Do you know the focal length of your lens? Have you ever wondered what that number means or how it differs from your camera’s zoom level? If so, read on for some helpful information.

Focal length measures the distance between the optical center and one side of an imaging system’s image sensor (or film). Focal lengths are usually expressed in millimeters, with lower numbers indicating wider angles (low-magnification) and higher numbers showing narrower angles (high magnification).

For example, a 200mm lens will create images with less magnification than when using a 50mm lens. Some lenses can be used at different focal lengths but provide the same field of view; these lenses are called “equivalent focal length.”

The focal length of a lens is significant when determining the camera’s field of view and magnification.

For example, if you were using a 200mm lens and wanted to produce an image with the same area of view as what your eye could see (also known as ‘35mm equivalent’), you would need to use a 16mm lens, since it has an equivalent focal length.

The same principle applies if you were shooting with a 35mm lens and wanted to achieve the exact image size that your eye can see (also known as ‘135mm equivalent’); you would need to use a 560mm lens since it has an equivalent focal length. The field of view and image size can also be affected by your camera’s sensor and CCD (charge-coupled device) size; the more significant the sensor or CCD, the smaller the ‘35mm equivalent’ crop facto

## Focal Length Formula

The lens’s focal length is equal to the center-to-center distance between adjacent pixels in a sensor (or film) divided by the diagonal measurement of one pixel. Since this relationship is true for most digital and 35mm imaging systems, you can use this formula to calculate the effective focal length:

Focal Length = (Sensor Diagonal / 2) / (Sensor Diagonal / 2)

For example, let’s say you have a 35mm camera with a 43.3mm diagonal sensor. The focal length = (43.3mm/2) / (43.3mm/2) = 21.65 mm . Note that the result is slightly different than the diagonal measurement of your camera’s imaging system.

Focal Length = (Center-to-center Distance Between Pixels / 2) / (Center-to-center Distance Between Pixels / 2)

## Types Of Lenses

### Convex Lens

Convex lenses are thicker in the middle and curve outwards to a focus point. These types of lenses can usually be found on magnifying glasses, telescopes, and binoculars.

### Concave Lens

Concave lenses are thinner in the center with sides that curve inward towards an opposite focal point. The standard household magnifying glass is an excellent example of this type of lens

## How To Find Focal Lenght Of Convex Lens

If you need to find the focal length of a convex lens, there are several simple ways to do it.

One is to take any object with straight edges, like an index card or piece of paper, and hold it at arm’s length in front of your eyes.

Now slowly move the object closer until you reach the point where you need to move the thing closer to keep it in focus.

Next, measure the distance between the paper and your eyes; this is how far away your focal point is from you. Using the formula below, you can then use that information to find out your lens’s focal length.

For example, let’s say you are holding the paper at 10 inches from your eyes, and you determine that is where you need to move the card for it to remain in focus.

We know that our focal length can be determined using this formula:

Focal Length = Distance Between Object & Lens / Distance between Object & Focal Point

So,

.Focal Length = 10 / 1

Focal Length = 10

This tells us that our lens has a focal length of 10 inches. You can also use this same method with any object as long as you know its size and the distance between it and your eyes; just remember to keep both those measurements in inches. This is why knowing your camera’s “35mm equivalent” is essential.

So, if you need to determine the focal length of a convex lens and have a 35mm equivalent camera, simply take that number and divide it by the size of your sensor or CCD (most people will not have a ‘1’ in front of their sensor size).

For example, if you have a 12.5mm lens on your camera, congratulations! You now know that’s equivalent to approximately 500mm! This information will be helpful when choosing lenses for bird watching or wildlife photography with the proper magnification and field of view.

## How To Find Focal Length Of Concave Lens

If you have a concave lens, it’s a little more challenging to find its focal length since it doesn’t have a focal point. Fortunately, there is a simple way to see this number if you know the object’s distance from your eyes or camera and its size.

For example, let’s say you place an index card six inches from your eyes and find that you need to move it to remain in focus.

We know that our focal length can be determined using this formula:

Focal Length = Distance Between Object & Lens / Distance between Object & Focal Point

So…

Focal Length = 6 / 1

Focal Length = 6

This tells us that our lens has a focal length of six inches. This information can also be used for any concave lens as long as you know its size and the distance between it and your eyes; just remember to keep both those measurements in inches.

So, if you need to determine the focal length of a concave lens and have a 35mm equivalent camera, simply take that number and divide it by the size of your sensor or CCD (most people will not have a ‘1’ in front of their sensor size).

For example, if you have a 10mm lens on your camera, congratulations! You now know that’s equivalent to approximately 30mm! This information will be useful when choosing lenses for macro photography with the proper magnification and field of view.

## How To Find Focal Length Of A Cylindrical Lens

If you have a cylindrical lens, it’s also more challenging to find its focal length since it doesn’t have a focal point.

Fortunately, there is a simple way to find this number if you know the object’s distance from your eyes or camera and its size.

For example, let’s say you place an index card two feet from your eyes (a large lens) and find that is where you need to move it in order for it to remain in focus.

We know that our focal length can be determined using this formula:

Focal Length = Distance Between Object & Lens / Distance between Object & Focal Point

So…

Focal Length = 2 / 1

Focal Length = 2

This tells us that our lens has a focal length of two feet. This information can also be used for any cylindrical lens as long as you know its size and the distance between it and your eyes; just remember to keep both those measurements in inches.

So, if you need to determine the focal length of a cylindrical lens and have a 35mm equivalent camera, simply take that number and divide it by the size of your sensor or CCD (most people will not have a ‘1’ in front of their sensor size). For example, if you have an 8mm lens on your camera, congratulations!

You now know that’s equivalent to approximately 25mm! This information can be useful when choosing lenses for close-up photography with the proper magnification and field of view.

In photography, different lenses have different focal lengths. It is essential to know the correct lens you should use to get the best photo possible since it’s impossible to carry all your lenses while on a field trip or something like that. You’d better learn to determine what focal length you need for your shot.

## Verdict

Focal length is the distance between the lens and the camera sensor.

This value allows you to calculate other matters associated with lenses, such as

the angle of view, depth of field, f-stop, etc.

If you have a concave lens, it’s a little more challenging to find its focal length since it doesn’t have a focal point.

Fortunately, there is a simple way to find this number if you know the object’s distance from your eyes or camera and its size.

FAQ's

### Q: How can I calculate the focal length from a photo?

A: To do that, you have to know the size of your camera’s sensor or CCD.

Then, simply divide this value by the object’s distance in the picture along with its size.

### Q: What is lens symmetry?

A: Symmetry means that light rays enter and exit the lens parallel to each other.

If you remove the asymmetrical lens from your camera, there won’t be any effect on your picture.

### Q: What is the focal length of a spherical lens?

A: Spherical lenses have spherical symmetry, which means that all light rays are symmetrical.

These lenses have a short focal length, and they reduce or distort the size of objects near the edges

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