. This is the same amount of light that reaches the sensor during the exposure, so the darker view is an honest representation of the final image.

Do mirrorless cameras need a depth of field preview button?

Mirrorless cameras can simulate depth of field in the electronic viewfinder or rear screen without moving the aperture, so the button is optional. Many mirrorless bodies still offer it as a physical control or as an assignable function.

Should I hold the button while taking the photo?

No. The camera stops the aperture down automatically at the moment of exposure. You only hold the button to preview, then release it before pressing the shutter.