Spiritmist
DNADoctor
Hello all,
For many photographers, even advanced ones, Depth of Field (DOF) can be a confusing topic. It need not be, however, and it is well worth taking the time to understand what DOF is and how it may be manipulated in order to make the kind of photos you want to make. This is especially true for those of us interested in shooting close-ups/macros of our favorite critters, since adequate DOF can be difficult to obtain in those kinds of shooting situations. With that in mind (and with some encouraging words from Connie), I thought I would post this DOF primer. I will talk briefly about all of the factors that affect DOF, but then discuss the three most important ones for most photographers (the ones that are camera-controllable) and how to use them.
First- DOF is dependent upon:
1) The maximum permissible size of the Circle of Confusion
2) Aperture diameter
3) Camera-to-subject distance
4) All factors that affect the image size as it is ultimately viewed, including focal length, print magnification and print viewing distance.
These factors are interrelated, their relationships are complex, and they do not all affect DOF equally. But for our purposes now it’s enough to know that they are the only things that affect DOF.
Circles of Confusion (and a definition of DOF):
When a lens is focused at any given distance, maximum sharpness is generated *only* at the plane of focus at that distance. This is called the “principal plane of focus” or the “critical plane of focus”. Objects on either side of that critical plane of focus will appear less sharp the farther away from it they are. The DOF is the extent of the distance between the nearest and farthest planes from the critical plane of focus within which objects appear “acceptably sharp”. So what does that mean? …
If you were to focus precisely on a point source of light, the light would appear as a point at the image plane. The farther your point of light was from the actual critical plane of focus, the more unsharp it would be; instead of registering as a point at the image plane, it would register as an increasingly blurred circle of light. These circles are known as “circles of confusion” (CoC). If these circles are small enough, our eyes will still see them as points, and the image appears acceptably sharp. If the circles of confusion get too large, our eyes see the blur; the image is no longer acceptable sharp, and that plane lies outside the DOF. DOF can thus also be defined as maximum permissible diameter of the CoC that is apparent in an out-of-focus photo. CoCs can be very useful, but a really thorough discussion of them is outside this article’s scope. I just wanted to mention them so that people would be familiar with them, but we’ll move on here.
Now we come to the important part: the camera-controlled factors affecting DOF and how to use them.
There are only three camera-adjustable factors affecting DOF: aperture, focal length and lens-to-subject distance.
Aperture:
DOF decreases as aperture increases (i.e., as f number get smaller), and increases as aperture decreases (i.e., as f numbers get larger). This is because the light cone passing through the lens is narrower at small apertures, and consequently is not as spread out even when not perfectly focused. As a result, more of the captured scene appears sharp.
Focal Length:
DOF decreases as focal length increases (more zoom), and increases as focal length decreases (less zoom). This is because image magnification magnifies differences in focus. When details are rendered larger in an image, it is easier to make out what is sharp and what is not. (This is also the logic behind how print size and viewing distance affect DOF, incidentally).
Lens-to-Subject Distance:
DOF decreases as distance-to-subject decreases, and increases as distance-to-subject increases. This is due to two things. The main one is the increased magnification of the image (see above). The other thing is that when you are focused close the lens is set further back from the focal plane, creating a small but real effective focal length increase (again, see above.)
So if you were trying for the theoretically shallowest DOF, you would want to shoot at max zoom, using your most open aperture (lowest f number) and at the minimum possible distance at which you can focus on your subject. It's also very useful to physically separate your subject from your background as much as possible to ensure a nice shallow DOF effect.
If you were trying for the theoretically deepest DOF, you'd want to shoot at max wide-angle, using your most closed aperture (highest f number) and from as far away (infinity) as possible to your subject. (In practice, there are some subtleties that slightly alter this deep DOF example. For example- rather than focusing at infinity, you could focus at something called the “hyperfocal distance”. This would give you a greater DOF in this case. But perhaps we'll leave that for another article.)
Hope this has been helpful, or at least interesting, for someone.
If anything is not clear or there are questions, I'll try to do my best to help.
