Sign convention: any measurement taken parallel and along the light rays is taken as positive and any measurement taken parallel and opposite the light rays is taken as negative.

Focal lengthvs zoom

Please don't mess a lens focal length, flange focal distance and back focal distance. These are three different parameters as you can see on the diagram below.

What isfocal length oflens class 10

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A person uses a spectacle lens of f = 25 cm to help focus at short distances. If her eye alone had focal length 2.1 cm (for closest vision), what is the focal length of the eye and spectacle lens combined, in cm? You may use the formula which assumes the lenses are close together. Note that the eye needs a focal length of about 1.88 cm (power of 54 dioptre) to focus on objects at 25 cm which is the typical nearpoint for a young adult.

Shorter focal lengths (wide-angle) tend to have a deeper depth of field, keeping more of the scene in focus. Longer focal lengths (telephoto) can create a shallower depth of field, isolating the subject from the background.

Wide-angle lenses have shorter focal lengths, typically less than 35mm, providing a broad field of view. Telephoto lenses have longer focal lengths, often above 70mm, allowing for magnification and bringing distant subjects closer.

Focal length oflens formula

Let us consider that light rays are falling on the lens from left to right. The focus point for the converging lens lies on the right side of the lens and we know that every measurement in the lens is taken from the optical center. The converging lens focal length is measured from the optical center to the focus point and the measurement is along with the falling rays therefore, it is taken as positive.

Crop factor, associated with cameras with smaller sensors, effectively multiplies the focal length of a lens. For example, a 50mm lens on a camera with a crop factor of 1.5x behaves like a 75mm lens (50mm x 1.5).

What isfocal length oflens

Given the (average) near point of a normal human eye can be taken to be 25cm, what must be the separation of the cones of the human retina?

The "normal" focal length is generally considered to be around 50mm on a full-frame camera. A 50mm lens provides a field of view similar to what the human eye perceives, making it versatile for various types of photography.

View angle of a lens is formed by the lines connecting center of the lens front element with border points of the space pictured by the lens with acceptable sharpness and brightness. The lens can be used for shooting on the frame size with diagonal that is less or equal to diameter of the image circle of the lens. Many cine lenses have larger image circle than size of the acceptable image quality circle, so actual view angle of most lenses is greater than view angle determined by the frame size.

Cameralens distance chart

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Shorter focal lengths (wide-angle lenses) capture more of the scene in the frame, making them suitable for landscapes and tight spaces. Longer focal lengths (telephoto lenses) bring distant subjects closer, making them ideal for sports and wildlife photography.

Focal lengthexamples

Camera focal lengthcomparison

Focal length is one of main parameters of any lens. This is the distance from the optical center of a lens to the camera sensor when the lens is focused to infinity. It is usually measured in millimeters (mm). Optical center of a lens is the light rays convergence point inside the lens. Focal length determines useful angle of view for specific frame size in vertical and horizontal planes. Shorter focal length means wider angle of view both in the objects plane and in the image plane. Longer focal length lenses give larger image of the object.

Understanding focal length is essential for photographers, as it plays a crucial role in composition, perspective, and the overall look of an image.

On the diagram below the object is located at the same distance from the camera sensor. But due to the lens optical center is farther frome the sensor, a 75mm lens produces larger image than a 35mm lens.

Focal length of a cameranikon

In the case of a diverging lens, the focal point lies on the left side of the lens. And the measurement of the focal length is taken from the optical center to the focus point and this measurement is opposite to the falling rays. Therefore, it is taken as negative.

Deviation of the actual focal lengths from the values listed above should not exceed ±2% for lenses with F<=22mm, and ±3% for all other lenses.

A zoom lens has a variable focal length, allowing the photographer to adjust the magnification level. For example, a 24-70mm lens is a zoom lens that can be set to 24mm for wide shots, 50mm for a normal view, and 70mm for a closer view.

Using a magnifying glass, one obtains an angular magnification for a relaxed eye given by 0.25/f where f is in metre. Using a magnifier with f = 0.08 m, A person sees an image which appears to occupy an angle of 8 degrees from top to bottom. What angle (in degrees) would the actual object occupy at the person's near point?

The choice of focal length depends on the intended use. Higher focal lengths are not necessarily better; it depends on the photographer's creative vision and the type of photography. Each focal length has its advantages in different situations.