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For a converging lens, it is common practice to form a sharp image of a distant light source on a screen. Since the object (light source) is far away, 1u\frac{1}{u}u1​ becomes negligible, and the focal length is approximately equal to the image distance.

Field of View or Field Diameter is very important in microscopy as it is a more meaningful number than "magnification". Field diameter is simply the number of millimeters or micrometers you will see in your whole field of view when looking into the eyepiece lens. It is just as if you put a ruler under the microscope and counted the number of lines.

This means that an object 20mm (2cm, or about 3/4 inch) wide would fill up the whole viewing area at 10x and an object about 6.7mm wide would fill up the whole area at 30x. As you can see, having the highest power may not be best for your particular application. When you move to greater magnifications, you sacrifice field of view.

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This page with pennies at different magnifications might be helpful to get a visual idea of what you can see at different magnifications.

In photography, it is desirable to eliminate the parameter image distance from the focal length equation by introducing the magnification of the image.

Magnification MMM refers to the enlargement or contraction of the image size compared to the object size. It is given by:

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If you're interested in learning how a lens is made for a given focal length using materials of different refractive indices, visit our lensmaker equation calculator without fail!

Get a metric ruler and place it on the stage of your microscope. Illuminate from above (if you are using a compound microscope, get a transparent ruler or illuminate it with a flashlight). See how many millimeters you can see from left to right. What would be the field of view of this image? (Answer located at bottom of page).

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High power microscopes usually have 10x eyepieces lenses, so there is only one column above the 4x objective. If you wanted to look at an amoeba that is 150µm long (0.15mm) then you would use either the 40x or 100x objectives. There are 25mm in an inch and there are 1,000um (micrometers) in a single millimeter.

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The field of view or angle of view refers to how much of the scenery can be captured by the lens. Wide angle lenses and ultrawide cameras are becoming household names thanks to many smartphones.

The chart below will tell you (approximately) what to expect when looking through a microscope with varying combinations of eyepieces and objective lenses. As an example (in green below), a dual power stereo microscope with 10x eyepiece lenses and 1x and 3x combinations of objective lenses, would have total powers of 10x and 30x and your field of view would be 20mm and 6.7mm respectively.

The focal length of a lens measures how sharply it can bend the light rays to converge at or diverge from a focal point. It is the distance between the center of the lens and the point where the light rays converge or diverge.

💡 Magnification is negative for real images and positive for virtual images. However, the traditional practice in photography is that the magnification is positive for positive focal length. Hence, the formula for magnification in photography is:

Our focal length calculator will help you figure out the focal length needed to capture a distant object through a thin lens. It will also calculate the magnification and field of view of the lens. This article will teach you the focal length equation and image distance formula. Additionally, you will learn how to calculate the field of view and magnification using focal length. So hop in!

Other considerations: The working distance is the distance from the bottom of the microscope (lens) to the part of the specimen that is in focus. As you increase the magnification, you decrease the working distance. If you need to work under the microscope, you will need a large working distance. Some special microscopes have extended working distances for these purposes. Zoom microscopes have a fixed working distance throughout the zoom range. When using a 100x objective lens (1000x total power) your working distance might only be 0.04mm (40µm). The lens will be extremely close to the specimen! The working distance and the amount of vertical motion of the microscope will also affect the maximum specimen height. Maximum specimen height is how tall an object you can put on the stage and still be able to focus on the top part of the specimen.