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Depth of Field Primer - Part I
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<blockquote data-quote="Joseph Bautsch" data-source="post: 794904" data-attributes="member: 654"><p><strong>A Depth of Field Primer &#8211; Part I</strong></p><p></p><p>One of the most confusing subjects for a beginner or even an intermediate level photographer to understand is Depth of Field (DOF). There are three basic elements of depth of field:</p><p></p><p>(1) F/stop</p><p>(2) Focal length</p><p>(3) Focus point distance</p><p></p><p>The problem with understanding and using depth of field lies with the interrelationships of these three elements. They each have an effect and changing any one or two in any combination and the depth of field changes. This means that changes are not readily apparent or easy to understand. So you have to have a good understanding of what these relationships are and how they affect depth of field before you can put it to any practical use. Once you understand their effect on the DOF you will be able to do a better job of combining changes in each to achieve the depth of field and exposure you want.</p><p></p><p>Simply put the depth of field is that distance in front of the subject focus point where everything will appear to be in focus and that distance behind the subject where everything will appear to be in focus. The two distances combined make up the total depth of field. In most cases the distance behind the subject will be the larger of the two. Sometimes as much as 98% of the total depth of field will be in back of the subject. Other times the two measurements will be almost equal. This will become apparent as we get into the calculations for DOF.</p><p></p><p>A lens will focus on a finite point at any given distance. Even the smallest of movement to the front or back of that point will cause focus to start falling off. So where does depth of field come from? It is primarily a function of our eyesight. Our eyes are not nearly good enough to detect this focus falling off until it reaches a certain distance in front of or to the back of the point of focus. As the distance from the point of focus gets larger it will reach a certain limit where the eye suddenly becomes aware of &#8221;fuzziness&#8221;. Those are the front and back points from the point of focus that are used for the measurement of depth of field. How far these two points are from the point of focus is determined by three lens functions, f/stop, focus distance, and focal length. </p><p> </p><p>There are two functions of DOF, which you may have heard of, that are beyond the scope of this discussion. These are Hyperfocal Distance and Circle of Confusion. Before you can understand either of these functions you have to have a good understanding of the DOF basics. </p><p></p><p><strong>Methods of Measurement: Point of Focus and Near/Far Limit</strong></p><p></p><p>To add to the confusion understanding DOF there are two methods of measurement. One method uses the camera as its starting measurement point, the lens focal plane, and gives the near limit and far limit distances. The second method uses the point of focus and measures the distance in front of and to the back of the subject. Both measuring methods actually do the same thing but from different points of view. </p><p></p><p>The measurements used in this explanation are the distance points to the front and back of the subject focus point. Doing it this way makes it easier to understand the relationships involved. The other method of measurement, near and far limits is more practical for field use. Trying to judge a distance in front of or behind a subject that may be 25 ft., 100 ft. or more in front of you is not going to be as accurate as judging those same distances from where you are standing. Hence, most all the field use devices for depth of field, such as printed charts, printed pocket calculators, and the ones used with devices such as the iPhone, will be based on the near and far limits. Go to figure one below for a graphic layout of these measurements.</p><p></p><p> Figure &#8211; One </p><p>[ATTACH]383270[/ATTACH]</p><p></p><p><strong>Depth of Field Calculator</strong></p><p></p><p>To get the most out of this explanation of depth of field you should open the DOFMaster Calculator and do the calculations as they are presented. The DOFMaster Depth of Field Calculator web site is listed below. There is a drawing there, similar to figure one, that demonstrates these measurements. The drawing in the DOF calculator is interactive with the calculator and displays the distance measurements resulting from the entry calculations. </p><p></p><p><a href="http://www.dofmaster.com/dofjs.html" target="_blank">Online Depth of Field Calculator</a></p><p></p><p>This Calculator needs some words of explanation. In the top left bracket of the calculator you will find the listing for your Nikon in the top drop down. Below that is the Focal Length entry (Actual Focal Length) and below that is the Selected F/stop. And last is the entry for distance. If you are using a focus distance of less than five feet, changing that measurement to inches rather than feet will compute a more accurate measurement. Over on the right it provides the calculations for both the near and far limits at the top and, just below, the distance in front of and behind the subject. </p><p></p><p>However, the main point of confusion to be cleared up is the actual focal length to be used in the calculation. Just below the main body of the calculator they show a picture of two zoom lenses with the longest focal length of each circled as the actual focal length. These are point and shoot lenses, which only have the wide-angle limit, and the telephoto limit printed on the front. For these point and shoot lenses the longest focal length or the shortest focal length, depending on which the camera is set closest to, is recommended as the actual one to use. There are no DSLR lenses listed and they are not needed as the actual focal length can be read directly on the lens, as a prime, or as it is zoomed in or out. I.e. if on a 70-300mm zoom lens you are zoomed to 125mm then that is the actual focal length of the lens to use. </p><p> </p><p><strong>The f/stop Relationship</strong></p><p></p><p>The f/stop is the primary, &#8220;tool&#8221; to change and control the depth of field. To demonstrate the f/stop effect on depth of field it&#8217;s necessary to isolate it from the effects of the other two elements. To do that fixed values are assigned to the focal length and to focus distance. Then the f/stop can be changed to demonstrate its effect. Let&#8217;s use the common shooting values of, 50mm focal length and a 12ft. focus distance and change the f/stop values from f/5.6 to f/22. Entering these numbers in the DOF Calculator produces the following numbers.