God's Palette

⑨ focal length

The focal length is a measure of the concentration or divergence of light in an optical system, which refers to the distance from the optical center of the lens to the focal point of light concentration.It is also the distance from the center of the lens to the film or CCD imaging plane in the camera.If you have seen "f=" in the English specification of the camera, or paid attention to the focal length on the camera lens, then the number following it is usually its focal length, that is, the focal length.For example, "f=8-24mm, 38-115mm (35mm equivalent)" means that the focal length of this camera is 8-24mm, and the angle of view of the diagonal line is equivalent to the focal length of 35-38mm of a traditional 115mm camera.Generally speaking, the standard lens focal length of a 35mm camera is about 28-70mm, so if the focal length is higher than 70mm, it means that it supports telephoto effect, and if it is lower than 28mm, it means that it has wide-angle shooting capability.

"Focusable range" is the extension of the focal length, which is usually divided into general shooting distance and close-up shooting distance. The general shooting distance of the camera is usually marked as "from a certain centimeter to infinity", while the products with advanced design are often still A close-up shooting function will be provided to make up for the inability to focus in general shooting mode.Some cameras put great emphasis on the magical ability to support 1cm close-up shooting, which is suitable for shooting fine objects.The lens of the camera is a set of lenses. When parallel rays of light pass through the lens, they will converge to a point. This point is called the focal point, and the distance from the focal point to the center of the lens is called the focal length.A lens with a fixed focal length is a fixed-focus lens; a lens with an adjustable focal length is a zoom lens.

When a beam of parallel light passes through the convex lens with the main axis of the convex lens, it will be converged into a point by the convex lens on the other side of the convex lens. This point is called the focal point, and the distance from the focal point to the optical center of the convex lens is called the focal length of the convex lens.A convex lens has a focal point on each side.

We can approximate the center of a convex lens as the optical center.The lens of the camera we use is equivalent to a convex lens, and the film (or the photosensitive device of a digital camera) is near the focal point of the convex lens, or in other words, the distance between the film and the optical center of the convex lens is approximately equal to the focal length of the convex lens.

Convex lenses can form images. Generally, when a convex lens is used as a camera lens, the clearest image it forms generally does not fall on the focus point, or in other words, the distance from the clearest image to the optical center (image distance) is generally not equal to the focal length. Rather, it is slightly larger than the focal length.The specific distance is related to the distance between the object being photographed and the lens (object distance). The larger the object distance, the smaller the image distance, (but in fact it is always greater than the focal length).

Because when we take pictures, the distance between the object to be photographed and the camera (lens) is not always the same. For example, when taking pictures of people, sometimes, if we want to take pictures of the whole body, we should stay far away;That is to say, the image distance is not always fixed. In this way, in order to get a clear image, the distance from the film to the optical center of the lens must be changed according to the different object distances. This changing process is what we usually call "adjustment". Jiao".

Like human eyes, digital cameras take in everything in the world through the lens. If the human eye has a focal length error (myopia), it will not be able to distinguish things correctly. Also as a digital camera lens, its most important feature is the lens focal length value.The focal length of the lens is different, and the breadth of the scene that can be photographed is different, and the effect of the photo is also very different.It is not difficult to see that for the same imaging area, the shorter the focal length of the lens, the larger the viewing angle; and for the lens with the same focal length, the smaller the imaging area, the smaller the viewing angle of the lens. The imaging area of ​​a 35 mm camera is equal to the photosensitive area of ​​a 135 film—the standard 36×24 mm. A digital camera uses a CCD sensor to replace the position of the film in a traditional camera, but its area has several specifications, ranging from high-end professional cameras to It varies from 18.4×27.6 mm to 2/3, 1/2, 1/3 or even 1/4 inches of ordinary digital cameras.

That is to say, the same lens has a wide-angle effect on some digital cameras, but it may become a standard lens on other cameras.It is not difficult to see that for the same imaging area, the shorter the focal length of the lens, the larger the angle of view; and for the lens with the same focal length, the smaller the imaging area, the smaller the angle of view of the lens. The imaging area of ​​a 35 mm camera is equal to the photosensitive area of ​​a 135 film—the standard 36×24 mm. A digital camera uses a CCD sensor to replace the position of the film in a traditional camera, but its area has several specifications, ranging from high-end professional cameras to It varies from 18.4×27.6 mm to 2/3, 1/2, 1/3 or even 1/4 inches of ordinary digital cameras.That is to say, the same lens has a wide-angle effect on some digital cameras, but it may become a standard lens on other cameras.

