Understanding “Depth of Field” in Photography

The depth of field is a photographic term that refers to the amount of space in front, behind, and on either side of the point where the camera is focused.

The greater the distance between this point and objects in front, behind, and to each side, the more shallow will be their appearances in our photograph. Photographers need to understand how this works so they can use it as an artistic tool when taking photos.

What is the depth of field in photography?

The depth of field is what will remain in focus and sharp in any given photograph. Everything that lies within the focal plane will be sharp. This plane corresponds to where we focus with our camera, which can either be at a very specific distance (in which case every object parallel to the axis of the sensor/film plane as well as those behind or in front of it will also fall on this plane) or, if it’s more practical, at an area – though still defined – covering a certain angular range around the point we have chosen for precise focusing.

If that zone is large enough then objects touching either side of the focal plane – but lying beyond a certain distance from it – will also appear very sharp. But if there isn’t sufficient room to accommodate the entirety of both, and objects beyond the far boundary come into play, then we will see their appearance growing less and less distinct as we look at them – even if our actual focusing was spot-on – until they eventually disappear from view altogether. In this case, they will appear as though out of focus.

The distance between what is closest to the lens/sensor plane that appears in sharpest detail and those elements which are blurred but visible varies from one scene to another depending on numerous factors such as how far away objects are from us or each other, the focal length of the lens used, aperture setting, etc. The greater apertures make it possible for a greater volume of space surrounding the main point in any given image – and thus extending beyond the focal plane – to appear sharp, whereas smaller apertures tend to pack more of this area into a reduced frame (and thus past the focal plane).

Depth of field and aperture size: an overview

The greater the distance between what is visible in focus and objects floating outside it, the shallower will be the depth of field. This phenomenon results from how light travels through space; if we consider light rays as straight lines (which they aren’t) then those elements lying further away from us will also fall on their extended trajectories resulting in them appearing less distinct than what lies immediately before or after them on either side.

This effect becomes even more pronounced when using small apertures, where the space beyond what is in focus will be so slim that it will appear blurred. If on the other hand, we use wide aperture settings then there will be much more room for light rays to fit into, allowing them to cover a greater volume of distance before they start losing their distinctiveness. Therefore the depth of field becomes shallower about aperture size, with objects lying even further away from us appearing increasingly in focus. The graphic here illustrates this effect by showing how depth of field varies depending on whether we’re using wide or smaller apertures; you’ll see that objects lying beyond the focal plane (beyond what is visible and sharp) become increasingly out of focus as we start making use of the smaller apertures.

Depth of field and focal length: an overview

The more powerful our lenses, the greater will be their ability to capture light. This results in such fast glass being able to offer both wide aperture settings as well as high optical performance across the entire focusing range – from near to infinity – which allows for objects lying at any distance from us to appear sharp. But if we want to maintain this quality of detail and definition across large swathes of space, there is very little room left on either side of what is in focus before it starts blurring… resulting in shallower depths of field.

This effect becomes particularly pronounced when using telephoto lenses (which are capable of capturing a much smaller volume of surrounding space) and with larger focal lengths (the difference between the shortest and longest frame size, as provided by our optics – making it possible to capture a more reduced angle of view). The graphic here illustrates this effect by showing how depth of field varies depending on whether we’re using wide or longer focal lengths; you’ll see that the more powerful lenses offer us greater capacity to retain sharpness across long distances.

Depth of field and aperture size: technicalities

Although deeper depths of field are typically associated with larger apertures, there is no direct correlation between their two effects:

  • The wider the aperture used, the shallower becomes a depth of field
  • But only up to a point, beyond which the aperture becomes too small, with this resulting in progressively deeper depths of field

So even if we continue to use wider aperture settings on our lens, there will come a point where it cannot go any further without becoming too small. And when that happens, depth of meaning immediately starts increasing again… which means that if you measure the depth of field at its smallest aperture size (typically around f/32) then you’ll get shallower results than if you were to calculate it at its widest aperture setting (usually ranging from f/1.4 through f/5.6 depending on your lens).

Depth of field and focal length: technicalities

It doesn’t matter whether we’re using a telephoto or wide-angle lenses: both can provide us with either shallower or deeper depths of field. The only factor that comes into play is the distance between our camera and what we’re photographing. If we want to capture both foreground and background objects at their absolute best, then our focus point needs to be positioned to lie close enough to them both (as much as possible). This typically means using shorter focal lengths for wide-angle shots – where there is little room left beyond what we’re focusing on – and longer focal lengths for telephoto images – where there is a lot more space around the main subject that will allow it to fit into a smaller frame.

