Digital Camera Lenses

2. Practical Examples of Depth of Field

Although tables and charts are very useful, where possible practical examples always help to illustrate a subject or technique much more easily. This is certainly true as far as photography itself is concerned in general, and particularly as regards the points being raised here. For depth of field is an observable visual effect. How much of an image viewed is sharp.

As with equations there are many permutations that are possible. In order to keep things as simple as possible we have used two camera types for the examples, a 1/1/8" sensor digicam and a APS-C 1.5x DSLR, with focal length being set to 'standard'. A couple of examples of still life are all we are giving as they are enough to show the main points.

Although we have tried to match the shots as much as is possible, one difference is the image sensor aspect ratios of 4x3 for the digicam and 3x2 for the DSLR, which we have not altered and will discuss the implications of on a subsequent page. 

We have deliberately chosen these particular shot setups, which are certainly not one's you would normally consider using for  images like these, to help illustrate the depth of field. Still life has been used as it's quite difficult to use anything else and get consistent and reliable comparisons. The stark background of light tiles interspersed with patterned ones and the clear distinction between the vertical wall and the flat surface due to the change in colour and surface type mean changes in depth of field and the effects can easily be seen. Had we set the chosen objects up against something more normally used, either a backcloth in a dark material, say perhaps black velvet, or a plain curved surface - so the background fades away without delineation - either a plain colour, perhaps white, or even translucent so you could light the subject from underneath [not for these objects perhaps, but especially good for glass], then the changes would not be so clearly seen.

Here is the first shot taken wide open at f 2.0 using the 3mp digicam. The focus point is exactly in the middle of the shot where the red cross is. As you can see virtually all the shot is in sharp focus. From the travel clock at the front to the wall tiles at the back. Very useful if you want a shot where everything in it is sharp and clear. Although in the centre of the picture, the barometer is not the sole point of interest. Your eye wanders over the scene as everything is clearly defined.

Now compare it with the following image.

Here is the identical shot, this time taken with the 6mp DSLR also wide open at f 2.8. The point of focus is the same. Both the travel clock and the tiles are clearly visible but out of focus, and the eye is drawn mainly to the barometer because it's in focus. To get a shot equal to the digicams where all the scene is in focus, stopping down to a very small aperture is needed. With still life, and using a tripod, this is not too much of a problem. Otherwise it might be, such as in a low light situation, where shutter speeds will be very slow.

Here are two more images for comparison.

This shots from a lower angle. Again the 3mp digicam at f 2.0. This time the travel clock is the point of sharp focus. The Barometer is still sharp as well. The focus on the tiles is slightly diffused, but they are still clearly visible. This is not what was wanted from this image. A much more diffused background was hoped for to bring attention to focus on the travel clock. However, even wide open, the depth of field is too great.

Compare this to the next shot.

The DSLR at f 2.8. The travel clock is sharp, the barometer isn't, it's well diffused, with the wall tiles being more so. The eye is naturally drawn to what's in focus, as intended. Selective focusing using depth of field to isolate a subject from the background is a technique used quite often in photography. It relies on the ability to adjust the depth of field by altering the aperture. This isn't really possible with the digicam because of the large depth of field it produces at maximum aperture, even though the focused distance was minimal.

Conclusions - Depth of Field

Although depth of field is only one of a number of factors to consider when taking a shot it's a very important one. If you consider once more the first examples and then imagine trying to take a shot of something outside, either handheld, or where there might be subject movement, an animal say, or plant or flower head on a windy day, then a high shutter speed becomes a necessity. Here a digicam has a distinct advantage. You can use one with the aperture wide open to ensure the highest possible shutter speed and be confident the depth of field will be good enough to keep most of the view sharp. With a DSLR, if the light levels are high enough, it might be possible to chose an aperture that will both freeze the shot and give good depth of field. If not then either flash will be needed or a higher ISO rating. Both have drawbacks. Flash is sometimes useful for freezing motion but doesn't do a lot at far as depth of field is concerned, and can introduced unwanted reflections. Using a high ISO might ensure a shot and give greater depth of field but the reduced quality might prevent the increased depth of field being used to it's full potential. Although its certainly true that the noise levels at high ISO's using a DSLR are much better than with a digicam, they aren't any better than film. 

The second pair of examples show the other side of the coin. Imagine once again you are outside taking a shot of a subject whose surroundings you have no control over and are distracting. Shallow depth of field is a great help in situations such as this. But what can you do with a digicam? If the depth of field is deep even wide open you have to rely on using software to reduce it. The only problem here is you have no way of judging what the shot will look like with a de-focused background and whether a different viewpoint or angle of view would produce a better result. Sometimes background colours which don't look out of place in a sharp shot with deep depth of field become a big distraction as part of a blurred background.

Having now discussed the differences between digicams and DSLR's concerning depth of field, the next page deals with some other aspects of digital camera lenses, fields of view, and the shutter speeds to use with them.