The Basics of Converting from an 800mm Lens to Inches
When you want to convert from an 800mm lens to inches, first decide what your conversion goal is. If your goal is print size or web size, then use the nearest common denominator. If you need a specific print size or web resolution, use the size needed in parentheses after the 800mm focal length. For example, if you have an 800mm lens and want to create a landscape print that is 12×18 inches, use 12 (800mm) as your focal length.
To convert from inches to mm:
1. Decide your conversion goal: Print size or web resolution?
2. Multiply by 3.14159 to get the focal length in millimeters: (800 ÷ 3.14159) = 245 mm
3. Divide by 100 to get the focal length in inches: (245 ÷ 100) = 24 5/8 inches
Tools You’ll Need
If you are shooting with an mm lens and need to convert to inches, there are a few tools that you’ll need.
The first tool is a conversion chart. This can be found online or in most photography books.
The next tool is a tape measure. You will need it to measure the inside of your lens from the front to the back (or vice versa).
The last tool is a caliper. You will need it to measure the width and height of your lens.
The Steps to Converting Your Lens
If you have a mm lens and want to convert it to an inch lens, there are several steps that you need to follow.
1. Find the conversion factor.
There is no one single conversion factor for all lenses, so you will need to find out what the conversion factor is for your particular lens. There are many online calculators that can help with this, or you can contact the manufacturer of your lens for more information.
2. Calculate the distance in inches between the focal point of your mm lens and the object being photographed.
To calculate the distance in inches between the focal point of your mm lens and the object being photographed, use this equation: D = (Focal Point – Object) / Conversion Factor.
3. Convert inches to millimeters using this equation: MM = (D*Conversion Factor) / 1000000mm
The Final Product
The final product looks great and the difference is very noticeable. If you are making a small to medium size change, using this method should be enough. If you are making a larger change, or if your lens is not round, you may need to find another way.