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- Thread starter CarterJamison578
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The convention of articulating aperture values retaining the f/number designation reinforces and reminds us of how the calculation is made. Given an f/number and a lens's focal length, you can calculate the size of the lens opening in mm in a second, should you need to. For example, f/8 on a 35mm lens says the diameter of the lens opening is 35mm divided by 8 or 4.375mm, and f/8 on an 800mm lens is 800 divided by 8 or 100mm. And vice-versa: if you have an unmarked barrel lens and have determined through measurement that the focal length is, for example, 165mm and the diameter of the optics measures out to 56mm, you can get the f/number of f/2.9 (just a wee bit faster than f/2.8) and plug that into your standard exposure calculating system to determine exposure.

Other systems of numerically representing aperture size existed before the f/number system was adopted and became convention, and they proved a lot harder to work with in determining exposure settings. Probably more information than you really want to know is available on Wikipedia: https://en.wikipedia.org/wiki/F-number

G

It is a really useful number as the angle of light cone from a lens is always proportional to the f-number, meaning you can calculate both depth of field (the area in front of the camera that appears sharp) and depth of focus (the area on each side of the image plane the sensor needs to be to make a sharp image (think focus tolerance)). You can also calculate the relative f-number for a system, for example when you are using a lot of extension for macro photography, to determine exposure.

The f-number is also proportional to diffraction, when your image becomes softer with the smaller aperture. The smaller the format or pixels, the more important and useful it becomes.

One thing the f-number has not become is a strange photography swear word, although...

But now we are just heading off topic, but that is a fine art around here.

Edit: I found this article that might shed some light on the subject :

In this blog post, I would like to share some insights with you regarding the connection between aperture and the inverse-square law of light, as well as

petapixel.com

Joel

thanks

ok

Edit: I found this article that might shed some light on the subject :

## Understanding the Inverse-Square Law of Light

In this blog post, I would like to share some insights with you regarding the connection between aperture and the inverse-square law of light, as well aspetapixel.com

Joel

ok thanks

It is a really useful number as the angle of light cone from a lens is always proportional to the f-number, meaning you can calculate both depth of field (the area in front of the camera that appears sharp) and depth of focus (the area on each side of the image plane the sensor needs to be to make a sharp image (think focus tolerance)). You can also calculate the relative f-number for a system, for example when you are using a lot of extension for macro photography, to determine exposure.

The f-number is also proportional to diffraction, when your image becomes softer with the smaller aperture. The smaller the format or pixels, the more important and useful it becomes.

One thing the f-number has not become is a strange photography swear word, although...

But now we are just heading off topic, but that is a fine art around here.

Yes there's a stop of light difference between f/1.4 and f/2 but it's not because of the inverse square law. It's because the area of the aperture - the opening the light passes through - at f/2 is half of the area at f/1.4

Edit: I found this article that might shed some light on the subject :

## Understanding the Inverse-Square Law of Light

In this blog post, I would like to share some insights with you regarding the connection between aperture and the inverse-square law of light, as well aspetapixel.com

Joel

The effect of the inverse square law is normalized by dividing by the focal length. f/2 is f/2 no matter how far the aperture is from the image plane and no matter what the aperture's physical diameter is.The apertures work similar by replicating distance by limiting the light into the camera. It's still by the square root of 2, but not exactly as the inverse square law.

Joel