Techniques to prevent lens from losing focus

Lead: How to evaluate whether a photo is scrapped, the biggest factor is whether the photo is in focus accurately. Only when the focus is accurate can the photos become clear. In actual shooting, there is a lot of knowledge about how to make our photos clear, which is not only related to the lens, but also closely related to the camera itself. The most important thing is our habitual operation. As a basic skill of taking pictures, if we don't master it thoroughly, it will often make our photos hazy.

Tips for preventing lens from losing focus 1. Generation principle

In the optical structure of the camera, correct focusing can make the subject appear clearly on the sensor, thus forming the clearest image. At this time, the contrast value is the largest, while the focus of other focuses will become blurred and the contrast value will decrease. In most digital cameras, autofocus is achieved by determining the accurate focus position by contrast or phase method. There is no need to delve into the specific technical principles here. What we are talking about today is the deviation between the camera's calculated focus and the actual focus, which is the key to the problem of weak focus of photos.

There are many reasons for inaccurate focusing, such as: mismatch between lens and camera, weak contrast value of lens, camera identification and calculation error, photographer movement, object position change, camera movement and so on. , will affect our actual shooting effect. There is a great possibility of out-of-focus and out-of-focus Except for random defocus, most defocus situations can actually be avoided. Here are six ways to avoid out-of-focus.

Second, the matching problem of the lens of the deputy factory

Let's talk about random defocus first, most of which will appear in the matching between the lens of the auxiliary factory and the camera body. Because camera manufacturers have many patented technologies and trade secrets in the focusing system, some focusing technologies can't be disclosed to the lens of the deputy factory, such as Sony's micro-single? For high-precision phase focusing, the camera has increased the restriction of following moving objects, and there is no open technology, so there will be some matching problems. At the same time, there will be no correction data of the auxiliary lens on the camera, but there will be some missing data such as edge luminous flux, color correction and distortion correction.

In addition to the problem of lens matching, it is considered that some original large aperture lenses occasionally lose focus when shooting at the maximum aperture. At the same time, some damaged lens optical axis is faulty, which will also lead to inaccurate autofocus and out of focus.

Third, avoid losing focus and be careful.

1, large aperture lens solves out-of-focus problem.

The lens is out of focus, which most often appears on the lens with large aperture, such as 50mm F 1.0, 85mm F 1.2, etc. When the maximum aperture is fully open, the lens occasionally loses focus. Because the aperture value of the lens is very large and the spirit is very shallow, the position of the automatic lens in the focus ring needs to be very fine, and the focus gap is only a few tenths of a millimeter, which will be magnified dozens of times.

100% has two solutions, that is, reducing the aperture or enlarging the focus in real time. After reducing the aperture, the depth of field becomes larger, the clear direction is wider, the focusing accuracy is not so high, and it is easier to focus, at the expense of a large aperture. If you want to focus accurately under the maximum aperture, the way is to turn on the manual focus mode, use the live view function, click to enlarge, turn the focus ring to focus manually, and adjust to the appropriate focal length.

2. Avoid camera movement

When taking portrait photos, it is often necessary to shoot from low to high, so as to absorb a higher number of people. However, if the midpoint focusing mode is selected, the midpoint is located on the human body and the face is not in focus, which becomes an out-of-focus image. Therefore, it is necessary to choose the focus position correctly when shooting, and set the focus at the position of the face to avoid defocusing.

3. Avoid foreground interference

When shooting in a responsible environment, the focus may be disturbed by other objects, resulting in inaccurate pictures, and the prospect is likely to affect our correct focus. In portrait photography, such dense and tiny distractions as hair are very easy to focus on the hair, but the face is really empty.

Auto focus is aimed at the front hair, and the eyes are out of focus.

The solution is to be extra careful about small objects when shooting, use single-point focusing, choose the right focus point, and avoid the camera automatically focusing forward. In addition, it is also a very necessary step to look back at the photos to check whether the focus is accurate.

4. High-precision focusing.

In SLR cameras, the accuracy of different focusing points is different, and different lenses will be different. Generally speaking, the focus of the middle point is more accurate than the focus of the edge, and it can have better focusing effect for some lenses with large aperture. Taking Canon 1Dx Mark II as an example, the position between the midpoints can support high-precision focusing below F2.8 aperture. Edge only supports precise focusing of lenses with aperture of F5.6 or F8.0, so we can avoid using mid-point focusing for high-precision focusing.

The above picture shows the focusing system of Canon 1Dx Mark II.

In addition, we need to choose the appropriate focusing method according to the shooting scene. It seems that when shooting sports, the correct choice of the motion mode of moving objects can be of great help to our shooting. High-end cameras can set different focus sensitivity and interrupt focus setting options for moving objects.

5. Avoid refraction affecting focusing.

When taking a picture, when encountering an object with specular reflection, there will be some deviations due to the incident light reflected by the specular reflection, which will make the camera's focusing system recognize the error, thus defocusing the image. The solution is to choose an object with the same lens distance to replace the detection position of focusing, and realize accurate focusing based on the principle of equal object distance.

6, lens focal length adjustment tool

In some sub-factory lenses, electronic focus correction tools will be supported to correct the inaccurate focus of the lens, and the fixed offset value of the lens will be manually corrected on the lens to make up for the defocusing problem.

abstract

No matter how high-definition the camera is, it is useless to focus incorrectly. Therefore, we can or need to form a small habit of looking at photos. It is an essential skill to check whether the exposure is accurate and the focus is accurate. The tips mentioned in this paper are all from the choice of lens and the setting of camera. I wish you all a good photo.