What is depth of field in photography?

After talking to you in detail about the three factors that determine the exposure in photography (shutter speed, aperture and ISO sensitivity), I will now address another very important subject to master for those who wish to learn photography, a basic notion: depth of field. You will often find it in the forums under the term “dof” (Depth of field). No need to be a professional photographer and have a 3000€ DSLR camera to master this notion.

We hear a lot about it in photography jargon, but it’s a parameter that’s not that simple to understand at first. Knowing how to use this depth of field is essential to give a creative effect to your photos. The blurred aspect in the background of the photos easily gives a professional look.

In any case, knowing how to control this parameter allows you in particular to highlight a subject on a specific scene (make it stand out in a way). That’s the difference with someone who shoots in automatic mode and lets their camera do the work. You will (and must) be the master of your artistic and photographic choices. This is what you’ll be doing when mastering this notion.

If you are new to photography, you may already be aware that using a large aperture (small “f/”) allows you to play with depth of field. However, this is not the only parameter to take into account. I explain in this article:

• What is depth of field in photography and the utility of controlling it,
• How you set it on a camera,
• What are the parameters that influence it?

Let’s get to the heart of the matter in a simple and not too technical way. What’s the depth of field then? To put it simply, I would define it as the area of the image in which the objects in your scene are in focus. In reality, you’ll have some of your objects in focus and others that are no longer in focus. So, you could say that it’s the distance between the beginning of your sharp objects and the beginning of the blurred objects. Is that clear or not? It’s not more complicated than that on paper.

You might wonder why mastering this notion is important in photography and in which cases it is used? Let’s start with the why of things. If you have understood the definition above, then you should realize that mastering depth of field on a picture essentially allows you to isolate and highlight a subject in a photo. But how? Simply by making it stand out sharp (or blurred) in the photo. If the subject to be highlighted is sharp (in the focus area), it will simply stand out better against other portions of your image that will be blurred.

Two examples of domains where the depth of field is usually short: macro photography on the left and food photography on the right.

Regarding the domains where depth of field is necessary, I would mention the following:

• Macro photography: this is a particular field, the idea being to highlight a detail,
• Portraits: the focus will generally be on a detail of the person (the eyes for example) and the rest will be slightly blurred,
• Food pictures: we often come across this type of photo with a precise detail on a portion of the plate,
• All scenes where you have a background (or foreground) that is not very aesthetic and you want to blur it,
• Finally, all situations where you want to highlight a detail (part of a landscape, a specific scene in a city, etc.).

As seen in the introduction, the aperture of the diaphragm you choose when shooting mainly influences the depth of field in a picture. It is in any case a parameter that you can play with quite easily as soon as you take a picture.

How do you change the aperture of a DSLR then? Mainly, by placing yourself in “A” mode (at Nikon) or “Av” mode (at Canon) to name the main ones. Nothing very complicated here. You rotate the knob of your camera to the desired mode and you now have to play on the aperture:

• The more you choose a small “f/”, i.e. a large aperture of the diaphragm, the more your depth of field will be reduced -> you have more blur…
• Conversely, if you want a large depth of field (e.g. on a landscape picture), you will choose a small aperture (ex: f/11 or f/14) -> the whole will be sharp.

To remember it, the smaller the number behind the “f/” (e.g. f/2.8), the shallower the depth of field. And vice versa. For the same picture in the same location with the same sensor size, the depth of field will be shorter at f/4 than at f/11.

This is the main parameter that you can easily manage. However, it is not the only one that influences the depth of field on an image.

We just saw that the aperture was one of the main parameters for choosing to blur for example the background and to highlight a subject on a photo. I present to you the 4 other parameters which can influence the depth of field.

This is one of the four parameters that influence the most the depth of field and the quality of its background (bokeh). Simply experience shooting the same subject at 30cm and at 2m, while keeping the same settings (aperture), the same background distance, the same body and of course the same focal length. The difference is simply noticeable at first glance. Judge rather the two pictures below.

