Basic principles for selecting zoom lenses
Although zoom lenses have the advantages of wide use, convenient composition, multi-purpose one lens, etc, there are many inferior to the fixed focus lens. For example, when shooting scenes with strong backlight, the zoom lens has a complex lens structure, which is easy to produce reflection and glare.
In close-distance photography, there will be asymptotic halo, image edge brightness than the center of the bottom, and other phenomena. The worse the quality of the lens, the more obvious these phenomena are. So there are some rules to follow when choosing a zoom lens, and let’s look at these basic principles.
1. Basic principles for selecting zoom lenses
(1) Zoom ratio is small
For image quality and manufacturing costs, zoom should be as small as possible. The zoom ratio is the ratio of the long end focal length of a zoom lens to the short end focal length. For example, a 35-70mm lens has a 2:1 zoom ratio. For zoom lenses starting from a wide-angle, its magnification is better to be small.
If you buy a zoom lens with a focal length less than 28mm, you should choose it carefully, because such a lens is more difficult to design and manufacture, and it is best to choose a lens with a smaller zoom rate.
If you buy a zoom lens with a focal length above 70mm, you can consider a larger zoom range.
(2) Fast is better than slow
The “fast-slow” of the lens refers to the size of the maximum aperture, and the faster the zoom lens, the better.
Generally speaking, the maximum aperture of the zoom lens should be able to reach F4. Of course, F5.6 can also be. Now a single lens reflective camera is in the case of full aperture focusing, so the larger the aperture, the brighter the viewfinder, the easier it is to focus.
At the same time, the size of the maximum aperture of the lens is also an important indicator to reflect the manufacturing level of the lens. Even if the same F5.6 is used for shooting, the lens with the maximum aperture of F2.8 is certainly better than the lens with the maximum aperture of F5.6.
(3) Simple is better than complex
Compared with the same type of lens, a lens with fewer lenses, the simpler the lens group structure, the better the shooting effect.
A typical 35-70mm zoom lens has about 12 lenses, divided into 10 groups. There are two “air-lens” contact surfaces between each group, so there are 20 surfaces in 10 groups, which form 190 axial reflections — commonly known as “ghost shadows”. A lens group with only 9 lenses has 18 contact surfaces, forming 152 corresponding “ghost shadows”; The lenses of the 11 lens groups have 22 contact surfaces, forming 231 “ghost shadows”.
In addition, if a filter is added, the number of “ghost shadows” can be dramatically increased. Therefore, in theory, the simpler the lens structure, the better. But with the improvement of optical technology, the lens “ghost shadows” phenomenon has been very little.
(4) Long is better than short
A 560g, 75x1045mm lens is a bit heavy to carry, but it’s worth the effort to get a good picture!
If the pursuit of high image quality is given priority, it is better to have a long lens tube. In order to ensure the imaging quality, the distance between the front lens and the rear lens of the zoom lens should be 2 times the focal length of the long end. That is to say, a 35-70mm lens, its length had better be about 140mm. Unfortunately, this means that today’s new lightweight zoom lenses are not as good as their predecessors at full open and medium apertures. Of course, whether this is important or not depends on the purpose of the photo.
If you want to print a picture to the size of a postcard, these tiny lenses are perfectly fine. If you want to make your photos very large, or mix pictures were taken with a prime lens and slides taken with a zoom lens, get a long lens.
(5) Far better than near
For the sake of image quality alone, try not to buy zoom lenses with very close focus. To keep the zoom lens in focus during the zoom process requires a very complex mechanical structure.
The closest focus distance should be at least 10 times the focal length of the long end of the lens. Therefore, a 35-70mm zoom lens can only focus at a distance of 0.7m. At this time, the image ratio is 1:8. If it is closer, it will produce a significant decline in sharpness. Therefore, many zoom lenses with 1:4 macro close-up functions are actually beyond the limit of optical quality, at the expense of lens quality.
When buying a lens, in addition to the imaging quality of the lens, but also to check the appearance and operation performance of the lens, but also to see whether it can be used with the camera. Only through a comprehensive inspection, in order to find faults and hidden dangers in time, help to select the desired lens.
2. Appearance inspection of mechanical system
The mechanical system of the lens is mainly: lens barrel, aperture blade, aperture adjustment ring, focusing ring, zoom ring, and related adjustment mechanism.
(1) Lens surface
The outer surface of the lens should be smooth and clean, without bump marks, scratches, wear, rust and mildew. The electric coating should not be yellow, with no rust, no peeling phenomenon. Adornment leather should level off beautifully, do not become warped. Surface spraying should be smooth, with no peeling and no obvious color uneven phenomenon.
(2) Inside the lens
The surface of the internal mechanical system of the lens should be black, dull and the black paint layer should not fall off. The screw and lens pressure ring on the surface of the screw thread should be intact without any traces of disassembly.
(3) Aperture blade
Rotate the aperture ring repeatedly to make the aperture smaller and larger. Check whether there are oil stains and rust marks on the front and back of the aperture blade.
Oil pollution will make the aperture blade inactive, even stick to death, but also flow to the lens to affect the light through, so that the definition of the photographic image, this is due to the assembly lens oil too much and overflow to the blade caused.
(4) Mirror tube and adjusting ring
There are various ruler lines, numbers, and letters on the tube and adjusting rings. Some of them are printed directly on the tube and adjustment ring, some are sunken, regardless of the process, the ruler line, numbers, and letters should be clear and beautiful.
In addition, shaking the lens gently with the hand should have no abnormal sound. If there is a sound of “card, card”, we should pay attention to whether it is due to the loosening of the lens pressure ring, resulting in the collision of the lens movement. Lens movement may cause the lens to not be coaxial, resulting in aberration significantly increased, lens imaging quality significantly decreased.
3. Visual inspection of the optical system
Lens optical system mainly includes lens, reflector, built-in filter.
(1) Transmission light inspection method
Open the lens aperture to the maximum so that light enters the lens from the front or back end, and looks through the lens at the other end to see its internal optical components. Each lens should meet the following conditions: clean, no cracks, no edge breakage, no glue, no mold, no bubbles, no scratches, no oil, no fingerprints, no debris.
(2) Reflected light test method
Turn the front end and rear end of the lens slightly to the bright place, and observe the reflected light on the lens surface from the side of the lens, so as to check whether the coating layer at the reflective part of the lens has scratches.