Table of Contents
Aperture Priority” and “Shutter Priority”
In order to obtain a more convenient operation, there is an “aperture priority” exposure method.
→Aperture priority:
It’s up to you to set the aperture value yourself, and then the camera will give a corresponding shutter value to suit you.
→Shutter priority:
If you want to control the shutter speed personally, you can use the “shutter priority” exposure mode, let the camera give you the corresponding aperture value to match you to get the appropriate exposure.
To control the error of aperture priority and shutter priority, “exposure compensation” is needed.
On digital SLRs, exposure compensation is generally set from -2 to +2. Some cameras even have exposure compensation ranging from -5 to +5. There are 4 levels of exposure compensation from -2 to +2, and each level represents one level of exposure. Simply put, it means a first-level aperture or a first-level shutter. When you use the “aperture priority” shooting mode, you set the aperture value, the camera will give the shutter value, then adjust the exposure compensation, the aperture value will not change, only the corresponding level of the shutter value is compensated.
For example, if you set the aperture F5.6, the camera gives a shutter of 1/250 second, and you adjust the exposure compensation by +1, the aperture is not affected, and the shutter speed will become 1/125 second.
Similarly, in the case of “shutter priority”, if you set the shutter to 1/30 second, the camera will give an aperture value of F8. If you set the exposure compensation to -2, the shutter will not be affected and the aperture value will be reduced to F16.
Focus
There are two reasons for the blurry photos: ①The shutter speed is too slow and the hands are shaking. ②The focus is not aligned.
Autofocus SLR cameras often have multiple focus points. When buying a camera, some friends think that the camera has the same focus points, and they think that their focusing ability is the same, and some people think that the more focus points the better, that is a big mistake. The type of focus point is more important.
“Straight-line” focus sensor:
Sensitive to vertical lines.
“Cross type” focus sensor:
Sensitive to both horizontal and vertical.
“M-shaped” focus sensor:
On the basis of the cross shape, it is also sensitive to diagonal lines, and of course it is more accurate and faster.
Among all the focus points, the “center” focus point is the most accurate and the focusing speed is the fastest. Therefore, many cameras are equipped with a cross or even a rice-shaped focus sensor in the center focus point.
Some high-end cameras also use high-precision “cross-type” focus sensors at other focus points. such as,
The high-end Canon 7D:
All 19 focus points are “cross” focus points, and the center is “double cross” focus points. This is a strong guarantee for 7D’s powerful focusing ability.
Canon 50D in the mid-range:
It is a 9-point full “cross” with a central “double cross” focus sensor.
Low-end Canon 500D:
Although the focus is also at 9 points, only the central point is a “cross”, and the other 8 points are just “one-word” focus sensors.
In this way, when dealing with low light, large aperture and dynamics, where the focusing ability is demanding, the Canon 500D is far behind the 50D, and it lags behind in focusing speed and accuracy.
Because of the difference in focusing ability, if a low-end camera chooses a large-aperture lens, sometimes the optical quality of a large-aperture lens cannot be used (low-end cameras have poor focus and can’t take advantage of the advantages that a large aperture can bring). For example, if you use 500D with Canon 50/1.2L, you may not be able to get a clear image when using an aperture smaller than F2.8, because the camera’s focus accuracy is not enough. This is determined by the positioning of the camera.
Some expensive professional quasi-zoom lenses mostly have an aperture of F2.8 or even larger. A larger aperture can ensure more light input, and can still obtain a sufficiently high shutter speed under low light conditions to obtain a clear image. But while the large aperture brings more light in, it also makes the depth of field of the photo subject become very shallow. If you are shooting documentary photography, the content of the background of the screen cannot be ignored.
Photosensitive element
High resolution and high definition are two concepts. Pixels only determine the size of your photo. A very blurry photo may also have high pixels.
In the film age, according to the size of the film, we can divide cameras into: APS format, 135 format, 120 format, and large format.
→135 format camera:
Use the most common 35mm film, the film size is 24*36mm. The full frame of today’s digital SLR era is 135 frames.
→APS frame:
Less than 135 frame system.
Full frame and APS frame only differ in the size of the photosensitive element. Most of the full-frame digital SLR lenses from the same manufacturer can be used on APS-frame digital SLR cameras.
The photosensitive element cannot record all the thresholds of natural light from the brightest to the darkest (just as the number of colors that a computer screen can display cannot record all the colors of the natural world), for example, suppose the light level of the natural world can be divided into 10 levels (1 The darkest, 10 is the brightest), then the photosensitive element can only record up to 6 levels. Therefore, when shooting, we have to consider where to put these 6 areas in the 1-10 level area. For example, suppose we want to record more levels of the bright parts, we put all the 6 levels of the photoreceptor on levels 4-10 of the 10 levels in nature, and abandon the 4 levels describing the levels of the dark parts.
