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What is Depth of Field

Over the last few months, many of my friends have asked me: ‘How do you make your backgrounds blurry?’. To answer this question I will need to explain what is Depth of Field in photography world. Before I can explain more about the way the depth of field works. I need to explain what your camera uses to work out the correct exposure for each photograph you take. All camera’s whether they are point and shoot, to mirrorless camera’s, to your lower end DSLR’s and then to your professional DSLR. They all use an Aperture, the Shutter speed and ISO.

Anyone can practice Depth of Field in photography by using some simple techniques. If you have a camera with interchangeable lenses and have a maximum open aperture size of either ƒ/1.4, ƒ/1.8 or ƒ/2.8. These lenses have large enough apertures which can generate soft enough images when the apertures are fully opened, also known as bokeh.

What is ISO?

What is ISO? The ISO stands for International Standards Organisation and has been used for many years as a way of identifying the light Sensitivity and the image sensor behind your camera shutter curtain. For photographers, the ISO key standard is Film Speed ratings. In the past, this was known as ASA or the American Standards Association. The aperture, the shutter speed and the ISO all make up the Exposure triangle.

What is Depth of Field - Haigh Hall
What is Depth of Field – Haigh Hall

Fig 1.1

In the image above, I was using a new lens, an AF-S Nikkor 50mm 1.8G. Because I need more space to make sure all of the house was in my visual range, I had to move back almost 100m from the front door. As a result of moving so far back, the trees are seen to be interfering with my photograph. I was using the 50mm prime lens for most of my photographs and I needed to use this lens to illustrate how Depth of Field works when using this lens as it has a large open ‘Aperture’ of ƒ/1.8. The ‘ƒ’ means focal ratio and is also known as the ‘F-stop’, is also referred to as the F-ratio in relation to size the aperture opening. Source: https://en.wikipedia.org/wiki/F-number

So What is Depth of Field?

What is Depth of Field? Basically, Depth of Field is the distance from the camera to the object/subject to the background. The best way to demonstrate how you can use your camera to for the best Depth of Field results is to have a steady hand or use a tripod. I use Aperture priority mode as the camera will automatically set your shutter speed for you. You can use manual mode if you wish, but I find using ‘Aperture Priority’ mode is the best way of the camera make the hard choices for you.

Photography and What is Depth of Field - DSC_7916

Decisions decisions: Now you have to decide if you want your background totally blurred or in focus. If you wish to have your background blurred, you will need to open your ‘Aperture’, to either the largest setting (in the case of the 50mm prime and depending if you have the ƒ/1.4 lens or the ƒ/1.8G Lens like I have). If it is a sunny day, your camera will automatically raise the shutter speed to the highest available shutter speed in order to compensate for a lot of light hitting the light sensor. If you are wanting all of your background to be in focus and sharp, you will need to close your ‘Aperture’ to around ƒ/11 to ƒ/16.

Can I use Older Cameras for Depth of Field?

Depending on the camera and how much available light you have outside, the camera will lower you shutter speed considerably, so the camera can capture what it thinks the best result for your photograph. If you have cloudy skies but very good visibility, you can maintain a reasonably high shutter speed with a small aperture size. Anything above 1/50th of a second and you reduce the chance of camera shake or a totally blurred photograph if you are holding your camera in your hands. Any shutter speed slower than 1/45th of a second and your photographs will start to become too blurry to recognise any features details without the use of a sturdy tripod.

Another way of thinking about your ‘Aperture’ works and acquiring the desired photographic result, is to think about your eyes work and how your pupils open more at night time when there is less light. When during the daytime, how your pupils close when looking at direct sunlight. Which is dangerous and I would never advise anyone to look directly at the sun. But you catch my meaning.

Demonstrating and Practicing Depth of Field

What is Depth of Field - Depth-Of-Field-Railings
What is Depth of Field – Depth-Of-Field-Railings

Fig 1.2

From Left to Right:

  1. I widened my aperture all the way up to 1.8 and kept my camera on aperture priority. The camera will then works out the shutter speed for you, so you don’t have to.
  2. In the second image, I adjusted my aperture to ƒ/2.4. The background is becoming less blurred and so are some of the railings around the centre.
  3. The aperture was increased to ƒ/3.3 and the background is gradually coming into focus. The railings either side of the centre is also coming into focus.
  4. The aperture at ƒ/4.8 and more and more features coming gradually into focus. Nearly all of the railing posts are nearly all in focus.
  5. The aperture at ƒ/6.7 and nearly all of the railings are in focus. The furthest railings are still out of focus due to the distance from the centre. The background is still blurred but you can work out more details in the background.
  6. 6 to 8. Aperture has been increased all the way up to ƒ/16 and the closest railings are now crisp and clearly in focus. The trees in the background are still slightly blurry but only very slightly. If you look at my shutter speed for no#8, you will see that I am at 1/15 of a second.

