Location: Mt. Pilkchuck trail, North Cascades, Washington // Time: Sunday hike
Being able to control which parts of a photograph are in focus and which ones are not is a very powerful way to catch the viewer’s eye. In this post I will talk about depth of field and how we can control it to achieve selective focus in your photographs.
Understanding depth of field
You have probably heard about it before: depth of field. Sounds almost mystic. Your camera may even have a button to pre visualize it, and you may not even know it’s there. So let’s start from the beginning. Let’s say you are in the fields and set focus point at a red flower, like the one in the diagram below. In reality an entire plane encompassing the red flower will be in focus, that is the focal plane. Now let’s also say that your red flower is in between a blue and a brown flower, so that the blue flower is closest to you, and the brown is the furthest. Depth of field can be defined as the region in front and behind the red flower (the focal plane) that appears acceptably sharp. And here is the first parameter that affects depth of field: aperture. In the example below aperture is set at f/2.8, which is a large aperture. As a result the focused region in front of and behind the focus plane is very narrow, so only the red flower is in sharp focus. In this case we say there is a shallow depth of field.
So what can we do to increase the depth of field? We can use a smaller aperture, such as f/16, like in the example below. Now the depth of field is much larger, and even though you still set the focus point on the red flower, both the blue and brown flowers are in acceptably sharp focus.
The second parameter that affects depth of field is the focal length of the lens you are shooting with. In this case, you just need to know that wide angle lenses have larger depth of field than longer focal length lenses.
And the last parameter I will mention is distance to the focal point: the shorter the distance, the shallower the depth of field. If you like macro photography I am sure you have experienced this.
Shallow depth of fields: large apertures, long focal distance lenses, short distance to focus point.
Large depth of field: small apertures, wide angle lenses, long distance to the focus point.
Now, be careful when using extreme aperture values. Why? Well, most lenses are soft or very soft at the corners when used wide open. This usually does not matter much as you want the corners out of focus anyways when shooting wide open. But how about with very small apertures? What if, let’s say you want to achieve the largest depth of field possible? Would you just close aperture to the maximum value? Let’s say f/32? Well, be careful with this as well. Turns out when using extremely small apertures, and you can probably consider any aperture smaller than f/22 quite extreme, you may actually achieve larger depths of field, but the overall quality of your image will degrade due to light diffraction. How does this happen? When using very small apertures the light is forced to go through a small circle in the middle of the image, so the rays of light that will hit he sensor will do so in a very oblique angle. As a result, more than one pixel may pick up that ray of light traveling sideways, and you will end up with a blurrier image. So be careful when using extremes!
Two examples of selective focus
And now, nothing better than talking about the two photos in this post, both good examples of shallow depth of field and selective focus.
I took the photo above during a hike up Mount Pilkchuk in the North Cascades. Along the way we passed a few small tarns surrounded by bushes covered in early morning dew. As you can see the depth of field is extremely shallow, probably a few millimeters, as only a few dew droplets are in sharp focus around the focus point. The fact that I was using a 90 mm lens and the close proximity to the leaf made it very easy to achieve the shallow depth of field by using an aperture of only f/7.1. I could have used the maximum aperture for this lens, f/2.8, but then the depth of field would have been too narrow.
The photo below, from a lavender farm in the Olympic peninsula, was taken with a 50 mm lens. So here, as I was using a shorter focal length I had to use a much larger aperture of f/1.8 to achieve the narrow depth of field and thus the selective focus .
Tip: Whenever you are taking photos with very shallow depth of field I highly recommend using a tripod. This will allow you to manually focus through the back screen. Most DSLRs will let you magnify the image, which will greatly help you get the focus where you want it.
Pre visualizing the depth of field
As I said at the beginning of the post, your camera, specially if its a higher-end DSLR (like the Nikon D300), may have a button that allows you to pre visualize the depth of field. How does this work? Before you take a photo the diaphragm in your camera stays always at its maximum aperture, wide open. This is to allow your camera to meter light and focus properly. Then, just as you press the shutter, the diaphragm closes to the selected aperture. So cameras that allow you to pre visualize depth of field let you close the diaphragm before the photo is taken, simply by pressing a button. If you select a small aperture you will notice that the image through the viewfinder gets darker. Very useful feature whenever not sure how much depth you have. Obviously, as an alternative you can also take the photo and check in on the back screen!
Well, this is it for now! I just realized I haven’t posted in a while. Well, as an excuse I will say that I have been very busy post processing photos from my first wedding. As a photographer I mean! Maybe I’ll tell you about what an experience it was in a future post! Until then have fun taking photos!
As usual, I would love to hear your questions, comments or critiques!
First photo: 1/400 sec @ f/7.1, ISO 500. Taken with the Tamron SP AF 90mm f/2.8 Di SP MACRO.
Second photo: 1/5000 sec @ f/1.8, ISO 100. Taken with the Nikkor 50mm f/1.8 D.
Location (dew droplets):
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