Your choice of aperture can make a big difference to the success of an image, as it affects the depth of field (DOF), i.e. the amount of the photograph that appears acceptably sharp from front to back. You may want front to back sharpness in a large landscape, so you would use a small aperture, such as f22.
Or you may be taking a portrait photo, and you want to throw the background out of focus, in which case you’ll choose a wider aperture, like f5.6. And in close up photography depth of field is shallower, so the choice of aperture can become really crucial.
However, it’s important to remember that when you look through your viewfinder, what you see is NOT what you get, in terms of depth of field! You can change the aperture setting all the way from the widest to the smallest, and the image in the viewfinder will stay the same. This is because the viewfinder image is always seen at the lens’s widest aperture, regardless of the aperture which has been set.
This is where the depth of field preview button comes into its own. When you hold the button down, the aperture will close down to the one that you’ve selected, so you will see accurately what will and will not be sharp in your photo. This sounds as if it solves all the problems – but there is a downside.
The smaller the aperture that you choose, the darker the image in the viewfinder will become when you hold down the depth of field preview button – so although it’s giving you an accurate image, it might actually be too dark to see it! It can be a really useful feature though at wide and mid-range apertures.
To overcome this, use live view mode on the LCD screen. Pressing the DOF preview button, while looking at the image on the back of the camera, will clearly show what parts of the image are sharp.
If you don’t have a depth of field preview button, and you’re concerned about how much of the image will be sharp from front to back, then try taking a series of photos at different apertures. This is definitely one of the benefits of digital – it would have seemed very wasteful with film!
When I photographed this delphinium bud, I wanted to keep as much sharpness as possible in the bud without any of the flower behind it coming into sharp focus. In this situation the depth of field preview button definitely helped me to find the best aperture for the shot.
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