How to manage depth of field, Part 2


When you take a photo, you need to set the aperture before you choose the shutter speed in order to control the depth of field. For a portrait, for example, you might want a depth of field of only a foot or so to isolate the face from the background. For a landscape you might want everything in focus from near to infinity. Obviously you would choose a wider aperture for the portrait, and a narrower aperture for the landscape. But which aperture exactly? The traditional way of doing this is to use the depth of field preview button on your camera to set the lens aperture to your chosen value while looking through the view finder (or at your live view image if your camera permits it- mine doesn’t). Without pressing the preview button you are seeing the image through the widest available aperture for that lens. Pressing the button reduces the aperture to the one you have selected. This can be a useful technique for choosing an aperture which is at the wider end of the spectrum of choices, such as when doing a portrait. You directly see what is in focus and what is not for different aperture choices. However, if you want to see the effect of smaller apertures, i.e. f/11 or f/16, the reduction of light to the viewfinder may make it very difficult to see well enough to see what is in focus and what is not. If you have this problem, as I do, then it is helpful to use a depth of field calculator to make this decision.

For many years I have been using the ExpoAperture2 depth of field calculator. Surprisingly, in the digital era, this is still available from large photography suppliers. As an analog device it doesn’t need to be turned on, doesn’t require a password, and won’t break if I drop it. It also easily demonstrates what we are looking at when we set the depth of field. To use it, first compose your image and decide what objects in the image need to be in focus. Imagine you are taking a landscape with a 24 mm lens where the nearest object in the image, a foreground rock, is 5 ½ feet away (measured from the camera). Otherwise, you want everything to the horizon to be in focus. First look at the back of the calculator. In the image below, there is a distance scale around the outer edge. The arrows point to the near focus point, 5 feet to be safe, and far focus point at infinity. To be certain of getting the near object in focus it is safer to choose a somewhat closer distance. My trick for doing this is to stand next to the camera and measure the distance from my feet to the nearest object. This will be a shorter distance than the distance from the camera itself.

Count the alternating grey and white zones between the near and far points. There are six zones. Also note the halfway point between them, i.e. three zones from the near focus point. This distance, 10 feet, is the hyperfocal distance. This is a very important distance and represents the plane of focus which will give adequate sharpness between 5 feet and infinity as long as you choose a sufficient aperture to include the six zones of field. Objects in the field of view which are an equal number of zones either in front of or behind the hyperfocal point will be equally out of focus. Note that you have less depth of field in front of the hyperfocal distance than behind it. For example 1 zone is 2 ½ feet in front of the focus plane, but 5 feet behind. As discussed in Part 1, depth of field increases with the distance from the camera. The objective is to determine the necessary aperture such that the degree to which the zones you selected are out of focus does not exceed the circle of confusion.

To determine the necessary aperture, turn the calculator over and look at the other side.



On the front of the calculator you first need to set the circle of confusion (CoC) by turning the outer dial. On this calculator you can simply set the sensor size (1.6 crop factor for my camera) and the CoC is set automatically (0.019 mm in this case). On the inner dial you set the focal length to 24 mm for the lens you are using. Look at the outer edge of the calculator for the f-stop which gives at least six zones for the depth of field (we determined this number above). We see that f/11 gives 6 ½ zones while f/8 only gives 4 ½ zones. Therefore, f/11 is the correct f-stop.

To take the picture, set the f-stop to f/11 in either aperture or manual mode. Focus on an object 10 feet (the hyperfocal distance) from the camera. Then turn off autofocus (if you had it on) so it doesn’t refocus on something else when you recompose to take the picture. You don’t have to focus on something in the field of view. I sometimes pace off 10 feet, or whatever the actual hyperfocal distance is for the lens I am using, from a tree trunk (use a tape measure if compulsive) and focus on that. Then I switch off autofocus, put the camera back on the tripod, and don’t touch the focus again. After you take the picture you can check it on your LCD screen to make sure the focus looks okay.

Since we noted that small apertures give greater depth of field, you might ask why not just use a very small aperture such as f/16 and not bother with all this. The reason is that there is a trade off in sharpness when you adjust aperture. For most DSLR lenses, apertures in the f/8 to f/11 range give the sharpest images. The reason is that wider apertures use more of the lens diameter. It is difficult to maintain perfect curvature to the edge of the lens (although expensive lenses do this better). So larger apertures may give suboptimal focus, particularly at the edges of the image. Smaller apertures have the problem that light is diffracted by the edge of the diaphragm. With very small apertures, the edge of the diaphragm affects a larger area relative to the size of the diaphragm opening, and therefore may also cause a blurrier image. However, apertures of f/16 or smaller may be needed to get everything in focus. The point is to use the best aperture for the image. For landscapes this is usually the largest aperture which is calculated to keep everything in focus. For portraits, you would follow the same procedure as above, except you would probably be using a lens with a longer focal length and would choose a smaller number of zones (perhaps only one or two) to get shallow depth of field.

The other alternative in the digital era is to use a cell phone app. I mainly use this for macro photography for which the analog device above is not suitable. For a wildflower portrait, I might be using my 70-200 mm zoom lens set at 100 mm with an extension tube for close focusing. As with human portraits, you want the flower to be sharp, but the background to be blurred to isolate the flower. You might think a wide aperture would also be best, but as you will see, this may not be the case. You are usually working at close range and with a telephoto lens, both of which will tend to reduce depth of field. Therefore, a smaller aperture may be needed, but how small? As always, you can use the depth of field preview, but again there is the difficulty of seeing clearly with small apertures. In this case, a depth of field app may be helpful.

With the one illustrated above, you first enter your camera model in order to set the default circle of confusion (professionals may want a more sophisticated app which allows you to specify CoC exactly). You choose the f-stop, focal length of the lens, and distance to subject, in this case 30 inches. Below you read the depth of field with near and far focus points. In this example I wanted a total depth of field of 1 inch and found that f/16 was needed, even at 30 inches distance. If I reset the aperture choice to f/13, the depth of field is 0.96 in (not shown) which may be close enough. At f/11 the depth of field is 0.85 inches. Therefore, I should shoot at either f/16 or f/13. It should be noted that these calculations are not perfect, and it is not possible to tell how far out of focus the background actually is by looking at the calculator alone. If you can’t see with the depth of field preview button on your camera, it would be wise to bracket the shot taking exposures at f/16, f/13, and perhaps f/11 (making appropriate adjustments to shutter speed) and choosing the best image after viewing on your computer screen. For some flower portraits, minimal blurriness at the near and far edges of the flower might be acceptable if the background was less intrusive. You may also need to experiment with changing the camera distance. A closer, tighter shot will eliminate intrusive background detail, but only the stamens and pistils may be in focus, even with a small aperture. Moving the camera back will show more of the background detail, but the entire flower will be sharp, although you may have to crop the image. It is a subjective decision. However, the depth of field app at least gives you a starting point for adjusting these parameters.

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