What Does 'Diffraction-Limited' Mean?

Discussion in 'Open Discussion' started by Amin Sabet, Jul 23, 2012.

  1. Amin Sabet

    Amin Sabet Administrator

    Apr 10, 2009
    Boston, MA (USA)
    Diffraction is a physical property of the lens. When a lens is set to a high f-stop (small aperture), light waves passing through spread out and interfere with one another in a way that limits the amount of detail in the image projected upon a sensor. A particular camera lens may, for example, project a very sharp image on the sensor at f/5.6, a less detailed image on the camera at f/11, and a less detailed image still at f/22. This is purely a function of the lens aperture and has nothing to do with the sensor resolution (megapixel count).

    Say that lens were mounted on a typical 16MP Micro 4/3 camera. That camera will have enough sensor resolution to show the differences in the images it is receiving at those three different f-stop values. The amount of detail in the final image produced by the camera-lens system will be limited by lens diffraction (f/5.6 print will show more detail than f/22 print).

    Now imagine that the camera has a 1-pixel (not one megapixel) sensor. With a single pixel, the sensor obviously cannot record any spatial information. The sensor is blind to the fact that the lens is delivering a less detailed image at f/22 than it is delivers at f/5.6. The resulting print will look the same in either case; ie, the final image in this case is not diffraction limited because it is limited by sensor resolution.

    Below is a series of simulated crops from equal-sized prints made by three different cameras: a 16MP camera, a 4MP camera, and a 1MP camera. For each camera, crops are shown with the lens set to f/5.6, f/11, and f/22.

    attachment.php?attachmentid=180&d=1364677356.jpg


    Some observations:
    • The 16MP sensor capture enough detail to clearly record the differences in detail which the lens delivers at each of the lens apertures. Thus, there is an easily observable drop in final image detail between f/5.6 and f/11 and another drop between f/11 and f/22.
    • The lower resolution 4MP sensor doesn't have enough sensor resolution to show a big difference between the f/5.6 and f/11 images but can capture enough detail to discern a significant drop in detail between f/11 and f/22.
    • The 1MP sensor doesn't have enough sensor resolution to show a big difference between even the f/5.6 and f/22 images.
    • As the f-number goes up and the effects of lens diffraction increase, less detail is delivered to the sensors, and therefore the differences in final output becomes less. For example, there is big difference between the 16MP and 4MP when the lens is delivering a very detailed image at f/5.6. The difference between final output between these cameras is noticeably less at f/11 and negligible at f/22. A high resolution sensor cannot capture detail which the lens doesn't deliver.
    Conclusions:
    • The observation that a given camera system becomes "diffraction-limited" at a certain f-stop is another way of saying that the camera system is "not sensor limited" for that f-stop. It's a good thing for a camera to be diffraction limited at a low f-number.
    • Diffraction is a property of the lens and not a disadvantage of having smaller pixels (higher pixel density).
    How does "crop factor" affect diffraction? Let's take an example of the same exact lens being used on a hypothetical Micro 4/3 camera and a 135 format camera. Micro 4/3 is a 2X crop from 135 format, meaning that it is about 1/4th of the format size. If we take two sensors consisting of the same pixel size and the Micro 4/3 camera is 6MP, the 135 format camera will be about 24MP.

    At the pixel level, if we were viewing all images at 100%, the effects of diffraction are the same for both formats at any given f-stop, meaning that if diffraction softening started to become noticeable at f/8 for one format, the same would be true for the other. However in practice, a Micro 4/3 sensor image has to be magnified 2X relative to a 135 format sensor image in order to get the same output size (ie, print size or display size). As a result, the diffraction effects are also relatively magnified with Micro 4/3, and diffraction softening which begins to become evident with 135 format at f/16 will begin to become evident with a Micro 4/3 camera at f/8.
     
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  2. biomed

    biomed Mu-43 Hall of Famer

    Aug 22, 2013
    Seattle area
    Mike
    Amin - Diffraction effects is a confusing subject. Researching on the web has not made it any easier. Understanding why it happens seems to boil down to a matter of the resolving power of the lens and the resolving power of the sensor. In your example the higher the pixel count the more the effect is seen due to the better resolving capability of the sensor. One article I read broke the diffraction limit for different 4/3 sensors. For a 5 megapixel sensor the diffraction limit is f/13, for a 12 megapixel sensor the diffraction limit is f/8 and for a 16 megapixel sensor the diffraction limit is f/7. The author stated that these were rough calculations.

