A short definition of what Depth-of-Field in photography is…
Acceptable sharpness
Depth-of-fields (DOF) is the area (depth) in which subjects in view of your lens appear to be acceptably sharp. The DOF area closest to you is called the DOF near limit, and the other end the DOF far limit. Within that interval, subjects appear to be sharp.
If you have not tried to focus manually, I encourage you to try so. Flick the little switch on the front left of your camera (typically) from AF to M, and try to turn the focus ring manually. Notice how different parts of the scene becomes sharp as you turn the ring. You are actually “pushing” the focal plane back and forth when you turn the focus ring – the focal plane being where you as photographer decide the image is to be sharp, but there is an element of “forgiveness” prior and after the focal plane and that is the DOF.
DOF illustrated
I think of the DOF being around 1/3 prior to the focal plane (the focus point) and 2/3 after the focal plane, but this is not a technically correct way to see it, but if you like me just want to have a drivers license to what DOF is, then this is a good and operational way to think about DOF.
Your DOF depends on many factors such as the distance to the subject, the sensor size, most notably the aperture and the length of your lens. Most photographers work with the aperture to control the DOF, but you can also use the distance to the subject.
Macro photography
Macro photographers suffer from the fact that when you are super close to your subject, the DOF shrinks to almost noting, even if you pump up the Aperture to something crazy high – it won’t help, as the distance is so small that you get a paper thin DOF no matter what. Therefore many macro photographers use focus stacking, where you take several images and change the focus point, and then in post merge the sharp parts together to create an image with a larger DOF.
Portrait photography
Portrait photographers use a relatively small aperture to shrink the DOF so that the background becomes very blurred and hence does not take focus from the subject. You can also get this effect at a higher aperture (and hence DOF) if you just make sure that the distance from the subject to the background is by factors way bigger than the distance from the lens to the subject.
Although the name sounds very advanced and almost like something from a galaxy far far away, it is really very simple: to make the most of your dept-of-field (DOF).
Notice the “distance” in the term “hyperfocal distance” – it is all about the distance between your camera and the focus point. When you focus at the hyperfocal distance, everything between that point and to infinity is in focus. Or I should say, appears to be in focus. There is a lot of technical details here that I omit, but I want to give you a drivers license to shooting at the hyperfocal distance – not turn you into an engineer.
Your depth-of-field (DOF) is an interval before and after the focus point where things appear to be in focus. I normally think about it so that 1/3 of the depth-of-field (DOF) lies before the focus point, and 2/3rds lies after the focus point. This is not very accurate, but a good operational way to think about it. So when your focus point is so that the far end of the DOF just reaches infinity, then there is also a good part before the focus point that is in focus. If your hyperfocal distance for example is 10 meters away, then the space between the focus point and halfway back to you also appears to be in focus. In other words, only what is between you and 5 meters out will be out of focus. From 5 meters out and to infinity is in focus.
Calculations are not necessary
So should you calculate the (DOF)? You can if you want to, but what I do is to focus to infinity with manual focus, and then pull the focus point back towards me until infinity becomes out of focus, and then revert just a little bit until infinity becomes sharp again, and then I have the focus point hyper focal distance. If you have a mirrorless camera with focus peak highlights, this is a brilliant illustration of how your focus plane and the (DOF) works you can find the hyper focal distance using the method I just described, but supported by the focus peaking highlights.
Learning from old lenses…
On old lenses, there was markings showing the (DOF). The black dot just above the blue “11” shows that the lens is at f/11 aperture . The same blue color as the “11” is used for the (DOF) markings on the zoom ring. You can see that to the left, the blue mark is at infinity, and to the right the other blue mark is between 3 and 5, i.e. around 4, the precision is not that great. The focus point is between 5 meters and infinity (the tilted 8 to the left just above the black dot). The blue mark to the left is the far end of the (DOF) whereas the blue mark to the right is the near end of the (DOF).
DOF indicator on the Nikkor 50mm f/1.2
Is the lens at the hyper focal distance? Yes, it is as the far end of the DOF touches infinity. Had I focused closer, then infinity would be out of focus (which every portrait photographer knows), had I focused further away, I would have wasted DOF beyond infinity.
The distance is not linear
Also notice the distance scale: You can see it (in meters) top right is at 1.2 meters, then 1.5, 2, 3, 5 and infinity! So it is almost is if the distance “explodes” when you get beyond 5 meters, i.e. a very small turn on the focus ring gives a big jump in the distance. That is why it is vital that you get the far end of the DOF to touch infinity, because you then work with the part of the distance scale where you cover a lot of ground!
DOF calculator
If you put the above example into a DOF calculator (credit: Photopills) then you will get a hyperfocal distance of 7.42 meters, DOF near limit to the half of that and the DOF far limit to infinity. In this example we have got the most out of the DOF and only the distance from the camera and 3.7 meters out is out of focus.
DOF calculator from Photopills
Notice that the DOF depends on several factors, such as the sensor size, the aperture you shoot at, the focal length of your lens, distance to the subject etc. But no matter what DOF you are working with, the hyperfocal distance is the focus distance where you make the most of what you got.
