Understanding Optical Power vs Focal Length in Photography

Introduction to Optical Power

In the world of photography, optical power and focal length are two key concepts that photographers and enthusiasts often need to understand. Optical power is a measure of the magnification of an image, which essentially compares how the image size changes with different lenses. It is a term used to compare the relative magnification between lenses, and is based on the number of times the subject appears larger or smaller than in a 'normal' configuration.

Defining Optical Power

The term 'optical power' is particularly useful when comparing different lenses. For example, if a lens makes a subject appear twice as large as another lens on the same sensor, the first lens has an optical power of 2X, while the second lens has 1X power, which is considered the normal power. Similarly, a lens that makes the subject half the size would have an optical power of 0.5X.

Focal Length Explained

Focal length, on the other hand, is a specific measurement that refers to the distance between the lens and the image sensor when the lens is focused at infinity. It is a key factor in determining the angle of view of the lens. Focal length is used to calculate the angle of view, which in turn determines how much of the scene can be captured in the image.

Key Differences

The main difference between optical power and focal length lies in their definitions and applications. While focal length is a specific measure, optical power is a relative measure comparing the magnification of different lenses. Focal length is a specific formula that takes into account the angle of view and the specific ability to focus at infinity, whereas optical power is based on the ratio of the angle of view, from wide to telephoto, in relation to the sensor size the lens is designed to be used with.

Optical Power in Different Sensor Sizes

Each sensor size has its own 'normal' focal length, which is the focal length that approximates the angle of view of the human eye. This 'normal' focal length is historically and somewhat inaccurately referred to as the 'standard' or 'natural' view for that particular sensor format. For example, on a full-frame sensor, 50mm is considered the 'normal' focal length. On a crop sensor, a 50mm lens would appear similar to a 75-85mm lens on a full-frame camera, due to the crop factor.

Examples of Optical Power and Focal Length

Let's look at a few practical examples to clarify these concepts. If a 35mm lens and a 105mm lens are compared, the 105mm lens has 3X the optical power of the 35mm lens because it provides a three times magnification compared to the 35mm.
If a zoom lens goes from 25mm to 100mm on a camera with a 50mm 'normal' focal length, it has a zoom power of 4X, as it spans from the wide-angle to the telephoto range. Similarly, a lens that goes from 100mm to 300mm would have a 3X zoom power, even if the 'normal' focal length of 50mm is not in the zoom range.

Conclusion

Understanding the difference between optical power and focal length is essential for photographers and camera enthusiasts. While focal length gives a specific measurement, optical power offers a comparative measure, which is particularly useful for understanding and selecting lenses. As different sensor formats have different 'normal' focal lengths, it is important to consider the specific sensor size when discussing these concepts.