When to blur the background of a photograph

April 3, 2022 Filippo Nenna

Everything in focus versus selective focus.
iPhones versus fast aperture lenses.
Ansel verses Leonardo?

I would bet most people buy their first serious camera because they want the blurry background that makes a shot look “professional”. Ironically, photographers historically have gone to great extremes to get everything as in-focus as possible. I guess we always desire the outcome that is most difficult to achieve.

How to blur the background

You can get a blurry background even with a compact point and shoot camera by using the macro mode and/or getting your subject as close to the camera as possible, and the background as far away as possible. The camera to subject and subject to background distances are critical
For the same camera to subject distance, the wider the aperture (lower f number) the blurrier the background
The larger the image sensor, the blurrier the background because the camera to subject distance changes to maintain the same subject size
Smart phone software can try to fake the blurred background (e.g. “portrait mode”), but it often easy to spot because focus is based on what the software considers to be the main subject, not the physics of relative distance from the lens

When not to blur the background

Ansel Adams and some of his peers formed a group called f64 - an aperture of f64 was needed to get most things in a landscape photograph in focus, even when cleverly using the Scheimpflug principle. With the large format cameras he was using, blurring out the background was so easy it was considered a nuisance. With landscapes, the long shutter speeds required to use these narrow apertures were not a problem. Ansel liked large format for the ability to develop one large negative at a time, but a smaller negative would have made the camera to subject distance larger and consequently had more in focus.

So why did Ansel not use 35mm (or micro 4/3 if it were today) to get more in focus? Because moving the camera changes perspective, and a normal large format lens would have to be replaced with a super-wide small format lens with all the distortions that go with it. In reality, the biggest reason was the size of the negative. Film grain is the same physical size on all formats, so a larger negative dwarfs the grain providing increased apparent image quality.

You probably want as much of the image in focus as possible when:

you have a great composition with a defined subject and no distracting elements
documenting a scene which might be scrutinized closely by the viewer (think press images and landscapes)
printing large and you want to avoid large blurry areas of uninteresting space

f8 on a 35mm camera, along with all elements at a relatively similar distance to the lens, keeps most of the image in focus

But blur can really help you out sometimes

What do Da Vinci and wedding photographers have in common? They think backgrounds should have less detail than the subject. Leonardo says atmospheric perspective naturally removed detail form the backgrounds, and a lot of his portraits had a landscape background.

The human eye only sees a small portion of the world in focus at any one time, but our eyes scan and send all these details to memory, creating the illusion that more is in focus at once than there really are.

Modern portrait photographers blur the background for different reasons. The main reason is to isolate the subject, or as an artistic decision (it can make a scene look dreamy). But caution; often this is to make up for other failures:

lack of tonal contrast between subject to background
poor composition
busy or ugly backgrounds
or lack of sufficient light

f1.4 on a 35mm camera, with a subject relatively close and the background relatively far away blurs distracting background elements. Notice how the color of the background plant remains a distraction regardless of how much it is blurred.

No doubt - shooting wide open can get you out of all these bad situations at a pinch.

Artistic decisions aside, should we brute-force these problems away, or think like a proper photographer and use good technique and problem solving skills? Intentionality is key.

You thoughts are welcome in the comments below.

Forget the exposure triangle! A simpler way to understand exposure.

December 2, 2020 Filippo Nenna

I’ve gone into depth on how to use a real exposure triangle, but there’s a much simpler way of than thinking in triangles:

FORGET ABOUT ISO

What?!

Here is the deal, ISO just gets in the way. And it doesn’t provide a creative effect when changed. Now we can go from juggling three relationships to just one: shutter speed versus aperture. Using a familiar and constant ISO is like finding middle-C on a piano or the bumps on the 'f' and 'j' keys on a computer keyboard.

The figure below shows a triangle with an example exposure of ISO 400, f5.6 at 1/125th of a second. If we always shoot at ISO 400 we can simplify to a line where all we need to know is f5.6 at 1/125th. All pairs of values give the same resultant exposure, so f4 at 1/250th = f5.6 at 1/125th = f8 at 1/60th.

