Macro Cameras Resolution and Sensor Size
All of the digital camera types are available with a number of different pixel resolutions, mostly in the range from 8 to 14 Mp, though several cameras are now available that have over 20 Mp.
How many pixels you need will depend on a number of factors, primarily how large you intend to reproduce the images. In general, the more pixels you have, the better the image quality will be, though there are a lot of other factors involved such as the quality of the lens used and any in-camera image processing.
The image from a 6-Mp mobile phone will almost certainly not be as good as that taken with a 6-Mp camera with a high-quality lens and saved as a RAW or high-quality JPEG file.
For most purposes, 8 Mp is adequate, and will give a high-quality A3 ink-jet print. For high-quality reproduction in books and magazines
(photomechanical reproduction), 10 Mp, 12 Mp, or more may be required, and are becoming the norm in DSLRs. Table 2.1 gives examples of reproduction sizes with various sensor resolutions. Larger sensors will obviously offer greater opportunities for cropping parts of the image.
Images can also be upsized, or interpolated, in image-processing programs, such as Adobe Photoshop (see Chapter 7), to enable larger reproduction sizes with minimal loss of quality.
The situation with sensor size can be quite confusing, however, whereby different sizes of sensors can have the same number of pixels, thus giving pixels of different sizes and densities. For example, the Nikon D300 camera has
12.3 Mp in a 23.6 × 15.8-mm DX sensor, while the Nikon D700 has 12.1 Mp in a 24 × 36-mm FX sensor. This will have an effect on the effective focal length of lenses, depth of field, and possible quality issues such as noise.
Compact and bridge cameras tend to have smaller sensors. For example, both the Nikon Coolpix S560 compact and the Nikon P80 bridge camera have a
10-Mp, 2 3 -in. sensor, with 3648 × 2736 pixels.
Sensors in DSLRs with a “crop factor” of 1.5× or 1.6× are often given the generic name of APS-C (referring to the size of the APS film system), but there are several variants between manufacturers.
effect of Sensor Size on Focal Length
Lenses can generally be divided into the broad categories of standard, wide- angle, and telephoto. The standard-length lens gives an angle of view similar to that of the human eye when viewing a scene in a fairly relaxed mode, without straining peripheral vision. With 35 mm film and digital cameras, the standard focal length of the lens is 50 mm, derived from the approximate length of the diagonal across the 24 × 36-mm film/sensor area. Wide-angle lenses have focal lengths shorter than 50 mm (e.g., 28 or 35 mm), while telephoto lenses have focal lengths longer than 50 mm (e.g., 100 or
Where a camera has a sensor smaller than 35 mm, the length of the diagonal is correspondingly shorter. The sensor in a Nikon D300, for example, is 23.6 ×
15.8 mm. The length of the diagonal for this particular sensor is approximately 30 mm. Thus, a 30 mm lens used with this sensor would give an angle of view similar to a 50 mm lens on a 35 mm film sensor. Putting it another way, the
50 mm standard lens on a full-frame sensor would act as a telephoto lens on the smaller sensor. Thus, there is a magnifying, or “crop,” factor of around 1.5× when lenses from 35 mm cameras are used with DSLRs with small sensors.
This can be very useful in some circumstances. For example, a 100 mm macro lens becomes effectively a 150 mm lens when used with an APS-C-sized sensor, allowing a greater working distance between camera and subject. A 300 mm lens becomes effectively a 450 mm lens, which is very useful for sport and wildlife photographers. The relative aperture remains unaffected—that is, an f/2.8 lens is still f/2.8 even though the effective length is longer. The main problem is with wide-angle lenses, where a 28 mm wide-angle lens becomes effectively a 42 mm lens, therefore no longer the wide-angle lens it was. You would need to use a 20 mm lens to give approximately the same angle of view as the 28 mm.
Most lens manufacturers now produce a range of lenses specifically designed for digital cameras with APS-C- and similar-sized sensors (e.g., Sigma DC, Nikon DX, Canon EF-S). They cannot be used with full-frame sensors as the circle they project is too small and severe vignetting will occur.
It is important that you know the size of the sensor in your camera; it will enable you to calculate the magnifying effect on the lens, and calculate precise magnification ratios, if this is important to you.
A simple rule of thumb to determine the maximum reproduction size of a digital image is to divide the largest dimension of pixels by the resolution of the printing device. Many high-quality magazines and books use a printing screen with a resolution of 150 lines per inch.
The usual recommendation when producing images for this type of photomechanical reproduction is to set the image resolution to 300 dots per inch (dpi)—twice the resolution of the printing device.
If you are using a 10-Mp sensor, this typically has dimensions of 3648 × 2736 pixels.