Similarly, suppose we want to record more of the dark parts of the scene, then we will assign 6 levels to the 1-6 levels in the natural world, and give up the 7-10 light levels.
If you want to show the details of the dark part, use the dark part as the metering basis. If you want to express the details of the highlights, use the highlights as the basis for metering. In practice, in most cases, more levels of highlights are preserved.
Scenery photography, usually morning and evening photography, 6-9 in the morning, 4-7 in the afternoon, in the middle of the time is a rest.
Why do ordinary photography often emphasize the choice of morning and evening as the shooting time? The reason is that the position of the sun in the sky in the morning and evening is lower, and the projection of objects is often longer. This kind of light and shadow effect can make the level of the picture richer and more durable.
Moreover, in the morning and evening, the difference in brightness between the sky and the ground is smaller than at other times, and the latitude of the photosensitive element can be fully utilized to bring rich layers to the picture. (Rather than the sharp contrast between bright and dark under strong light, the camera exposure can not take into account both the bright and dark parts. Either the bright part details or the dark part details are lost.) In other words, in order to make the picture richer, we must make the main body of the picture and The background is controlled within the range that the photosensitive element can express.
Sensitivity ISO
Photos are blurred, often because the shutter speed is too slow. We can adjust the size of the aperture, that is, the larger the aperture, the more light enters, and the shutter speed can be increased. But sometimes even at the maximum aperture, sufficient exposure cannot be guaranteed, and the shutter speed cannot be increased, resulting in blurry photos. Then, we need to use “sensitivity ISO” to increase the shutter speed.
“Sensitivity” is a quantitative value of the ability of photosensitive materials to perceive light, and the international unit is ISO. Common sensitivity classifications are: ISO100, 200, 400, 800, 1600, 3200, 6400, 12800. Every increase of 1 level (for example, from 100 to 200), the light sensitivity is increased by 1 time, which is equivalent to the light intensity increased by 1 time, so that we can also increase the shutter value by 1 level accordingly.
| Low sensitivity
|
Medium sensitivity
|
High sensitivity
|
Ultra high sensitivity |
| Below ISO100 | ISO 100~200 | ISO 400~800 | Above ISO1600 |
Therefore, when the aperture is the same and the shooting scene is the same, the exposure time (shutter speed) is inversely proportional to the sensitivity.
So, now we know that the three major elements of digital SLR control exposure are: aperture, shutter, and sensitivity.
Some cameras can subdivide every two levels of sensitivity into 1/3 levels of adjustment. For example, between 100 and 200, divided into 3 levels, there are more fine sensitivity values such as 100, 125, 160, and 200. The purpose of this design is actually to use “low sensitivity” as much as possible under a stable shutter. Because the higher the sensitivity, the side effect is that it will increase the noise of the picture, and the sharpness of the picture details, color saturation, color deviation, picture level and picture contrast will be severely affected.
For ASP frames, below ISO400, the noise problem is not obvious. But if you can use low sensitivity, don’t use high sensitivity. For full-frame digital SLRs, the high-sensitivity effect is much better than that of APS frames, so the scope of application can be increased by 1-2 levels.
Of course, for handheld shooting in low light conditions, you should first ensure the clarity of the picture (the value of high sensitivity is here), and secondly consider the loss of image quality caused by high sensitivity. Because the picture quality is not perfect, it can still be used, but if the picture is blurred, it will be “useless”.
Shooting flowing water for a silky effect is to extend the exposure time, but the lowest sensitivity (ISO50 or 100) should be used (because the exposure time itself is already very long.)
Lens
The structure of a digital SLR lens is more complicated. No lens has the function of a single lens. A lens is usually composed of multiple lenses. The lenses are grouped according to design needs. Therefore, manufacturers usually use “several groups of several lenses”. Way to describe the internal structure of the lens.
Product positioning of the lens:
In order to cater to different consumers, manufacturers usually provide multiple lenses for selection at the same or similar focal lengths, which correspond to beginners, enthusiasts, and professional photographers. Therefore, according to different levels, the functions of the lens are differentiated. Professional lenses for high-end users often have larger apertures and volumes to load more lenses, obtain larger apertures and better image quality. At the same time, new technologies such as anti-shake and ultrasonic motor focusing are also preferentially installed on these lenses.