It seems that it is very inconvenient for us to rely on the focal length value to distinguish the angle of view of the digital camera lens, so digital camera manufacturers usually provide a relative value that is easy to compare, that is, mark the 35mm camera lens with the same angle of view as the digital camera lens Focal length, such a corresponding focal length value is easy for us to understand.For example, the focal length of the Fuji MX-500 lens is 7.6 mm, and the diagonal angle of view is 70 degrees, which is equivalent to a 35 mm lens, which is a small wide-angle; Fuji’s MX-600 is equipped with a small wide-angle zoom lens equivalent to 35-105 mm.

⑩Viewfinder

Viewfinders can be divided into optical viewfinders and electronic viewfinders.Optical viewfinder, as the name implies, is to complete the work of framing through optical components.According to different working principles, it is divided into rangefinder and single-lens reflex coaxial.

In consumer-grade digital cameras, rangefinder viewfinders are the most common. This method of framing is very simple. It is to open a hole above the lens and install glass on the front and back, so that the photographer can see the person to be photographed through this hole. Or something.Although the current rangefinder viewfinder is not so simple, there are functions such as zoom glass and focus auxiliary lines, but the overall structure is very simple.It is because of the simple structure that the cost is relatively low, so it is widely used in low-end digital cameras.But the rangefinder viewfinder also has its disadvantages, because it does not directly view the view through the lens, so the image seen by the photographer from the viewfinder and the image on the final photo will have a certain degree of deviation. Especially obviously, this is not conducive to the photographer's composition and framing of the photo.

The structure of the single-lens reflex type is just much more complicated, so the manufacturing cost is also relatively high, and generally all is used on the high-end product, just usually said digital single-lens reflex (DSLR).The single-lens reflex viewfinder is a viewfinder directly through the lens. The light enters from the lens, passes through a mirror, refracts to the upper focusing screen for imaging, and then refracts into the eyepiece, so that the photographer can see what he wants from the viewing frame. The captured image, because it is directly viewed through the lens, solves the problem of image deviation, and truly achieves the effect of "what you see is what you get".

(11) White balance

Although the word White Balance is not a "hot word", it is still necessary to know as a beginner.The basic concept of white balance is "regardless of any light source, white objects can be restored to white".This involves some knowledge of color science.

White refers to the visual reaction formed by the light reflected into the human eye due to the same proportion of blue, green and red colors and a certain brightness.The popular understanding of white is the brightness that does not contain color components, and the proportions of the three primary colors of red, green, and blue in the reflected light are the same.

There is almost no such concept in traditional photography. At that time, color filters were used for color correction.It's just that the digital camera does not rely on changing the color filter to adjust the color temperature, but uses a circuit to change the gain of the electrical signal generated by different color lights.

There are three CMOS electronic coupling elements inside the camera, they sense blue, green, and red light respectively. In the preset situation, the electronic amplification ratio of these three photosensitive circuits is the same, which is the relationship of 1:1:1. The white balance Adjustment is to change this proportional relationship according to the adjusted scene.

For example, the ratio of blue, green, and red lights of the adjusted scene is 2:1:1 (the ratio of blue light is more, and the color temperature is higher), then the ratio of adjusted white balance is 1:2:2, and the adjusted circuit In the magnification ratio, the proportion of obvious blue is reduced, and the proportion of green and red is increased. In this way, the adjusted scene will pass through the white balance adjustment circuit to the captured image, and the proportion of blue, green and red will be the same, so that the color of the resulting image show normaly.

There are generally built-in white balance setting options in the camera, including automatic white balance, daylight, shade, cloudy sky, tungsten light, fluorescent light, flash and custom, and we can choose the white balance option suitable for the current shooting environment.Of course, the built-in white balance settings of the camera may not be comprehensive. In some complex light source environments, we can also use custom white balance, which requires some white balance correction tools. Common white balance tools include white balance cards, white balance mirrors, White balance filter etc.

(12) RAW format

Most photography beginners have no impression of RAW. They only know that if you choose RAW format in the photo format, there will be a large number of unopenable files in the memory card.In fact, RAW is also a picture format. We can simply think that it is the "negative" of a digital camera.

