One topic that I have discussed during many classes that I instruct, and defined to many people that have come to my shop when they think they have are lens or focus problems, and more than once written about is “depth of field”, but it still seems to be an elusive concept for many.
I pondered about this last Thursday when a local photographer showed me an 8×10 print from photographs he had made inside a church during a wedding the previous weekend. He showed it to me proudly, but commented he wished that his lens was sharper at its wide-angle focal length.
The print displayed a view down the central isle with church pews left and right, leading to a pulpit in the distant centre, likely about 30 feet from where the photographer was standing. The overall exposure was fairly good, and he told me he had shot at an aperture of f/2.8. However, the fact that his photograph wasn’t in focus, except for the pulpit, had very little to do with using his zoom lens at its wide-angle focal length.
The definition of depth of field is “that area around the main subject, in front of and behind, that is in acceptably sharp focus”. In application the wider the lens’ aperture is the less the depth of field, or that area of sharp focus, around the main subject will be.
Wide aperture lenses are very popular these days and using a lens at a wide aperture like f/2.8 when making a portrait isolates the main subject and produces a soft, out-of-focus background by reducing the depth of field.
Using a wide aperture can increase the exposure in limited lighting conditions, but along with the benefit of additional light reaching the camera’s sensor the resulting effect is reduced depth of field. Creating a field of focus behind the subject of twelve inches or so might look really good when making a portrait, but in that photograph of the church isle with pews on both sides and the distant pulpit, everything in the foreground, from the photographer to the pulpit, looked out-of-focus.
Many photographers unwittingly rely too much on their photography equipment to (magically?) make good images and blame faults in their photographs on that same equipment. Understanding the basic concept of depth of field would have made that photograph work easily.
I have photographed weddings in that same church and instead of relying on a wide aperture to bring the necessary light into the dim environment, I use a flash and an aperture opening that will give me lots of depth of field so the foreground as well as the background will have reasonable sharpness.
Yes, subjects closer to the flash will be brighter and those further away will be darker, but I use an off-camera flash cord and hold the flash at arms-length above my head, and direct the light over and above the closer pews and onto more distant subjects. With film this technique is more hit and miss, but with digital technology I just check the camera’s LCD (liquid crystal display) and make any light intensity corrections on the flash menu. I can also make further corrections to the image in seconds in postproduction using PhotoShop.
The smaller the lenses aperture is the more the area of focus, or depth of field, will be. I prefer using a small aperture for scenic photography and, as in this instance, interiors. In both types of photographs I am concerned with all elements in the photograph, front to back, of being sharp and in “acceptably sharp focus”.
Assuming the lens isn’t sharp when the real problem is with photographic technique can get expensive if the photographer goes so far as replacing a lens. My recommendation is to spend time learning the basics of photography like depth of field instead of blaming equipment when problems occur.