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Thread: f ratio?

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    Default f ratio?



    I have read about the science and definition about f#'s but what is better a lower or higher f#?

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    Lower means faster pictures - less star trails and brighter images
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    so a lower f # is more desirable?

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    Certainly from the astrophotography point of view -
    Sometimes plantetary viewing is too bright and needs filtering.
    Longer focal length mirrors are cheaper to make I think and lenses less prone to spherical aberration (colour fringing)
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    for larger lenses and mirrors, a longer focal length means higher magnification
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    so a 130mm f/8 wont bring in as much light as a 130mm f/6.. but the f/8 will be capable of higher magnification?

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    hey gappa

    the f/ratio in telescopes makes it so that the higher the number the slower the ratio, so by exposure wise you would have to expose twice as much as a lower f/number , ex. an f/10 at 1 min and an f/5 at one min, you would see more detail in the lower f/ratio, basicly the ratio of a telescope is the ratio between the focal length and the aperture.

    also the more aperture you have the more the detail your mirror can gather, like a bucket of light if you will.
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    Quote Originally Posted by Gappa View Post
    so a 130mm f/8 wont bring in as much light as a 130mm f/6.. but the f/8 will be capable of higher magnification?
    Yea , But another trade off with a slower scope (f8) is a narrower field of view as opposed to a f6 ...Faster scopes will be good for wide fields of view like faint nebulosity but lower Magnifications.
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    so for viewing with your eyes a lower # will be brighter

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    Actually, f numbers are only valuable reference for photography, visual performance will not be notably effected. For photography, lower numbers mean generally a wider field of view (for a given aperture) and less exposure time.

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