Depth of field (DOF) is a very important issue in photography. In many cases, a very large DOF is desired. For instance, this is often the case in landscape photography.

However, this immediately causes technical problems. The usual solution is to stop the lens down to its smallest aperture. This maximizes the DOF, but the small aperture also significantly degrades the quality of the image due to diffraction.

Wouldn’t it be nice if it were possible to get an extended DOF without any lose of image quality? Well, with a little bit of planning, an infinite DOF is possible. The trick here is that several shots of the subject are taken (preferably with the camera on a tripod). Each shot is focused at a different distance. That way, every point in the scene is in sharp focus in at least one of the images. This can all be done at apertures that suffer little image degradation due to diffraction (the middle apertures for most lenses). When using this technique, it is important to lock down the exposure and white balance so that they are the same for every shot.

At first, it might appear that it would be difficult putting all those images together, carefully aligning the images, figuring out which image is sharpest for each point in the scene, and creating a final image. Actually, this is a very simple workflow. Photoshop will do almost all of the hard work.

The Process

The process of combining the images in Photoshop is a six step process.

1. Each image is opened in Photoshop (if using a raw converter, each image must be processed in exactly the same way).

2. All of the images are moved into the same file. This can be done by dragging and dropping the images into one of the open files. The result is a single file with each of the images as a separate layer.

3. Next, it is necessary to make sure that all of the images are perfectly aligned. If the images were shot on a tripod, it might be tempting to think that the images are already aligned. However, this might not be the case (especially if the camera was touched for manual focusing). Luckily, Photoshop can align the images. To do this, the layers are selected by pressing Control+Alt+a on a PC or Command+Option+a on a Mac. Then, the layers are aligned by choosing Edit/Auto-Align Layers (the Auto option is used).

4. The images are now ready to be blended. In this step, Photoshop will analyze the layers to determine which layer is sharpest at each point in the image and will create masks for each layer based on the analysis. With the layers still selected, choose Edit/Auto-Blend Layers and choose Stack Images. It is also suggested that Seamless Tones and Colors be left unchecked. Yes, it does sound like a really good thing to check. However, if this box is checked, Photoshop will try to match the colors of the layers on an area by area basis determined by the masks that get created. In other words, areas that are not masked out get affected but the other areas are not affected. This may create localized color shifts that may cause color problems later. So, it is best to leave the box unchecked.

5. After the blending, there may be a bit of white along some of the edges that is a result of aligning the images. Consequently, the next step is to crop the image. This can be done by fitting the image on the screen by pressing Control+0 on a PC or Command+0 on a Mack, drawing a rectangle around the image with the Crop tool, and hitting the Enter key.

6. The last step is the cleanup. This is necessary because, while the Photoshop blending is pretty good, it is not perfect. There may be a few places where the image detail was not taken from the sharpest image. When this is the case, the best way to clean up any problems is to identify the layer with the sharpest detail in the problem area. Then, paint white on the mask of this layer in the problem area. It will also be necessary to paint black in the problem area on the masks of all of the higher layers. One other point is that it is usually easiest to determine which layer has the sharpest detail for a problem area by first disabling all of the layer masks. Then, the layers can be clicked off and on until the sharpest layer is found.

There are a few caveats with this technique. Obviously, the technique can only be used with objects that are not moving. Also, the camera will need to be manually focused for each of the multiple shots. The last caveat is that it is better to take several shots, each with the focus point moved only slightly from the previous shot, rather than take only a few shots with significantly different focus points. This will guarantee that there will be no out of focus points in the final image.

When we, as photographers, plan a shot, one of the fundamental decisions that we must make is what we want to be in sharp focus. What we desire to be in focus can vary radically from one image to another. For an expansive vista of the Grand Canyon, we may desire that everything from the foreground to the farthest recesses of the canyon be sharp. For a macro shot of a flower, we may want a very narrow range of focus so that the viewers’ attention is concentrated at a specific point in the image.

The issue of what is and is not in focus is referred to as depth of field. From a practical point of view, depth of field can be defined as the nearest point to the farthest point that appears sharp in an image. So, the question now becomes, “How can we control the depth of field?” Actually, there are several factors that we can control that will affect the depth of field.

Aperture

The camera aperture has a very large impact on depth of field. Basically, the smaller the aperture is, the greater the depth of field that will result. So, if you want a large depth of field, you should use a small aperture. For a small depth of field, you will need a large aperture.

Focal Length

Increasing the focal length reduces the depth of field. Conversely, decreasing the focal length increases the depth of field. Thus, long lenses typically have small depth of fields, and wide angle lenses have large depth of fields. This is one of the reasons that landscape photographers often use wide angle lenses. A wide angle lens combined with a small aperture produces a very large depth of field.

Distance

The greater the distance is from the subject, the greater the depth of field that will result.

Lens Sharpness

Lens sharpness has an impact on depth of field. Sharp lenses are sharper throughout the field of view. This increases the depth of field.

Sensor or Film Size

Okay, this one is a little bit more complicated. In one way, larger sensors in digital cameras or larger film in film cameras produce greater depth of field because the image from a larger sensor or piece of film does not need to be enlarged as much in order to create a print. The less an image is enlarged, the sharper it will appear. This increases the depth of field. On the other hand, cameras with larger sensors or film tend to use longer lenses which decreases the depth of field. So, what is the overall impact? Usually, the larger sensor or film cameras will produce a smaller depth of field. However, that is not a guaranteed thing as much depends on how the camera is used.

Print Size

The general, the larger the print is, the smaller the depth of field that results. This is because the print has to be enlarged more from the original size of the sensor or film in order to create a large print. This softens the print detail and reduces the depth of field.

Focusing Distance

When it comes to the focus distance, there is one very special focusing distance called the hyperfocal distance. The hyperfocal distance is the shortest distance from the camera such that everything from approximately half that distance to infinity will appear sharp. Focusing the camera at the hyperfocal distance results in the greatest depth of field possible for a given combination of aperture and focal length.

There are a number of ways to determine the hyperfocal distance. Probably, the most common method is to use a depth of field guide. A depth of field guide is an inexpensive, small device that allows a photographer to determine hyperfocal distances as well as other depth of field parameters.

In addition, photographers that have older lenses may find that their lenses have depth of field scales engraved on the lenses. To set the hyperfocal distance, one simply aligns the infinity symbol with the color coded depth of field lines. Unfortunately, most modern lenses no longer have depth of field scales.

Summary

With the above information, you should be ready to head into the field with full control over the depth of field in your images.

If you are interested in learning even more about depth of field, please check out Ron’s full length depth of field article at Depth of Field