Spot size and focal length lens

[edmondng]

why does the spot size increase with lower f#.

Lets says you have 2 lens of the same material, and diameter, just different focal length. F/2 has higher spot size than F/3 but F/2 is faster. Is spot size a characteristic of a lens? So if your detector is fixed in size you need to get a lens that fits about the size of your detector or less otherwise it will result in blurry image?

 

[n0_3sc]

why does the spot size increase with lower f#.

f# = f/D, where f = focal length, and D = diameter of aperture. This diameter IS essentially your spot size. Therefore bigger D = lower f#

Is spot size a characteristic of a lens? So if your detector is fixed in size you need to get a lens that fits about the size of your detector or less otherwise it will result in blurry image?

Yes and No. It depends on the aperture-lens configuration - If we're talking about camera's its not something major to worry about because it's always designed optimally.

So if your detector is fixed in size you need to get a lens that fits about the size of your detector or less otherwise it will result in blurry image?

Your lens doesn't need to "fit" the size of your detector but rather focus down to within the detectors area.

 

[oswaler]

Spot size and focal length are important parameters to consider when using lenses in scientific experiments or imaging. The spot size refers to the area of the focused beam of light, while the focal length is the distance from the lens to the point where it focuses the light. These two parameters are related, and as the focal length decreases, the spot size increases.

This is because the spot size is determined by the angle of convergence of the light rays as they pass through the lens. With a lower f-number (focal length to diameter ratio), the angle of convergence is larger, resulting in a larger spot size. This also means that the light rays are more spread out, which can lead to a decrease in the resolution of the image.

The speed of the lens, represented by the f-number, is also affected by the focal length. A lower f-number means a faster lens, as it allows more light to pass through and reach the focal point. However, this also means that the spot size will be larger, which can impact the quality of the image.

Spot size is indeed a characteristic of a lens and is an important consideration when choosing a lens for a specific experiment or imaging purpose. If the detector size is fixed, it is important to choose a lens with a focal length that is appropriate for the size of the detector. Using a lens with a larger spot size than the detector can result in a blurry image and a decrease in resolution.

In summary, spot size and focal length are closely related parameters, and a decrease in focal length will result in an increase in spot size. It is important to consider both of these factors when choosing a lens for a specific application, in order to achieve the desired image quality and resolution.