Friday, 14 March 2008

Depth of Field (DOF)

Depth of Field is another area where some beginners to macro photography begin to feel confused. After reading through some of the explanations available on the internet this is understandable. You would need a degree in mathematics and physics to understand some of the information available in some books and websites. I hope to explain Depth of Field in the simplest way possible so that my readers are still with me at the end of the next paragraph! Depth of Field is the area you see in your view finder and resulting photograph that is in sharp focus. In macro photography depth of field is always going to be small and can be determined by the amount of magnification of your subject, the distance to your subject and the aperture value used. Remember that the aperture is measured in f numbers or stops. The area that is considered to be in sharp focus will be indicated by your cameras auto focus points. The camera will do this even when you have set the camera lens to manual focus. This is a good way to double check that you have manually focussed correctly. Now you may be wondering why it is better to use manual focus, especially after buying a lens with an expensive super sonic auto focussing motor. It is simply quicker and more reliable to focus manually then to use auto focus. If your subject is very small it is difficult to prevent the camera from auto focussing on the surroundings. In outdoor photography this could be a branch, stone or blade of grass!

Shallow Depth of Field (DOF)
It has already been established that there is never going to much depth of field in macro photography. This has to be accepted at the beginning and used to your advantage. It is this lack of depth of field that makes macro such a challenging form of photography. To keep this as simple as possible the closer you get to your subject the lower the depth of field. In macro photography the distance between the subject and lens could be very small indeed. If you increase the magnification the depth of field will decrease further. A small area of the frame will be in focus. This is why it is difficult to hand hold at high levels of magnification. At very high levels of magnification it can be difficult to actually find the subject in the frame!

Plane of Focus
The film or sensor in your camera is considered to be a plane. This is where all the light is brought together to form the image. The Plane of Focus is a parallel plane to the film or sensor in front of the camera. The size and exact position of the plane of focus can be altered by using different lenses on the camera. Some poor quality lenses have a tendency to produce a curved plane of focus which causes spherical aberrations. This is known as curvature of field. In other words it will cause straight lines to look curved in your images.

The basic principle to learn from plane of focus is that you can use it to get the most out of your small depth of field. To do this all you have to do is align your camera focussing plane (film or sensor) parallel with your subject. I will not dwell on plane of focus too long. This is all I wanted to say about it for now. Remember that when you set up your composition to align the camera with the subject. This is another important law of macro photography that can have an enormous effect on your results. This is one reason why setting up a macro shot can take such a long time. This is a very intricate business and every millimetre counts. You have to be precise when setting up the camera equipment to get a quality end product.

Circle of Confusion
I really do not want to add to the confusion! There are some people in the world who believe that to fully understand Depth of Field you first need to get to grips with the circle of confusion. Yes, it is a real name and not something I made up for a joke. The circle of confusion is actually more to do with human vision and optics than photography. I have brought it up here so that it has been mentioned. At some stage if there is little else going on in the world I might return to this subject.

Working Distance
Working distance is the shortest workable distance between the lens and your subject. This is 150mm (15cm) on my lens. This is a benefit when photographing insects because I can work without scaring them away. This is one good reason why you need to think about what you would like to photograph before buying your macro lens. The working distance is simply the space between the end of your lens and the subject. If you have a very small working distance there will be less light available and more need to use some type of artificial lighting or flash. Moving the camera and lens further away from the subject and using lower magnification will increase the depth of field resulting in a larger area of the frame being in focus.

Try the Technique for Yourself
This theory is easy to test with your own camera and lens. Take a picture in each position and compare them afterwards. Make sure that you have a subject that can be measured from the front of the frame to the back of the frame. I often use children’s toy figures for this experiment. Another way is to lay a measuring ruler along the frame. Every website that shows depth of field uses a plastic ruler or a sheet of graph paper. I decided to use something a bit more colourful, a procession of “little people” toys. Technically these pictures are quite poor but they show the depth of field quite well. The camera was set up on a tripod and focussed on the front toy. Only the aperture was changed between each shot. The camera was pointing towards the toys at an angle and therefore not parallel to the subject.

Figure 1. At f2.8 most of the front toy is in sharp focus, everything else is blurred.

Figure 2. At f11 more of the image is in sharp focus but the toy in the background is blurred.

Figure 3. At f22 a much greater area appears to be in sharp focus.

The images could still be improved by aligning the camera so that it is parallel to the toys. This was just a simple test to show the depth of field and how to control it using different aperture values. Figure 3 also shows that my camera sensor is still in need of a clean! When using small aperture values any dirt, grit, grime or dust on the digital sensor of your camera will be visible in the resulting photographs. I will address this problem in a subsequent post. It is possible to remove the specks of dirt from digital images with image editing software such as Coral Paint Shop Pro or Adobe Photoshop. This can be less expensive than damaging the sensor or sending the camera to be serviced. It is a dilemma that faces us all, even though new camera’s now have self-cleaning sensors I feel sceptical as to how effective they will be once the more extensive grime and grot finds its way into the camera.

I could have done this experiment differently and focussed on the toy in the centre of the frame. I would recommend that you copy my experiment using anything you have to hand. This set of pictures was taken indoors in poor light and without the use of flash. The camera was approximately a metre away from the toys. This is just a quick example and is not intended to show the maximum or minimum depth of field. There are lots of books and websites that show the depth of field in much the same way. A page of text such as a newspaper column or any type of measurement or scale are popular (but bland) ways to show the depth of field.

Summary of Depth of Field

  1. For a small depth of field move your camera closer to the subject and use high magnification and a large aperture. (f2.8)
  2. For a large depth of field move the camera further away form the subject and use low magnification and a small aperture. (f22)

This is clearly a very basic explanation of Depth of Field. It is important not to under estimate the importance of Depth of Field to macro photography. To blur the background of your subject use a large aperture (example f2.8) and to make the background and foreground sharper use a small aperture (example f22). Using the correct aperture setting for your subject and magnification will yield much improved macro photography. Make sure when setting up your macro shots that you have aligned the plane of focus with your subject. This will increase your chances of getting the subject in sharp focus in the frame.

I hope you found this explanation of Depth of Field informative and useful. It is a difficult subject to portray in basic terms. If you feel it is too basic or not covered fully enough please add a comment and I will address all reasonable requests in my next post or a relevant subsequent post. In my next post will cover shutter speed. This is much less complicated subject than depth of field and should be a much more enjoyable introductory tutorial. There are some good effects that you can gain by understanding how to use shutter speeds. I plan to revert to some general photography basics to help illustrate how the correct shutter speed can improve your macro photography. Hopefully I can shed enough light on this subject to make it easier to comprehend. I am still thinking about the best way to illustrate shutter speed on here. I used to have a really good illustration for this but it has gone missing! With all this new found knowledge of aperture and depth of field you should be already seeing some better results in your macro photography. If you have made it through all to the end of this post you may need to sit quietly in a darkened room for at least a couple of hours!

Thank you for visiting my macro photography for beginners website. If you have any questions, comments, queries or suggestions please contact me via the comments box. I am always interested in the camera equipment other people are using, particularly for their outdoor wildlife macro photography. What do you use for a light box? I’m thinking about building a light box from a clear plastic storage box. This would serve two purposes as it could also be used for carrying photographic equipment and/or my sandwiches. I will let you know how I get on with building this light box nearer the time.

Marvin Africa


Anonymous said...

Mate, this is really useful stuff. Thanks a lot!

Marvin Africa said...

Thanks for reading it! I'm currently re-writing all these articles for my new website

Depth of Field is a bit of mind-bender...