As you can see, the closer you are to the subject, the shallower the depth of field will be. Conversely, the further you move away from the subject, the greater the depth of field. Yes, I have a 3-year-old child so I took one of his toys 😎.) You can indeed notice that my son’s car is entirely sharp on this picture taken at a distance of 2m, while only part of the same car is in focus when I took the picture at 30cm.

Splendid masterpiece in my living room: on the left I am 30cm away from the subject, on the right I am 1.5m away.

You might be wondering, what if you use a zoom? You can redo the test. I.e. take a subject at a distance of 1m then move back 3m and take the same subject while zooming in (to obtain roughly the same framing as the first one). You will eventually realize that the results are almost identical.

In other words, if you move back further but use a longer focal length (by zooming in), the results in terms of depth of field are the same or almost. By zooming in, you also reduce the depth of field. I detail this point below.

This is another important point to understand in photography. Remember this: when you take a picture, you will be focusing on where you want the picture to be sharp. So far so good. When you look in detail, if you’ve chosen a large aperture, you’ll actually find that the blurred area in the background is not uniformly blurred. Let me explain. Look in detail and you will see a sort of gradient of blur.

The elements just behind your focus will be very slightly blurred, and the further you move away from the focus area, the more blurred these elements will be. This is particularly noticeable on a close-up portrait shot at full aperture (understand with a small f/ to blur the background as much as possible).

In the picture above, you can see that the focus was on the eyes. As soon as you move away from this focus area, the rest of the face will start to be slightly blurred (the cheeks here or the nose for example). If you move further back (e.g. at the temple), you will see even more blur appear. Finally, look at the hair or ears, they are even blurrier.

Another example to illustrate this is a crate of cherries (I dream of it by the way) shot at a market. You can clearly see the focus area in the center of the picture. As you move away from this sharp area, the cherries become more and more blurry, almost unnoticeable at the back of the image (only blurry circles…). Note also that the blur does not only exist in the background. You can clearly see on the picture that the foreground, just before the sharp cherries, is also more and more blurry the further you move back.

You simply come to the conclusion that the farther your background is from your main subject (actually from the focus area to be precise), the more blurred that background will be. It’s the same for a portrait, a landscape or a photo in a market. If you understand this element, you should be able to apply it in your next photo shoot. To help you: Remember that if you want to isolate a subject and highlight it with a large aperture for example, consider choosing a composition where the background will be as far away as possible from the area you’re going to focus on.

Third point that influences the depth of field: the focal length used. Indeed, this point is related to the two presented above. You will realize that long focal lengths (e.g. telephoto lenses 100mm, 200mm, 300mm, 600mm), make it possible to obtain very reduced depths of field even with small apertures (f/8 for example).

Keep in mind the following: the longer the focal length becomes, the shallower the depth of field will be. The shorter the focal length, the greater the depth of field.

Here’s another great test done on one of my son’s stuffed animal. I’m with a 250mm focal length and at f/8, which is a relatively small aperture. Nevertheless, I manage to produce a nice blur in the background. Do the test shooting at f/8 with a 15mm, you’ll never get such a blur. The longer the focal length, the more the depth of field is reduced.

Be careful though, I’m talking about if you shoot from the same location. Let’s take a concrete example to illustrate this with a proof by associated calculation! You decide to place yourself at a specific place and take a picture of your child located 10m from you with a 100mm let’s say at f/2.8 (for the example), you get a depth of field of 1.69m. Simply put, your picture will be sharp over 1.69m (a little in front and a little behind as shown by the numbers “Depth of field in front” and “Depth of field behind”). So far so good. Now you decide to use a shorter focal length, a 50mm, to take a picture of your child, still from the same place. Your depth of field will increase to 7.62m, well beyond the 1.69m (which you get with your 100mm). This proves that the longer you use a long focal length, the shorter the depth of field will be (and vice versa).

However, you have to take into account that obviously, if you shoot from the same place, your field of view with a 100mm will be much smaller than with a 50mm. On the latter, the framing of your child will be much wider. Of course, if the subject (your child) occupies the same proportion in the image, the depth of field will be the same with a 50mm or a 100mm. Except that to get the same framing with a 50mm as with a 100mm, you will have to move much closer to your child. You know what I mean? I give you an excellent depth of field simulator (below) in conclusion on which you can have fun seeing the differences depending in particular on the focal length used. I often use this one (from which the two images below are taken).