Aperture is the iris of your lens. You can regulate the area that lets light pass, just like the iris in your eye. The larger the area, the more light it lets through. Typically on a modern camera, you can regulate the aperture via controls on the camera, but often you can also operate the aperture manually directly on the lens.
Just to confuse things, the aperture is measured in f-stop numbers, and the larger the f-stop the smaller the aperture.
Fast and slow
A fast and a slow lens. This is not a reference to sending your lens to the race track, which of course would be pointless, but rather it is about how much light your lens lets through to the sensor under a given set of conditions. Fast = lets through a loft of light. Slow = less so. Using a fast lens at open aperture can be used to highlight a subject due to the background blur that is achievable with a fast lens. So we all want fast lenses in the best of worlds. But there is no such thing as a free lunch. It comes at a price, literally. And not only your wallet feels the weight, also your camera bag will be burdened with more glass if you insist on a fast lens. And more so if you do so with a full frame camera body. But let’s have a look at your lens speed and what the impact is on your photography.
Aperture measures
Aperture is measured in f-stop numbers, so that the lower the f-stop number, the more light is let through. A very fast lens has an aperture of f/1.4, whereas a not-so-fast lens has an aperture of f/5.6. The aperture numbers are odd and difficult to remember, but it is a good thing to understand the scale, as walking one step up and down the scale is referred to as “stopping down one stop” and “stopping up one stop”. An amazing fast lens has a minimum f-stop of f/1.0. The next f-stop on the scale is f/1.4. If you just remember these two numbers, and that the f-stop scale is a doubling of these numbers for every stop, then you can remember the entire scale. The f/1.0 series is hence: f/1.0 f/2.0 f/4.0 f/8.0/ f/16 f/32 Similarly the f/1.4 series is: f/1.4 f/2.8 f/5.6 f/11 f/22 If you then weave these two series together you get the following f/1.0 f/1.4 f/2.0 f/2.8 f/4.0 f/5.6 f/8.0/ f/11 f/16 f/22 f/32 The series can be extended beyond the above, i.e. some lenses are below f/1.0, but it is rare. On an old lens like this Nikon 135mm, you typically find the aperture scale in the bottom of the lens (closest to the camera) with a ring to turn in order to change the aperture. Notice how the numbers on the lens match the f-stop numbers above. Here the lens is set at f/8: If you try to look into your lens with the aperture wide open, you typically get a very good idea about how fast your lens is. This 135mm lens can stop down to an (relative to the length of the lens) impressive f/2.8, and as you can see, this lens is all glass, that uses every inch and mm of the inside of the lens to let through light:
Ambient light
Ambien or available light is important if you shoot in low light conditions where a flash or artificial light would ruin the scene or seem odd to those in the picture. Take this picture from evening in the summer cottage – the scene would simply not have been the same with a flash light added:
Bringing a flash to this scene would ruin it all…
So in low light situations a fast lens is key.
If you are so lucky to have lots of light available to your photography, no matter if it is ambient light created by the sun, or artificial light created with flashes, steady light, reflectors and what not, then the speed of your lens is less of a concern. But only less. If you have a slow lens, then artificial light can get sufficient light to your sensor so that your Shutter speed is fast enough to prevent both camera shake and subject move without raising your ISO to a “grainy” level.
However, a solid depth-of-field effect where your subject is clearly separated from the background (“pop out”) can be difficult to achieve with high f-stop numbers, as the depth of field increases as you reduce the aperture.
If you want to play with the options, this depth of field calculator shows you exactly how narrow your depth of fields gets when you work with a fast lens and get close to your subject:
So a fast lens is more important for ambient light shooters than artificial light shooters, but faster is also better in terms of background separation. If you are willing to pay the price and weight penalty that comes with the a fast lens is of course entirely up to you.
Your lens and the speed of it
Either on the front of the lens or on the side of it, the fastest aperture it will go to is clearly written on the lens. The picture shows a lens that will go down to f/2.8 as the fastest. You may also be able to see that this is a prime lens (it cannot zoom, the length is fixed), and that it is a 135mm lens.
The longer the lens is, the more difficult it is for manufacturers to construct the lens so that it lest through a lot of light. Physics simply dictate that a long lens requires more glass – this is also why long fast lenses (that sports photographers are so dependent upon) are super expensive.
Non-linear relationship
Now, you may think that the difference between a lens that is f/2.8 and f/1.8 is a relative measure indicated by the size of the numbers. It is not. If you stop down from f/2.8 to f/2.0 you have reduced the light that hits your sensor to 50%!! Every time you stop up or down one step on the aperture scale, you double or half the amount of light! That is why the speed of your lens is so important – we are not talking a marginal improvement as as a consequence of the last stop – we are talking an overwhelming doubling of the light. And that is why serious photographers can get a heated debate about how fast their nifty fifty (50mm prime lens) should be: f/1.4, f/1.8 or f/2.0?
As you can see, I found the f/1.8 version to be appropriate for my budget and need, but of course I have a dream to get a 50mm f/1.4 lens some beautiful day!
Now, just to illustrate how dramatically the amount of light is reduced when you stop up in the f-series, here is an illustration of how fast the amount fades upwards:
So, in my mind, the speed discussion is very fair – we are at the important end of the scale!
Questions and comments
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