APS-C vs. a Camera with 1/2.3" Sensor
For outdoor portraits, it is often desirable to soften the background to reduce the distractions with the subject. This has been accomplished by using a telephoto lens with a large lens aperture to reduce the depth of field (DOF). (Lens theory shows that a telephoto lens does not actually reduce the DOF but enlarges the background image relative to the subject which enhances the desired effect.)
With the introduction of compact digital cameras, with their tiny sensors, it has become seemingly impossible to produce the blurred background desired. The depth-of-field (DOF) of a camera depends on the lens f-stop, and the size of the sensor, which affects the object magnification and the allowable circle-of-confusion (allowable blurriness). Cameras with larger sensors require more magnification to fill the sensor, but can tolerate more blurriness as the image does not have to be enlarged as much in printing.
M=Magnification, which varies with object size
C=Circle of confusion
C=0.00578mm for 1/2.3" sensor
C-0.02mm for APS-C sensor
Depth-of-field predictions included in this study come from "Depth of Field vs. Magnification"
Here, I have compared a camera with an APS-C sensor and a compact camera with a 1/2.3" sensor, which has a diagonal dimension that is 29% of the APS-C sensor. (The APS-C sensor is in itself only 67% of a 35mm-size sensor, so it is not capable of producing as blurry a background as a camera with a 35-mm size sensor.)
The following three images are made with the small-sensor camera where the magnification ratio is only 1/396. (The lens focal lengths do not correspond to those used to make the images of the lamp alone, with the same camera, which is an oversight. But, since the DOF is not affected by focal length, it is immaterial. Again, what matters is the magnification and the aperture.)
1/2.3" Sensor M=1/(396) 5-100mm, f2.8-5.7 zoom
5mm, f2.8 DOF~29 ft
13.8mm, f4 DOF~162 ft
APS-C M=1/31 with 35-70mm, f3.5-4.5 zoom
23mm, f4.5 DOF~Infinite
There is little blurriness for this small sensor because the magnification is so small (1/396), which results in large DOF. In fact, the DOF is infinite at f4.5.
The compact camera has far too much DOF to produce a blurry background when filling the sensor with a 6-foot tall object, such as the post lamp. The surprising result, however, is that the APS-C camera also has too much DOF to cause the background to be blurry when photographing the entire post lamp. Thus, it would not be expected that a portrait of a standing person would have the desired soft background effect. It is suggested that a lens with an f2 or larger aperture be used with the APS-C format camera to produce the desired effect. Compact cameras do not have interchangeable lenses and their lenses do not have apertures as large as f2, so there is no way to obtain the blurred background effect except to limit the subject to head or head-and-shoulders composition.
The next two images were made with an f2 manual focus lens that was originally shipped with Pentax 35mm film SLR's. It is fitted to the APS-C sensor digital SLR, Pentax K100D that is used for the previous APS-C format images.
APS-C M=1/31 with 50mm, f2.0, fixed focus lens
35mm, f3.5, DOF~0.4 ft
50mm, f4, DOF~0.5 ft
The DOF for these three images is only influenced, slightly, by the f-stop change. There is considerable blurriness of the background, which is accentuated by the longer focal lengths. (DOF is not actually affected by focal length, but the perspective change associated with the use of a long focal length causes enlargement of the background, which tends to make the background look softer.)
Do not be confused by the fact that the DOF gets larger with increasing focal length. It is not a consequence of the focal length but rather that of the reduced aperture that accompanies longer focal lengths on these and most zoom lenses, whose f-stop number gets larger with increasing zoom.
70mm, f4.5, DOF~0.6 ft
50mm, f2 DOF ~0.23 ft
This image has a very blurry background, due to the large aperture, f2, and the high magnification, 1/31
APS-C M=1/124 with 50mm, f2.0, fixed focus lens
The following images were made by the camera with the small sensor (1/2.3").
1/2.3" Sensor M=1/99 5-100mm, f2.8-5.7 zoom
50mm, f2 DOF~4.67 ft
The image of the entire post lamp still has too much DOF to produce a blurry background, although the DOF has been reduced from 10.7 ft at f4 to 4.67 ft, here, by the f2 aperture.
The only way to obtain a blurry background is to fill the frame with a head-and-shoulders subject. The use of a large aperture in itself, is not sufficient, as the last image reveals.
APS-C M=1/124 with 70-210mm, f4 zoom
9.9mm, f3.5 DOF~1.33 ft
21mm, f4.5 DOF~1.96 ft
70mm, f4 DOF~10.7 ft
The background is still quite blurry here because the magnification (1/99) is still relatively large. Note, again, that, although the background is not actually blurrier for the longer focal lengths, it is more subdued, as it was with the longer focal lengths on the APS-C camera.
47.5mm, f5.0 DOF~2.34 ft
135mm, f4 DOf~10.7 ft
The following three images show the effect of reduced magnification on the images made with the APS-C camera. It requires less magnification to fill the sensor with an image of the entire post lamp than when filling the sensor with the lamp, alone. The magnification ratio is reduced from 1/31 to 1/124. This reduced magnification is reflected in the large increase in DOF, at the same f-stops.
210mm, f4 DOF~10.7 ft
APS-C M=1/31 with 70-210mm, f4 zoom
APS-C M=1/124 with 35-70mm, f3.5-4.5 zoom
70mm, f4 DOF~0.5 ft
35mm, f3.5 DOF~8.7 ft
135mm, f4 DOF~0.5 ft
50mm, f4 DOF~10.7 ft
210mm, f4 DOF~0.5 ft
These six pictures were made with a Tokina 70-210mm, f4, manual focus zoom lens, a rare constant-aperture zoom. Unfortunately, the background is not distant from the subject and the out-of-focus is minimal. (The lens focal length is too long to allow me to stand in the same position as for the other shots.) Nevertheless, the series confirms that a magnification of 1/124 is too small to produce significant blurring of the background. The magnification of 1/31 has more blurring, but again, the background is too close to reveal it well. The effect of focal length is apparent in the magnification of the background at the longer focal lengths.
70mm, f4.5 DOF~14.8 ft
There is little background blurriness for M=1/(124), and it is not accentuated much by the longer focal length of 70mm.