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Depth of Field
Depth of Field, or Why isn’t everything in focus?
So it’s been almost two years since my last “how photography works” post, and I know y’all have been waiting with bated breath, but Depth of Field is a much easier subject to talk about when there are examples to look at, and until recently, I didn’t have any useful examples. But now I do. I actually wrote most of this post two years ago, so if there are disjunctions, paragraphs dropped halfway through, or any other problems with the text, do let me know!
Depth of field
The short answer to “Why isn’t everything in focus?” is that a *point* in the field of view is only represented as a *point* on the film or sensor when that point is exactly in the plane of focus. Points closer to or further from the sensor than the plane of focus are represented on the sensor as small, blurry circles. The size of these circles is related to the distance between the point and the plane of focus, and to the aperture chosen. The closer to the plane of focus, or the smaller the aperture (larger f/number), the smaller the "circle of confusion" and the sharper that point appears to the audience. The range of distances where the circle of confusion is small enough to appear sharp is called the depth of the field.
Other factors that affect the range of acceptable focus are:
1) the size of the enlargement (because the bigger you blow up the picture, the bigger the circles get)
2) the viewing distance of the final print (because the farther you get from the print, the smaller the circles look)
3) the focal length of your lens (wide-angle lenses have much more depth-of-field for any given f/stop than longer lenses)
4) the distance between the camera and the subject (the farther away the subject, the greater the distance between closest and farthest acceptable focus for any given f/stop).
The primary tool you have in controlling depth of field is your selection of f/stop. The bigger the f/stop number, the smaller the aperture, and the smaller the circle of confusion for any given point in your field of view. Rule of thumb is that acceptable depth of field extends twice as far beyond the plane of focus as in front of it. Put another way:
P_________A___B______C______________________infinity
Where P is the photographer, B is the focus distance, A is the closest point of acceptable focus, and C is the furthest point of acceptable focus.
If the distance between A and B is one foot, the distance between B and C will be somewhere around two feet. This “rule” holds only in the middle range of camera-to-subject distances; for very close shots, the range of acceptable focus is so small that A-B and B-C approach equivalence, and for distant shots, it is possible to set things up such that C stretches out to infinity.
For a quick example of how depth-of-field actually works in practice, take a look at these two images, in which the only difference is the aperture I chose and the corresponding alteration in shutter speed:
The left image was shot at f/2.8, the right at f/8.0.
Note the significantly smaller area of sharp focus in the f/2.8 image. I could have dialed the aperture down to f/22 or even f/32, and gotten even more of the image sharp, but take a close look at the bottom-left corner of both images. At a narrower aperture, that gravel would have started to be an ugly distraction, and nobody wants that. (Which nicely shows one reason why you might want your images to have less total depth of field than is technically achievable*.)
In both frames, I have focused the camera on the exact same spot: the middle of the foreground blossoms. The camera was locked on a tripod to eliminate shift between exposures and shake during them.
You are free to form your own opinion, but in this case, I prefer the f/8 image. Cactus needles should be sharp, methinks.
Wider-angle lenses and most macro lenses have a depth-of-field guide built right in, at least for the higher f/stops. To the left and right of the focus mark on the lens should be pairs of hash marks labeled with f/stop numbers. These hash marks show you the near-and-far range of acceptable focus (under ideal conditions) for each listed f/stop--the focus mark points at the distance of the focal plane, each DOF hash mark points at the distance of one end of the range of acceptable focus.
Your camera also might have a depth-of-field preview button (fairly likely with an SLR, very unlikely for a point-and-shoot**). Generally, the camera holds the lens aperture wide open while you're composing your image, which shows you the absolute minimum depth of field achievable by your lens. Pushing the DOF preview button closes the lens aperture down to where it's going to be when you actually release the shutter, which does two things for you. First, it makes the viewfinder very very dark, which makes it kind of difficult to actually see the second effect. Second, it shows you exactly the image that you will be capturing, with all circles of confusion the same size they will be on the captured image. The DOF preview button has other uses for helping your composition, but those are a nice long post all on their own.
I have an iPhone app called “Photo Buddy” which looks to have a pretty good DOF calculator, which I have never yet remembered to actually use when I am out in the field with my camera.
*Other reasons include, but are not limited to:
- isolating a subject from a confusing background
- showing the viewer what the really important part of the image is
- boosting shutter speed to capture a moving subject
**A P&S is unlikely to let you adjust the aperture anyway, and even if it does, it won’t have a wide range of apertures to choose from, but most P&S cameras have a *very* short lens with a small sensor, so you’ll have pretty good depth of field even shooting wide open.
Related topics (will add links when associated articles are written, or you can search on my “photography” tag):
Exposure
Focal lengths, f-stops, photon density, and autofocus
What the hell is "bokeh"?
Depth of Field
Camera shake and motion blur
Using the flash (Yeah, right! As soon as I figure it out myself…)
Film: Reciprocity failure
Expanding depth of field with Photoshop
Digital: The histogram is your friend
Feel free to suggest topics as well, though I make no commitment to address them in a timely manner. :)
Disclaimer/acknowledgments: Except as noted, all the above text is original to me, composed without direct reference to any other source. However, there are some people who have had a profound impact on my knowledge of photography, and it is possible that what came out of my head is very similar in language to what they tried so hard to put into it, so it would be remiss of me if I did not acknowledge their prior assistance.
I am deeply and directly indebted to Rod Barbee, Scott Bourne, Don Mammoser, and David Middleton for their in-person assistance and instruction. I have also been significantly influenced by the written works of Ansel Adams, Art Wolfe, David Gardner, and Galen Rowell. Years of articles in _Outdoor Photographer_ magazine provide some of my background as well.
