What are the factors that determine lens quality? How to choose a lens?The lens quality referred to here mainly refers to the imaging quality of the lens. The function of the lens is to clearly image the subject and record it on the imaging carrier. The imaging quality of the lens determines the image quality of the photo. The lens has high resolution, small correction aberrations, small dispersion circle, large diameter, and multi-layer coating on the lens, the lens quality is high; otherwise, the quality is poor.
The following factors determine the quality of the lens:
• Resolution, also known as resolution. The resolution of a lens refers to the ability of the lens to image transfer details, expressed in line pairs/mm on the focal plane. At the maximum aperture of contemporary lenses, 200-150 line pairs/mm can be resolved at the center of the field of view (from the center to the outside), and 150-100 line pairs/mm at the edge of the field of view (from the inside to the outside), even if it is very Good lens (such as Canon EF50mmF1.8 lens field of view center 180 line pairs/mm, full field of view average 146 line pairs/mm).
• Edge resolution. The resolution outside the circle with 20% of the diagonal of the frame as the diameter. Generally, the resolution at 0.3, 0.5, 0.7, 0.85 of the diagonal of the frame is measured, and the average, maximum, and minimum are the basis for distinguishing the quality of the lens.
• Aspheric lens. The same from the center of the lens to the peripheral mirror is called a spherical lens, and the curvature of the mirror from the center of the lens to the periphery is different, and it is called an aspheric lens. The aspheric lens can effectively correct aberrations, reduce the number of lens sets, increase the lens aperture, and improve the resolution.
• Coating, also known as coating, interference coating. The purpose of adding the film is to interfere with the imaging light. The earliest use of coating technology for lens lenses was the EKTRA camera produced by Kodak in 1942. The lens of the lens emits and absorbs light while transmitting light. When light passes through an uncoated lens, it is reflected 5% when incident, 2% is absorbed by the lens, and 5% is reflected when it exits, with a total loss of 12%. If no film is added, the more the number of transparent sheets, the lower the transmittance and the more reflected light. The reflected light will be reflected twice, and finally irradiated on the film or sensor, which will inevitably cause blurred images.
• Color difference. Because the refractive index of the colored light of different wavelengths is different, the focal point behind the lens usually cannot be concentrated at one point (such as purple, blue, cyan, green, yellow, orange, red), resulting in different dispersion images, causing the lens The color difference and color distortion.
In summary, when choosing a lens, first consider the focal length. The focal length is a functional mark of the lens. The fixed focus lens has a small number of lens elements, small aberrations, and good image quality, but you need to prepare a few when shooting. The image quality of the zoom lens is relatively poor due to the large number of lens elements. Secondly, we must also consider the brand. For the same camera, different brands or models of lenses can be used (the interface needs to be the same), but the image quality is far different. Generally speaking, the quality is basically proportional to the price. However, the ratio is only very different. If the quality is 1/10 higher, the price is often several times higher. The body of a digital camera depreciates quickly, and the lens generally does not depreciate. The distinctive lens that has been discontinued can even add value.