A deeper look into Depth of Field

A Guest post by Reader Josh Wells

A shallow depth of field is highly sought after due to it’s ability to separate the subject from it’s background and is found in many professional photographs. By now your probably know that larger apertures (f/2.8 and below) correlate to a shallower depth of field whereas small apertures (f/16 and above) will render almost the entire frame in focus. In this tutorial I will explain further factors into controlling depth of field, how much is too much and why different sensor and film sizes give different depths of field.

F/ Stops

F/ Stops are generally the hardest of the three elements contributing to exposure to grasp. Both because of their inverse relationship with brightness and because it’s often hard to understand what is actually happening when you change apertures. To understand this fully it is important to understand what an F/ Stop really means. An F/ Stop meaning Focal stop, is the fraction of a lens’s focal length as measured (Widthwise) passing through the lens, in order to keep the same amount of light. Still confused? Here are some diagrams of how an aperture of F/2 is measured for a 50mm lens and a 100mm lens.

Whilst scoring no points for artistic merit, I hope these help explain how F/ Stops are measured and how each of these lenses will give the same exposure at each aperture setting. More advanced lenses such as those used in cinematography use T/ Stops or Transmission stops, which factor in the amount of light a lens loses through the elements to give the true amount of light input from the lens. However since this tutorial is about depth of field we do not need to worry about T/ Stops so much.

This aperture as it gets wider effectively baffles the light, so eventually it can only be focused to one small point, bringing the rest out of focus. As it becomes smaller (For example a 20mm lens will have only a 10mm aperture at f/2, meaning a deep depth of field) the light will be less baffled and the lens will have better control focusing it in and around the required focal point.

So how do we get depth of field?

Depth of field is ultimately effected by two things: The aperture of the lens as we have just covered and the distance between subject and background. If you have a lens with the focus distances marked on it have a look at it now, you’ll notice that the distance the focus ring needs to be pushed to go from 7 to 15 feet (Or at 2 to 5 metres for us using the metric system) is about the same as from 2.5 to 3 feet (Or at 0.8 to 1 metres). And then there is hardly any distance from 25 feet to infinity (Try with 15 metres too). So at f/4 on a 50mm lens I could have from 25 feet and everything behind that in focus, or I could have almost the distance between 6 and 4.5 feet.

To utilize this for the shallowest possible depth of field, try to bring as much separation as possible between the subject in focus and the background, for example a headshot will render a distant background out of focus at almost any aperture. If the lens you grabbed also has marks for the depth of field at each given aperture you’ll be able to see how this distance effects each F/ Stop.

Crop Sensors

This can get a bit confusing when you account for different sensor sizes such as APS-C as used by many consumer DSLRs, Four Thirds, and Medium format from some film cameras and some very expensive digital ones. Many people make the correlation between focal length and depth of field and assume that a cropped or four thirds sensor will equal a longer focal length and therefore shallower depth of field. This is not the case due to a cropped sensor working more as the name entails, and literally shooting from an equivalent crop of what would appear on a full frame sensor. Lets explain this with math.

We’ll use a 50mm f/1.8 lens.

So this lens currently has a maximum aperture of 50/1.8 – 50mm/1.8= 27.7mm Aperture

Now lets put this on an APS-C cropped sensor with a crop of 1.6x – 50mm*1.6= 80mm Equivalent Focal length

However the maximum aperture is still 27.7mm. – 80mm Equivalent focal Length / 27.7 Aperture = ~2.8 New equivalent aperture.

We can now think of the lens rather than as an 80mm F/1.8 but more accurately as an 80mm F/2.8.

This also works the other way with larger sensors, which have reversed crop factors and can thus have for example a 90mm F/2.8 lens which will give a crop factor around 0.6.

90*0.6= 54.

Meaning with a medium format sensor of film one can have the depth of field of a 90mm F/2.8 lens however with the angle of view of a near 50mm lens.

Too much Bokeh?!

Whilst in many cases when we want to throw a background out of focus we want as shallow a depth of field as we can afford, there are several cases in which one must find ways to deepen depth of field.

One of the reasons one may have too shallow a depth of field is in Wildlife photography where focal lengths in excess of 300mm are required in order not to disturb the animals, especially when shooting birds. Avian photography can sometimes have so shallow a depth of field that it becomes impossible to accurately focus, or difficult to keep the entire bird in focus due to excessive depth of field. This is avoided through stopping down to smaller apertures and sometimes a flash extender is required to sufficiently light the subject. Such as this Visual Echoes FX3 Better Beamer Flash Extender for Use FX3 B&H or more simple DIY ones.

Another is in Macro photography in which our other variable; Distance, is pushed to an extreme. In macro photography it is not uncommon to require stopping down to f/ 32 in order to gain the necessary depth of field. The light loss from this is aided again with flashes specifically for Macro, often ring flashes.