Mullimburra Point, NSW, Australia. Phase One XT 150MP with Rodenstock 32mm lens, f11 @ 1/30 seconds, ISO 50. With wide-angle lenses like this, you may keep both the foreground and background in sharp focus, but the higher the resolution and the longer the focal length, the harder it becomes.
Depth-of-field is a wonderful thing because it ‘appears’ to extend the area of sharp focus. Strictly speaking, there is only a single plane of sharp focus, but depth-of-field is that area behind and in front of the plane of sharp focus that ‘appears’ to be sharply focused as well.
High resolution photographers are sometimes disappointed when they look at their files at 100% because parts of the image aren’t as sharp as they’d like. However, if you view that same file at, say, 20% or 10% resolution (like you’d view an image on social media), it looks perfectly sharp! What’s happening?
Depth-of-field relies on the human eye’s limited ability to see details. Areas that are slightly blurred can appear perfectly sharp to us. And when viewing an image, the smaller it is or the further away we are, the better depth-of-field seems to work. The first observation is that you shouldn’t always worry if areas in your image seem slightly out-of-focus because, if you reproduce them at smaller sizes, they will look perfectly fine. However, what if this image is to be produced as a large print which people will inspect up closely? What do we need to know?
Depth-of-field will continue to work, but not as extensively. Because high resolution sensors resolve more detail, you don’t get as much bang for your buck when it comes to depth-of-field. This may require you to use a smaller aperture to compensate, or you may need to focus-stack the image to achieve the degree of sharpness you require.
And there are situations where you simply can’t get the amount of depth-of-field you want, so you have to make a decision about where you focus.
As an example, landscape photographers like to focus part way into the scene, perhaps on the hyper focal focusing point where depth-of-field is supposed to extend out to infinity. What you can find with high resolution sensors is that depth-of-field doesn’t actually extend as far as you expected, so the solution is to focus further into the scene to ensure the horizon is also sharp. The consequence is that less of the foreground is visually sharp – it’s a compromise.
Note, this isn’t a reason for not using a high resolution sensor. Exactly the same phenomenon happens with lower resolution sensors, it’s just that we can’t see it. If we use our high resolution sensor images in the same way that we’d use the lower resolution sensor photos, we are no worse off. The problem only become apparent when we enlarge our images – which is why we’re using a high resolution sensor, of course!