Kind Regards,
Brian
For many photographers, even advanced ones, Depth of Field (DOF) can be a confusing topic. It need not be, however, and it is well worth taking the time to understand what DOF is and how it may be manipulated in order to make the kind of photos you want to make. This is especially true for those of us interested in shooting close-ups/macros of our favorite critters, since adequate DOF can be difficult to obtain in those kinds of shooting situations. With that in mind (and with some encouraging words from Connie), I thought I would post this DOF primer. I will talk briefly about all of the factors that affect DOF, but then discuss the three most important ones for most photographers (the ones that are camera-controllable) and how to use them.
First- DOF is dependent upon:
1) The maximum permissible size of the Circle of Confusion
2) Aperture diameter
3) Camera-to-subject distance
4) All factors that affect the image size as it is ultimately viewed, including focal length, print magnification and print viewing distance.
These factors are interrelated, their relationships are complex, and they do not all affect DOF equally. But for our purposes now it’s enough to know that they are the only things that affect DOF.
Circles of Confusion (and a definition of DOF):
When a lens is focused at any given distance, maximum sharpness is generated *only* at the plane of focus at that distance. This is called the “principal plane of focus” or the “critical plane of focus”. Objects on either side of that critical plane of focus will appear less sharp the farther away from it they are. The DOF is the extent of the distance between the nearest and farthest planes from the critical plane of focus within which objects appear “acceptably sharp”. So what does that mean? …
If you were to focus precisely on a point source of light, the light would appear as a point at the image plane. The farther your point of light was from the actual critical plane of focus, the more unsharp it would be; instead of registering as a point at the image plane, it would register as an increasingly blurred circle of light. These circles are known as “circles of confusion” (CoC). If these circles are small enough, our eyes will still see them as points, and the image appears acceptably sharp. If the circles of confusion get too large, our eyes see the blur; the image is no longer acceptable sharp, and that plane lies outside the DOF. DOF can thus also be defined as maximum permissible diameter of the CoC that is apparent in an out-of-focus photo. CoCs can be very useful, but a really thorough discussion of them is outside this article’s scope. I just wanted to mention them so that people would be familiar with them, but we’ll move on here.
Now we come to the important part: the camera-controlled factors affecting DOF and how to use them.
There are only three camera-adjustable factors affecting DOF: aperture, focal length and lens-to-subject distance.
Aperture:
DOF decreases as aperture increases (i.e., as f number get smaller), and increases as aperture decreases (i.e., as f numbers get larger). This is because the light cone passing through the lens is narrower at small apertures, and consequently is not as spread out even when not perfectly focused. As a result, more of the captured scene appears sharp.
Focal Length:
DOF decreases as focal length increases (more zoom), and increases as focal length decreases (less zoom). This is because image magnification magnifies differences in focus. When details are rendered larger in an image, it is easier to make out what is sharp and what is not. (This is also the logic behind how print size and viewing distance affect DOF, incidentally).
Lens-to-Subject Distance:
DOF decreases as distance-to-subject decreases, and increases as distance-to-subject increases. This is due to two things. The main one is the increased magnification of the image (see above). The other thing is that when you are focused close the lens is set further back from the focal plane, creating a small but real effective focal length increase (again, see above.)
So if you were trying for the theoretically shallowest DOF, you would want to shoot at max zoom, using your most open aperture (lowest f number) and at the minimum possible distance at which you can focus on your subject. It's also very useful to physically separate your subject from your background as much as possible to ensure a nice shallow DOF effect.
If you were trying for the theoretically deepest DOF, you'd want to shoot at max wide-angle, using your most closed aperture (highest f number) and from as far away (infinity) as possible to your subject. (In practice, there are some subtleties that slightly alter this deep DOF example. For example- rather than focusing at infinity, you could focus at something called the “hyperfocal distance”. This would give you a greater DOF in this case. But perhaps we'll leave that for another article.)
Hope this has been helpful, or at least interesting, for someone.
If anything is not clear or there are questions, I'll try to do my best to help.Kind Regards,
Brian