So you just used your whole paycheck to get this extremely fast, wide aperture lens of your dreams.
But what in the world..?! You’re not happy with the results. They’re not as sharp as you expected for this price – there must be something wrong with the lens! Hold on now, let’s take a step back and see if it’s possible that you’re not using the lens like you should.
Fast lenses are great for a lot of things, shooting in low light being one of the major advantages, because the wider aperture allows more light into the imaging sensor, which in turn allows you to use a faster shutter speed. This is why lenses like the 70-200mm f/2.8 are such a workhorse in a photojournalist’s camera bag. Another advantage to the wider aperture is the ability to create shallow depth of field in your image, which can make your subject really stand out from the background. The85mm f/1.4 is one of my go-to lenses for almost any portrait situation for just that ability.
Understanding Depth of Field
It’s very important to understand that depth of field works differently dependent on the lens you are using. For instance, a lens such as the Nikon 85mm f/1.4 will have paper thin depth of field when used at f/1.4, at close distances to the subject. As the subject moves further away from the camera, that depth of field becomes a little greater. But at the minimum focusing distance, you could focus on an eyelash on your subject, and still have the eyeball be out of focus, despite the fact it’s only half an inch behind the eyelash. Assuming you don’t want to back up and change the framing on your subject, the best way to ensure sharp focus on the eye, is to choose a focus point on the eye and be careful it does not accidentally focus on an eyelash.
If you are willing to sacrifice a bit of that shallow depth of field, simply stop down a bit to give yourself some leeway on where you focus. While at f/1.4 or f/1.8 you may not be able to get both the eyelash and the eye in focus, but at f/2.2 or f/2.8, you’ll likely have enough depth of field to achieve sharp focus on both.
Unfortunately, it’s not always possible to focus at the hyperfocal distance. For instance, photographing at a party in a dimly lit room, you may notice some areas out of focus due to the shallow depth of field. The fix here is to stop down a bit if you can. If you need more light, consider using a flash if appropriate, and stopping down the lens to give greater depth of field. Another option is to raise the ISO a bit to allow you to stop down the aperture more.
Choosing Your Focus Point
One of the best things you can do as a photographer is take control of where your camera is focusing by selecting the focus point that you want, and ensuring that the focus point you choose is on the subject you want to be sharp. Cameras today have multiple focus points, with some having as many as 61 AF points. While it’s true that generally speaking the center point will be the most accurate of those points, technological advances have made the points along the outer edge much more accurate than in the past. This means that you can choose those outer focus points with confidence when composing your image.
Another related problem to the camera choosing the wrong AF point, is photographers employing a technique known as focus and recompose. This technique came about back when cameras only had a few AF points bunched around the center of the viewfinder. In many situations, it’s not a problem, as long as you are using an aperture that will provide adequate depth of field to maintain focus on the object or person you’ve focused on.
However, when using a fast lens, at a wide open aperture, focusing and then recomposing your shot becomes a real problem. This is because when using a fast lens at a wide aperture, the depth of field is so thin, that recomposing the shot will actually shift the plane of sharp focus away from the subject you initially focused on. So while you may have focused on the correct subject, using the center AF point, in adjusting your composition you knocked your subject right out of focus again. The solution for this issue is the same as above: manually select an AF point that you can place right on top of your subject, without recomposing your shot.
Embrace the Bokeh
Lastly, in answer to those who bought fast glass to shoot it wide open, I say –embrace the bokeh! Bokeh is defined as the visual quality of the out of focus areas of an image, and each lens renders these areas a bit differently. Fast lenses typically have beautifully smooth bokeh.
Read the full article over at Digital Photography School.
Source: Digital Photography School