Despite everything (I have to illustrate this in a comparative photo), you can get the same depth of field with a 50mm and a 100mm if the framing is the same, which will only be possible by moving around. In this case, with a 50mm, if you move forward only 5m from the subject, you get the same depth of field as with your 100mm at 10m. But you will still realize by comparing the two backgrounds that the one made with a 100mm will be softer, blurrier. This is simply due to the longer field of view/focal length which tends to “enlarge” the blurred areas.

The last point that is sometimes forgotten but which also acts in a rather important way on the depth of field: the size of the sensor. Indeed, it is always necessary to compare what can be compared and very often people do not take into account the variation in the size of sensors, in particular between a full frame sensor body (24x36mm), an APS-C body (15,7 x 23,7) and a MFT body (m4/3) of 13 x 17,3mm. The dimensions are very different, from a single to double between a full-frame body and an MFT sensor found in the sensors of the Panasonic and Olympus mirrorless cameras among others.

I’m not going to get into big calculations to show you this, but I’ll try to explain the idea in the simplest way possible with just two copies of the calculation screen. The size of the sensor influences your field of view of the subject. For example, from the same place, to get the same framing of your child with a full frame body and an MFT body, you will have to use a focal length twice as short with the MFT body (it is half as small in size). Example: use a 100mm with a full frame and a 50mm with an MFT. Using a shorter focal length will simply make you increase your depth of field (as demonstrated in the paragraph above).

You’re going to tell me, that’s all well and good, but so what? The idea is just to show you that you can’t compare depths of field and thus background blurs between different sensor sizes, without taking into account the size of the sensor. If I take my example from earlier and keep the same distance from the subject:

• On a full frame body, with a 100mm at f/2.8, you have a depth of field of 1.69m,
• If you use an MFT body with a 100m f/2.8, you get a depth of field of 0.84m (therefore shorter),
• However, the result in terms of framing is not at all the same and the picture taken with the MFT body will be much more zoomed,
• To get the same framing with your MFT body as with your full frame body, you must use a 50mm focal length (twice as short),
• However, in this case, you get a depth of field of 3.48m (so more than twice as much as with your full frame body),
• Thus, for the same framing (focal length) and the same parameters, you will get a sharper picture overall and less blur in the background with your MFT body than with your full frame body.

I’ll stop here so as not to complicate things, but there would be more to say in reality, such as how to find the equivalent depth of field between a full frame body and a MTF body if you shoot the same subject from the same location (for example a singer at a concert).

If you had to remember only one thing, it would be that the larger the size of the sensor, the shallower the depth of field, for the same parameters and framing. This is the reason why in difficult light conditions (concert, indoor portrait, etc.), shooting with a full frame body and bright lenses will be a plus. Example: you’re trying to capture a portrait of a person at a concert and you’re shooting with an 85mm f/2 (to reduce the depth of field and produce a nice background blur) on a full frame body. To get the same result with an MFT body and the same framing, you’ll have to choose a focal length of about 42.5mm (twice as short), but also an aperture of f/1 (to get the same depth of field). However, this lens does not exist, even if there is a 42.5mm f/1.2, but which costs more than 1000€… (while the 85mm f/1.8 is almost three times cheaper). Want to know more? I wrote an article about the size of camera sensors in the “Photography equipment” section of the blog.

To understand and refine your learning on the notion, I invite you to spend some time on this depth of field simulator.

I am coming to the end of this article on depth of field, a complex notion for learning photography. I hope I have been clear enough on the definitions and examples. If you would like more information or clarification on the subject, please feel free to leave a comment at the end of the article. Here is a table that summarizes the article in a simple way.

If you haven’t yet mastered all the techniques of exposure, a little reminder of the aperture in a photo is essential, isn’t it?

See you soon and good photo

Sylvain