Digital SLRs and Depth of Field
Digital SLR (DSLR) cameras use lenses designed for 35mm SLR cameras. The physical performance of a lens doesn't change when you mount it on a DSLR. And yet, we see more depth of field (DOF) in photographs taken with the DSLR. Why? Because we change the DOF with our feet; we move further away from the subject.
Let's see how this works:
Focal Length Multiplier and Field of View Crop
Lenses designed for 35mm SLRs project an image circle large enough to fully encompass the 35mm frame. The same lenses on DSLRs project the same image circle. As shown in Figure 3, the sensor crops a smaller portion of the circle. In other words, it crops the field of view of the lens.
When the digitally recorded image is enlarged, the photograph will appear to have been taken with a longer lens, as shown in Figure 4. We use the term "focal length multiplier" to describe the effect. It appears this way only because a cropped image was enlarged to the same size as the photograph enlarged from the 35mm image.
A focal length multiplier of 1.5 indicates that the width of the 35mm frame is 1.5 times the width of the sensor in the DSLR. Thus, a lens used on a DSLR will appear to be 1.5 times longer than it does when it is mounted on the 35mm SLR. We see the field of view crop, or focal length multiplier, as we look through the viewfinder of the DSLR. A 50mm lens on a DSLR will show the same image on the viewfinder as will a 75mm lens mounted on a 35mm SLR.
The DOF changes because of the way we handle the recorded images. The CCD records a smaller portion of the projected image. Thus, that image must be enlarged more to produce the final photograph. This, by itself, would yield less DOF. But, we move further away from the subject so we can get identically framed photographs. The longer subject distance increases the DOF. Subject distance has a stronger influence on DOF than does image enlargement. Thus, the DOF is greater in the DSLR photograph.
Depth of Field Comparison
Let's look at some examples to see how the DOF changes when we use a DSLR. DOFMaster for Windows was used to produce the DOF scales shown below. For the 35mm SLR DOF scales, 0.030mm was used for the circle of confusion. For the DSLR DOF scales, 0.020mm (0.030/1.5) was used for the circle of confusion. Thus, the examples compare DOF for a 35mm SLR and a DSLR with a focal length multiplier of 1.5.
Depth of Field with 50mm Lens and Subject Distance of 10 Feet
As shown in Figures 5 and 6 below, the DSLR photographs will exhibit less depth of field when the subject distance is the same for each camera. The total DOF at f/5.6 for the 35mm SLR is about 4.25 feet, from about 8.25 feet to 12.5 feet. The total DOF at f/5.6 for the DSLR is about 2.75 feet, from about 8.75 feet to 11.5 feet. So, the DOF is about 1.5 feet shorter, at f/5.6, with the 50mm lens focused at 10 feet on the DSLR.
However, the photographs aren't framed identically; the DSLR photograph is a cropped version of the 35mm photograph (Figure 7).
Depth of Field with 50mm Lens, 35mm SLR Subject Distance of 10 Feet, DSLR Subject Distance of 15 feet
As shown in Figures 8 and 9 below, the DSLR photographs will exhibit more depth of field when the subject distance for the DSLR is adjusted so the framing is identical for the two cameras (Figure 10.) The total DOF at f/5.6 for the 35mm SLR is about 4.25 feet, from about 8.25 feet to 12.5 feet. The total DOF at f/5.6 for the DSLR is about 6.5 feet, from about 12.5 feet to 19 feet. So, the DOF is about 2.25 feet longer, at f/5.6, with the 50mm lens focused at 15 feet on the DSLR.
Depth of Field Using 35mm SLR with 50mm Lens and Subject Distance of 10 Feet,
Figure 11. Depth of Field for 35mm SLR. 50mm lens, focused at 10 feet
Figure 12. Depth of Field for DSLR. 33mm lens, focused at 10 feet
Figure 13. SLR and DSLR Photographs.