</p><p></p><p>F/stop of 5.6; DOF Total 4.03 ft.; In Front 1.68 ft.; In Back 2.34 ft.</p><p>F/stop of 8.0; DOF Total 5.85 ft.; In Front 2.25 ft.; In Back 3.6 ft.</p><p>F/stop of 16; DOF Total 14.1 ft.; In Front 3.8 ft.; In Back 10.3 ft.</p><p>F/stop of 22; DOF Total 27.3 ft.; In Front 4.7 ft.; In Back 22.6 ft.</p><p></p><p>This chart demonstrates the increase in total depth of field as the f/stop number increases. Most photographers learn early on that increasing the f/stop from a lower number, f/5.6 to a higher number, f/22 will increase the depth of field and decrease it going in the reverse direction. Look at the changes in front of the subject focus point from f/5.6 to f/22, it goes from 1.68 ft. to 4.7 ft. Not a lot of change, only 3 ft. But look at the change in back, it goes from 2.34 ft. to 22.6 ft. a change of just over 20 ft. At f/22 the distance in back of the subject is 83% of the total depth of field distance. At f/5.6 it&#8217;s 58% of the total. But there is a lot more going on here that needs to be understood. </p><p></p><p>There are four basic relationships that need to be understood from the f/stop effect on depth of field that are very important in learning its use. </p><p></p><p></p><p>(1) Where are the distance limit points of the DOF both in front and in back of the subject focus point located? Knowing where these distance points are located will tell you what will be in focus and what will not. These point locations can also be expressed in near limit and far limit distances as measured from the camera. </p><p></p><p>(2) Changing the total depth, with an increase or decrease in f/stop, allows you to put objects, background or foreground, in or out of focus as needed. </p><p></p><p>(3) An increase in depth of field will result in greater changes in back of the subject focus point relative to the changes in front. This allows you to control background focus. </p><p></p><p>(4) You don&#8217;t necessarily need to focus directly on the subject. You can move the focus point forward or back to shift the total depth of field to place it where it&#8217;s needed. (This also changes the size of the DOF so a re-calculation may be needed to be sure it still covers the area to be in focus).</p><p></p><p>The general rule applying to the f/stop effect on depth of field is: &#8220;Any increase or decrease in f/stop will expand or contract the depth of field for any given setting of focus distance and/or focal length&#8221;. </p><p></p><p> <a href="http://nikonites.com/blogs/joseph-bautsch/37-depth-field-primer-part-ii.html" target="_blank">Part II - The Focus Distance and Focal Length Relationship</a></p></blockquote><p></p>
[QUOTE="Joseph Bautsch, post: 794904, member: 654"] [B]A Depth of Field Primer – Part I[/B] One of the most confusing subjects for a beginner or even an intermediate level photographer to understand is Depth of Field (DOF). There are three basic elements of depth of field: (1) F/stop (2) Focal length (3) Focus point distance The problem with understanding and using depth of field lies with the interrelationships of these three elements. They each have an effect and changing any one or two in any combination and the depth of field changes. This means that changes are not readily apparent or easy to understand. So you have to have a good understanding of what these relationships are and how they affect depth of field before you can put it to any practical use. Once you understand their effect on the DOF you will be able to do a better job of combining changes in each to achieve the depth of field and exposure you want. Simply put the depth of field is that distance in front of the subject focus point where everything will appear to be in focus and that distance behind the subject where everything will appear to be in focus. The two distances combined make up the total depth of field. In most cases the distance behind the subject will be the larger of the two. Sometimes as much as 98% of the total depth of field will be in back of the subject. Other times the two measurements will be almost equal. This will become apparent as we get into the calculations for DOF. A lens will focus on a finite point at any given distance. Even the smallest of movement to the front or back of that point will cause focus to start falling off. So where does depth of field come from? It is primarily a function of our eyesight. Our eyes are not nearly good enough to detect this focus falling off until it reaches a certain distance in front of or to the back of the point of focus. As the distance from the point of focus gets larger it will reach a certain limit where the eye suddenly becomes aware of ”fuzziness”. Those are the front and back points from the point of focus that are used for the measurement of depth of field. How far these two points are from the point of focus is determined by three lens functions, f/stop, focus distance, and focal length. There are two functions of DOF, which you may have heard of, that are beyond the scope of this discussion. These are Hyperfocal Distance and Circle of Confusion. Before you can understand either of these functions you have to have a good understanding of the DOF basics. [B]Methods of Measurement: Point of Focus and Near/Far Limit[/B] To add to the confusion understanding DOF there are two methods of measurement. One method uses the camera as its starting measurement