So it’s been almost two years since my last “how photography works” post, and I know y’all have been waiting with bated breath, but Depth of Field is a much easier subject to talk about when there are examples to look at, and until recently, I didn’t have any useful examples. But now I do. I actually wrote most of this post two years ago, so if there are disjunctions, paragraphs dropped halfway through, or any other problems with the text, do let me know!
Depth of field
The short answer to “Why isn’t everything in focus?” is that a *point* in the field of view is only represented as a *point* on the film or sensor when that point is exactly in the plane of focus. Points closer to or further from the sensor than the plane of focus are represented on the sensor as small, blurry circles. The size of these circles is related to the distance between the point and the plane of focus, and to the aperture chosen. The closer to the plane of focus, or the smaller the aperture (larger f/number), the smaller the "circle of confusion" and the sharper that point appears to the audience. The range of distances where the circle of confusion is small enough to appear sharp is called the depth of the field.
Other factors that affect the range of acceptable focus are:
1) the size of the enlargement (because the bigger you blow up the picture, the bigger the circles get)
2) the viewing distance of the final print (because the farther you get from the print, the smaller the circles look)
3) the focal length of your lens (wide-angle lenses have much more depth-of-field for any given f/stop than longer lenses)
4) the distance between the camera and the subject (the farther away the subject, the greater the distance between closest and farthest acceptable focus for any given f/stop).
The primary tool you have in controlling depth of field is your selection of f/stop. The bigger the f/stop number, the smaller the aperture, and the smaller the circle of confusion for any given point in your field of view. Rule of thumb is that acceptable depth of field extends twice as far beyond the plane of focus as in front of it. Put another way:
P_________A___B______C______________________infinity
Where P is the photographer, B is the focus distance, A is the closest point of acceptable focus, and C is the furthest point of acceptable focus.
If the distance between A and B is one foot, the distance between B and C will be somewhere around two feet. This “rule” holds only in the middle range of camera-to-subject distances; for very close shots, the range of acceptable focus is so small that A-B and B-C approach equivalence, and for distant shots, it is possible to set things up such that C stretches out to infinity.
For a quick example of how depth-of-field actually works in practice, take a look at these two images, in which the only difference is the aperture I chose and the corresponding alteration in shutter speed:
The left image was shot at f/2.8, the right at f/8.0.
Note the significantly smaller area of sharp focus in the f/2.8 image. I could have dialed the aperture down to f/22 or even f/32, and gotten even more of the image sharp, but take a close look at the bottom-left corner of both images. At a narrower aperture, that gravel would have started to be an ugly distraction, and nobody wants that. (Which nicely shows one reason why you might want your images to have less total depth of field than is technically achievable*.)
In both frames, I have focused the camera on the exact same spot: the middle of the foreground blossoms. The camera was locked on a tripod to eliminate shift between exposures and shake during them.
You are free to form your own opinion, but in this case, I prefer the f/8 image. Cactus needles should be sharp, methinks.
Wider-angle lenses and most macro lenses have a depth-of-field guide built right in, at least for the higher f/stops. To the left and right of the focus mark on the lens should be pairs of hash marks labeled with f/stop numbers. These hash marks show you the near-and-far range of acceptable focus (under ideal conditions) for each listed f/stop--the focus mark points at the distance of the focal plane, each DOF hash mark points at the distance of one end of the range of acceptable focus.
Your camera also might have a depth-of-field preview button (fairly likely with an SLR, very unlikely for a point-and-shoot**). Generally, the camera holds the lens aperture wide open while you're composing your image, which shows you the absolute minimum depth of field achievable by your lens. Pushing the DOF preview button closes the lens aperture down to where it's going to be when you actually release the shutter, which does two things for you. First, it makes the viewfinder very very dark, which makes it kind of difficult to actually see the second effect. Second, it shows you exactly the image that you will be capturing, with all circles of confusion the same size they will be on the captured image. The DOF preview button has other uses for helping your composition, but those are a nice long post all on their own.
I have an iPhone app called “Photo Buddy” which looks to have a pretty good DOF calculator, which I have never yet remembered to actually use when I am out in the field with my camera.
*Other reasons include, but are not limited to:
- isolating a subject from a confusing background
- showing the viewer what the really important part of the image is
- boosting shutter speed to capture a moving subject
**A P&S is unlikely to let you adjust the aperture anyway, and even if it does, it won’t have a wide range of apertures to choose from, but most P&S cameras have a *very* short lens with a small sensor, so you’ll have pretty good depth of field even shooting wide open.
Related topics (will add links when associated articles are written, or you can search on my “photography” tag):
Exposure
Focal lengths, f-stops, photon density, and autofocus
What the hell is "bokeh"?
Depth of Field
Camera shake and motion blur
Using the flash (Yeah, right! As soon as I figure it out myself…)
Film: Reciprocity failure
Expanding depth of field with Photoshop
Digital: The histogram is your friend
Feel free to suggest topics as well, though I make no commitment to address them in a timely manner. :)
Disclaimer/acknowledgments: Except as noted, all the above text is original to me, composed without direct reference to any other source. However, there are some people who have had a profound impact on my knowledge of photography, and it is possible that what came out of my head is very similar in language to what they tried so hard to put into it, so it would be remiss of me if I did not acknowledge their prior assistance.
I am deeply and directly indebted to Rod Barbee, Scott Bourne, Don Mammoser, and David Middleton for their in-person assistance and instruction. I have also been significantly influenced by the written works of Ansel Adams, Art Wolfe, David Gardner, and Galen Rowell. Years of articles in _Outdoor Photographer_ magazine provide some of my background as well.