The Chinese interpretation of RAW is "raw material" or "unprocessed stuff". The RAW file contains all the photo information of the original picture file after it is generated by the sensor and before it enters the camera image processor.It should be noted that the suffix of the image file taken after we set the photo to RAW format is not necessarily xxx.RAW, and RAW is just a general term for such unprocessed images.Each camera manufacturer will use different encoding methods to record RAW data, so the corresponding suffixes are also different.

The advantage of the RAW format is that it records all the information of the camera when shooting, and provides a wider post-production space. For example, parameter information such as white balance, sharpness, and saturation can be finely adjusted in post-processing, and these adjustments are not necessary. It will damage the picture quality.

The difference between the two can also be seen from the size of the file. For the same photo, the file size of the RAW format is obviously much larger than that of the JPEG format, and the difference between the two is almost 4 times.

Usually we can use the official software in the CD that comes with the camera to open and export RAW format files, which is a better choice.Of course, if you own more than one brand of camera, you can also choose to use third-party application software to open RAW format files. Here, I recommend using Lightroom software developed by Adobe.Of course, how to use these software is also a science, and there are many related tutorials on the Internet. Friends who are interested can learn it by themselves.

(13) Depth of field

Depth of field refers to the object distance range measured along the axis of the imaging depth of field camera that can obtain clear images in front of the camera lens or other imagers.After the focus is completed, a clear image can be formed in the range before and after the focus. This distance range before and after is called the depth of field.

There is a certain length of space in front of the lens (before and after the focusing point). When the subject is located in this space, its image on the film is just between the two circles of confusion before and after the focus.The length of the space where the subject is located is called the depth of field.In other words, for the subjects in this space, the blurring degree of the image presented on the negative plane is within the limited range of the permissible circle of confusion, and the length of this space is the depth of field.

The so-called depth of field is when the focus is on a certain point, the front and rear are still clear range.It can decide whether to blur the background to make the subject stand out, or to capture a sharp background.We can often see that in photos of flowers, insects, etc., the background is blurred (called a small depth of field).However, when taking commemorative photos or group photos, photos of scenery, etc., the background will generally be taken as clearly as the subject (called a large depth of field).

When the lens of the camera is focused on a certain object, points on the same plane perpendicular to the axis of the lens at the center of the lens can have a fairly clear image on the film or receiver, and in front of this plane along the axis of the lens Points in a certain range with the back can also form a clearer image point acceptable to the eyes, and the distance between the front and back of this plane is called the depth of field of the camera.

When the light parallel to the optical axis enters the convex lens, the ideal lens should gather all the light at one point and then spread out in a cone shape. This point where all the light is gathered is called the focal point.

Before and after the focal point, the light begins to gather and diffuse, and the image of the point becomes blurred, forming an enlarged circle, which is called the circle of confusion.

In reality, viewing and shooting images are observed in a certain way (such as projection, zooming into photos, etc.), and the image perceived by the naked eye has a great relationship with the magnification, projection distance and viewing distance. If The diameter of the circle of confusion is smaller than the discrimination ability of the human eye, and the blur produced by the actual image cannot be recognized within a certain range.This unrecognizable circle of confusion is called the permissible circle of confusion.Before and after the focal point, there is an admissible circle of confusion.

Taking the person holding the camera as the benchmark, the distance from the focal point to the near allowable circle of confusion is called the foreground depth, and the distance from the focus to the distant allowable circle of confusion is called the rear depth of field.

Aperture, lens, and the distance of the subject are important factors affecting the depth of field:

1. The larger the aperture (the smaller the aperture value f), the shallower the depth of field, and the smaller the aperture (the larger the aperture value f), the deeper the depth of field.

2. The longer the focal length of the lens, the shallower the depth of field, and vice versa.

3. The closer the subject is, the shallower the depth of field is, and the farther the subject is, the deeper the depth of field is.

(14) Calculation formula for depth of field (useless)

δ——allowable diameter of the circle of confusion

F - lens focal length

f - the shooting aperture value of the lens

L - focus distance

ΔL1 - deep foreground

ΔL2——rear depth of field

ΔL——depth of field

Foreground depth ΔL1=FδL^2/(f^2+FδL)(1)

Rear depth of field ΔL2=FδL^2/(f^2-FδL)(2)