What is the circle of blur in photography?

The meaning of the blur circle

The point light source passes through the lens and forms a point on the image plane. Keep the distance between the lens and the film unchanged and move the point light source back and forth along the optical axis. The image formed on the plane will become a circle with a certain diameter. The size of the circle depends on the lens aperture and the degree of deviation of the point light source. As long as the diameter of the circular image is small enough, the photo will still be clear enough. The point light source is circular. If the image is larger, the photo will appear blurry. This critical point of the circular image of the light source is called the circle of blur.

Generally, the blur circle diameter of a 35 mm camera lens is 1/30 mm, the blur circle diameter of a medium format camera lens is 1/15 mm, and the blur circle diameter of a micro camera is 1/60 mm. However, the diameter of the blur circle specified by each camera manufacturer will be slightly different, so you should refer to the camera manual.

The concept of the circle of blur was first proposed by an article signed by T.H in the British Photography Magazine in 1866. He found through experiments that the human eye can distinguish the distance between the pictures at a photopic distance of 25 cm. Two lines of 1/4mm. A 35 mm camera film is enlarged into a 20x30 cm photo (that is, enlarged 8 times). The circle with a diameter of 1/4 mm on the photo should be a circle with a diameter of 1/32 mm on the film. This is what later cameras such as Leica The factory takes the circle of blur as 1/30 mm.

The intuitive reason why the image on a picture looks clear or unclear lies in the eye's ability to distinguish various details on the picture. If it can distinguish, it will be clear; if it cannot be completely distinguished, it will not be clear; if it cannot be completely distinguished, it will not be clear. Distinguishing is fuzzy.

The image is composed of countless light points of different light and shade. The smaller the light points that make up the image, the higher the definition of the image. The lens focuses on a certain point on the subject, and that point creates a focus on the film. The focus is the smallest point of light that makes up the image. This smallest point of light is actually an extremely small circle whose diameter can be measured. Relative to the smallest spot of the focal image, all other image spots are larger than it. The larger the distance from the focus point (including further or closer to the lens than the focus point), the larger the circle (light spot) formed on the film.

Within a certain range, although the circle (light spot) formed on the film by the scene before and after the focus point is increasing, it can still produce a clearer image in terms of visual effect. When the circle (light spot) that forms the image increases to a certain extent, it begins to form an unclear image. The larger the circle that forms the image, the blurry the image becomes.

In photography, the largest circle that can produce a clearer visual image is called the "circle of blur." When the circle forming the image is larger than the circle of blur, a blurry image will be produced; conversely, as long as the circle forming the image is smaller than the circle of blur, a clear or relatively clear image will be produced. (Range of depth of field ≤ circle of blur)

*Practical points of the circle of blur

The maximum allowable diameter of the circle of blur depends on the viewer's vision, the distance at which the photo is viewed, and the impact on the film Request to enlarge size related.

Experiments have shown that people with normal vision can still have a clearer perception of an image with a blur circle diameter of 0.25 mm when viewed 25 cm away from the photo under sufficient light conditions; while for blur circle diameters larger than The 0.25mm image looks unclear or even blurry. Therefore, the maximum diameter of the blur circle allowed by the image on the film can be calculated using the formula "maximum blur circle diameter = 0.25mm/magnification". For example, if you want to enlarge an 8-inch × 10-inch photo using a 24mm The maximum diameter of the circle of blur allowed for the image on the negative when using an inch x 10 inch photo.

Based on the meaning and practical points of the blur circle, it is not difficult for us to understand why the image clarity of the same film will decrease when enlarged at high magnification.