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Depth of field photography guide for beginners

Depth of field in photography is essentially what's in focus. It could be +/- "y" feet from point of focus. There are a lot of discussions on the internet about bokeh vs background blur which makes depth of field a confusing topic for beginners. Terminologies such as bokeh, background blur, shallow depth of field and deep depth of field are all interlinked with the concept of depth of field. Commonly abbreviated as DOF, depth of field forms a very important part of modern-day digital photography. It can tremendously help to achieve creative shots in literally any field of photography. In the next section, I will describe the above terms in simple words skipping any involvement of physics.
Shallow depth of field means everything except the focused subject is blurred out. This can include an object either from foreground or background. A deeper depth of field means everything is sharp and in focus. A good example of deep depth of field photographs would be landscape photography. Bokeh represents the aesthetic quality of blur produced by a lens for out of focus areas. Some photographers perceive bokeh as the way a lens renders out of focus pinpoint sources of light. The quality of bokeh achieved in a photograph is dependent on the camera lens used and it is hard to quantify it. But how do you achieve a decent background blur with limited gear? In order to do this, we need to understand what factors affect the depth of field.  Let's focus on them one by one with examples.
1. Distance between subject and DSLR camera:
The closer you get to your subject, the easier would be to achieve a blurred background or shallow depth of field. This is the reason why macro images have a very shallow depth of field. The miniature airplane image below was taken with a kit lens at F/3.5.

Distance between subject and camera

2. Distance between subject and background:
Distant light sources or mountains appear out of focus in portrait shots due to this reason.  The animation below shows the effect of background distance on the depth of field. All the shots were taken with same camera settings, only the background cardboard box was moved between shots. Notice how the text written on box becomes blurred out at when the distance is maximum.

Distance between subject and background

3. Aperture:
Wider apertures (F/1.4, F/1.8, F/2, F2.8) produce shallow depth of field which means everything except the subject is blurred out. As the aperture becomes smaller and smaller (F/4, F/5.6, F/8 and so on) the depth of field becomes deeper and everything starts to appear sharp and in focus. A short animation to show the effect of aperture on depth of field live in action.

Effect of aperture on depth of field

4. Focal length:
Longer your focal length, shallower the depth of field would be. This is why longer telephoto lens makes it easy to achieve out of focus background shots. An extremely oversimplified reason is that the distance between camera and subject appears to be reduced with longer focal lengths.
5. Sensor size: 
This point has always been so controversial in photography discussions,  the sensor itself directly does not change the depth of field. Sensor size changes the field of view (effective focal length) which then impacts the DOF.  Using the same 50 mm F/1.8 lens on a given cropped sensor camera and full frame camera will offer a different field of view.  Let's say you want to create an image with same relative subject size and composition on two different sensor cameras. For the full frame camera, you will physically have to get closer to the subject or use a longer focal length. Getting closer produces shallow depth of field as mentioned in point 1. Change in depth of field due to longer focal length is discussed in point 4. This is how the depth of field changes with sensor size, simple.

Shallow depth of field:  Let's focus on why do we even need shallow depth of field and possible side effects? Basically, shallow depth of field allows achieving better subject separation which means hiding all the distractions in a given image. Due to this, the image looks aesthetically pleasing. Example: Portrait and fashion photography. But the shallow depth of field shouldn't be used everywhere, say landscape shots where it is highly preferable to have everything in focus. Also, achieving a shallow depth of field comes with its own risks too. It becomes easier to miss focus when using wider aperture values. With aperture values like F/1.4, the depth of field becomes so shallow that it becomes challenging to get the whole face elements in focus.  Hence it is sometimes beneficial to use a comparatively smaller aperture for a given lens. Example: F/2 or F/2.2 on a Canon 50 mm F/1.8 lens.
How can F/1.4 or F/1.8 or F/2.8 have deep depth of field as well?
So far we have read and understood that smaller aperture values (F/4, F/5.6 and so on) will produce a deeper depth of field. So how do some photographers make everything look so sharp even at F/1.8 or F/2.8? The answer is distance, and the one major example of this would be astrophotography. To achieve a deeper depth of field with wider apertures, photograph a subject at a reasonable amount of distance with the background being not so far away from the subject. The image will be in focus with no blur.
Hope the examples were helpful to understand how the depth of field varies and how to achieve a good background blur shot. Happy photography. :) 

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