    In real world use the balance between DOF and diffraction is always been a compromise. More than one article suggested it is a good idea to test all of your lenses to see where diffraction limiting becomes an issue. Theoretically the diffraction limiting should be the same for all lenses ( except for some special lens designs). I think I will run tests with each lens I use just to see if this does hold true. This is probably why the smallest aperture on my first digital camera (Minolta DiMage 7) was f/8. This issue and others like shutter shock seem to be getting more and more attention. I can say that I have been blissfully ignorant of these issues in he past 59 years of practicing the art of photography. Thank you for your post!

    Mike
     
  3. Chris5107

    Chris5107 Mu-43 Top Veteran

    866
    Jan 28, 2011
    USA
    Thanks Amin,

    We spend a great deal of money on these tools for photography and it is important to understand how they work so we can get the most out of them. This type of thread is helpful for that.
     
  4. T N Args

    T N Args Agent Photocateur

    Dec 3, 2013
    Adelaide, Australia
    call me Arg
    In theory it is not a property of the lens at all, and purely a function of aperture. Assuming a circular aperture. If we have a given size that we do not want the diffraction circle to exceed, then, there is a fixed f number that we cannot exceed.

    But in practice we read reports of lenses with different 'diffraction limit' f-numbers, tested on the same camera. The Leica DG25 is an example that reportedly allows higher f numbers without diffraction. I would like to know how this happens; I don't understand it.
     
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  5. biomed

    biomed Mu-43 Hall of Famer

    Aug 22, 2013
    Seattle area
    Mike
    I have found that to be true with the 25/1.4 too. I often shoot at apertures of f/8 or smaller. Most often I use the 25 at f/4 or f/5.6 where it really shines. There must be other factors that come into play. Of all the articles I have read on diffraction limiting there haven't been any that completely agree. Most rely heavily on mathematics which is fine, but I feel it is more important to know the individual diffraction of each lens I own. Theory and real world experience often differ. I would rather just take photos, but this has peaked my curiosity. I am not sure when I will get the testing done or how scientific it will be.

    Just wandering in a dark tunnel and hoping that distant light is not an approaching train.

    Mike
     
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  6. biomed

    biomed Mu-43 Hall of Famer

    Aug 22, 2013
    Seattle area
    Mike
    I also want to add something about the DG 25/1.4 - I don't seem to notice any significant softening on 13x19 prints of images shot at f/8 with this lens. These are from RAW files processed in LR/PS using sharpening (unsharp mask or smart sharpening). I believe the sharpening in PP and the higher contrast I prefer probably do a lot to mask any diffraction limiting to a degree. IMHO diffraction limiting is not going to prevent any of us from capturing great images. Yes it is a factor we need to be aware of but let's not let it hobble our creative efforts.

    14280600383_128a8bca2a_o.jpg
     
  7. Robstar1963

    Robstar1963 Mu-43 Top Veteran

    942
    Jun 10, 2011
    Isle of Wight England UK
    Robert (Rob)
    Thanks Amin interesting and informative thread
    Regards
    Rob
     
  8. dhazeghi

    dhazeghi Mu-43 Hall of Famer

    Aug 6, 2010
    San Jose, CA
    Dara
    Yes, and well illustrated. Although it is puzzling that it got no responses when it was originally posted 2 years ago, and has suddenly gotten a bunch now!
     
  9. phigmov

    phigmov Mu-43 Hall of Famer

    Apr 4, 2010
    Dumb question (bear with me), do diffraction limits apply to macro photography in the same way ?
    Particularly where you may want to get as much in focus or sharp as possible.
     
  10. barry13

    barry13 Super Moderator; Photon Wrangler

    Mar 7, 2014
    Southern California
    Barry
    Hi, yes they do... However it may not be very noticeable unless you need large print sizes.

    Some macro photographers use Focus Stacking, where a bunch of pictures are taken at different focus depths, and then 'stacked' in software.
    Hard to do with live animals or with plants in a breeze though.

    Barry
     
  11. Robstar1963

    Robstar1963 Mu-43 Top Veteran

    942
    Jun 10, 2011
    Isle of Wight England UK
    Robert (Rob)
    I hadn't noticed the date of the original post !
     
  12. phigmov

    phigmov Mu-43 Hall of Famer

    Apr 4, 2010
  13. biomed

    biomed Mu-43 Hall of Famer

    Aug 22, 2013
    Seattle area
    Mike
    I only found the post while search for "diffraction limit" on Google. Very well done, Amin.