Introduction to photography: what is the aperture in 5 minutes? Relationship between aperture size and depth of field

Aperture is a component in the lens that controls the aperture size. It consists of several diaphragm blades, which are mainly used to adjust the amount of light entering the camera. The larger the aperture and the shallower the depth of field, the more shots we can take. The smaller the aperture, the deeper the depth of field and the fewer lenses, so the depth of field and lens are actually the same concept. ?

I believe that people who study photography must have heard of exposure three-element aperture, shutter and ISO, but they may not be familiar with their principles and applications. It is recommended to refer to the three elements of exposure: aperture, shutter and ISO sensitivity.

1. What is the aperture?

1, say aperture

Aperture is a component in the lens that controls the aperture size. It consists of several diaphragm blades, which are mainly used to adjust the amount of light entering the camera. When other parameters remain unchanged, increasing the aperture will let more light enter the camera and make the photo brighter; Narrowing the aperture is the opposite, which will make the photo darker.

More specifically, the action principle of aperture is very similar to that of human pupil. In order to cope with different light and different visual environments, human pupils will be narrowed in bright sunlight to avoid discomfort, and enlarged in the dark to get more light. The same is true of aperture, which is why people say that "large aperture has better night shooting ability".

The larger the aperture → the larger the opening → the greater the amount of light entering → the brighter the photo.

The smaller the aperture → the smaller the opening → the smaller the amount of light entering → the darker the photo.

2, the structure of the aperture

The aperture is composed of aperture blades, and the aperture is controlled by the openings of these aperture blades. Generally speaking, the more aperture blades, the higher the accuracy, and the more circular aperture holes can be realized. At present, mainstream lenses generally have 5-9 aperture blades.

So does the number of diaphragm blades have an effect on the actual shooting? The more blades, the rounder the aperture, and the smoother the scattered scene. On the contrary, the fewer leaves, the more likely it is to have a polygonal scattered scene (as shown in the upper right, the scattered scene is 7 sides). Therefore, the high-order lens will emphasize the reason why the number of blades is 9 or more.

3. Aperture value and exposure

Generally speaking, we will use f/ value to represent the size of the aperture, such as f/ 1, f/ 1.4, f/2, f/fF2.8, f/4, f/5.6, f/8, f/1/kloc-.

As for the formula or source of f/ value, it is not important (please refer to paragraph 5 if you are interested, but you must know that "the smaller the number, the larger the aperture", so the aperture of f/2.8 is even larger than that of f/8. As for how big? Double, triple or 2.85 times? Actually, it's not. The correct answer is eight times. How did you know?

Therefore, when all parameters and environment are the same, the aperture of f/4 will be twice as dark as that of f/2.8 (the amount of light entering is half less). If you want to get the same exposure as f/2.8, you need to double the shutter time or double the sensitivity. In the same way, if you want to lower the brightness of f/4 photos by another level with the aperture today, you will naturally know to adjust the aperture to f/5.6 instead of f/2.8 or f/8.

The smaller the f/ value → the larger the aperture → the brighter the photo.

The larger the f/ value → the smaller the aperture → the darker the photo.

Second, the relationship between aperture and depth of field

Relationship between aperture and depth of field

After understanding the basic principle of aperture, let's talk about the application of aperture in photography, in which the depth of field is most closely related to aperture. I believe you should often hear the concept of "large aperture shallow depth of field, small aperture deep depth of field". What is depth of field?

1. What is the depth of field?

Generally, when we take pictures, besides the subject, there are also backgrounds and prospects, with girls as the leading role and trees and dogs as the backgrounds and prospects respectively. Today, when the camera is in focus, how deep the image is in the front and back range of the focus is called the depth of field. The clear range of the scene (1) is wide, so we will say that the depth of field of the picture is deep; In case (2), the clear range is narrow, so we will say that the depth of field of the picture is shallow.

It can also be seen from the actual example pictures that the deeper the depth of field, the larger the range that can be seen clearly, and conversely, the shallower the depth of field, the narrower the range that can be seen clearly.

2. The relationship between aperture, depth of field and lens.

We set the focus position in the green frame (koala), and tested the real shooting effect of large aperture f/2.8 and small aperture f/8.0 respectively. From the blur degree of foreground (dark candlestick) and background (kitchen utensils in the distance), it can be seen that the clear range is small and the depth of field is shallow under large aperture, while the clear range is large and the depth of field is deep under small aperture.

In addition, in addition to the terminology of depth of field, we also call this "blurred imaging outside the focus range" a stray scene. In other words, the larger the aperture and the shallower the depth of field today, the more obvious the scattered scene we can get; The smaller the aperture, the deeper the depth of field and the fewer lenses, so the depth of field and lens are actually the same concept.

The larger the aperture (the smaller the f/ value) → the narrower the clear range of the picture → the shallower the depth of field (the more dispersed the scene).

The smaller the aperture (the larger the f/ value) → the wider the clear range of the picture → the deeper the depth of field (less shots).

Large aperture shallow depth of field, small aperture deep depth of field

3. What are the advantages and disadvantages of large aperture and small aperture, and when to use them?

Now that we know the basic principle and application of depth of field, when should we use large aperture and small aperture?

