Depth of field: create a background blur

The depth of field is the area of ​​the image in which objects are in focus, that is, the area of ​​sharpness of the image. First important point: this zone of sharpness does not contain just the plane where you are focusing (it is called the focal plane ): it is distributed in front of and behind this focal plane.Take a look at this little diagram to understand:

Focal plane and position of the focus area. You will notice that the focal plane is not centered in the area of ​​focus, but rather positioned at 1/3 – 2/3 (that’s a detail yes)

You can also define the depth of field as the distance between the sharp foreground and the last sharp plane of the image.

It is quite rare to be so precise indeed, but you have every right to say “my depth of field is 2.57 cm“!

Now is the time to go through the parameters that will change the size of the depth of field together.

Depth of field: how to control background blur?

You already know, it’s not a scoop, the aperture of the diaphragm of the camera is the first lever to vary the depth of field.

Open the lens diaphragm

So let’s review how the depth of field is modified by the aperture, thanks to 2 photos of my immense fortune taken at f / 1.8 and f / 8.

My little fortune, on the left at f / 1.8, on the right at f / 8. We see that the area of ​​focus is smaller at f / 1.8 than at f / 8.

The depth of field is all the more weak as your aperture is large, and as the number f / is small (yes I know you have to reread my sentence twice to fully understand ^^)

Seen from above, my little fortune goes like this:

Here, you can see that my diagram at the beginning was a little too simplified. The limits of the sharp area are not clearly defined by lines: this is why I chose to represent the sharpness to you by a gradient.

And that’s how you have to imagine it to understand it properly. The border between the clear zone and the fuzzy zones is progressive, it would be more correct to speak of transition.

If you’ve ever taken a wide-aperture photo where your subject isn’t quite in focus, but not completely out of focus either, maybe it was just in that famous transition zone!

To top it off (otherwise it would be too simple), perceived sharpness also depends on how far away you look at the image (the farther away you are, the less you will see the differences in sharpness between areas), the size at which it is displayed or printed the image, etc…

Concretely, if you go back to the photos of the parts taken at f / 1.8 and f / 8, and that you gradually move back from your screen, you will see that from a moment the two photos will appear identical to you!

The other classic example that everyone knows is the metro poster that looks sharp from the opposite platform but looks ugly when you stick your nose on it.

Anyway, you get the idea.

What is important to note is that the blurred background is increasingly significant as and as we s’ away from where you made the development, that is, the area of ​​sharpness. Check if you want.

You might be starting to see where I’m going with this, as we’re going to touch on the second parameter that affects your depth of field.

Increase the distance of the background from the subject

I just said just before that the further away you are from the sharpness area, the more the background blur is important and visible, let alone if the aperture is large. Which seems quite logical saying so.

Corollary : the further your background is from your subject, the more this background will be blurred. Logic.

Warning  : this does not mean that the further the background is, the shallower the depth of field: the depth of the zone of focus remains the same. The background is just further away from the focal plane and therefore more blurry.

Let’s call our friend Darth Vader to show us what happens in practice (he agreed to remove his cloak so that we could well see the books behind).

On the left the books are far from Darth Vader, on the right they are just behind him.

Both images were taken at the same aperture  : f / 1.8, and I was the same distance from Lord Vader. The only parameter that has changed is the distance from the book that constitutes the background  : in the first photo it is relatively far away. On the second it is very close. Let’s see what it looks like from above to give you an idea:

You see ? So I changed my background blur without touching the aperture, and without moving myself: I simply changed the distance from the background.

It’s all well and good, but if the background is a wall, how do I move it?

You are right. Well first of all, if you can move your subject (if you’re doing a portrait, for example), you can move your subject away from the background, which is the same thing. You can also change your perspective by rotating around your subject to find a more distant background.

When you think about it, this technique is awfully smart, as it allows you to have a more blurry background without having to buy into a lens that opens more!

But if you don’t have that option, there is another way to decrease depth of field without changing hardware. Did you think you could get away with this?

Decrease the distance of the device from the subject

This secret parameter which affects the depth of field is your distance to the subject. Vader will therefore still help us to understand (he still very nice for someone past the dark side!)