We can lighten exposure by shifting the aperture values to the right (e.g. f4 at 1/125th), or darken by moving them to the left (e.g. f16 at 1/125th).

Take ISO out of the equation, and you only need to worry about the creative variables of shutter speed and aperture

You can take ISO out of the equation two ways. (1) You can act like a film shooter with a fixed speed like 400 and make it work in both dim and bright conditions (or use a limited palate of ISOs like 100 for sunny conditions and 800 for indoors) or (2) There is the possibility of ‘auto-ISO’ on digital cameras. This can make life easier in rapidly changing conditions, but in more stable conditions, a fixed ISO will prevent the camera’s meter from getting fooled by light or dark elements in the frame.

Now the ISO is removed from the equation, we can just worry about aperture and shutter speed, which both can control the amount of light and provide some creative decisions for your image. Using this information and locking-in ISO400 I can use 1/500th at f16 outdoors on a sunny day, or 1/60th f2 indoors on an evening. If conditions get slightly darker or lighter, the shutter speed can be raised or lowered a stop or two. Doesn’t have to be a perfect guess either. Film is very forgiving of incorrect exposures and it’s not really a problem for digital cameras anymore also. All this said, with a simpler way to estimate exposure, your guesses will become very accurate very quickly.

Of course there are always exceptions to rules of thumb, so you might want to return to ISO variations if you want to use extreme shutter speeds or apertures.

Single exposure, many shutter/aperture combinations

There are equivalent shutter/aperture combinations, known as reciprocals that result in the same resultant exposure.

A great way to see reciprocals in action is on an old Hasselblad lens - exposure is determined as an 'exposure value' (EVs) - a single number which represents all equivalent shutter/aperture combinations assuming a fixed ISO. EVs can be determined on light meters such as the Sekonic 758. On the lens, the shutter speeds and aperture value rings are locked - meaning that if you want to get a faster shutter or decrease depth of field, you can do so without changing the amount of light hitting the film.

Hasselblad 500 shutter and aperture scale

Hasselblad 500 EV scale

Digital cameras won't let you set exposure this easily. P mode almost does it, but it re-calculates for every picture. It would be great to use one control wheel to scroll through EVs, and another to pick the desired combination.

exposure variables quick reference

It’s useful to think in full stops (doubling and halving of light values) to further simplify the process. Half stops are overkill for most types of photography and third or tenth stops are for precise scientific, studio and product photography. For most people, using less than full stops are an illusion of precision that is not important for your images.

Shutter Speed

Slow shutter speeds (e.g. 1/15th, 1/30th) let in lots of light, but any movement of camera or subject might cause motion blur
Fast shutter speeds (e.g. 1/250th, 1/500th) doesn’t allow as much light to the sensor, but can freeze motion
Full stop speeds: 1s, 1/2s, 1/4s, 1/8s, 1/15th, 1/30th, 1/60th, 1/125th, 1/250th, 1/500th, 1/1000th.

Aperture

Wide apertures (e.g. f1.4, f2) have a narrow depth of field (blurred background) and lets in a lot of light
Narrow apertures (e.g. f11, f16) have a wide depth of field (lots in focus) and restricts light to the sensor
Full stop apertures: f1, f1.4, f2, f4, f5.6, f8, f11, f16, f22, f32

ISO

Low ISOs (e.g. 100, 200) are for high light levels and have minimal digital noise/film grain
High ISOs (e.g. 1600, 3200) are for low light conditions at the expense of more digital noise/film grain
Full stop ISOs: 100, 200, 400, 800, 1600, 3200, 6400

What's inside the exposure triangle?

November 24, 2020 Filippo Nenna

bad examples of complicated exposure triangles by Filippo Nenna

The benefit of manual exposure over an automatic one is consistency between shots. Manual overrides the camera’s metering system which is easily tricked by light reflected from the subject. A black cat or white snow will be forced to middle-gray unless you take control. The exposure triangle is an attempt to introduce the mechanics of manual exposures.

I’ll simplify the triangle to three line-relationships in this post, or you can forget about ISO completely as I explain in this post on simplifying exposure calculations.

