# Spot size and focal length lens

## Main Question or Discussion Point

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?

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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.