Depth of Field – Deeper than you thought

Depth of Field – Deeper than you think

Continuing my tutorial tradition, I publish the next chapter in the series – Depth of Field – Deeper than you think.

Contents:

Introduction
What is Depth of Field (DOF) and what is it good for?
Circle of Confusion
How is DOF created?
Hyper-focal distance
Digital cameras
How focus length affects the DOF.
Summary

1. Introduction

We all love to photograph and we all attempt to create nicer more artistic photos. Shooting up close and very up-close (Macro shots) is usually accompanied by a blurry background which isolates the subject from the background and makes it more prominent to the viewer. Many photographers have a rough idea what depth of field (DOF) is and most of them know that the larger the aperture is, the smaller the DOF is, but few know why this is and how it all works. This is why I am publishing this article to help everyone to get a deeper understanding of the issue.

2. Depth of Field – “DOF is the distance in front of and beyond the subject that appears to be in focus”. This distance is dependant on several factors that we will elaborate on later. DOF allows us to isolate the subject or to create a picture where the focus is very wide and covers all parts of the photo. Understanding this will help us take better photos and reach the results we are looking for.

3. Circle of Confusion – In order to understand DOF better, let's try and explain its opposite, which is the circle of confusion. The circle of confusion is a circle that is formed in the area of the photo that is out of focus. Meaning, in a place where one pixell should have been, due to the lack of focus, a blurry circle of pixels is formed – this is the circle of confusion. Since for every distance from the lens a focus is determined, what doesn’t fall in that distance will appear out of focus and in a circle of confusion. See illustarion:

4. How is DOF created?

After understanding what the circle of confusion is, it is easier to understand what DOF is. DOF is the distance in front of and beyond the subject where the image is still sharp. The two factors that determine DOF are the relative distance from the lens and the lens aperture.

A. Lens aperture – The larger the aperture (lower F number) so the DOF is shallower. The smaller the aperture (higher F number) so the DOF grows. The reason for this is the way the light breaks inside the lens due to the the aperture setting. See illustration:

Large Aperture

Small Aperture

Examples of different apertures in a compact digital camera

Examples of different apertures in a reflex digital camera (DSLR) (Nikon D70 focal length of 200 mm)

B. Distance from the lens – The closer an object is to the lens, so does the DOF become shallower in proportion, and the farther the object is, so does the DOF become deeper. This is the rule, see explanations later on. This is the reason why in macro shots the DOF is sometimes very small, sometimes even a few millimeters deep, and on the other hand, is infinite in scenery shots. See illustration:

A more profound example – in Macro photography we can see that one eye of the fly (rhymes) is in focus while the other one isn't. now that's a shallow DOF of less than a mm across.

5. Hyperfocal distance

Is a distance beyond which all objects can be brought into an “acceptable” focus, or the distance from the lens in which infinity falls into focus for a given aperture. Meaning, shooting in a certain aperture at a given distance for which the DOF is so great that even infinity is in focus. We use this technique especiall for scenery shots and by closing the aperture as much as possible we can, inevitably, bring this distance closer. For example, at F/2 we can shoot in an infinite DOF from a distnace of 5 meters, and at F/8 we can shoot to infinity from a distance of 1.5 meters (these figures are for compact digital cameras).

6. Digital cameras

You’ve probably heard many photographers complaining that if they had an SLR camera they could reach much more impressive DOFs. Why is it that a compact digital camera can’t reach the same DOF of a professional camera? The answer lies in the size of the sensor. If regular film cameras are in a 35 mm format and the professional digital cameras come close and sometimes even match them, then in the compact digital cameras the size of the sensor is barely the size of the nail on you pinky. This is the reason compact digital cameras use shorter focus lengths to reach the matching focus length of a 35 mm camera and this is why they can only reach much less impressive DOF and why the lens apertures are different, and an aperture of F/8 usually suffices, while in the SLR camera you need an aperture of F/22 to reach the same size DOF.

7. How does focus length affects the DOF

It’s customary to think that for a given aperture, the longer the focus is, so does the DOF narrows in comparison. It looks like it in principal, but the following website claims differently and I wanted to bring its explanation to your attention: Does DOF grow smaller as the focus grows longer? Basically no. The DOF will grow smaller only if the object will grow bigger and then the DOF will be shallow because the “distance” or the relative size of the object in the frame grows. If you maintain the object in the same relative size in the frame for a 35 mm lens and a 200 mm lens you’ll see that the DOF remains the same. Proof of this claim can be found in the next link – http://www.luminous-landscape.com/tutorials/dof2.shtml .

In order to save unnecessary confusion – The DOF appears shallower because the objects in the background get bigger as a result of the change in perspective and this is why the DOF appears shallower in lenses with a longer focal length and also why an F/2.8 200 mm lens will give us a shallower DOF than F/2.8 18 mm lens when the object should, in principle, remain the same relative size in the frame.

If it's not clear, then yes, larger focal length lenses will create apparent shallow DOF.

The next photo was taken with a Nikon D70 with a focus of 200 mm (parallel to 300 mm) with an F/2.8 aperture.

Summarize

There are 4 elements affecting DOF: aperture, focal length, distance from the subject and sensor size. The first three are (usually) easy to control and with them we'll master the art of DOF. DOF is a very important tool in photography that enables us to reach good results. We can use the skills we learned to reach a shallow DOF and isolate our subject or our portrait from the background or we can create a picture with very deep DOF and use it to create a sharp background in addition to the subject. I haven’t discussed much of the issues regarding DOF so as not to overload with information.

The following is a link for t DOF calculator – http://www.dofmaster.com/dofjs.html

I hope this tutorial helped you understand what DOF is, how it is created and how it can be controlled.

Thanks for your attention, and I hope everything is clear,

Sincerely yours,

Roie Galitz

www.galitz.co.il

Other articles in english– www.galitz.co.il/en/articles.html

Translated to English by Yael Shapira-Galitz