# Why Does Tube Length Affect Microscope Focal Length?

• akinokoe
In summary, the tutorial discusses the importance of the tube lens and intermediate image distance in determining the effective focal length of a microscope system.
akinokoe
I am trying to design a microscope and I am confused about some concepts. I plan to buy a Olympus 4x infinite-corrected objective lens and 6 inch long tube and a tube lens (f= 75mm) and then a CCD camera. By chance I found a tutorial online

http://www.olympusmicro.com/primer/java/microscopy/focallength/index.html

I have questions about the tutorial: why is the focal length of the objective lens changing? Since the light beam come out of the objective lens is parallel, why does the tube length still matter? Really confused ... Need help!

Last edited:
The focal length of the objective lens does not actually change; it stays the same. The tutorial is referring to the effective focal length of the entire microscope system, which changes with the addition of the tube lens and camera. The tube length matters because it affects the distance between the objective lens and the tube lens; this is known as the intermediate image distance. The intermediate image distance sets the size of the intermediate image, which is then magnified by the tube lens and then further magnified by the camera. All of these components come together to form a complete microscope system with a specific magnification and effective focal length.

## 1. What is the definition of objective lens focal length?

The objective lens focal length refers to the distance between the center of the lens and the point where all the light converges to a single point. It is typically measured in millimeters (mm).

## 2. How does the focal length affect the magnification of an image?

The focal length of an objective lens directly affects the magnification of an image. A shorter focal length will result in a larger magnification, while a longer focal length will result in a smaller magnification.

## 3. What is the relationship between the focal length and the field of view?

The focal length and the field of view have an inverse relationship. This means that a shorter focal length will result in a wider field of view, while a longer focal length will result in a narrower field of view.

## 4. How does the objective lens focal length impact the depth of field?

The focal length of an objective lens also affects the depth of field, which is the range of distances that appear in focus in an image. A longer focal length will result in a shallower depth of field, while a shorter focal length will result in a deeper depth of field.

## 5. What is the ideal objective lens focal length for different types of microscopy?

The ideal objective lens focal length will vary depending on the type of microscopy being used and the specific needs of the experiment. In general, higher magnification objectives will have shorter focal lengths, while lower magnification objectives will have longer focal lengths. It is important to choose the appropriate focal length for the desired magnification and resolution needed for the specific experiment.

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