Have you ever noticed the small button near the lens on your camera? Do you know its use? Well, it certainly does not function to remove the lens out from the camera (as you would have imagined) but this button is actually known as depth of field or DOF preview button.
What is DOF?
Depth of field is the distance between the areas in the photo that look clear and sharp. Typically, the image is said to have deep depth of field if the picture is crisp and the main subject is in focus. The pictures that have only the main subject in focus and the background is dull have shallow depth of field.
Depending on the DSLR camera model you are using, the DOF preview button is usually placed adjacent to the lens. In most of the models it is placed strategically so that the photographers can easily press it with their thumb so that they can easily remove it without any hassle.
The best way to know how the button works is to set to the aperture on the priority mode. Begin with widest maximum aperture setting possible and then press the depth of field preview button. Increase the aperture by gradually increasing the f-stop and press the DOF button every time you increase the f-stop and see the effect on the image.
When you close the aperture, you may notice that the scene that you see through the viewfinder gets darker. However, when you click the picture, the resulting image wouldn’t be dark. This is because the camera slows down the shutter speed to make-up for the narrow aperture setting and thereby gets the perfect exposure.
Helps in detecting hotspot
Often times, while doing a portrait photography or taking close-up shots, one of the major concerns faced by photographers is the bring background. Bright spots can be disturbing and affect the quality of the picture but more often than not they go unseen while viewing through the viewfinder. To ensure that you get the best image output, it is best to push the DOF preview button, the image may become dark but it would help you to find the distraction hotspots easily.
Identify lens fare problems
While clicking outdoor pictures, especially during the day and when the sun is low, there is always a risk of facing lens flare problem. This happens because of the direct bouncing of the light around the lens. By previewing the image with the DOF button you can clearly detect the lens flare and do something to mitigate the problem like setting a wider aperture or use a shading to cover your lens. Such minor changes can make a huge difference to the final image.
Optimize the focus on the main subject
While shooting in the outdoors especially if you are doing wildlife or nature photography you can use the depth of field preview button to ensure that the subject you want to click is in focus. It is greatly helpful if you want to click a close-up photo or if you want to highlight the minute feature of the subject like the eye of the animal or the stigma on the flower.
Detect exposure problems
By looking through the viewfinder and setting the lens one or two stops smaller than the widest aperture, press the DOF preview button to get a clear idea of how the image would look like if it is underexposed. Typically, most photographers reduce the exposure of the image to give the image a unique feel. Just as you can detect the problems of underexposure, you can also do the reverse and check what the image would look like if it is overexposed by pushing the DOF butting and opening the lens to maximum extent.
If your are shooting in a place that is not well lit or not so bright, the DOF preview button can hinder clear view of the things that you can see through the viewfinder. If you face any problem with your camera while shooting at particular situation, you can try pushing the button by changing your camera setting to the live view mode. Depending on the setting you have adjusted on your camera, the preview button wouldn’t darken the image in focus.
(Video courtesy : Ultimate PhotoGuide)