Whenever you're considering buying a new lens, you need to consider image quality. This is crucial for successful photos and long-lasting satisfaction with the lens. What makes a good lens?
- Brightness distribution over the image
- image sharpness and contrast
- low stray light
- no color fringes
- geometrical rendering
- good light transmission
These 6 points are crucial for good optics, and you should definitely research them for every single lens. Now I'll explain each of these factors in detail so that nothing can go wrong 🙂
Brightness distribution over the image The brightness in a photo should be the same across the entire area. For example, you want to take a photo of great cloud formations in the sky. With such unstructured motifs, it is important to avoid a drop in brightness. This would have a very disturbing effect on the whole photo. However, a certain drop in brightness cannot be avoided with wide-angle lenses, as many physical reasons speak against it. A drop in brightness of up to 25 % is not that bad.
image sharpness and contrast In order to achieve a good result with these two properties, I would recommend you a fixed focal length (fixed focal length: lens with a focal length that cannot be adjusted; for example: 70 mm). This can be designed more precisely than a zoom lens due to the few lenses. This means that with a fixed focal length, the lenses are optimized in terms of sharpness and contrast. This is only partially possible with a zoom lens, as it is made up of many lenses. You can read sharpness and contrast from the "MTF curves" that are usually given. How to read these curves? There will soon be a separate blog entry for this, because the topic is simply quite extensive.
Little stray lightStray light is what photographers call diffuse light, which is caused by reflections in the camera and lens. The diffuse stray light is normally distributed evenly over the entire image and is therefore not really visible. However, it makes the image brighter and lowers the contrast. The aperture setting also has an influence on the image quality in connection with stray light. The wider the aperture, the worse the image quality. The choice between fixed focal length and zoom lens also plays a role here. With a zoom lens, the light breaks through the many lenses very often and so there is more scattered light than with the fixed focal length, which is built with fewer lenses. This shortcoming arises primarily through direct irradiation of light and through different refractions of the light. What can you do against stray light?
- You should pay attention to a good compensation of the lenses.
- The "inner walls" and all camera and lens parts that are in the beam path are matted and painted black by the manufacturers.
- The ribbing of surfaces in the lens reduces the reflections (for example, also in old cameras with the bellows)
- It also makes sense to use the lens hood (also called lens hood).
- Use shades (e.g. black cardboard).
no color fringes (chromatic aberration)Color fringing is an appearance at light/dark or dark/light edges, manifested by colored pixels at these edges. It can be caused by the mosaic filter of the image sensor in the camera, by the lens, and by digital color interpolation. What can be done to reduce color fringing?
- choose a small lens aperture to sharpen contrast edges
- select the focus level so that the contrasting edge is in the focus area
- Achromats or apochromats can be built into the lens by the manufacturer
Geometrically accurate reproduction means that, for example in architectural photography, all lines, surfaces, and shapes are reproduced in the image as realistically as possible. A distinction is made between barrel and pincushion distortion. Pincushion distortion: With this type of distortion, all straight lines are distorted inwards. A straight line thus becomes a slightly oval shape. Barrel distortion: With this type of distortion, slightly oval lines are also produced, but these are distorted or curved outwards. Distortion is not dependent on the aperture, but rather on the position of the aperture in the lens. If the aperture is located in front of the lens elements, the distortion is barrel-shaped. If the aperture is located behind the lens elements, the distortion is pincushion-shaped. The problem of distortion is exacerbated in zoom lenses because the aperture moves when the focal length is changed.
Good light transmission (spectral transmission) Light transmission is affected by reflections and absorption on the inner walls. Good light transmission is achieved thanks to the very good coating (anti-reflective coating of the lenses) from the manufacturer. The maximum aperture stated on each lens does not take into account the value of good or bad light transmission. The spectral transmission is specified as a percentage depending on the light color (wavelength).
