In this short blog I will give a non-scientific layman terms explanation of how I see the focal plane in photography. You can find more technical and advanced explanations out there, but this one focuses on what you need to know to make use of the focal plane.
A big umbrella
The focal plane is the plane where your lens and camera in combination has your subject in focus. So when you zoom in and out, different things will appear sharp in your viewfinder. If you take a picture of a person, you risk that the eyes are out of focus if you focus on the ears or the nose. Eyes are super important in portrait photography, and hence all portrait photographers and advanced auto focus systems make sure to zoom in on and detect the eyes. It is less of a problem if the nose or ear is slightly blurred or out of focus.
You can think of the focal plane as one big virtual umbrella that you push back and forth as you turn the focus ring on your lens. Exactly where the fabric from the umbrella touches the subject, the subject will be sharp.
In the picture below, you can see the glasses that I have put in focus are almost the only thing in focus. Both before and after the glasses, everything is out of focus:
Only one focal plane
It is important to understand that there is only one focal plane. No matter how advanced a camera you have and how much intelligence there is in the auto focus system of your camera, the physics and mechanics do not change. There is only one focal plane. You can only push one “umbrella” back and forth. You can do this with auto focus or manual focus, but there is only one.
I say this because when you see or read some of the camera manufacturers adds for the latest and greatest in auto focus technology, you get the impression that the intelligent systems have overcome all focus and sharpness issues. They have not, because they cannot fundamentally change the fact that only one plane can be in focus at a time.
There is some good news…
The good news – and what has saved me a lot of times – is that a little before and some after the focal plane, there is an additional area where the subject appears to be sharp. This is known as the depth of field. Good news is that you can increase (or decrease) the depth of field if you change the aperture. A small aperture (large f-stop number) gives large depth of field. In other words, there is some room for slack if you have a small aperture (large f-stop number like f/16) .
There is no such thing as a free lunch in photography. So the price for a small aperture is that it lets in very little light, which means you need to bump up either the ISO or leave the shutter open for a longer time or both. This introduces the risk of camera shake and subject blur, meaning your pictures will appear not-so-sharp anyway. So you have to strike a balance, find a compromise. General advice is hard to give in this regard, but try to start out with a middle-of-the-road aperture (say f/5.6) and work your way up or down until you find a good compromise.
Depth of field is also depends on your distance to the subject, so the closer you are to your subject, the more shallow the depth of field will be in absolute terms. This is what causes a lot of headache for for example flower photographers. This is where focus stacking comes into play, but that is the subject for a different blog.
Questions and comments
Thank you for reading this far. I hope you found this blog useful. Questions and comments (and likes!) are more than welcome!