Whether you have a camera with an elaborate focus system like the Canon 1DX Mark II or Canon 7D Mark II or an entry level camera like a Rebel T5, focus works the same. Knowing the basics of focus can help you understand how to ensure multiple subjects or multiple people remain in focus. It comes down to understanding depth of field and the focus plane. These are two concepts that work hand-in-hand to determine focus and can be fully within the photographer’s control.
First let’s look at the focal plane as it’s much simpler to examine. If you point the camera at a person and acquire focus, the focus point or area that is in focus is the focal plane. Imagine it as a flat plane perpendicular to the camera’s lens or image sensor. Everything on this plane is in focus, depending on depth of field which we’ll discuss next. Anything in front of or behind the focal plan can become out of focus. In other words, it is best to keep subjects on the same focal plane to remain in focus.
Depth of field can either be a photographer’s best friend or worst enemy. It’s mostly associated with “professional photos” and almost always seen from DSLR photos. It can define someone’s style, but it can also be a sign of an amateur compensating for knowledge and skill by over-using shallow depth of field — but that’s another topic entirely. Depth of field (DoF) can be wide or shallow and this refers to the area in focus.
To understand what DoF is all about, imagine a sphere. The center of this sphere is your focus point or the subject in focus. The further from the center an object becomes, the more out of focus it becomes. However, the area that remains in focus can be wide (wide depth of field) or shallow (shallow depth of field). A wide depth of field could indicate that the are that remains in focus from the center is large. you may not lose focus until something moves to the edges of the frame, for instance. A shallow depth of field can present an area of focus so tiny that moving just a few inches from the center and the subject is blurry. The good news is that by adjusting exposure and focal length, depth of field can be controlled.
One way to control DoF is by adjusting the aperture. A wider aperture, represented by a smaller number called an f-stop (for example, f/2.8), will produce a shallower depth of field than a narrow aperture such as f/8. This can be set in camera either by switching to AV Mode (Aperture Priority Mode) or manual mode. The adverse effect of this is that a narrower aperture like f/8 will not capture as much light as f/2.8. This means that by widening the depth of field, the image becomes darker unless shutter and ISO are also adjusted. This is where understanding the 3 components of exposure will assist in capturing the best possible image.
Another way to adjust depth of field is through the focal length. A longer focal length, or zooming in, will cause a shallower depth of field than a wider shot. For instance, more of an image will remain in focus at 18mm than one shot at 200mm. Shooting at the widest angle possible isn’t always the best option either, though. Keep in mind that various focal lengths can cause changes in perspective and also distortion. For instance, taking portraits at 18mm can be unflattering to the model or subject as it may stretch portions of the face.
Mastering focus, especially for multiple subjects, is a balancing act. Aperture and focal length need to be kept in mind to maximize the depth of field for the area your subjects are in. Too narrow of an aperture will require a higher ISO (more noise) or slower shutter (more motion blur) — or both. Choosing the right focal length is also part of the balancing act. Too wide and the subject may appear distorted, but too close and the depth of field becomes narrow. All of this must be considered to find the ideal depth of field. It’s also important to keep in mind the focus plane as well.