Depth-of-Field and Small(er) Sensor Digital Cameras

The image sensor housed in a traditional compact point-and-shoot digital camera is very small.

In the image above, most compact digital cameras have image sensors that range in size from the yellow to light-blue boxes, or smaller. The full-sized, gray box is “full-frame” and is considered so because it represents the full-frame of a traditional 35mm negative. Although there are not many digital cameras that currently use a full-frame image sensor, many of the more expensive, and higher image quality digital cameras like the Canon 1Ds series, Canon 5D Mark II, Nikon D3 series, Nikon D700, and Leica M9, to name a few, have full-frame digital image sensors.

When using a camera with a smaller image sensor the lens has to have a shorter focal length to achieve the same field-of-view as a lens mounted in front of a full-frame image sensor. For example, the popular Olympus micro 4/3rds digital cameras have an image sensor, in the image above, of 4/3″ in size, which is 1/2 the area of a full frame. Due to the area being 1/2 that of the full-frame, these cameras have what is called a 2x crop factor. Essentially, in order to achieve the same field-of-view as full-frame, a lens with 1/2 shorter focal length is required. Or, when using the micro 4/3rds, or the 4/3rds, Olympus, you can calculate the focal length field-of-view equivalent for full-frame by doubling the lens’s focal length that is mounted to the camera. If a 14mm lens is being used, it’s effective field-of-view in terms of a 35mm full-frame focal length is equivalent to 28mm. Taking this a step further, due to the very small size of many compact digital point-and-shoot cameras, in order to achieve a a 24-28mm equivalent field-of-view with a full-frame camera, the lenses on the smaller-sensor cameras start at around 4-5mm in focal length.

This is very important in thinking about depth-of-field because shorter focal length lenses produce images with a greater perceived depth-of-field at an equivalent aperture and distance to subject. Using a 100mm lens set to an aperture of f/4 on a full-frame camera with a subject distance at 10 meters, the depth-of-field with acceptable sharpness starts at approximately 8.9 meters and ends at 11.5 meters. Whereas, the same setup using a 28mm lens set to an aperture of f/4 has acceptable sharpness from approximately 3.8 meters to infinity. If you are using a micro 4/3rds camera with a 14mm lens, although the perceived field-of-view is 28mm, the depth-of-field is guided by the absolute focal length, in this case 14mm. The depth-of-field will, therefore, be greater with the micro 4/3rds camera using a 14mm lens than a full-frame camera using a 28mm lens, even though the overall perspective (field-of-view) of the images will look the same. When thinking about compact point-and-shoot cameras, this becomes even more dramatic considering the short 4-5mm lenses currently beings used.

Take a look at the images below. These were quickly taken to illustrate the differences in depth-of-field noted above.

Nikon D700 (full-frame) with 70mm lens, set to 1/10 of a second, f/5.6 and 400 ISO

Olympus E-PL2 (micro 4/3rds) with 36mm lens, set to 1/10 of a second, f/5.6 and 400 ISO

The first image was a D700, which is a full-frame sensor camera, mounted on a tripod with a 70mm lens attached. White balance was auto, but doesn’t matter anyway as we are only looking at the depth-of-field. The second image was an Olympus E-PL2, which is a micro 4/3rds camera and therefore has a smaller image sensor than the D700, with a 14-42mm lens that I set to what I thought was 35mm on the focal length scale, but the EXIF data told me was actually 36mm. Everything else was set the same on both cameras. As you can see the auto white balance of both cameras yields very different results. In any case, what is important, however, are the differences in depth-of-field. The perspectives of both images are slightly off, but the D700 does sit higher than the E-PL2 on a tripod, and the E-PL2 has a different aspect ratio (hence why the left and right edges are cut-off by the E-PL2). But, you do get the idea that you are looking at the same scene without any real major differences in the composition. For both images, the autofocus point for the cameras was set to the center dot, and it was pointed on the PE of the Pentax box in the center of the frame. Lets take a close up look at the images:

Nikon D700 Crop

Olympus E-PL2 Crop

Nikon D700 Crop

Olympus E-PL2 Crop

As you can see in the cropped images, the E-PL2 overall shows a greater area of sharpness even though the apertures were set to same f/5.6. Due to the differences in the sensor sizes the overall composition remains the same, even though the focal lengths of the lenses were different, 70mm and 36mm. When thinking about or trying to decide on a digital camera, it is important to keep in mind the differences in sensor sizes as the size will effect the lens choices you make and how the images are ultimately rendered.