by, 09-27-2010 at 10:30 PM (4549 Views)
Depth of Field Primer - Part II
The Focus Distance and Focal Length Relationship
At this point you should have a basic understanding of depth of field using the f/stop element. If not you should go back and go over it again until you do. If you don’t have that understanding adding the remaining elements will only add to the confusion.
These elements, focus distance and lens focal length, have a similar effect on the depth of field as f/stop. With changes in either one, the depth of field can get larger or smaller depending on the degree of change. Like f/stop there is a general rule for each regarding its effect on depth of field. These general rules are stated as to their direct effect only on the changes in depth of field.
The general rule for focus distance can be stated as follows: “The further the focus point distance is from the lens the larger the depth of field becomes”. Of course the reverse of that is true as well. To get a shallow depth of field use a focus distance as close to the subject as practical. To get a deeper depth of field use a focus distance as far back as practical.
The general rule for focal length can be stated as follows: “The longer the focal length the smaller the depth of field”. The reverse of that is true as well. For a deeper depth of field use a lens with as short a focal length as practical to get the shot. For a smaller depth of field use as long a focal length as practical.
At this point getting into a lot of calculations to demonstrate the rules for focal length and focus distance would probably just add to the difficulty in understanding what they do. If you want to prove the effects of the general rules you can enter several changes in the calculator and see what they do. For now let it serfice that they work as stated. The primary thing to remember is what they do to the depth of field.
Focus distance and f/stop are the two primary elements of depth of field that are used to determine its size in a composition. Focal length is primarily considered in planning the composition and is not usually an element used to change the depth of field after the composition is set. Let’s look at a demonstration of depth of field using the elements of focus distance and f/stop.
Six Photos Demonstrating DOF
Students in photography classes are most always given an assignment to take a series of photos that demonstrate depth of field. The instructions usually involve using a lens with a focal length in the range of 50mm and placing three small objects, one behind the other, on an open book or open file folder with print on it for the demonstration photos. Also all the demo shots are to be taken with the camera at the same fixed distance from the objects. This will limit getting the photos to the use of the focusing and setting of the f/stop on the lens.
To set this up open the file folder with two sheets of 8.5”x 11” paper inside. Shooting across the open folder will give us 17” of depth. Use three salt and peppershakers about 1” in diameter and place them one on the closest top edge of the two papers, one in the center of the two and one on the back paper bottom edge. That will give you an 8” distance between each. Set the camera up on a tripod perpendicular to the 17” depth and centered on the middle salt and peppershaker. Let’s call the closest salt and peppershaker object one, the center one object two, and the one on the back corner object three. Refer to figure one below for a graphic layout of the set up.
Figure – One - Shooting set up
Now we need to determine the distance we need to set up the camera from the subject objects. To do this we have to go to the DOF Calculator and put in some numbers. Part of the requirements is that we use a 50mm focal length. Let’s use the Nikon 50mm, f/1.8 prime. We need to find the minimum distance needed to get a depth of field that will bring all three objects in focus and within the DOF. That means we have to use the center object as the main focus point to get all of the shots. We will need a camera distance that will give us a large enough depth of field in front of object two to bring the object one into focus. Remember that the depth of field in back of the center object will be larger than the one in front. So the only one we need to look at is the one in front. When we find that, the object in back will also be within the depth of field. We will be dealing with inches so change the Subject Distance calculation from feet to inches. Enter the camera body being used. In this case I’ve used the D90. Enter the required focal length, 50mm.
The two variables we have to work with is f/stop and focus distance. What combination of these two will give us the depth of field needed to cover all three objects? It’s now a matter of entering different number combinations until you find the one that gives you the depth of field you are looking for. (I’ve already done this so I’ll go right to it). The combination is f/22 and 49” (see figure one for that measurement). That will give you a total DOF of 22.2”. Remember you need a DOF of at least 8.5” in front of object two to include object one. These numbers calculate an in front DOF of 8.7” just enough to do the job and have the camera as close to the subject as possible.
Now we have to set the camera up 49” from object two. This has to be an exact measurement. Because we are dealing with inches an error of the smallest amount will change the depth if field and throw everything off. The first point of the measurement is easy, it’s the centerline of object two. But where is the measuring point on the camera? It’s the focal plane of the lens on the sensor. Fortunately for us Nikon has put a focal plane indicator on the outside of the camera. Look on the top of the camera. It’s a circle with a line through its center. It’s very small and hard to see. It could be either black or in white depending on the model camera. This is the measuring point for the 49” on the camera.
Note: At this point turn off the auto focus. All focusing will be done manually.
There are six levels of depth of field that can be demonstrated with this setup.
Since we are already set up for having all three objects in focus let’s go ahead and get that photo. Center object two in the viewfinder and lock the camera in that position. Manually focus the lens on that object and take the photo. The settings for this photo were described above.
For photo two we will have object two in focus with objects one and object three out of focus with both being outside of the depth of field. We already have the focal length entered. The focus point distance remains on object two. We need to reduce the lens f/stop number to get a smaller DOF. Let’s change it from f/22 to f/4.5. That f/stop will give us a total depth of field of 4.21”, 2.02” in front of object two and 2.2” in back. That puts the center object in the DOF but excludes objects one and three.
In shot three we will put the first object in focus with objects two and three out of focus. To do that we will need to change the lens focus to object one (41”). There is no need to measure it just refocus the lens to object one and the focus point distance falls into place. We only need to look at the DOF behind the first object. If we use f/8 it will give us a depth of 2.78” in back of object one and that will put the other two outside the DOF and out of focus.
Now we put object three in focus and the two in front of it out side the DOF and out of focus. Now we need to refocus on object three (57”). In this case we only need to know the DOF in front of the third object. If we use f/2.8 that will give us an in front distance of 1.75”. That’s small enough to put the first two objects outside the DOF.
Lets now put object three outside the DOF and out of focus with the first two in the depth of field and in focus. To do that we can use the DOF on the backside of the first object. Let’s refocus the lens on object one (41”). Now we need to find an f/stop that will give us a DOF in the back of object one that will include object two but exclude object three. With f/22 we have a DOF behind object one of 9”. That brings object two with in the DOF with object three outside and out of focus.
In photo six we will put object one out of focus and have objects two and three in focus. In this case we need to refocus the lens on object two (49”). Now we need to find a DOF in back of object two deep enough to bring object three into focus but still leave object one out of focus. If we use f/16 we have a DOF of 8.8” behind the object two, which just puts object three within the DOF. That also gives us an in front DOF of 6.5”, which just barely puts the first object out of the DOF. Just by one and one half inch. The first object will probably just be a little fuzzy but it still qualifies. You can use another element of depth of field and move the focus point to just behind object two. (Refer to Part I, basic relationship four). The shifting of the DOF further to the back will keep object two and object three in focus but will make object one even more out of focus but more apparent for the demonstration photo.
By now you should not only have a good understanding of depth of field but also how to manipulate it, move it around and place it where you want it. Doing these calculations in the field is not very practical unless you have some type of calculator or printed chart with you and you have the time to use it. Understanding depth of field and how its three elements change the distances involved will improve your photos and take out a lot of guesswork.
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