The triangle

The exposure triangle explains the relationship between the exposure variables of ISO, aperture and shutter speed. It visually shows these three variables are related, but it is rare to see how a given resultant exposure plots in the triangle and what happens to that point as settings change. Look to the bottom of this post to brush up on how the exposure variables affect your image and what the full-stop values of each variable are. Oftentimes the diagram just has meaningless arrows guiding you in a circle, or telling you that the middle of the empty triangle is the “perfect” exposure. And when we add strobes to the mix, the triangle is no longer a useful tool at all. And where do you plot a 10-stop ND filter?

Google search for Exposure Triangles and you get a lot of empty shapes…

First things first, the exposure triangle (and light meter readings for that matter) provide a relationship between exposure variables, not a “correct exposure”. Correct exposure depends on the scene and is either artisticly subjective, or rigorously objective using something like the zone system. We talk about neither here.

Second things second, when we use the triangle as a real ternary plot we see that it represents not a range of exposures, but only one resultant exposure. Below in Fig.1a we see that all intersecting points share the same exposure value, each point being a different combination of the exposure variables. We can explore the plot by moving along straight lines. As you move from point to point, you’ll find you will keep one variable constant, increase another variable by one stop of light, and decrease the last variable by one stop.

In Fig.1b (same base as Fig.1a) we start at “x” with ISO 400, 1/500th at f8. If we stay at f8, we can only move along the blue f8 line to somewhere like “y” where we loose 2 stops of light to get to ISO 100, but gain 2 stops with 1/125th of a second. Alternatively, if we stay at ISO 400 we can only move along the red ISO 400 line to somewhere like “z” where we loose 2 stops of light to get f16, and gain 2 stops with 1/125th of a second.

To get a darker or lighter final image you need to shift the range of values on an axis. In Fig.1c the aperture range is shifted to wider apertures. Compared to Fig.1a, every point is exactly 3 stops brighter. You could also shift the ISO or shutter speed axes, but the values already represent a range you would use in everyday situations.

An Exposure Triangle That Works

All Points Give The Same Exposure

Shifted Aperture Range Changes Exposure

Figure 1. Usable exposure triangles.

So the triangle is in fact a playground to explore the infinite ways to get the same resultant exposure.

From a triangle to three lines

It is easier to think of the relationships as three separate linear plots, like in Fig.2. To maintain a certain exposure value, shutter speed increases with decreasing aperture (constant ISO), aperture needs to increase if ISO decreases (constant shutter speed), and ISO decreases with decreasing shutter speed (constant aperture). An action that increases exposure needs to be countered with an action that decreases exposure if the resultant exposure value is to remain constant. Just like in the triangles, each combination is reciprocal so they all give the same resultant exposure. You need to shift the values of one variable left or right in order to make your image lighter or darker.

Shutter Speed vs Aperture

ISO vs Aperture

Shutter Speed vs ISO

Figure 2. Linear relationships between exposure variables.

Understanding how to maintain or change exposure is critical for a good image. Once this is internalized and second-nature, you can worry about more important things like ideas, light and interaction with your subject.

exposure variables quick reference

Shutter Speed

Slow shutter speeds (e.g. 1/15th, 1/30th) let in lots of light, but any movement of camera or subject might cause motion blur
Fast shutter speeds (e.g. 1/250th, 1/500th) doesn’t allow as much light to the sensor, but can freeze motion
Full stop speeds: 1s, 1/2s, 1/4s, 1/8s, 1/15th, 1/30th, 1/60th, 1/125th, 1/250th, 1/500th, 1/1000th.

Aperture

Wide apertures (e.g. f1.4, f2) have a narrow depth of field (blurred background) and lets in a lot of light
Narrow apertures (e.g. f11, f16) have a wide depth of field (lots in focus) and restricts light to the sensor
Full stop apertures: f1, f1.4, f2, f4, f5.6, f8, f11, f16, f22, f32

ISO

Low ISOs (e.g. 100, 200) are for high light levels and have minimal digital noise/film grain
High ISOs (e.g. 1600, 3200) are for low light conditions at the expense of more digital noise/film grain
Full stop ISOs: 100, 200, 400, 800, 1600, 3200, 6400