Welcome to Part 2 of my depth of field tutorial. In Part 1 we looked at circles of confusion and how the size of the aperture influences these. We also learnt that essentially there really isn’t such a thing as depth of field, as only objects on a single plane can ever be truly sharp. Depth of field is really just an individual perception which changes depending on the enlargement of the image and the distance it is viewed from - something no depth of field chart or app will ever take into account.
So we now know that for a given focal length at a given distance, the smaller the aperture we set on the lens, the less out of focus anything not on the plane of focus appears. Many photographers try to increase depth of field even further by applying something called the hyperfocal technique. This states that we should focus a third of the way between the closest and farthest objects we want to be sharp. It is however a little more involved than this, and is often misunderstood by many photographers who think they should be focusing a third of the way into the scene, which is really not a good thing to be doing.
To understand the theory of the hyperfocal distance, we first need to understand what infinity is when referring to its distance on a lens. All lenses reach infinity at a certain point, and once reached, anything beyond this point is on the same plane of focus. The shorter the focal length, the sooner a lens will reach infinity. A 14mm lens will reach infinity at just a few metres, whereas a 300mm lens won’t reach infinity until a couple of hundred metres or more. It is essential you know at what distance your lens reaches infinity if you are to try to use the hyperfocal technique, so here’s how to work it out. Focus your lens at infinity by bringing into sharp focus a subject which is a long way in the distance (i.e. something you know is at infinity). Then without altering the focus, get an assistant to hold an object with lots of detail in front of the lens (a newspaper is a good thing to use). Now get them to walk away from the lens and watch the object come into sharp focus. When it does, ask them to stop. If you have Live View on your camera, you will be able to zoom in to the object to check precise focus, although this is far easier on Canon cameras than on Nikon and other brands - one of the many reasons my entire team are Canon users. If on closer scrutiny the object still looks a little blurred, then you need to get your assistant to move slightly further away. When you are happy that the object is precisely focused, you have reached infinity. For argument’s sake, let’s assume we are using a 50mm lens and we have worked out that infinity is reached at 35 metres.
With this knowledge, if we find ourselves using this lens in a scene where we want to maximise depth of field, and our closest object we want to render sharp is 5 metres away, we need to focus 1/3rd of the way between 5 metres and infinity, which in this case is 35m. The difference between the two is 30m, so a quick calculation says that a third of the way between these two is 10 metres further than the closest object. So in this situation we would need to focus at 15 metres to maximise depth of field. However, as we discussed in Part 1 of this tutorial, only objects at precisely 15 metres will now be truly sharp. Anything closer or further away will now be rendered as a circle of confusion onto the sensor. So if our main subject is at 23m from the camera, then all we have done is focused away from this - the last thing we want to do.
Scenes like this are the easiest of all to get pin sharp, as there is no depth of field required. The entire scene is at infinity, so it wouldn’t make any difference to the depth of field if I had shot this at f/4 or f/22. All you need to do is focus on one part of the scene, and the rest will automatically also be perfectly in focus. When faced with this luxury it is always best to use a wider aperture, as this avoids diffraction - something we will look at in the final part next week.
A common mistake is to see photographers focusing one third of the way into their scene, which is very different from focusing a third of the way between the closest subject and infinity. For example, if we have hills in the distance which are 2 miles away, and our closest subject is 5 metres away, then 1/3rd of the way into our scene is about 0.6 of a mile! That’s about 0.6 of a mile past where infinity starts.
Even when the hyperlocal technique is correctly applied, it can often result in poor sharpness. Let’s take the example where we have two objects which we want to render sharp. The closest is at 4 metres, and the other is at 19 metres. Let’s also assume there is nothing between the two other than clear air. The hyperfocal technique suggests we focus at 9 metres (1/3rd of the way between 4m and 19m). We have now successfully focused on thin air, and have ensured neither object is sharp! So as you can see, the hyperfocal technique is a dangerous theory to follow and is often the cause of poor definition in an image.
This is a good example of when using the hyperlocal technique would be a huge mistake. In order to get the main foreground stone as well as the background stones sharp, it would be tempting to focus between them to bring both equally close to the plane of focus. However, all we would achieve is to ensure that neither would be pin sharp, and little of interest exists at the point where we would now be focused. The result would be a soft foreground rock and soft background rocks. Here I focused on the foreground rock as if this looks sharp it gives the illusion that the others are also sharp, even though they are not.
So where should we focus in a scene? My advice is to decide where your focal point is (the subject you want the viewer’s eyes to be drawn to), and focus precisely on this point. Then, using aperture, and if appropriate focal length or distance to subject (but not both), try to render all other distances as sharp or as soft as you desire for the intended end usage of the image. The important thing to understand and remember is that detail which is not precisely on the plane of focus cannot ever be truly sharp. Once we accept this, we will be far more satisfied with the sharpness of our work.
For this image there is no single focal point, as the ice steps and the mountains have equal appeal. The closest use step is less than 1 metre from the camera, and the mountain is over a mile away. As there is also lots of detail between the two, this is a good example of when the hyperlocal technique can be used effectively. Here I focused on the hoar frost on either side of the steps, as this is about 1/3rd of the way between the closest ice step and where infinity starts, which is just past the large boulder on the left. Applying the hyperfocal technique here has ensured the hoar frost is pin sharp, and due to my short focal length and small aperture (f/16) the ice steps and mountains have also been rendered sharp enough to appear as though they are also on the plane of focus.
For this shot I focused on the far edge of the foreground grass. Even though this is not the focal point of the image, I chose to focus here to ensure the foreground was bitingly sharp. As a result, the background is slightly soft, but due to it being a long way away, it looks natural being slightly soft as our eyes cannot see fine detail at such distances anyway. Had I focused at infinity, the foreground would be noticeably soft, and due to the background being so far away it would also appear soft (even though it is sharply focused). The end result would have been an image which lacked any area of proper sharpness.
In the final part of my depth of field tutorial, we will be looking at why you don’t want to use small apertures, and the advantages of a tilt lens.