How Focal Length Changes Depth of Field: The Ultimate Beginner's Guide

Focal length can have a major effect on how much of a photograph appears sharp, but the simple rule that “longer lenses always create shallower depth of field” leaves out an important part of the story. The result depends on what else stays constant when you change lenses.

If you keep the camera in the same position and focus at the same distance, a longer focal length generally gives you shallower depth of field and greater subject magnification. If you move backward with the longer lens so the subject remains approximately the same size in the frame, however, the difference in total depth of field can become much smaller.

That is why focal length, aperture, focusing distance and framing need to be considered together. If aperture is still confusing, start with our guide to aperture in photography before comparing focal lengths.

What Is Depth of Field?

Depth of field is the range of distances around the point of focus that appears acceptably sharp in the finished photograph. It is not a perfectly defined wall where an image suddenly changes from sharp to blurry. Sharpness gradually decreases as objects move farther in front of or behind the focused distance.

A shallow depth of field keeps a relatively narrow range acceptably sharp. This is commonly used when a photographer wants a portrait, animal, product or detail to stand out from its surroundings.

A deep depth of field keeps a larger range of distances acceptably sharp. Landscapes, architecture, interiors and group photographs often benefit from greater depth of field when several parts of the scene need to remain readable.

Aperture is one of the strongest controls over this effect. A relatively wide aperture such as f/2 or f/2.8 generally reduces depth of field, while a narrower aperture such as f/8 or f/11 generally increases it. For a deeper explanation of the numbering system, see how f-stops work.

How Does Focal Length Affect Depth of Field?

The easiest way to understand focal length and depth of field is to separate two different experiments.

Changing Focal Length Without Moving the Camera

Imagine a person standing several metres in front of you. You photograph them first at 35mm and then at 85mm while keeping your camera position, aperture and focus distance unchanged.

The 85mm image captures a narrower angle of view, so the person occupies much more of the frame. Under these conditions, the longer focal length normally produces noticeably shallower depth of field. Nikon's photography glossary describes depth of field as varying with focal length, aperture and shooting distance, while Sony's depth-of-field guide also lists focal length, aperture and camera-to-subject distance as key variables.

This is one reason zooming from the wide end to the telephoto end of a lens can make the background appear dramatically softer when you remain in one position.

Changing Distance to Keep the Same Framing

Now perform a different test. Make a portrait at a shorter focal length, then switch to a longer focal length and step backward until the person's head occupies approximately the same percentage of the frame.

This changes the comparison considerably. With similar subject magnification, the same camera format and the same f-number, total depth of field can become surprisingly similar across different focal lengths in many normal shooting situations.

This is the source of much of the confusion surrounding focal length and depth of field. A same-framing depth-of-field comparison from PhotoPills shows that when focus distance is adjusted to keep a similar field of view, total depth of field can remain very similar across different focal lengths under many ordinary shooting conditions.

The photographs still will not look identical. The amount and appearance of the background can change substantially even when the numerical depth of field around the subject is similar.

Depth of Field and Background Blur Are Not the Same Thing

This distinction is especially important when comparing a wide-angle portrait with a telephoto portrait.

Depth of field describes how much distance around your focus point appears acceptably sharp.

Background blur describes how far out of focus a particular background object appears and how large that blurred detail looks in the photograph.

A long lens captures a narrower part of the background and renders that area at greater magnification. Therefore, a telephoto image can produce a background that looks larger, softer and less distracting even when the difference in depth of field around the subject is smaller than expected.

The distance between your subject and the background matters too. Moving a person farther from a wall does not directly change the depth-of-field zone around the focused person if the other relevant shooting variables remain the same. It does give the wall more distance in which to fall out of focus, so it can look substantially blurrier.

What About Perspective Compression?

Telephoto lenses are often said to “compress perspective,” but focal length should not be treated as an independent perspective control.

Perspective depends primarily on where the camera is positioned relative to the objects in the scene. Focal length determines how tightly that view is framed.

This matters because photographers normally move when changing focal lengths. If you replace a 35mm lens with an 85mm lens and want the person to remain the same size in the frame, you need to move farther away. That new camera position changes the apparent size relationship between the subject and more distant objects.

The result is the familiar telephoto look in which mountains, buildings, lights or other background elements appear larger relative to the subject. Fujifilm's focal-length guide similarly explains that the way telephoto lenses are commonly used can make them appear to compress perspective.

If you photograph from exactly the same camera position and simply crop a wider image to match a tighter composition, the underlying perspective does not change because the viewpoint never moved.

Why Wide-Angle Lenses Often Look Like They Have More Depth of Field

Wide-angle lenses such as 16mm, 24mm and 35mm frequently produce photographs in which much of the scene appears sharp. This makes them useful for landscapes, architecture, travel images and environmental photographs.

At the same aperture and focusing distance, a shorter focal length generally provides greater depth of field than a longer focal length. Wide lenses also make distant objects smaller in the frame, which can make remaining blur less visually obvious.

This does not mean a wide-angle lens automatically keeps everything sharp. Move close enough to a foreground subject, use a wide aperture and place the background far behind it, and even a wide lens can produce obvious background separation.

For landscapes, shorter focal lengths can also make it easier to obtain a useful near-to-far focus range. Our guide to aperture for landscape photography explains why simply stopping a lens down to its smallest available aperture is not always the best solution.

