Depth of field, Differential Focus and Perspective

Peter Flower – October 2018


The purpose of this article is to give general guidance on these topics, and then to suggest ways in which they may be used creatively to produce interesting photographs. The extent to which each individual will be able to achieve all the aims will depend upon the camera equipment that they have.

The diagram shows examples of the two extremes.

Acknowledgement: Illustration from photographylife web site

The terms depth of field and differential focus are used to indicate opposite ends of the technique to capture images that appear to be in focus over a range of distances, or to concentrate attention on a selected object by blurring surrounding detail.

Note: The majority of photographs illustrating this article were taken with one of two cameras – Panasonic GX8 (Micro 4/3) and a Panasonic TZ90 compact camera. The focal lengths are shown as equivalent to that of a full-frame camera for consistency. The images are generally shown unedited.

All photographs, except those annotated otherwise, are the copyright of Peter Flower.

It is necessary to understand the some of the technical terms used and the reasons why they affect the ability to achieve the desired result, and the limitations of your own equipment.

First, an explanation of the terms used -

Lens aperture – This is a measure of the size of the iris through which the light passes in the lens. It is adjustable and the size is expressed in f-stop values. The actual value is obtained by a calculation that involves the radius of the aperture relative to the focal length. You do not need to know the details, but because of this relationship lenses of different focal lengths can achieve the same f-stop value.

The calculation of f-stop values results in the largest aperture having the smallest numeric value, and the largest number representing the smallest aperture. For example, the widest lens aperture might be shown as f/2.8 (or F2.8 or 1:2.8) and the smallest as f/16 (or F16 or 1:16)

Acknowledgement: Illustration from photographylife web site

The term f-stop term originated from the era of mechanically controlled lenses, when there were click-stop settings for the range, such as f/2.8, f/4, f/5.6, f/8, f/11, f/16 etc. Modern cameras with their electronic control may well set the aperture at any point in between these values.

Focal length – This is the measure of the distance of the lens from the sensor in order to bring parallel rays from a source into focus. In the case of a simple lens, like that of a magnifying glass bringing the rays of the sun to a focal point, it is easy to comprehend and measure. However, camera lenses are composed of multiple elements contained within an outer shell. This is further complicated by zoom lenses where internal elements are adjusted, not only for focus but also to vary the focal length.

Prime lenses – This is the term used to refer to lenses that have a fixed focal length.

Zoom lenses – These are able to vary the focal length between two points. (Note: It is important to remember that, with the few exceptions of very expensive lenses, the maximum aperture shown on the lens will become smaller as the focal length is increased. This information is normally shown on the lens – e.g. 1:4-5.6 indicating that as the focal length increases to its longest setting the maximum aperture will decrease from f/4 to f/5.6.

Sensor size – The size of the electronic sensor that measures the electrons of light entering the camera. These vary considerably, but with the exception of expensive and specialist cameras are generally regarded as being relative to the original 35mm film camera. The most popular variations are detailed below. These can be very confusing due to the descriptive terms which are applied.

Full-frame – The sensor is 36x24mm, the same size as the picture area of 35mm film.

APS-C – Originating from the term Advanced Photo System type-C, which was a smaller version of cartridge film introduced by Kodak. The approximate sensor size is 23.7 x15.6mm (Canon 22.2x14.8mm).

4/3 – This is the sensor size used by Micro 4/3 cameras, mainly from Panasonic and Olympus. Unlike the previous two sensors, with a dimensional ratio of 3:2, these cameras have a native ratio of 4:3, hence the name. The frame size is roughly half that of the full-frame camera.

1” - This is a smaller size again, used in numerous compact cameras.

1/2.3” – This is one of the smallest sensors, widely used in compact cameras. These are popular because their small size allows a greater zoom range whilst still retaining relatively small bodies. (Note: The smallest aperture available on cameras of this sort tends to be something like f/8. The reason for this is that diffraction from the small iris tends to degrade the otherwise sharp image if the aperture is closed beyond this point)

The relative sizes of the various sensors are indicated in the following diagram.

Focal length equivalents – Because the sensor sizes vary so widely so do the focal lengths of lenses required. As an example, a 4.3mm focal length lens on the smallest sensor mentioned above will give the equivalent angle of view as a 24mm lens on a full-frame camera. In a similar way, a camera with a 4/3 sensor (being half-size of full-frame) would require a 12mm lens for the same angle of view.

The following generic terms apply to lenses on full-frame cameras.

Wide-angle lens – Generally in the range of 12 to 28mm focal length.

50mm – Generally regarded as the standard focal length. Often referred to as the 'nifty fifty'. This is regarded as the lens that provides the most representative field of view relative to our own natural view. (Although we have much wider optical vision than this our concentration tends to be on this central perspective.

75-90mm lenses – Lenses in this range tend to be most popular for portrait photography. The reason is that facial images can be recorded without the distortion that comes from lenses that are used closer up. (Especially smartphone 'selfies' !)

