Stopping down and IQ

Discussion in 'Open Discussion' started by MRM, Apr 22, 2016.

  1. MRM

    MRM Mu-43 Regular

    112
    Jun 9, 2013
    Seattle, WA
    Matt
    I was wondering if anyone knows why a lens, especially a very fast lens, is generally softer wide open. I know some m43 lenses are very sharp wide open but especially lenses that are f2 or wider, what causes them to be softer wide open? I've been looking in google searches and havnt found a consensus answer. The technical the better and I know there are probably several answers.
     
  2. wjiang

    wjiang Mu-43 Hall of Famer

  3. Turbofrog

    Turbofrog Mu-43 Hall of Famer

    Mar 21, 2014
    Yeah, I think basically the long-story short is "it's much easier to correct for optical aberrations with a smaller aperture."

    As with so many things in life and engineering, the "good / fast / cheap" triangle applies, where you can pick only two.

    I'm trying to think of another analogy from product design that is comparable, but I'm drawing a blank right now.
     
  4. MRM

    MRM Mu-43 Regular

    112
    Jun 9, 2013
    Seattle, WA
    Matt
    Thanks. That gives some of the answer. Why however does it improve stopping down? Does it minimize the use of the outer portions of the lens causing the center and easier to correct portion to be used? I saw something stating that it part had to do with the increased distance from the light at the edges of the lens being in focus in a slightly different location then the center due to increased travel distance. would sensor advancements such as improved incidence angle or more efficient micro lenses improve lens performance?
     
  5. wjiang

    wjiang Mu-43 Hall of Famer

    It's more than that. At smaller apertures there is less of the glass actually being used for imaging, and it happens to be that it's the more central and less curved area of the glass that is involved, with light rays travelling more consistently perpendicular to the sensor plane. All that would reduce the effect of any aberrations.

    That's for a given lens, stopping down. Why do ultra fast lenses tend to perform poorly wide open? They tend to need very big curved glass to get light from that big hole down to a smaller area. The aberrations just end up being more prominent as a result. You can of course use low dispersion, high refractive index glass, add corrective elements etc but that all adds complexity, size, weight and cost.

    Compare the Canon 50mm f/1.8 to the PL 42.5mm f/1.2 Nocticron - the Nocticron is faster but is very well corrected so is way sharp wide open, unlike the plastic fantastic. On the otherhand the Nocticron is noticeably bigger, heavier and way more expensive.
     
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  6. wjiang

    wjiang Mu-43 Hall of Famer

    The differences in angle would impact chromatic aberration and vignetting most noticeably. Better micro lenses would deal with the vignetting but not aberrations. Now curved sensors on the otherhand...
     
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  7. tkbslc

    tkbslc Mu-43 Hall of Famer

    M4/3 (and other smaller sensors) seem to have an advantage in that even at fast apertures, the lens elements are not massive. So it's not that difficult or expensive to correct. Look at the 20mm f1.7 or 25mm f1.8. Both are really sharp right from wide open and are relatively fast and affordable. But a 20mm f1.7 has very small lens elements. Theoretically they could be as small as 12mm in diameter. So there isn't that much glass to perfect or as wide a variety of light paths to correct for. If we were comparing an f1.8 normal prime on FF, then we are talking double the diameter, which is 3.14x as much glass. So it goes up exponentially.

    You will also find that telephotos have sharper corner than wide angles. Telephotos have light coming in at shallow angles, which makes correcting optical problems easier. Typically a fast 85mm is going to be sharper wide open (and cheaper) than a fast 24mm (in FF terms). There must be something of a sweet spot right around 40-100mm equivalent where making a fast lens that is corrected is easier and cheaper. Because that is typically where you find the cheap, fast lenses that are very sharp even at wide aperture.

    Another nice thing we have going for us these days is that computers can model all this stuff ahead of time and find the best designs, so we can mostly have our cake and eat it too. Newer lens designs are very sharp, even wide open - and even the cheap ones!
     
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  8. MRM

    MRM Mu-43 Regular

    112
    Jun 9, 2013
    Seattle, WA
    Matt
    Thanks! I got started thinking that will future sensor advancement improve current lenses and then that changed into what does cause differences in lenses in regards to sharpness. The problem with curved sensors would be that you would have to design all new lenses or so i would think. granted they could be cheaper and smaller.
     
  9. PacNWMike

    PacNWMike Mu-43 Hall of Famer

    Dec 5, 2014
    Salish Sea
    guess?
    Remember that as you stop down diffraction kicks in. f5.6-f8 is probably the sweet spot getting some dof but not venturing into diffraction too far..
     
  10. Turbofrog

    Turbofrog Mu-43 Hall of Famer

    Mar 21, 2014
    I think it's an open question whether it's easier to design for smaller formats vs. larger ones. Larger formats have more glass and larger lens elements which add process challenges. But M4/3 literally needs to resolve at twice as high a level as FF to be optically comparable. And in many cases it almost achieves that goal (maybe 70-80% better, not 100%). So it's all a series of tradeoffs.
     
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  11. tkbslc

    tkbslc Mu-43 Hall of Famer

    That is a fair point, but I don't know if absolute resolving power is directly relevant to the discussion at hand.
     
  12. Turbofrog

    Turbofrog Mu-43 Hall of Famer

    Mar 21, 2014
    Perhaps. I tend to think that the main consequence of stopping down to reduce those optical aberrations is to improve resolution. Spherical aberration, chromatic aberration, coma, field curvature, astigmatism...most of those issues that tend to be improved when stopping down have the effect of reducing perceived resolution and contrast in the final image.
     
  13. fredlong

    fredlong Just this guy...

    Apr 18, 2011
    Massachusetts USA
    Fred
    At those focal lengths lens designs are usually neither telephoto ( shorter than the actual focal length) nor retrofocal (shorter than the actual focal length). The extra glass for optical trickery adds size and weight and it's own problems that need to be corrected.
    Fred
     
  14. obsolescence

    obsolescence Mu-43 Rookie

    21
    Jan 24, 2016
    Los Angeles
    John Gaylord
    The big Zuiko FT SHG lenses are unsurpassed wrt sharpness across the frame at all apertures. They don't need much software correction, either. If maximum IQ is your goal and you don't mind the size and weight, these lenses definitely should be considered (especially for the expected EM-1 Mkii).