Blurring the background–even with a point and shoot or cell phone camera

In this post we’ll talk about controlling the “depth of field” the parts of the picture that is in sharp focus and the parts that are blurred – and I will show examples taken with a point and shoot camera and with my iPhone.

When the camera is focused on an object that object and everything at the same distance from the camera is in sharp focus, there is a range before and after that object that is also sharp and everything outside that range is blurred (more blurred the farther it is from the focused object).

The sharp range can be small (that’s what’s called a “shallow depth of field”) causing the background and objects close to the camera to be very blurry or it can be large (usually called wide or deep depth of field) up to the point everything is in focus and nothing is blurred at all.

There are just 4 factors that control the depth of field (that’s actual physics, backed up by math and formulas) and with most cameras we can control 3 of them, and they are:

Focal Length (How zoomed in you are)

Note that this only works with real optical zoom that is created by moving pieces of glass inside the camera not with digital zoom, so this works all but the cheapest point and shoot cameras but not on most cell phones.

To show the effect I’ve took my lovely Canon 550D with the 18-135 lens and shoot two pictures without moving, one fully zoomed in at 135mm and the other at 32mm (not fully zoomed out at 18mm because that would show you too much of the mess in my garden on the sides of that nice fence ).

You can clearly see that the background is much more blurred in the zoomed in image (you can also see the ridicules construction I used to position the bear, but that’s another subject), but what would happen if I did the same experiment with a point and shoot camera? let’s try, I took my old Fuji S700 (a relatively high end point and shoot from 2007, maybe earlier) and shot the same pictures again, one fully zoomed in at 63mm and one zoomed out at 13mm.

Because the very different sensor size between DSLR and point and shoot the “zoomed-in-ness” at the same focal length is very different, the physical focal length in the point and shoot is much smaller (that’s one of the reasons point and shoots can’t get as shallow a depth of field as DSLRs).




Again, we clearly see the difference in background blur, juts to make it easier to see here is a small part of the upper left corner of all four pictures:


The top left is the DSLR zoomed in clearly showing the blurred pattern, the top right is the point and shoot zoomed in it’s so blurred it’s almost a solid color (some of the blur is caused by the lowest quality lens of the smaller camera, if you don’t look in the corner you can see the blurred pattern very clearly), the bottom left is the DSLR zoomed out and the bottom right is the point and shoot zoomed out (the difference in color is simply because the DSLR got a wider area in the picture and caught a piece of the fence that is under a roof while the entire point and shoot image is in direct sunlight).

The focal length have such a big effect on depth of field that at wider focal length (lower number, zoomed out) it’s completely impossible to blur the background, even with a good camera and lens.

One thing that is painfully obvious is that changing the focal length seriously changes the entire composition of the picture and not just the background blurriness – and this brings us to:

Aperture Value (Size of the hole in the lens)

The aperture value is the only option that changes the depth of field without otherwise changing the composition, this is easy to use in cameras that support this, just switch the camera to A or Av mode and set a lower number for smaller depth of field and an higher number for a larger depth of field (if you want to know what those numbers mean look at my previous post understanding f numbers).

But remember all 4 factors always effect depth of field, you can open the aperture wide open (smallest number) but if you are zoomed all the way out you still won’t get a shallow depth of field, especially with point and shoot cameras you have to set the aperture value and zoom in (and get close, we’ll talk about that below) to get a nice shallow depth of field.

To demonstrate I’ve used the Canon 50mm f/1.8 lens on the 550D (because that lens opens all the way to 1.8 it’s easier t o see the difference) the first image the aperture value is set to f/11 to get an extremely wide depth of field:

And in the second picture  I’ve gone all the way to f/1.8 for extremely shallow depth of field (if you look closely you will see that while the bear’s nose is very sharp the ears are blurry).

The aperture value also effects other things in the image (see my previous post on aperture) mostly the amount of light coming into the camera and so the required shutter speed but also how light sources look.

This is all good and nice on higher end cameras but I did mention this is also possible with the iPhone that has a fixed focal length and a fixed aperture – so this brings us to the third factor that is always under your control no matter what kind of camera you have:

Distance to subject

Simply put the closer you are the smaller the depth of field, here are two pictures to demonstrate: in the first, we are near our bear of a model

and than a picture with the same camera settings but from farther away:

For smaller cameras you may have to be very close to get the effect (for the iPhone 3GS, it’s just a few inches), just be aware there’s a something called “minimum focusing distance” – this is, unsurprisingly, the minimum distance  the lens can focus to, everything closer will be out of focus, in point and shoots turning your camera macro mode (icon of a flower) may help, here’s a picture from my iPhone (this time using a toy monkey as a model, with an extra cluttered background – just to make a point):

You can only see the eye because this is how close you have to be to get a shallow depth of field with an iPhone.

Now, depth of field is one thing where a nicer camera really makes a difference, just compare the previous picture – that is the shallowest depth of field I could get with the iPhone and I had to place the iPhone so it was almost touching the monkey to get the shallowest depth of field.

I can easily get much shallower depth of field with my entry level DSLR and lens (Canon 550D with 18-135 at 135mm, wide open at a non-impressive f/5.6, minimum focusing distance):

Or with the cheapest lens Canon make (50mm f/1.8 II wide open at f/1.8, minimum focusing distance)

The difference in depth of field is very obvious, the difference in colors is also obvious – and a large part of it is due to the difference in lighting – the iPhone 3GS doesn’t have any flash at all while the DLSR used an external flash bounced on the ceiling.

Circle of confusion

Circle of confusion is 3 things: 1. the coolest name ever for a technical concept. 2. not important because we can’t control it. 3. A way to describe how out of focus something has to be before we consider it blurry (that is, how big the blur have to be before we notice it).

Circle of confusion is determined by the physical size and resolution of the camera’s sensor and is only important to you if you want to calculate the depth of fields manually or create your own depth of field calculator (or in other words, not at all to most people). 

Using this in practice

Depth of field is one area where a good camera and lens makes a huge difference, with a camera with a larger sensor (like all DSLRs) and a “fast lens” (a lens that can reach lower aperture values) controlling depth of field is easy, just set the aperture and you’re set, for point and shoots you mostly have to work with focal length and distance.

Focal length and subject distance really effect the composition of your image, you can’t just dial them in to set the depth of field – they also interfere with each other, if you want to reduce depth of field by zooming in you have to step back to get the same image and that increases the depth of field due to the change in distance to subject.

If you do want to get a shallow depth of field with a point and shoot try to use all the factors, zoom all the way in, set the camera to A or Av mode (if it has one) and set the lowest number the camera let you and get as close as possible to you subject.

If you have a DSLR you probably want a depth of field calculator so you know what to set in order to keep your entire subject in focus while blurring the background, there are many free ones on the internet and there’s also many free apps for every smartphone so you can carry one with you.

Next week I’ll write about the opposite problem, how to get a widest possible depth of fields (for example, so both the person next to you can the mountain in the background are both sharp).

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  1. Pingback: 10 tips for taking better Christmas photos | Weddings from Whole Picture

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