So let’s start from the previous image where Vader was close to the background (left here). I am then about 1m from him. I will then get closer to him about 50 cm (on the right).

On the left at 1m, on the right at 50cm (yes the frame necessarily changes since I get closer)

I remind you that on these two images, I did not change the aperture, nor the distance of the background from the subject. I just took the first one about 1m from Vader, and the second one 50cm.

Obviously we see Vader in full on the first and not on the second, but what is important to see is that on the first the background is also in the zone of sharpness (or focal plane). While on the second, the background is out of the focus area. All this just by getting closer!

Translation: the closer you are to the subject, the shallower the depth of field.

Note  : don’t forget the minimum focusing distance of your lens (here 0.45m
With the 50mm f / 1.8), so if you can’t focus, you are too close of the subject
  !

Yes but you, as you are here, you use your favorite 50mm f / 1.8, and therefore you cannot zoom. But what if I use a zoom?

Well, I admit, I was at 50mm. So, for the occasion, I brought out the big caliber: the 70-200mm Which brings us naturally to the next parameter that affects your depth of field: the focal length.

Increase the focal length of the lens

As good scientists that we are, I suggest that you vary only one parameter at a time. Let us leave Vader in the same position and vary only the focal length. So I start from 70mm at f / 4 and I step back a little to have the same wide framing as with the 50mm. Then, I don’t move the camera position anymore and I zoom in up to 200mm (staying at f / 4), which gives us the photo on the right.

You will notice that the background is more blurry! The depth of field is so small that only the bottom of his helmet is perfectly clear, and his hands are a little blurry.

Translation: the larger your focal length, the shallower the depth of field.

Now, a little bonus, let’s complicate things a bit!

By this I mean that I am going to do two simultaneous actions which vary the depth of field in opposite directions:

  • On the left, I frame Vador at 70mm f / 4 (as before)
  • On the right, I go to 200mm f / 4 and I step back to have more or less the same framing
To the left at 70m, to the right at 200mm (by stepping back)

We see that the background blur is more or less the same.

What happened here? I moved away, which increased the depth of field. I zoomed in, which reduced the depth of field. In the end, we do not see any difference, but we must be aware that these two phenomena occur in parallel.

To be completely exact, the two framings are not the same in terms of the relative sizes of the elements in the frame (take a good look at Vader’s feet). The detail is in this article on the secrets of the prospect.

Photographing with a large sensor camera

Here is the last criterion which plays on the depth of field. I purposely put it last in the article because you probably aren’t going to change devices just for that.

But be aware that for the same equivalent focal length, and all the other parameters which are otherwise equal (aperture, distance to the subject, distance between the subject and the background), your depth of field will be even lower than the size of your sensor will increase.

We simplify, in reality it is above all that a larger sensor encourages you to choose a longer focal length to have the same result, and we have seen that the depth of field decreases with the increase in the focal length. But basically what you have to remember is that it’s easier to have a shallow depth of field on a large sensor. 

So I’m going to immediately defuse a fear that might sprout in your mind by reading this: no, you don’t need a Full Frame camera to create background blur.

If you’ve read my article on choosing a camera, you know that the micro 4/3 format is already a great sensor, enough to create a shallow depth of field in your photos (and besides it has other benefits when traveling, but that’s another topic ^^)

A photo taken with a Full-Frame micro 4/3 camera at 19mm f / 2.8

To tell you, it’s even possible to create a shallow depth of field with a one-inch sensor (that of most expert compacts), even smaller than the 4/3 mic.

Of course, it will take a little goodwill by maximizing all the parameters that we have seen above: use the largest aperture, zoom in, get closer, and finally move your subject away from the background: the magic should happen !

Calculating the depth of field: how to do it in the field?

I will tell you a story that I hope will de-stress you.

When I started shooting, like almost everyone else, I really understood what depth of field was thanks to the 50mm f / 1.8 lens. But I had a lot of questions about how to know in the field what aperture value I should use.

I imagined that experienced photographers had some sort of sixth sense to mentally preview what the blur they were going to create. So much so that they never hesitated between f / 5.6 and f / 4, a bit as if they had a small app grafted into their brain.

So I will disappoint you because I am not.

In most cases, it is experience in certain photographic situations that will help you know what value will do. Let me explain.