Telephoto lens – These are regarded as being in the range from 135mm up. Depending upon their actual length they are sometimes referred to as 'short' or 'long' telephotos.

Important Note: In the context of focal lengths it should be remembered that actual focal length is the deciding factor when depth of field is considered. This is the reason why, at the same aperture, the compact camera will give a greater depth of field.

Photographs taken from the same position. Note that the image taken with the GX8 (M4/3 camera) shows more differential than that from the small sensor of the TZ90.

Depth of field

Does not exist! - the captured image is either in focus or not.

We rely on the perception of things being almost in focus. Apparent depth of field relies on the distance at which we view the image. If the image is enlarged, or looked at too closely, out-of-focus elements becomes evident. There are scientific measures of what is acceptable, but for simplicity of this article I do not attempt to include these. The important thing to remember is that slightly out-of-focus objects within the image, viewed at the right distance, will be accepted as being in focus.

Light from an out-of-focus source will cause what is known as a circle of confusion. If this point is small enough (indicating that the source is almost in focus) it will not be noticed when viewed.

The following diagram shows the reason for circles of confusion.

Diagram showing how circles of confusion are formed from inaccurately focused sources

Example photographs showing circles of confusion -

© Gavin Hoey The background highlights come from out-of-focus fairy lights

© Peter Flower Grasses taken against the backdrop of trees. The bright spots, varied in size by the f-stop settings, are caused by light filtering through the leaves.

As will be explained later, there are technical reasons why cameras with different sized sensors and lenses will have limitations in the extent to which they are able to capture depth of field.

In a slightly different context, this limitation also applies to us. Our vision adapts to the subject we are looking at. This is why paintings (generally) do not have out-of-focus backgrounds. As the artist scans around the scene he or she changes the eye's focus to make everything clear.

Images from internet source. It will be obvious that such a depth of field, from close foreground objects to the far horizon, would be virtually impossible photographically

Returning to the subject of photography, there are three factors to take into consideration – distance of the subject from the lens, aperture, and lens to sensor distance (focal length). Perhaps the subject where depth of field is most important is in landscape images. The chances are that you want the foreground, middle ground and distant features to be pin-sharp. The following two images are representative. In the first it was desirable to keep the foreground slipway and boat in focus, as well as the historic ship and background of Portsmouth Harbour. In the second, of a beach in Canada, the foreground log, boat, lake and distant hills have all been kept in focus.

© Peter Flower – Portsmouth Harbour Jill Flower - Canadian Lake

As a general rule, expect the depth of focus to be less in front of the focus point and more beyond. This can be useful, particularly in landscape photographs, when it is wished to show a great depth of field, from a foreground object to the far horizon. Focusing beyond the foreground subject can still render it (apparently) sharp, whilst retaining detail of a distant mountain range. (However, remember that a slightly hazy distant view may not spoil the picture)

Regarding calculation of depth of focus, it is possible to research information, either on the web or phone apps, that enable this to be obtained. In the days when manual focus lenses were the norm it was likely that depth of field markings would be engraved on the lens barrel. An example of this is shown in the following images where it is possible to gain an indication of the likely depth of field available from a particular aperture and focus setting.

Differential focus

This is where other subjects such as portraits, plus details of plants and flowers may preferably use this technique. The objective is to isolate the main subject sufficiently from intrusive other details so that it becomes the main focus of the viewer's attention. The level to which this carried out is important.

This technique requires the use of a wide aperture and/or the maximum difference in distance between the desired subject and the surroundings. However, remember that it may be desirable to retain some degree of surrounding detail in order to show the context within which the image was taken.

Note: In this instance cameras with larger sensor sizes will be at an advantage over those with smaller ones, because the lenses used have actual focal lengths that are longer.

Some examples of this are shown in the following images.

The comparison photographs of autumn berries show the value of differential focus. I did not notice the man in a white shirt in the far distance. Despite the fact that the camera was very close to the berries the small f-stop still rendered him fairy clearly. Changing to the maximum aperture eliminated this problem.

Reinforcing the message that the actual focal length is the controlling factor, the following photograph, taken up close to leaves in Priory Park, shows the limitations of the small TZ90 sensor. Although the f/4 aperture was selected this still did not render the distant goal sufficiently out-of-focus.

However, utilising the TZ90 long zoom range (where depth of field is less) enabled the red rose in the following picture to be presented against a suitably blurred background. The same applied to the pink rose. Despite being taken at f/8 (sufficient to keep both front and rear petals sharp) the long focal length threw the background out-of-focus.

The value of careful choice of aperture is demonstrated in the following photographs. These were taken at the two extremes of the available apertures, where the shot at f/2 is much more successful.

The following portrait shots were deliberately taken with telephoto lenses, showing the advantage of focal length choice and wide apertures.

These two photographs illustrate the improvement that choice of aperture makes. The second one retains the context of the scene, but directs interest to the central figure.