Factors affecting depth of field

There are several factors that can also play a role in the way depth of field behaves. In most cases, these are controllable by us and will often allow for both greater control over what is in focus as well as providing us with specific creative advantages:

  • The circle of confusion limit
  • Aperture setting
  • Focal length
  • Distance to subject

The circle of confusion limit
The circle of confusion limit is a factor that’s directly linked to how much detail we’re going to be able to render within the image. Essentially this refers to how large objects appear when viewed from a given distance – i.e. on-screen or via print which can vary according to our equipment and the way we choose to display them. For example, if you have a large screen at home or enjoy looking at prints of your photographs on the wall, then you’ll need to ensure that all major details are rendered cleanly enough for them to be able to stand out without getting lost in the overall image. To determine what size objects need to be within an image for this to happen, you can use something called the “circle of confusion limit”. This is based on our intended viewing distance, with shorter focal lengths requiring smaller limits than telephotos due to their reduced angle of view (and therefore scale). The standard value for 35mm film photography typically ranges from 0.025mm – although computer projection systems tend to capture smaller images – through to 0.10mm for high-end digital screens.

Distance to subject

As we can see from the depth of field table, given a specific aperture size and circle of confusion limit, depth of field varies according to both focal length and distance to the subject:

  • Short focal lengths require close distances if the depth is to be retained across large image scales
  • Longer focal lengths allow for greater subject distances when working at wide angles while also maintaining deep depths of the field throughout the frame

Aperture setting

The wider the aperture used, the shallower becomes a depth of field – but only up to a point beyond which it becomes too small this resulting in progressively deeper depths of field. Using smaller apertures reduces diffraction interference from light traveling past our aperture blades.

Focal length

As focal length increases, so too does apparent subject distance – meaning that more of the background will be captured within deep depths of field. However, the trade-off is that as we increase the focal length, this also reduces the size of the image on the screen (or via the print) and therefore the amount of detail visible to us. This makes it harder to accurately judge DOF with longer lenses; it’s for this reason that most photographers choose shorter ones instead.

Object-field calculation methods

There are two different methods for calculating depth of field:
The first method is based on the assumption that when viewed from a distance, the human eye will be able to take in all of an image at once without any part of it becoming obscured or hidden. This area then becomes our “image format” which we’ll want to match to the circle of confusion limit size to determine the depth of field. To do this, simply divide 1 by the focal length being used.

The second method only takes into account what falls under acceptably sharp focus within our frame – i.e. everything between half and double the hyperfocal distance depending on whether critical sharpness is required across both near-far planes or just one (as with portraits).

How to overcome DOF limitations

Depth of field is an important tool when it comes to taking creative control over the outcome of our images, but achieving it can be challenging. Here are five ways in which you can ensure that as much of the foreground and background as possible, appear sharp:

  1. Open up your lens as much as possible.
    For example, there is little point in having a maximum f/stop of f/2.8 if you can only shoot at an equivalent focal length of 35mm or shorter for this to be effective. As the “f” number decreases (i.e., increasing your aperture), so too does the actual depth of field; giving you greater creative control over what appears sharp and what doesn’t in each image captured. When shooting with longer lenses that inherently show more distortion around the edges, open up your lens even further (e.g., try an f-stop of f/4). This will increase the apparent size of any subject within that frame; bringing it into focus along with the rest of the image. This is a particularly effective technique when photographing people.
  2. Ensure that you extend your lens’s focal length to the maximum allowed by any given adapter/teleconverter combination.
    Let’s say for example, that you have a Pentax K-mount compatible Sigma 300mm f/2.8 AF lens and a 1.4x teleconverter mounted on a Pentax Q7 body – which has an APS-C sized sensor with a crop factor of 5.5x (and no ability to change lenses). In this instance, because the crop factor increases the focal length to almost 400mm, you can effectively increase your f-stop to f/4 (while retaining the same 400mm “equivalent” focal length). You can also use a smaller value of teleconverter; such as a 0.7x model (for an effective focal length of over 700mm), and still achieve an f-stop of f/4 at its maximum aperture. This has the added benefit that it will allow you to avoid issues with auto-focus while hand-holding your camera; which may not be able to achieve proper focus when using such long lenses and their more narrow depth of field.
  3. Avoid changing lenses between shots: for example, if you’re shooting wildlife or sports on a long lens, consider buying two copies of the same lens in different mounts (e.g., Nikon & Canon) – or stick with interchangeable-lens models, which already have all their lenses built-in. Doing so sits you in good stead to accommodate for future changes in sensor size before making any further financial commitments. Then choose an adapter that will maintain infinity focus when transitioning from one lens mount to another.

You could also consider buying two of the same lens, but with different focal lengths; say a 300mm prime for your full-frame DSLR and then a 200mm prime for your Pentax Q7 body (with its smaller 1/1.7″ image sensor). Whichever way you go, be sure that what you’re fitting has been subject to rigorous quality control and is up to the job. A way to work with DOF and still enlarge your image enough for a print without cropping is by utilizing a medium format camera system.

  1. Use manual focus with your lens set at its widest aperture: In this instance, depth of field plays less of a part in what appears sharp or otherwise in each image – because you’ll get more of it. Plus you can usually achieve faster-focusing speeds on the fly when using an autofocus-disabled DSLR; especially when working with long lenses (which may be too sluggish to switch back and forth between different focal ranges) – or more likely, their mirrorless interchangeable lens cousins which offer instant switching between AF/MF modes on the fly.
  2. Ensure that your camera sensor is parallel with subject matter when shooting macro-style images: If you tilt the actual image plane away from what it’s pointing at, even by a very small amount, you’ll end up rotating any objects within your frame relative to each other; causing one part to be more or less blurry than another depending upon its distance from the focal point – which in turn will result in your final image being out of focus for no good reason whatsoever.

