What is depth of field exactly?
Depth of field is the phenomenon where a part of the photo in front of and behind the focus point is also sharp. The amount of depth of field is determined by the lens used, the focus distance, and the aperture. The longer the focal length, the smaller the depth of field. The closer we focus, the smaller the depth of field (think macro!). And the smaller the aperture opening (thus the larger the f-number), the greater the depth of field.
Portrait photo, taken with a Panasonic G6 and 100mm. Due to the relatively long focal length, the background is quite out of focus. This is usually the intention with portraits.
In landscape photography, you often want to have maximum depth of field so that the entire landscape is sharp from front to back. Many landscape photographers use an old school trick for this. They focus (manually) about one third from the bottom and then use a small aperture like f/11 to ensure everything falls within the depth of field. I call this 'old school' because you used to read in photography books that the depth of field always neatly follows that rule: one third of the depth of field would be in front of the focus plane and two thirds behind it. However, that is a myth. For some combinations of focal length and focus distance, it may be correct, but certainly not always.
Let's take a look at the depth of field of a 24mm lens, popular for landscapes. To determine the depth of field, I use the handy PhotoPills iPhone app. If you focus at 1 meter and use an aperture of f/8, the depth of field ranges from 71cm to 1.69m. That aligns quite well with that 1/3 in front and 2/3 behind. But if you focus at 2 meters, the depth of field ranges from 1.21 meters to 11.32 meters, and that old wisdom no longer holds. When focusing at 3 meters, the depth of field ranges from 1.34 meters to infinity! In short, the depth of field increases much faster towards the back when you also focus a bit further back.
A little jellyfish on the beach. Despite using 24mm and f/11, the depth of field is very shallow. This is because the focus had to be set so close.
Bizarre landscape in the White Desert, part of the Sahara (Egypt). With 21mm and f/13 everything is sharp, but where do you set the focus?
Hyperfocal distance
From the previous little test, we see not only that the textbook trick doesn't apply, but also another interesting phenomenon. If you want everything from, say, 1 meter to infinity to be sharp, you shouldn't focus at that distance of one meter, but a bit behind it. In that sense, the textbook wisdom is somewhat correct. Because part of the depth of field falls in front of the focus distance, you can achieve a depth of field that runs exactly from one meter to infinity. But where exactly you need to focus and which aperture you should use follows another rule of thumb. We call this the hyperfocal distance, but you can forget that term right away. Just remember how it works. Using the hyperfocal distance means you need to focus on 2x the distance you want to be sharp in front of you, so in this example, 2 x 1 meter = 2 meters. Then you choose an aperture that gives a depth of field extending back to infinity when focused at 2 meters. For that, we again use PhotoPills (or a simple depth of field table) and conclude that this is f/10. In practice, you set f/11. The depth of field now runs from 0.92 meters to infinity.
Harbor in Lanzarote (Canary Islands) By using the 'hyperfocal distance setting', everything is sharp from front to back (20mm, f/11).
Smaller sensor
The used 24mm focal length in our story is of course based on a full frame sensor. If you use a camera with a so-called crop factor (a smaller sensor than full frame), then 24mm is already a bit too long for landscape photography. But what about depth of field? Of course, the depth of field of a 24mm lens does not change when you use such a lens on a camera with a smaller sensor. However, there is a catch that you need to take into account. If you use a camera with a crop factor, you will practically need to use a correspondingly shorter focal length to achieve the same field of view. With a camera that has a 1.5x crop factor, like many consumer digital SLRs, you would then use a 24mm / 1.5 = 16mm focal length, and with a Micro Four Thirds camera (2x crop factor) even a 12mm focal length. And that does ensure that you get a greater depth of field, because we have already seen that the shorter the focal length, the greater the depth of field.
For landscape photography, this is usually not a problem; on the contrary. If you want a lot of depth of field, you are at an advantage with a camera with a smaller sensor. Just do the math: if you want everything in focus from 1 meter to infinity with a 16mm focal length, focusing at 2 meters at f/5.6 is more than enough. In this case, that means two stops less than full frame. For wildlife and bird photographers, who want such a nice blurry background, it’s a different story. You might think that a Micro Four Thirds camera with a 4.0/300mm is the ultimate for wildlife photography due to its crop factor. As if you are using a 4.0/600mm lens on a full frame camera! But then you forget the depth of field effect of that 300mm. In terms of depth of field, it is not like a 4.0/600mm on full frame, but like an 8.0/600mm. That makes a significant difference when it comes to a blurry background. Keep that in mind before you hastily swap your equipment!...
An elephant portrait taken with 600mm f/4.0 on full frame. With a 300mm f/4.0 on Micro Four Thirds format, you would get the same image crop, but not the same shallow depth of field.
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This article about depth of field was written by Johan Elzenga and published in digifoto Starter 2.2018. Do you want to receive the basic courses and other interesting articles and reviews in your mailbox from now on? Then take out a subscription to digifoto Starter. You can also easily reorder individual editions in our revamped webshop.
