As many of you realize by now, I often choose our weekly assignment topics based on what I am seeing in the previous week's assignments. I've run several assignments now that emphasized the use of "near, middle and far" elements in the scene in order to convey a sense of depth. During these assignments (and during our workshop in Crested Butte as well) I observed that not very many images included elements as "near" as I thought they should be. So, I searched my archives for images that I could use as examples, only to find out that I didn't have very many images that included really "near" objects, or at least as near as they should be in order to really emphasize depth. I've seen these types of images in galleries and books, but obviously my own photography falls short in this department.
I think there are a couple of reasons not many of us get close enough to really emphasize the "near" objects in the scene. First, we tend to photograph from standing height, and have to consciously think about lowering the tripod and getting into a less comfortable position in order to change our perspective. Second, and probably the primary reason for me, is lens choice. My "normal" lens is a 28mm - 70mm zoom lens. Although 28mm is considered "wide," it isn't quite wide enough to allow the depth of field I need to get really close to the "near" objects in my scene (without causing the distant objects to fall outside the depth of field).
Here's an example I found in my files of "near" elements photographed at the 28mm end of my zoom lens. You can see how using a wide angle lens and getting close will help emphasize the difference in size between near and far elements, accentuating the perception of depth in an image. However, If I zoom into this image at 100%, I can see that my depth of field wasn't quite sufficient enough to keep the distant elements in focus. The image is OK for web display, but would "fall apart" if I tried to make a large print from this file.
Lupine Road, Crested Butte, Colorado
By using a much wider lens, such as 18mm (my widest lens) and getting much closer to the "near" element, I could emphasize this element in relation to distant objects, while still obtaining enough depth of field to keep everything in the scene reasonably sharp. So why don't I do this more often? First, I would have to overcome the inertia of having to change lenses. Second, it is much more challenging to compose a successful image using an ultra-wide, since more of the surrounding area is included in the scene, and needs to support the composition. In the scene shown above, an ultra-wide lens would have included more of the bank on the right (in shadow), which would have detracted from the image. I would have needed to find a different vantage point to successfully compose an image with a wider-angle lens. Visualizing and composing an image with an ultra-wide angle lens is a skill I need to practice and get better at.
Let's compare depth of field and focus distances for the 28mm and 18mm lenses. For the 28mm lens, the closest I can focus and still have objects at "infinity" within the depth of field (at an aperture of f/11) is 7'8". This focus distance is defined as the "hyperfocal distance." If (for ease of comparison) I focus at 8 feet, the depth of field extends from 3' 11'' to ∞. For the 18mm lens, if I focus at 8 feet, the depth of field extends from 2' 3.2'' to ∞. However, I can (and want to) focus much closer than 8 feet when I am using the 18mm lens. If I move to 1/2 the distance from the "near" element (making the "near" object appear larger than it would with the 28mm lens) and focus at 4 feet, the depth of field will extend from 1' 9.3'' to ∞.
Many people have been taught the rule of thumb that "depth of field is the same for all lenses when the size of the subject in the frame is held constant (by adjusting your distance from the subject) and the same f-stop is used," and would argue that wider angle lenses do not provide more depth of field. However, this rule of thumb is only true when the focus distance for the shortest lens is less than about 1/4 of the hyperfocal distance for that lens. The hyperfocal distance for the 18mm lens at an aperture of f11 is 3' 2.3'', so the "rule of thumb" is only true for focus distances less than 9.6 inches (1/4 the hyperfocal distance). For any focus distances greater than 9.6 inches the depth of field available from the 18mm lens becomes significantly greater than that from the 28mm lens. This rule of thumb ("depth of field is the same for all lenses...") applies more to telephoto lenses than it does to wide angle lenses, and unless you are focusing VERY closely, wider angle lenses can provide significantly greater depth of field when used in grand scenic "near, middle, far" compositions.
The assignment for the week of 3 - 9 August is "Wide and Close." You should use your widest angle lens to compose your images, getting as close as possible to the "near" element in your scene, while still keeping the farthest objects in the scene within the depth of field of your lens. You can reference this chart to determine the depth of field available for your specific camera and lens:
http://www.dofmaster.com/doftable.html (Type in the focal length of your lens, use the drop down menus to select your camera and units, then click "calculate.")
I encourage you to ask questions if my explanation of the assignment is not clear.
Please upload your images to the "Wide and Close" album in the Weekly Assignments category of the Gallery no-later-than midnight Mountain Time (GMT -07:00) on Sunday 9 August 2009.
I'll look forward to seeing your images.
Keith