point, the lens focal plane, and gives the near limit and far limit distances. The second method uses the point of focus and measures the distance in front of and to the back of the subject. Both measuring methods actually do the same thing but from different points of view. The measurements used in this explanation are the distance points to the front and back of the subject focus point. Doing it this way makes it easier to understand the relationships involved. The other method of measurement, near and far limits is more practical for field use. Trying to judge a distance in front of or behind a subject that may be 25 ft., 100 ft. or more in front of you is not going to be as accurate as judging those same distances from where you are standing. Hence, most all the field use devices for depth of field, such as printed charts, printed pocket calculators, and the ones used with devices such as the iPhone, will be based on the near and far limits. Go to figure one below for a graphic layout of these measurements. Figure – One [ATTACH]383270._xfImport[/ATTACH] [B]Depth of Field Calculator[/B] To get the most out of this explanation of depth of field you should open the DOFMaster Calculator and do the calculations as they are presented. The DOFMaster Depth of Field Calculator web site is listed below. There is a drawing there, similar to figure one, that demonstrates these measurements. The drawing in the DOF calculator is interactive with the calculator and displays the distance measurements resulting from the entry calculations. [url=http://www.dofmaster.com/dofjs.html]Online Depth of Field Calculator[/url] This Calculator needs some words of explanation. In the top left bracket of the calculator you will find the listing for your Nikon in the top drop down. Below that is the Focal Length entry (Actual Focal Length) and below that is the Selected F/stop. And last is the entry for distance. If you are using a focus distance of less than five feet, changing that measurement to inches rather than feet will compute a more accurate measurement. Over on the right it provides the calculations for both the near and far limits at the top and, just below, the distance in front of and behind the subject. However, the main point of confusion to be cleared up is the actual focal length to be used in the calculation. Just below the main body of the calculator they show a picture of two zoom lenses with the longest focal length of each circled as the actual focal length. These are point and shoot lenses, which only have the wide-angle limit, and the telephoto limit printed on the front. For these point and shoot lenses the longest focal length or the shortest focal length, depending on which the camera is set closest to, is recommended as the actual one to use. There are no DSLR lenses listed and they are not needed as the actual focal length can be read directly on the lens, as a prime, or as it is zoomed in or out. I.e. if on a 70-300mm zoom lens you are zoomed to 125mm then that is the actual focal length of the lens to use. [B]The f/stop Relationship[/B] The f/stop is the primary, “tool” to change and control the depth of field. To demonstrate the f/stop effect on depth of field it’s necessary to isolate it from the effects of the other two elements. To do that fixed values are assigned to the focal length and to focus distance. Then the f/stop can be changed to demonstrate its effect. Let’s use the common shooting values of, 50mm focal length and a 12ft. focus distance and change the f/stop values from f/5.6 to f/22. Entering these numbers in the DOF Calculator produces the following numbers. F/stop of 5.6; DOF Total 4.03 ft.; In Front 1.68 ft.; In Back 2.34 ft. F/stop of 8.0; DOF Total 5.85 ft.; In Front 2.25 ft.; In Back 3.6 ft. F/stop of 16; DOF Total 14.1 ft.; In Front 3.8 ft.; In Back 10.3 ft. F/stop of 22; DOF Total 27.3 ft.; In Front 4.7 ft.; In Back 22.6 ft. This chart demonstrates the increase in total depth of field as the f/stop number increases. Most photographers learn early on that increasing the f/stop from a lower number, f/5.6 to a higher number, f/22 will increase the depth of field and decrease it going in the reverse direction. Look at the changes in front of the subject focus point from f/5.6 to f/22, it goes from 1.68 ft. to 4.7 ft. Not a lot of change, only 3 ft. But look at the change in back, it goes from 2.34 ft. to 22.6 ft. a change of just over 20 ft. At f/22 the distance in back of the subject is 83% of the total depth of field distance. At f/5.6 it’s 58% of the total. But there is a lot more going on here that needs to be understood. There are four basic relationships that need to be understood from the f/stop effect on depth of field that are very important in learning its use. (1) Where are the distance limit points of the DOF both in front and in back of the subject focus point located? Knowing where these distance points are located will tell you what will be in focus and what will not. These point locations can also be expressed in near limit and far limit distances as measured from the camera. (2) Changing the total depth, with an increase or decrease in f/stop, allows you to put objects, background or foreground, in or out of focus as needed. (3) An increase in depth of field will result in greater changes in back of the subject focus point relative to the changes in front. This allows you to control background focus. (4) You don’t necessarily need to focus directly on the subject. You can move the focus point forward or back to shift the total depth of field to place it where it’s needed. (This also changes the size of the DOF so a re-calculation may be needed to be sure it still covers the area to be in focus). The general rule applying to the f/stop effect on depth of field is: “Any increase or decrease in f/stop will expand or contract the depth of field for any given setting of focus distance and/or focal length”. [URL="http://nikonites.com/blogs/joseph-bautsch/37-depth-field-primer-part-ii.html"]Part II - The Focus Distance and Focal Length Relationship[/URL] [/QUOTE]
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