The smaller the aperture size (I.e. ƒ/1.6), the camera will need to compensate with the shutter speed to ensure that enough light is hitting the image sensor at the back of the camera. Meaning the shutter speed will be considerably lower, which takes in to account for the aperture is a ƒ/16. As I had my camera in Aperture priority mode, the camera was calculating how light was available, then setting the shutter speed the camera required to take a photograph when the aperture was at ƒ/1.8.

Practice Depth of Field

To best demonstrate or practice what is Depth of Field and how it works. I came across a funny looking tree stump as seen in Fig 1.3 below. I set up my tripod and kept the camera on aperture priority. I moved my focus point to the tree stump and the background was totally blurred out. I had my aperture on its maximum opening of ƒ/1.8. The camera chose my shutter speeds for me automatically.

What is Depth of Field - Tree-Stump-DOF
What is Depth of Field – Tree-Stump-DOF

Fig 1.3

In Fig 1.3, this image demonstrates how ‘Depth of Field’ works in same way as in Fig 1.2 but using a different subject. The tree stump is the subject of my focus and the train tracks and the tree line is a part of the background. Without having to move the camera or the tripod, I can instead move my focus point on either the tree stump of the background. Remember what I said about the aperture size and the amount of light coming into the camera. Because I opened up the aperture all the way up to ƒ/1.8. I have the maximum amount of light hitting my image sensor.

The shutter speed was set automatically or selected by the camera to 1/500th of a second (or 1/500’s). My camera focal point was set to the tree stump making the entire background completely blurred out. This is because my aperture is now fully open and my shutter speed will automatically compensate for the maximum opened aperture. Because my camera is closer to the tree stump than the Railway tracks, my field of view is more narrow, making all of the back ground totally out of focus.

There is simply too much light for the camera to cope and it can’t sharpen on the background for now. Even if my shutter speed was under manual control and I slowed the shutter speed down, the photograph would then be over exposed. Letting the camera automatically choose my shutter speed is the best option while in the Aperture Priority Mode (A:= Nikon/Av:= Canon Mode).

For the second part of the exercise, I set my focal point to the tracks. The camera’s shutter speed jumped to 1/1000th of a second. In the lower part of fig 1.3, the tree stump is now blurred and although it is recognisable, the tree stump is outside of the focal range. My focal range is still the same but it has shifted, so instead of the tree stump being in my focal range. My focal range is now in front of the tree stump, so everything in front of the tree stump out of focus and now totally blurry.

Depth of Field Diagram

Let’s use another image to demonstrate how the focal range changes depending upon how much your aperture is open or closed.

what is Depth of Field Depth-of-Field-Diagram
what is Depth of Field Depth-of-Field-Diagram

Fig 1.4

In Fig 1.4, I have drawn a small diagram to illustrate how Depth of Field works with the aperture and the focal range, between camera to the central focal point then to the background.

  • No#1 Of Fig 1.4  shows camera position to the focus point (in the middle) and then to the background. The shaded area’s before and behind the focus points, will be the areas which are the clearest and sharpest, as seen through the lens after the photograph is taken.
  • No#2 Represents how the Aperture is opened almost the it’s maximum. The focal range will be more shallow and so everything before and after the subject is less likely to be sharp. The background is also less likely to be sharp while still being blurry, with no identifiable features. This is what happens when your ‘Aperture’ is opened all the way up to ƒ/1.8. No#3 is meant to illustrate the aperture being closed, so everything before and after the central focus point will come into focus, with more of the background also becoming more clear and sharp.
  • No#3 is meant to illustrate the aperture being closed, so everything before and after the central focus point will come into focus, with more of the background also becoming more clear and sharp.

Conclusion

I have done the best I can to explain and illustrate what is Depth of Field in the best lamens terms as possible. The best way to learn of ‘Depth of Field‘, is to go out and practice for yourself. That is the way I learned, by opening up my ‘Aperture‘ to ƒ/1.8, then closing the ‘Aperture’ right down to ƒ/16 on my 50mm prime lens. There are other lenses which will help, like the 24-70mm ƒ/2.8G lens and the 70-200mm ƒ/2.8G VR. These lenses can open their aperture fairly wide and they are good for portrait and landscape photography.

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John M