Shulou(Shulou.com)11/24 Report--

The title of the original text: "the knowledge of camera focus is explained in detail. Do you really understand the focus?" "

According to the principle of optics, the focus is only one plane, that is, the focal plane, and the scene on the focal plane is the clearest. When taking a picture, the subject happens to be on the focal plane, that is, it is focused.

The object on the focal plane of the depth of field is the clearest, and some objects outside the focal plane are also clear, which is the depth of field. The depth of the foreground in front of the focal plane is called the depth of field behind the focal plane, which is generally greater than the depth of the foreground.

The farther away from the focal plane, the lower the definition, up to a certain boundary, no matter how far it is blurred, it can also be said to be virtual, which can be understood as beyond the depth of field. If you want to take virtual photos, reduce the depth of field, otherwise increase the depth of field.

With regard to depth of field, there is a formula for calculating the depth of field, and there is also an App that directly generates the calculation result. Put a formula for calculating the depth of field, it doesn't matter if you can't remember it, as long as it looks good.

There are three common factors that affect the depth of field: aperture, focal length, focus distance (object distance).

The larger the aperture, the smaller the depth of field.

The longer the focal length, the smaller the depth of field.

The shorter the distance, the smaller the depth of field.

02. The first step in autofocusing and manual focusing is to adjust the position of the focal plane so that it fits into focus correctly with the main body. There are two kinds of focusing methods: manual focusing (MF) and automatic focusing (AF).

When focusing manually, turn the focus ring on the lens by hand, and the focal plane changes and can be moved from the nearest to the farthest.

Photography in the digital age pursues convenient keys. Manual focusing is too troublesome and can only be used when autofocus fails, for example, the ambient brightness is lower than the working brightness of focus, and for example, there is no contrast where you want to focus. Another example, the lens aperture is smaller than the focus aperture, which will lead to autofocus failure.

In AF mode, the lens will focus automatically by half pressing the camera shutter button. Autofocus pursues convenience. In order to make it more convenient, there are three common modes: single autofocus, continuous autofocus and intelligent autofocus.

It's the same old rule. If you use this name at home, my family will definitely have a new name. The three focus modes on different brands of cameras are different. Canon SLR cameras have ONE SHOT single point focus, AI FOCUS artificial intelligence focus and AI SERVO artificial intelligence servo focus. While Nikon SLR is autofocus autofocus, autofocus-single single autofocus and autofocus-continue continuous autofocus, but the working principle and operation method are roughly similar.

Single autofocus: the camera only focuses once and does not change after alignment, even if the subject runs away and the camera moves, the focal plane distance will not change. This technique is suitable for shooting stationary objects, and this focus mode is used in most cases.

Continuous autofocus: as long as you half-press the shutter, the focus system will always work, and if the subject moves, the focus will follow to ensure that the subject is always on the focal plane.

Intelligent autofocus: it can be adjusted freely between single and continuous according to the state of the photographed object, and it is also an autofocus mode for the moving subject to continuously focus. This kind of focusing mode is also suitable for shooting moving objects.

03. The farthest focus is infinity, and the farthest place is the focal plane. At this time, there is no depth of the background, only the depth of the foreground. Areas as deep as infinity in the foreground are clear.

The nearest is the nearest focusing distance of the lens. If your nearest focusing distance is 0.5m, you can focus on objects within 0.5m and beyond. Objects within 0.5m must not be in focus. This 0.5m calculation method should be noted that it is the distance from the "focus" to the "camera sensor", not the distance from the front end of the lens. The camera usually uses the symbol in the following picture to mark the location of the sensor.

As the focal distance of the hyperfocal length changes, so does the depth of field. When shooting scenery, you often need to be clear both near and far, in other words, you need the maximum depth of field.

When is the depth of field greatest? When focusing on a certain point, the rear boundary of the depth of field just falls at infinity, when the range of depth of field is the largest. This point of focus is the "hyperfocal length", the aperture and focal length will affect the depth of field, so different combinations of aperture and focal length will produce different hyperfocal length.

When shooting the starry sky, turn the focus ring back after focusing infinity, in order to find the hyperfocal length, in order to get the maximum clear range and take a clear picture of the foreground.

05, the continuous upgrading of focusing manufacturers are also constantly improving the ability of camera and lens focusing systems, in short, to make autofocus "faster" and "more accurate". As soon as you half press the shutter, the machine can quickly find the object to focus on and successfully focus on it.

The first is the principle of focusing, which is divided into phase focusing and contrast focusing.

Phase focusing: the focus mode used by the general SLR camera, the light entered by the lens is refracted to the focus module, resulting in a phase difference, and the focus module judges the focus mode according to the phase difference.

Contrast focus: the light entered by the lens is projected directly onto the camera's photosensitive chip, and the main photosensitive chip determines the focus mode of the focus position by calculating the contrast of the image.

The second is the recognition ability, which is divided into linear focus, cross focus and double cross focus.

Linear focus: the linearity on the focus module is divided into vertical and horizontal lines, and the camera can recognize and focus when the lines of the object intersect the vertical or horizontal lines. If it's parallel, it's out of focus.

Cross focus: linear focus is one line, cross focus is two lines, forming a "ten" word, which improves the focusing ability.

Double cross focus: on top of the original cross, another cross is added, just like the meter word, so the focusing ability is stronger.

The last part is the focus working area, which is mainly shown in the number of focus and the area that can be identified by each focus.

1. Auto focus of all focus

If you enable full focus, it is tantamount to giving the right to focus to the camera, which will choose where it is clear according to the whole picture. This focusing mode is suitable for tracking shooting, and used in conjunction with the continuous autofocus mode, so that the focus of the camera can keep changing with the moving subject.

2. Single point autofocus

You can choose a focus by yourself through the arrow keys, so that a specific place in the picture is clear. For example, make sure the character's eyes are clear when taking a portrait, or focus on a single flower in a sea of flowers.

This method is accurate and fast, but you need to choose the position of focus carefully. In single autofocus mode, half press the shutter to start and lock the autofocus so that you don't have a focus shift when you recompose.

3. Automatic focus group alignment

Some of the more advanced cameras allow you to choose a set of focus, thus increasing the speed of autofocus. It is suitable for tracking and shooting a small thing, because it is difficult to track and focus if there is only one focus. When you manually select an autofocus, the camera automatically uses the autofocus around you to track things in motion.

This article is from the official account of Wechat: photography Research Society (ID:shoujiPhotography), by Zhang Xiaozhang.

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