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Understanding Field Stops in Imaging Systems

What are Field Stops?

In the field of imaging systems, the field of view is limited by an optical aperture known as the field stop. This aperture restricts the angular range within which objects can be effectively imaged.

Types of Field Stops

Field stops can take various forms:

• In digital cameras, the size of the image sensor often acts as the field stop, limiting the field of view.
• A field stop can be a diaphragm positioned in an intermediate image plane.
• At times, the instrument’s design itself creates an optical aperture that serves as the field stop.

Role of Field Stops

When an optical system incorporates multiple apertures, the field stop is the aperture that most severely constrains the field of view.

Vignetting Effect

It’s important to note that an optical aperture not in a plane conjugate to the object plane does not create sharp edges in the field of view. Instead, it causes a vignetting effect, leading to a gradual decrease in image brightness at the edges.

Example: Keplerian Telescope

In a Keplerian telescope, the field of view is limited by the ratio of the ocular lens diameter to the objective focal length. The barrel, which is essentially the ocular lens diameter, serves as the field stop. Due to the barrel entrance being behind the intermediate image plane, some vignetting occurs.

Conclusion

Understanding field stops is crucial in optimizing the performance of imaging systems, ensuring that the field of view is effectively managed to capture clear and accurate images.

Source: Cloudy Nights
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