Thread: Optics question about image circles and optical formulae

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  1. #1 Optics question about image circles and optical formulae 
    Senior Member Karim D. Ghantous's Avatar
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    Let's say you have an arbitrary lens. The front element is, for argument's sake, 1" wide. Its image circle covers 4-perf 35mm.

    But let's say you want to cover an image circle exactly twice as large, which we will assume is 8-perf 35mm. The simplest option is to us a 1.4x TC.

    Now, here's the question: could you just take that lens formula and just make it bigger? In that case, the front element would be 2" wide, and the rest in proportion. You won't change a single thing about the optical formula, you'll just make it bigger. Is it that simplistic or is there more to it?
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  2. #2  
    Laowa just announced the f0.95 Argus line. The similarities and differences are interesting:

    https://photorumors.com/2021/01/11/v...pecifications/

    Because glass is a three dimensional thing, uniform scaling (in all three dimensions) goes as the cubic of the scaling. And various aberrations scale by as much as the 5th power of distance from the center. Which is why so often lens designers make different tradeoffs depending on the actual physical and image characteristics they are trying to achieve. The Argus line shows:

    35mm FF vs 33mm APS-C (image circle and FOV difference at nearly identical image magnification)

    33mm APS-C vs 25mm m43 (same FOV but proportionally different image circle and image magnification) using very similar-looking optical designs (scaled only?)

    Finally, contrast 33&25 against 45mm lens which has different optical configuration instead of simple scaling.
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  3. #3  
    Senior Member Karim D. Ghantous's Avatar
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    Quote Originally Posted by Michael Tiemann View Post
    Laowa just announced the f0.95 Argus line. The similarities and differences are interesting:

    https://photorumors.com/2021/01/11/v...pecifications/
    I see... So they basically scaled the same optical formula, but tweaked it a little each time.

    Quote Originally Posted by Michael Tiemann View Post
    Because glass is a three dimensional thing, uniform scaling (in all three dimensions) goes as the cubic of the scaling. And various aberrations scale by as much as the 5th power of distance from the center.
    So, if I understand this correctly, scaling up a design magnifies aberrations, and so you probably have to tweak the formula afterwards. Correct?

    BTW just in case you know the answer: is optical plastic cheaper than optical glass?
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  4. #4  
    Moderator Phil Holland's Avatar
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    Quote Originally Posted by Karim D. Ghantous View Post
    I see... So they basically scaled the same optical formula, but tweaked it a little each time.
    Nope. Look at the elements used. Though some use 14 or 13, there's also different elements and groupings involved.
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