This point is further reinforced by the photograph of the someone doing some artwork on Brighton beach. The differential focus successfully brings attention to the drawing activity. At the same time the background, whilst not detracting, is sufficiently detailed to show the context within which the picture was taken.

The following photographs were taken in a lavender field at the peak time of its colour. Numerous visitors had been attracted, including the two young people shown. The long focal length lens was chosen to concentrate on them, showing the setting, but not swamping them with detail of the lavender bushes.

Perspective (and viewpoints)

Angle of view is another factor that is closely associated with the ability to capture interesting images. There is a perception that telephoto lenses 'compress' the perspective. In fact it is the taking position relative to the scene that governs the perspective. This is demonstrated in the following photographs. Which is the wide-angle shot and which is the telephoto one?

In fact they are both the same photograph. If you look closely at the first you will see two ladies on the pedestrian crossing. You will see them more clearly in the second. This is the same photograph, with the second image cropped to give the appearance of being taken with a telephoto lens. This third image, taken from the same point, is the real one taken with a telephoto setting.

The only way that you change perspective is by moving yourself. A wide-angle lens will give a more panoramic view of the scene, but as demonstrated above the narrower angle of the telephoto is recording an identical image, just concentrated on a more central area. Another street scene (of Guildford High Street) shows the impact that a different viewpoint can give. Two shots show it from different points above ground level.

In comparison to the Reigate photograph it will be seen that, being closer to the pedestrians, they are much more dominant in the scene.

It is an unfortunate habit that, using cameras with fixed viewfinders, there is a tendency to take photographs from head height.

The effects of height relative to the subject are summarised below -

High viewpoint makes subject appear 'weaker'

Low viewpoint makes subject appear more dominant

High viewpoint not ideal for babies, dogs, cats, low-growing flowers.

Viewpoint at the same level is often best

The different viewpoints show the advantages. The portrait is taken at the small girl's level. In the case of the flowers they also demonstrate the advantages of avoiding distracting backgrounds, either using the blue sky or background in shadow.

Summing up

It is desirable to control the aperture manually, setting a large f-stop for shallow depth of focus, or a small one to give the maximum desired depth of field.

As explained earlier, choice of the exact desired focus point (in combination with aperture) is necessary to achieve the desired result.

There are guides to assist in obtaining acceptable depth of field. In general terms the shortest focal length lens will be capable of producing the greatest depth of field (and telephoto the least), whilst the largest lens aperture will produce the least and the smallest aperture the most.

If you use the automatic settings of the camera, such as Program mode and/or Automatic Focus the software within the camera will decide the settings. For example, Program mode will decide the combination of aperture and shutter speed control. By the same token, automatic focus is likely to set the focus on the nearest object within the focus zone. (On most cameras it is possible to select a variety of focus point groupings) The Spot focus setting, if available, is one that can be relied upon to pinpoint the desired position accurately. (Note: Some cameras enable 'eye focus' and selective focus point settings, sometimes controlled by touch-screen or a toggle switch) Manual focus may be a problem with some cameras. If so, you should familiarise yourself with the autofocus option that gives you the most reliable result that you desire.

This article set out to give you a comprehensive understanding of the factors involved. It is not necessary to remember all the details, but to remember the general principles. With practice this will enable to you to improve your photography. Experiment. Try different focal lengths, apertures, focus control and point of view to change perspective.

Technical Stuff

As explained before, sensor sizes vary enormously. It is not possible to provide scale diagrams in this article because display screen sizes on pads, laptops and PCs vary so widely. However, to give an impression of the vast difference between full-frame and compact camera sensors I took the following photograph.

The colour slide shows the equivalent size of a full-frame sensor. The tiny pink sticker on my smallest finger-nail shows the approximate size of a 1/2.3” sensor.

In order to try to equate the lens focal lengths that will provide identical frame-filling images on different cameras the term equivalence was adopted. This took into account the diagonal measure of the sensor relative to the full-frame (35mm) sensor at 43.3mm. By this means it is possible to do a simple calculation relating to your own camera. (Note: The figures relating to these diagonal measurements are approximate, but give a reasonably accurate comparison)

Camera Diagonal

Full-frame 43.3mm Ratio 1:1

APS-C 28.2mm Ratio 1.5:1 (Canon different, normally regarded as 1.6:1)

4/3 21.6mm Ratio 2:1

1” 15.86mm Ratio 2.7:1

1/2.3” 7.6mm Ratio 5.6:1

Under normal circumstances these figures are not important. But, if a lens from another manufacturer (or lens not specifically intended for this model) is fitted via an adapter it does become significant. As an example, Canon EF lenses (intended for full-frame models) can be fitted to smaller APS-C models. However, the smaller sensor will, in effect, crop the image. On the other hand an EF-S lens (intended for their APS-C range cameras) would not provide a wide enough image for the full-frame model. For this reason, EF-S lenses cannot be fitted to the larger models.

With the increasing popularity of fitting legacy lenses from one manufacturer's range to the cameras of another, using adapters, the focal lengths (equating to angle of view) have become a more significant factor for consideration.