Does the frame size affect the depth of field?

Lead: Many photographers may lack knowledge of county-level operation and shooting skills when they are just getting started, so I will share some knowledge about depth of field with you.

Does the frame size affect the depth of field? First of all, I want to say that this title is not very rigorous. This sentence is taken from the video of photographer Tony Northrup. He said that many small card machines, such as f/2.8 large aperture lenses, can't actually capture the zoom effect of f/2.8 aperture of full-frame cameras, so they should be played? Equivalent focal length? And then what? Equivalent aperture? Add the calculation to get the corresponding much finer aperture value, which makes it easier for consumers to compare. This film has aroused considerable repercussions, especially in the concept of physics, which deserves to be clarified again to understand the improper explanation.

Maybe you are confused, don't worry, let's explain some basic physical imaging concepts first.

1. What is the focal length?

I believe everyone has done an experiment in primary school. As long as you put the magnifying glass on the paper at a suitable distance, the infinite sun will focus on the paper and become a little bit. Is this the scorched distance? Focal length? . Different magnifiers have different focal lengths according to the curvature of their lenses.

Although the current camera lens structure is very complicated and consists of multiple lenses, the fixed-focus lens is actually a magnifying glass with a fixed focal length. What is the significance of different focal lengths in photography? It turns out that the longer the focal length, the larger the image will be, which is why sports photographers use 600mm? Cannon? You can take a clear picture of the athletes in the distance. The above figure compares the image sizes of the 50mm lens and the 150mm lens.

2. What is focus?

We know that in order to shoot a clear image, we must focus when shooting. For example, if I want to shoot a forest, if I aim the camera at a tree two meters away, the tree two meters away will be clear. If I focus on the tree four meters away, the tree four meters away will be clear.

3. What is the depth of field?

Although we choose a distance when focusing, the scenery at that distance will be very clear, but in fact, the closer and farther the focusing distance is, the clearer it will be. What is the clear depth from near to far? Depth of field? Yes For example, focusing on two meters, in fact, the place from one meter to four meters is clear, and the distance from one meter to four meters is the depth of field.

4. What is aperture?

Have you ever wondered why the description of aperture is always? f/？ A numerical value? That's because the amount of light entering the lens is related to the focal length of the lens and the diameter of the aperture. ? f/？ The focal length is divided by the diameter of the aperture. So it's also an f/2.0 lens. The aperture diameter of the 50mm f/2.0 lens is 25mm, while the aperture diameter of the 100mm f/2.0 lens is 50mm.

But the focus of aperture value is the measurement of the amount of light entering. In most cases, we usually use the value of f to calculate, rather than deliberately considering the aperture diameter.

5. What will affect the depth of field?

Here comes the point. Only Jing is deeply affected? Focal length? 、? Aperture? And then what? Lens focal length? The size of the photosensitive element does not affect the depth of field. There is a simple reason. When the camera focuses and the light passes through the lens and aperture, the image is actually formed, no matter what size photosensitive element you use. In the above example, focusing on two meters means that the clear range is one to four meters. Whether you use a full-frame camera or an M43 camera, the result is the same. The only difference is the size of the image, not the depth of field.

Then why does everyone say that a big camera will have a shallow depth of field? ! This is because of the problem of image cropping. Take a look at the picture above. For example, compared with a camera in Quan Huafu, the M4/3 has a small frame, uses a 50mm lens, and the imaging range on the sensor is only half that of Quan Huafu (because the' cropping factor' of M4/3 is 2X), so you can't see the remaining blurred area. Naturally, you feel the depth of field? Not shallow enough? Yes

If we explain it from another angle, we usually introduce it? Equivalent focal length? The concept of M4/3 uses a 50mm lens, and Quan Huafu uses a 100mm lens (because the clipping factor of M4/3 is 2X). Of course, the depth of field in Quan Huafu is very shallow! The longer the focal length of the lens, the shallower the depth of field of the image.

6. What is 6? What is Tony Northrop talking about?

For example, he said that the equivalent focal length of M4/3 12-35mm f/2.8 lens is 24-70mm, but the depth of field shot with f/2.8 at the equivalent focal length of 70mm will not be as shallow as that shot with F/2.8 at Quan Huafu's 70mm, so the manufacturer is misleading and the lens should be renamed as f/5.6.

We disagree with this statement, because the focus of aperture value is the calculation of photometry, not the comparison of depth of field. In fact, we don't need to convert all the data into the standard 135 system (Quan Huafu). As early as the movie era, there have been many different frames, such as 1 10,135,645,67,69,612,617,4? 5、5? 7、8? 10 and so on. We all know that different pictures will have different depth of field.

The equivalent focal length is mentioned today simply because many people are familiar with the 135 system, so it is easier to associate with the corresponding photographic effect, but it has no special significance in the calculation of actual shooting, because most calculations need to use the real aperture and focal length. Therefore, as long as everyone understands that different pictures have different depth of field due to different lens choices, it is enough.

However, it is preferable for Tony Northrop to demonstrate the corresponding depth of field calculation methods in different formats, such as 35mm f/2.8 under m4/3 and 70mm f/5.6 under Quan Huafu, which should be the same (focal length multiplied by 2, aperture divided by 2). If we want to stick to Tony Northrop's logic, it should be said that the equivalent focal length of M4/3 12-35mm f/2.8 lens is 24-70mm, and the equivalent aperture value is f/5.6, which is more reasonable. (I just feel a little redundant)