The following table roughly compares the advantages and disadvantages of large and small apertures and wind, and the suitable scenes. But there is no standard answer to this question. It is not that a large aperture is only suitable for taking portraits, and a small aperture is only suitable for taking scenery. Everything depends on the current photo environment and the picture you want to express as a photographer, so don't be limited by these concepts.

4. The smaller the aperture, the better the image quality?

As mentioned above, when the maximum aperture of the lens is used, its image quality performance is usually not the best, and it usually needs to be reduced by 2-3 levels to reach the optimal aperture of the lens, that is to say, reducing the aperture is helpful to improve the overall performance of the photo, and generally speaking, the optimal aperture of the lens will be in the range of f/5.6-f/ 1 1 (not absolute). But if it is a high-order lens, even if the aperture is fully open, the imaging quality is still excellent. Although it is sharp and easy to use, it is still not the best aperture.

In this case, as long as the aperture of the lens is minimized, can we have the best picture quality? In fact, due to the diffraction phenomenon of physical optics, when the aperture is too small (such as f/22), the photo will be not sharp enough and a little hazy. Therefore, when you take landscape photos, remember not to shrink the aperture indefinitely for deeper depth of field. It is best to control the aperture in the best aperture range!

Third, what do you think of the lens aperture?

After understanding the use skills and application of aperture, how can we know how big the aperture of the lens is at hand? Or how to distinguish the aperture size when buying a lens?

1, what is the maximum aperture?

There are fixed-focus lenses and zoom lenses. As the name implies, a fixed-focus lens has only a single and fixed focal length (such as 24mm, 35mm, 50mm, 85mm, etc.). ), and you can only change one lens at a time with different focal lengths. The zoom lens can flexibly change the focal length (such as 24-70mm, 70-200mm, etc. ). As long as the focal length is within the lens range, you can zoom freely without changing the lens.

Then if the zoom lens sounds so convenient and easy to use, why do you need a fixed focus lens?

This is because the optical design of the fixed-focus lens is relatively simple, which not only makes it easier to optimize with fewer optical elements, but also has better imaging quality. The most important thing is that the aperture can usually be made larger. The zoom lens, on the other hand, has a complex optical design, which makes the aperture usually not very large (generally f/2.8), and some even shrink with the increase of focal length. So how do we read the size of the aperture? The following is an example.

2. What is a constant aperture?

Does the aperture of zoom lens have to shrink with the change of focal length? Don't you have a zoom lens with a fixed aperture? The answer is yes, the so-called constant aperture zoom lens. In order to keep the aperture constant in zoom environment, the price and weight of constant aperture zoom lens are usually high. For example, the common big ternary lenses of various brands are all f/2.8 constant aperture lenses. What do you think of this kind of lens?

Fourth, how to adjust the aperture?

If you want to adjust the aperture, you must first have a camera that can manually control the aperture, shutter and ISO (SLR, micro-single, category list). Then set the camera to manual mode or aperture priority mode, so that the aperture can be adjusted. As for the functions of these modes, Dai Feng will write another article to explain the differences between the four modes of the camera and their suitable use opportunities.

5. Calculation and principle of aperture series

This section will introduce the mathematical principle behind the f/ value of the aperture in detail. This paragraph is not directly helpful for learning photography itself, and even a bit boring. If you are not interested, you can skip it without understanding. But if you are a curious baby like Dai Feng, I believe this passage can make you understand the physical meaning behind its numbers!

What are the definitions of 1 and f/ value?

Aperture f value = lens focal length mm/ aperture diameter

For example, the f/2.8 lens has an aperture diameter of about 8.6mm; The focal length is 24mm Similarly, when the focal length is 70mm, the aperture diameter is 25 mm..

Therefore, from the definition of aperture, we can know why the aperture of a lens with a longer focal length is rarely f/2.8 or even larger, because the aperture diameter will become very large, which will not only sacrifice the weight and volume of the lens, but also be expensive. So we see that 200mm, up to 400mm, has an aperture of f/2.8 in the market, while we hardly see that 800mm/ 1200mm has an aperture of f/2.8 or even f/4.

In addition, from the definition, we can also know that the aperture diameter (aperture size) is inversely proportional to the f/ value, which also explains why the smaller the f/ value, the larger the aperture. Same focal length (eg50mm): The larger the aperture, the larger the volume and weight. Same aperture (egf/2.8): The longer the focal length, the greater the volume and weight.

2. Why does the brightness increase or decrease by one time and the f/ value increase or decrease by non-multiple?

As mentioned in the first paragraph, to double the brightness, the aperture needs to be increased by one level to double the incident brightness. But why does the aperture increase by one level and the f/ value not decrease by several times? Let's go back to the definitions of brightness and f/ value.

∝ Aperture area = π× (aperture radius) 2 Description: The light enters the camera after passing through the aperture, so the amount of light entering will be affected by the aperture area (∝ means it is directly proportional). Aperture f value = lens focal length mm/ aperture diameter→ aperture diameter = lens focal length mm/ aperture f value aperture area = π× (aperture radius) 2 = π.

Note: aperture radius = aperture diameter /2.

Summary: incoming light ∝ aperture area ∝ (lens focal length mm/ aperture f value) 2

Therefore, under the same focal length, if the incident light quantity is doubled (×2), the F value needs to be (1/√2) times of the original. This is why the sequence of aperture series is f/ 1, f/ 1.4, f/2, f/2.8, f/4, f/5.6, /8, f/1,f//kloc-0.