The importance of depth of field

Depth of field is a fundamental property of all photographic images and the wider you can make yours, the more creative options you’ll be able to explore. Once your images appear sharp from front to back – giving them that “3D” feel – this will help to ensure that viewers are engaged with what they’re seeing on-screen or print, rather than focusing on any faults such as camera shake or focus errors which otherwise might have been present.

The hyperfocal distance & depth limits: A useful tool for anyone trying to find a balance between focus and background/foreground detail is called the hyperfocal distance. This figure represents the point at which focusing on anything beyond will place it beyond our depth of field boundaries – ensuring that everything from half this distance through to infinity is captured within perfect focus.

Having said that, unless you’re shooting with an extremely wide-angle lens or are photographing a scene with little detail in the foreground or background, then there’s no guarantee that every part of your image will be sufficiently sharp for this to be useful (if you want to know more about why this happens, check out my article on the circle of confusion limits). As such, taking multiple images and stitching them together can work better despite requiring more time and effort; but if nothing else, helps ensure that all parts of the frame are captured in sharp detail.

Focal length versus subject distance: By way of summary, I’d just like to add that when taking images at wide angles it’s always better to ensure they’re sharp from foreground through to background if you want a greater impression of depth. This can be done by increasing focal length and reducing the distance to your main subject – but only up to a point as this also decreases image size on-screen (or via the print) and therefore detail visible. As such, some photographers will use wider than normal lens lengths to gain greater apparent subject distances while retaining as much foreground/background detail as possible.

How to work with depth of field: a step-by-step approach

When capturing images with a shallow/small depth of field, you need to be careful not to include anything in your frame that may distract from your main subject. This includes making sure any background distractions are sufficiently out of focus so as not to take attention away from them. As such, the aim when working within this range is always going to be about minimizing clutter and distraction while carefully choosing which elements should appear sharpest. Although there isn’t a right or wrong way to go about this, I would recommend following these steps:
1) Ensure everything is in focus
2) move closer
3) Reduce the focal length
4) Increase aperture size
5) Reposition yourself
6) Don’t give up.

1) Ensure everything is in focus
Everything you see within the frame should appear to be sharp, even if it’s far off in the distance. As such, I would recommend using a focusing method that allows you to easily change what’s currently being focused on – either manually or via autofocus.

2) move closer
By getting physically closer to your main subject, you can dramatically decrease the distance between it and other elements in the frame – which should help ensure that everything appears sharp, even if it’s a considerable distance away.

3) Reduce the focal length
Using a wider angle lens means that there is more detail visible within the scene as well as the extending depth of field into areas that would normally be out of focus. Of course, this also has the consequence of narrowing angles as well as reducing image size on screen/printed media – so working with wider lenses isn’t always best for those wanting greater levels of detail.

4) Increase aperture size
As well as helping to increase apparent subject distance (depending on focal length), using a larger aperture will provide greater levels of detail compared to smaller ones. Aperture size can be adjusted manually or by using an aperture-priority mode, although the latter will typically offer greater levels of control over what’s being used.

5) Reposition yourself
As your subject becomes more dominant within the frame, it may be necessary to move closer to eliminate any distractions that are ruining an otherwise perfect shot. Even if you’re currently taking a landscape image that doesn’t include people, I would recommend considering all elements carefully before doing anything else – including potentially altering focal length and/or camera position.

6) Don’t give up.
I’ve mentioned this throughout several steps but great images aren’t always made in one go; so don’t be afraid to experiment with various images and techniques to get the most out of your equipment. If you have a high-resolution camera, it can be easy to think that everything should appear sharp when composing an image but that isn’t always the case – particularly under scrutiny. This is why I would recommend carrying spare batteries; as well as extra memory cards.

Deep vs narrow depth of field

Using a large aperture will provide greater levels of detail compared to smaller ones. As such, I would recommend using this method as long as you’re comfortable with the focal length being used – which needs to be considered concerning camera body size and weight. The opposite is true when working with a narrow depth of field. In these cases, use a smaller aperture size so that everything within the frame remains sharp – even if it’s far away from your main subject. You may also need to reposition yourself based on how close or far away from your subject you are.

Ultimately, what matters most is the look and feel of an image – not just how it appears on screen/ media. Working within a larger depth of field may not be suitable for capturing a great portrait but that doesn’t mean it’s not worth attempting. In the same way, working within a narrow depth of field can be challenging – especially if you’re working at a distance from your subject – so don’t be afraid to experiment with different levels of sharpness or use other methods to achieve greater detail.

Depth of field: Bottom-line

When thinking about what you’re trying to achieve, it’s also worth noting that something appearing sharp doesn’t necessarily mean that it is. This can be especially true when working with large apertures as smaller objects may not remain in focus for the benefit of larger ones – which are on closer inspection more defined. As long as you have some level of control over your environment, being able to extend the depth of field will always provide greater levels of detail compared to working within a narrow or shallow one. However, this comes at the expense of aperture size so it’s important to consider these factors before getting started.

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