All you wanted to know about Depth of Field (DOF) but were afraid to ask.

An understanding of depth of field is creatively one of the most useful tools at our disposal. Unfortunately, if your photographic knowledge is somewhat limited it could be quite intimidating. This article has been presented with you in mind, the absolute beginner.

What exactly is depth of field? Simply stated, DOF is the area of the subject that is in focus in-front-of and behind the focused plane. (The very area that I am focusing on) A photograph is viewed and displayed in a two-dimensional plane that occupies length and breadth only–such as is observed on the front of your smart-phone or the print that you could be holding in your hand. Therefore the image has to be recorded in such a way that the sense of depth is created. This is principally achieved by the focal length of the lens in use. There are other factors that come into play that will require a better understanding, however, the field of vision of the wide or ultra-wide angle lens is considered to be one of the most important.

We need to understand and to accept that wide-angle lenses give a much greater depth of field than a standard to a telephoto lens and a greater sense of perspective. Primarily the field of vision of the wide-angle lens has been the domain of the smartphone camera, up until the latest generation that is now being made available to us, in my humble opinion they still have some very serious limitations. Smartphones have been relying on this principle and are now including more than one lens or lens combination in their latest, more expensive generation of smartphones. I know a complete article on smartphone cameras needs to be addressed as this will give us a better understanding as to whether we have made the right choice or not.

Telephoto lenses magnify the subject and tend to compress perspective. We also need to accept that thankfully camera manufacturers are still using and making reference to the 35mm film camera. So when we mention a standard lens we will be referring to the 50mm lens that was used on the 35mm ‘DSLR’ full frame single lens reflex film camera. The standard lens is as close to the perspective and vision of the human eye when viewing a scene normally and a bit further on I have a brief explanation that will definitely aid your understanding of the 50mm standard lens.

The photograph was taken by Peter Bendheim Using a Nikon D50, aperture priority f/4.8 set to 48mm on a 24-120 Nikkor lens, (200 ISO f/4.8 @1/30th sec) equivalent to 72mm when using a 35mm film camera. 

It is important that you understand that this diagram was based on the use of film before digital capture was introduced. The explanation related to the exposure of 35mm film that we still term ‘full frame’ today. I would suggest that you test your particular sensor, aperture and lens that you are using as there will be differences between sensor sizes and pixel resolution, megapixels. Depth of focus would be somewhat different, including depth of field, but use the explanation as only a guide to aid your understanding.

Depth of focus extends over distance ‘A’ when ‘b’ represents the maximum permissible, and what is termed, circle of confusion. Depth of focus can be increased between distance ‘A’ when a lower standard of sharpness and larger circles of confusion are acceptable. This will happen when the lens is stopped down or when the subject is closer or a telephoto lens is being used. I have learnt and have come to understand that un-sharp images are composed of a confusion of these circles when image points have been spread to circular discs of light when the sensor has been placed in any position other than the plane of sharp focus. This will be observed and understood and can be seen in the enclosed photograph of the young boys in this article taken by Peter Bendheim.

 

 

If the sensor is placed in any position other than the plane of sharp focus, image points, as an example, specular highlights will spread to a circular disc of light and all the images that are unsharp will be composed of a confusion of these circles. This will be far more pronounced with the lens wide open than when it is stopped down–smallest aperture.

With DOF, there is always only one plane of sharp focus in a scene or subject being photographed. The choice of lens influences the ability to control DOF and to reiterate – wide-angle lenses have a greater DOF than telephoto lenses. As your love of and knowledge of creative photography increases, you may want to experience the magnificent and incredible ability of the macro lens. Macro lenses offer extreme magnification, life-size and closer, but are more problematic to control with regards to DOF.

I adopt a general rule; the more you magnify, the closer you get to your subject, the shallower the DOF becomes. The depth of field does not abruptly change from sharp to un-sharp but instead occurs as a gradual transition. In fact, everything immediately in front of or behind the focused plane begins to lose sharpness—even if this is not perceived by our eyes or by the resolution of the camera’s lens.

 

Here are some of the most important steps at our disposal. Practical steps that will aid you in controlling the DOF of the subject being photographed. (You will notice that most of these steps are not available to us on our smartphones and will be addressed in a future article.)