Why Telephoto Lenses Create Such Strong Subject Separation

Longer focal lengths such as 85mm, 135mm and 200mm are popular when a photographer wants to isolate a person or distant subject from a complicated background.

Several effects work together:

Narrower angle of view: Less of the background is included in the frame.
Greater magnification: Both the subject and visible background details are rendered larger.
Shallow depth of field at a fixed shooting distance: Increasing focal length while remaining in the same position can considerably reduce the acceptably sharp range.
Different working distance: Maintaining similar framing with a longer lens often requires the photographer to move farther away, changing the perspective of the scene.

This combination explains why a portrait made with a longer lens can have a smooth, simplified background even when the lens is not used at an extremely wide aperture.

For portraits, focal length should still be balanced against how much of the person needs to remain sharp. See our guide to choosing an aperture for portrait photography for practical starting points.

The Four Variables to Think About When You Want More Background Blur

Instead of relying on focal length alone, check these four variables.

Use an appropriate aperture. A smaller f-number generally creates shallower depth of field. Do not automatically use the widest setting if important parts of the subject begin falling outside the usable focus range.
Consider your focusing distance. Moving the camera closer to the focused subject generally reduces depth of field.
Increase subject-to-background distance. A distant background can become much more defocused than a wall immediately behind the subject.
Choose focal length and framing together. Longer focal lengths can magnify out-of-focus areas and simplify the background, but the camera position required for your composition also changes the final look.

Canon's depth-of-field reference similarly identifies focal length, aperture, focus position, shooting distance and subject-to-background distance as important factors in the final appearance.

Try This Focal Length Experiment Yourself

A zoom lens makes it easy to see these relationships without relying on formulas.

Choose a stationary subject with a detailed background several metres behind it.
Select one aperture and keep it unchanged throughout the test.
Photograph the subject at the wide end of your zoom without moving the camera.
Zoom toward the telephoto end and make another photograph from exactly the same position.
Compare how much of the frame the subject occupies and how the background changes.
Return to the wider focal length and move closer until the subject fills a large part of the frame.
Switch to the longer focal length and step backward until the subject is approximately the same size in the frame.
Compare the second pair separately from the first pair.

The first comparison shows what happens when focal length changes while camera position remains fixed. The second shows what happens when focal length and shooting distance change together to maintain similar framing.

Pay attention not only to whether the subject remains sharp, but also to the size of objects in the background, how quickly focus falls away, and how much of the environment is visible.

Common Focal Length and Depth-of-Field Mistakes

Assuming a telephoto lens automatically has shallow depth of field: The result depends on aperture, focus distance, framing and subject magnification as well.
Confusing depth of field with background blur: Two photographs can have comparable depth of field around the subject while their backgrounds look very different.
Assuming focal length alone creates perspective compression: Changing the camera position to obtain the desired framing is a major reason wide-angle and telephoto portraits show different perspective.
Thinking wide-angle lenses cannot blur backgrounds: Close focusing, a wide aperture and a distant background can create clear subject separation even at short focal lengths.
Standing a subject directly against the background: Even a fast lens may leave a nearby wall relatively recognizable. Increasing the subject-to-background distance can often improve separation substantially.

Does Sensor Size Change the Result?

Camera format matters when comparing equivalent compositions. A larger sensor generally requires a longer focal length to produce the same field of view that a smaller sensor captures with a shorter lens.

When photographers compare similar framing at the same f-number, these different lens and distance requirements can make shallow depth of field easier to achieve with a larger format. That does not mean smaller-sensor cameras cannot create strong background blur.

A suitable aperture, focal length, shooting distance and background distance can produce strong separation on many camera formats. For a more detailed comparison, see full frame vs APS-C cameras.

How to Choose a Focal Length for the Look You Want

Choose focal length primarily according to the composition, working distance and perspective you want rather than chasing one supposedly perfect depth-of-field number.

If you want to show a subject within a large environment, a wider focal length can include more context. Move reasonably close when you still want some separation.

If you want a cleaner portrait with less distracting background information, a normal or telephoto focal length can narrow the field of view and make background elements appear larger and softer.

If you need extensive foreground-to-background sharpness, choose your composition first, then combine an appropriate focal length, focus distance and aperture. Do not automatically assume the smallest available aperture gives the best photograph.

The Simple Rule to Remember

Focal length does affect depth of field, but the effect cannot be separated from magnification, framing and camera distance.

From the same camera position, a longer focal length generally produces shallower depth of field than a shorter focal length at the same f-number and focus distance.
With the same subject framing, changing camera distance can make total depth-of-field differences between focal lengths much smaller than many photographers expect.
A telephoto photograph can still show much stronger-looking background blur because it magnifies a smaller portion of the background and is commonly photographed from a different camera position.

The best way to understand the relationship is to test it with your own lens. Keep one variable constant, change another deliberately and compare the photographs on a larger screen.

Once you have created the depth of field and subject separation you want in camera, editing can refine color, contrast and tone, but it cannot reliably recover original scene detail that was not sharply recorded. If you edit people photographs in Lightroom, you can also browse our Portrait Lightroom Presets for color and tonal starting points after capture.

Written by Asanka Dilshan — founder of AAAPresets, serving 10,000+ customers.