Imagine trying to shoot portraits at 50mm and f / 1.8, and find that your subject’s nose is sometimes out of focus.

To correct this, you might say to yourself that it would be wiser to shoot at f / 4 next time. And if there’s not enough light and you’re forced to switch to f / 1.8, you’re going to step back a bit more than usual for safety.

What I mean by this anecdote is that “  good parameter values  ” to have a given depth of field do not come out of Houdini’s hat They come from an experience itself nourished by your photo experience, rather than a calculation by a bionic chip in your brain ^^

So obviously, there are specific cases like in product photos, where the size of the sharpening zone is often adjusted to the millimeter (I anticipate the comments.

But in the majority of cases, the difference between f / 4 and f / 5.6 won’t drastically change what your photo gives off (unless of course it gets blurry at f / 5.6, but that’s a whole different subject ^^)

So remember that the choice of parameters to obtain a level of depth of field is above all based on the photographic experience with a given material. A bit as if you were building up a bank of reference images that you have produced (close portrait, landscape, stolen portrait, street scene, etc.) to which you unconsciously refer.

However, at the beginning, we need to play with the parameters to understand them well. This is completely normal ^^

There are online calculators and smartphone apps, which allow you to simulate the blur produced based on the various parameters described in this article.

If you want to take a break, take a look at the “Lens Adapter” on DZOFILM’s site. It is particularly well done because you can enter 4 of the 5 parameters we saw (yes, except the distance from the background about), and visualize the corresponding background bishop!

Even if you don’t often have fun with it in the field, it can at least help you understand the impact of the various parameters that we have seen. And also, you will discover that ultimately, you may not need a Full Frame to make more blur, or ”  that lens which opens at f / 1.2 absolutely essential to express your creativity  ” (yes I mimics an ad with irony).

Let’s talk about the objectives right away to make some pretty blurry!

What lens for a shallow depth of field?

This is a fairly common question but one that can easily be answered with the information seen above. Good continuation.

I’m kidding. More seriously, since the depth of field decreases as the focal length and the aperture increase, you will have more easily blur by maximizing these two parameters.

A fixed focal length can be relevant to learn how to manipulate the depth of field, because at the equivalent maximum aperture, they are less expensive to manufacture than zooms.

For example the 50mm f / 1.8 equivalent in micro 4/3, APS-C or Full Frame will already allow you to indulge yourself.

There are two corollaries to this:

  • For longer focal lengths (typically 70-200mm equivalents), you will have less need to maximize the aperture to create a shallow depth of field, and often a maximum value of f / 4 is sufficient (and the lens will be less heavy). ). Look on the lens simulator, and you will see that the depth of field is quite small at 200mm even at f / 4. Exception once again for micro 4/3 which can offer 70-200 f / 2.8 equivalents for 400g (always stand out those ^^)
  • For shorter focal lengths (especially wide angle ones) you will need a larger aperture to create some blur (compared to a larger focal length lens. But beware of fashionable 24mm f / 1.4 equivalents because even if you have the budget, you may not really need it. Often the point of a wide-angle lens is to show more context around its subject, and therefore the need to blur the background is less present.

If you want to see concrete models, that’s good, we’ve made a selection of the best fixed focal lengths under 500 euros in the article on choosing a lens!

Conclusion

It all sounds like a bit much, so let’s summarize the different settings that affect depth of field:

  • more the aperture is high (f = small), the more the depth of field is small.
  • more your background is far away from your subject, the more the background will be blurred.
    Please note: the distance from the background has no effect on the depth of field itself  !
  • you are more close to the subject, the depth of field is low.
  • the longer the focal length is important, the greater the depth of field is small.
  • Over the sensor in your device, the greater the depth of field is small (micro 4/3 but it is already very well ^^)

Depth of field is a complex effect to grasp and master, but I hope this article has helped you see more clearly and allow you to better understand the effects of your choices when shooting on the final rendering. In any case, just writing it helped me to integrate these notions myself!

One last thing: I have nothing against lenses at 2000 € which open at f / 1.2, they are magnificent objects. But I think it doesn’t get you wrong in thinking that they’ll take your photography to the next level thanks to their shallow depth of field.

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