 

  • The choice of aperture.
  • The point of focus. (The area on which you are focusing and where we are focusing, the closer we focus the shallower the DOF and visa versa)
  • The focal length of the lens.
  • The amount of light available to be able to achieve maximum DOF. (With the advent of digital cameras we can increase the ISO setting on our camera making it possible for us to achieve the desired effect. Setting the ISO ‘sensitivity to light value’ is important and ISO is covered in an artice, Knowing you DSLR in this edition.)
  • The aperture used: Generally considered to be the most important setting at our disposal. To recap, the standard 50mm DSLR lens is designed to reproduce the subject we are photographing in almost the exact way we see it normally–it neither magnifies the subject (telephoto) nor does it reduce it (wide-angle). The rule is quite simple; the larger the aperture, F/1.8 on our standard lens, the shallower the DOF. The smaller the aperture, f/22 on our standard lens, the greater the DOF.

 

This can be somewhat confusing for the beginner. How come a large aperture has a small number? (f/1.8 is wide open) and a small aperture (f/22 tightly closed) is a big number. To make it easier to understand turn these numbers into fractions by simply putting a one (1) over the number. It is easier to understand that 1/22 is smaller than 1/1.8 or 1/2. Keep in mind that this is not the case in reality but an aid to help your understanding.

 

 

  • The point of focus: A major problem with most compact digital cameras is that the moment you point the camera at your subject and half depress the shutter button the focus mechanism is activated and the camera focuses on infinity. This is particularity true when taking landscape photographs. Most of the subject in the foreground is therefore out of focus. This can be corrected by considering the hyperfocal distance and by simply moving the focus position back to the correct point/place will now result in more of the foreground being brought into focus without affecting the background at all as the background is already at infinity focus. This is may appear to be somewhat confusing if you are a complete novice, however, in a future edition, I will have a complete tutorial on hyperfocal distance, as this topic needs to be considered on its own.

 

  • The focal length of the lens: The basic rule is that a wide-angle lens has a large DOF and records much more of a scene but reduces the magnification in the process. A telephoto lens has a much shallower DOF but can greatly magnify but reduces or substantially reduces the image or scene that we may be photographing. The longer the lens the shallower the depth of field. 

 

 

• The amount of available light. Exposure is controlled by a combination of aperture and shutter speed. Thus lighting conditions may preclude a small aperture if we are trying to obtain the maximum depth of field as the exposure may require a very slow shutter speed. This may necessitate the use of a tripod to achieve the required exposure or dialling up the ISO setting as was previously mentioned.

Peter was quite close to his subjects and elected to focus on the eyes of the second boy. Note how shallow the DOF is in front of the camera and much longer behind the second boy. It is also interesting to note that we communicate with our eyes. If the eyes that we are looking at are in focus the rest of the picture also appears to be normal (in focus) as we aren’t distracted by the background. A very useful technique that appeals to the human psyche and works almost every time. The shutter speed that was used was also very slow so you must be particularly careful and support the camera or keep it very steady or you will end up with a blurred picture caused by camera movement (shake) if the camera or lens does not have a built-in image stabilizer.

Depth of field determines the extent to which things in front of and behind the actual focus point will appear in focus. In this shot, the photographer focused on the second glass from the left.

Depth of field is a term that refers to the extent to which things in front of and behind the actual focus point will be in focus. When the depth of field is extremely shallow, only things that are in the same plane as the focus point will be in sharp focus; foreground and background elements will appear blurry and out of focus. When depth of field is relatively deep, foreground and background elements within a certain range of the main subject will also be in focus. It’s also important to note that. Generally speaking, approximately one-third of this zone of sharp focus is in front of the focus point, and approximately two-thirds of it are behind the focus point. The aperture setting, focal length, and shooting distance affect depth of field. In the case of the aperture setting, smaller apertures (larger f-numbers increase depth of field, and larger apertures (smaller f-stop numbers) decrease depth of field. On a lens with an aperture range of f1.4 to f22, opening the aperture up to f1.4 results in a shallow depth of field and narrowing the aperture to f22 results in a deeper depth of field. In the case of focal length, depth of field is deeper at shorter focal lengths and shallower at longer focal lengths. And assuming that the aperture and focal length remain constant, the depth of field is deeper at long shooting distances, and shallower at short shooting distances. (This insert courtesy Sony Alpha Lenses)

In conclusion, DOF generally extends 1/3rd in front of the point at which you have focused and 2/3rds behind the point at which you have focused on. Depth of field is an experience. You must dicover, understand and correctly use it. Digital photography has made it very simple to experience DOF first hand. You have nothing to lose so go out and enjoy using your newfound knowledge and your creative talent. Don’t be afraid to share some of your pictures with us. Mail them to our mail address and we will offer you some constructive comment and may even share them with our readers.

 

info@digitalphotosa.com include DOF in the subject line.