​What is shift focus

​What is shift focus

The problem of shifting focus generally exists in the photographic life of many friends, when pressing the shutter focus, canvas prints it is clear that the focus is closed, but when looking at the picture taken, it is paste? If you have confirmed that it is not your technical problem, I am afraid that it is really the problem of camera focus shifting. The problem of Focus shifting can be divided into two main categories. One is the forward or backward Focus caused by the displacement of the camera body and the structural mechanism inside the lens, which is called "Front Focus/Back Focus" in English. The first is the Focus Shift caused by the optical characteristics of the lens itself, which is called "Focus Shift" in English.

Front /Back Focus/Back Focus

The single-eye camera may shift the focus of the lens and the distance between the AF module in the camera body and the sensor due to wear of the material used or the original design error. As it is related to the focus of the lens, canvas prints australia some fuselages will also be shifted due to different lenses.

Generally, the single-eye camera works. After the light enters the lens, it can be simplified into three routes. In the picture, L1 passes through the reflector, reflects through the focusing screen, and refracts into the eyes of people behind the viewing window in the penta prism. L2 is the light path that the mirror turns up and leads directly to the sensor when the shutter is later pressed to expose. L3 is a half - press shutter time part of the light led by the secondary reflector autofocus module to focus. In theory, L1, L2 and L3 paths of the same length are in the best condition. The focus judged when the shutter is half-pressed to focus does indeed look clear in the viewing window, large canvas prints so the result is also clear when the shutter is pressed and exposed.

In the case of focus shift, the length of the 3 paths is different. When L1 is not the same length as the other two, even though the camera is in focus, the viewing window looks blurry. When L2 is not long, it will appear clearly half press the shutter to see the focus in the viewing window, but the picture is blurred, which is a more common situation of focus shifting. When L3 is different from others, it is impossible to confirm the focusing state in the viewing window. The paths of L1, L2 and L3 are different, which may be caused by some errors in the design of the camera itself, or by the wear and tear of the camera after a long time of use, especially for the part of the reflector.

lens wear, may also lead to the problem of focus shifting.

Aiming at this common problem, some camera manufacturers have developed the function of "fine adjustment of autofocus", which mainly appears on the body of some medium and high order cameras. It can realize the adjustment of autofocus within a certain range, so as to eliminate the focus shifting caused by the inclusion error of the body and lens in the design. The automatic fine tuning function on some cameras is for zoom lens. It can also be adjusted at the wide Angle end and the telephoto end, respectively, to ensure the accuracy of autofocus in the variable focus section, and adjust data saving according to different lens models. This automatic fine tuning feature can also be combined with some lens correctors (edit: correctors can also be DIY! To determine the camera adjustment, the operation will be easier.

What is shift focus

Focus Shift

Excluding the camera body problem, it is also possible for the lens itself to have a focus shift due to the optical design, which is most common in large aperture lenses, especially f1.0-1.8. Due to the spherical aberration problem of the spherical mirror itself, when the aperture is large, although it can absorb more light, but the light entering from the center of the lens and the surrounding will be different due to the Angle of refraction, resulting in uneven focus and image blur, which is also called spherical aberration. Although most lenses now use aspheric lens in lens structure to correct aberrations in order to suppress aberrations, different designs and materials will be used according to the production and positioning of each lens, thus affecting its restraining force on spherical aberrations.

After understanding the cause of spherical aberration, let's look at the relationship between spherical aberration and autofocus. Previously is mentioned in the article about the aperture, general camera autofocus, is the largest aperture to focus, then quickly to use set of aperture, and large aperture lens, even itself in aberration of suppression, but as for structure design of precision and the relationship between the lens materials, and the focus of the judgment in open aperture with actual focus on A narrow aperture after migration, resulting in A focus point becomes B obviously points clear.

If encounter such a situation, many predecessors will tell you can shrink aperture method, to see if the situation is getting better. Why does taking a reduced aperture approach improve the problem? Isn't it true that cameras always focus on the open aperture of the lens? Even if the aperture is reduced, it is the same with the maximum aperture to judge the focus ah! Is good, the largest aperture judging focus does not change, even if stop down there as the focal point of the migration will exist, in A point of focus is still the same run to point B, but don't forget, A narrow aperture, the depth of field is darker, can clear the broader picture, is the focus offset tolerance also can greaten, that is to say, even if the focus to drift, but also within the scope of the depth of field, the user wants to focus effect is little.

Of course, the focus shift according to different lenses, shooting distance and other conditions will be different, generally shooting in the viewing window is also difficult to check, often in the shooting zoom in to check only found. Therefore, if you are using large aperture lenses like f1.0-1.8, you should pay special attention to the focus deviation. If a camera has the function of depth of field preview or LiveView, cheap canvas frames you can also make good use of it to increase the shooting success rate.

2nd Nov 2019

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