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Gear & Apps

Lens Diffraction | Understand It To Find Your Lens’ Sweet Spot & Make Better Buys

By Kishore Sawh on February 12th 2015

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Lens diffraction is one of those terms that fly out of people’s mouths at D4s speeds. (*Good God that was geeky*) – but it’s so often apparent to those who really understand it that many of those people are grossly mistaken about it. Which is a shame, because with a little clarity on the subject and the fuller understanding it affords, you’ll see the use of that knowledge apparent across the board; you’ll be able to get clearer, sharper images, pick better lenses for certain scenarios, be more well equipped and know what to look for when actually purchasing a lens.

Steve Perry from Back Country Gallery does a good, concise job of explaining it in the video featured here. He points out at the beginning that when most are referring to diffraction, they are referring to the softening of the image, particularly as you stop down your aperture. This is true in my experience as diffraction tends to appear as you stop down. It has to do with the way waves of light bend around obstacles/corners and mesh together, especially when traveling through a small opening. This causes a loss of sharpness, and it’s this very thing that usually gets measured when we speak about finding the sharpest point or ‘sweet spot’ of a lens.

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One of the major points of contention and confusion often comes from two places; the tendency for many people to associate zone of focus and depth of field as interchangeble with sharpness. A larger ‘Zone of Focus’ (for example, as afforded with an aperture of f/22 vs a smaller zone of focus f/5.6) is often spoken in the verbiage ‘more in focus,’ and I believe this is where some amount of confusion comes in. ‘In focus’ does not equal sharpness or clarity, and thus ‘more in focus’ does not mean ‘more sharp’ or ‘more clear.’

[REWIND: Diffraction, Aperture, and Starburst Effects]

lens-diffraction-aberration-nikon-D810-D4-resoution-photography-slrlounge-1

lens-diffraction-aberration-nikon-D810-D4-resoution-photography-slrlounge-5The second thing that often causes confusion is that most of us know from use and experience, reading about fast lenses or buying them, that most fast lenses aren’t as sharp wide open, say at f/1.4 than they are at f/5.6 or f/8. There’s this immediate association that the lower the f-number the more diffraction and softer the image is, but it doesn’t quite work that way.

The lack of clarity and sharpness you’re acquainted with at your lowest f-stop is a product of lens aberration and not diffraction, and lens aberration, inversely to diffraction, tends to become less apparent as you stop down a bit. So in a way, the sweet spot of your lens is the intersection point of lowest lens aberration and lowest diffraction. If you understand these two points, you’ll ‘get it,’ and life, along with your images, will be better for it.

See the video for more details, and visual representations of the information, which will help you visualize and internalize it with ease. There is afterall, more to diffraction than I’ve noted above, and the video does a really fine job of going through it all.

You can find out more great content from Steve and Back Country Gallery on their site.

Source: ISO1200, images are screen captures from featured video

About

A photographer and writer based in Miami, he can often be found at dog parks, and airports in London and Toronto. He is also a tremendous fan of flossing and the happiest guy around when the company’s good.

Q&A Discussions

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  1. Jacques Brierre

    Good subject and thanks for the link to a very informative video.
    I wonder if this very applies in the very same way when lenses designed for FF cameras are used with crop sensor cameras.

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  2. Ralph Hightower

    Informative article. I’m glad that SLR Lounge added the capability to bookmark or save articles.

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  3. Tosh Cuellar

    great article, loved the explanation

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  4. Gurmit Saini

    This was one great article with in depth explanation and had cleared some doubts.

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  5. Sean Goebel

    It’s not even as simple as “f/8 is the best aperture for sharpness on your D800.” Diffraction effects will become visible before then, at f/5.6 or so. The optimal aperture for sharpness will vary depending on the lens/body combination and what part of the image you’re looking at. See here for a good discussion of diffraction for different camera/lens combinations:
    http://www.lensrentals.com/blog/2013/03/overcoming-my-fentekaphobia

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  6. Basit Zargar

    great

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  7. Stephen Jennings

    I got a little confused with the lens vs sensor bit. With the d800 I find many 3rd party glass or “enthusiast” grade glass to come off way softer than if I used it on a smaller sensor like the d610 or crop sensor body. So it would seem with better quality lenses the sensor’s level of diffraction would be mitigated? I’ve been trying to find the “sweet spot” for the Nikon 14-24 for a few weeks now on the d800 and has so far illuded me so this post i hope will clear some stuff up for me.

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  8. Duy-Khang Hoang

    The article is titled Lens diffraction (as is the video it appears), the thing is, lens diffraction is independent of sensor size, resolution or pixel pitch. There is no escaping diffraction softening, it just so happens that there are other aberrations that mask the softening that diffraction contributes to the image at the wider apertures for most lenses. The point at which diffraction softening becomes the dominant contributor to image “softness” is the diffraction limit to put it simplistically. There is more to it than that since different colours ~ with their different wavelengths, will soften at different points (red suffers more than blue). The article and the video are discussing the perceived effect of diffraction with lens and sensor combinations. It should be noted that more pixels (for same sensor size and lens) will still produce more details – e.g. 36MP sensor vs 16MP sensor at f/11 still favours the 36MP sensor. What the article means to suggest is that the advantage of 36MP at f/11 vs 16MP at f/11 is not as great as the advantage of 36MP at f/8 vs 16MP at f/8.

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  9. Ben Perrin

    Great article. I didn’t realise that pixel size affected diffraction. I usually never go beyond f13 for fear of diffraction unless I want the star effect on lights but with the 5Ds R I’ll have to be even more careful. I’m probably going to have to refine my focus stacking techniques where possible. Once again, great article.

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  10. robert garfinkle

    ok – How bout “sweet resolution” (like “sweet spot”, but in reverse), right?

    He states, that diffraction starts on a D810 (D800 family) @ f8, ok; and a D4 will get diffraction around f11, yes?

    Would it be fair to say that a D750, may start experiencing diffraction around f9.5 to f10 and because it’s resolution is 24MP (thereabouts), lands you a camera which strikes a balance yielding you a higher detail and a greater depth of field before “noticeable” diffraction starts. is this right thinking. Now, take into account I am not trying to confuse Depth of Field and Resolution Detail.

    But, I have to ask, is the D4 not a good camera to use as an example because of a fatter pixel vs. a D810 or D750 – does overall pixel size come into play… If a 16mp camera, like the D4, has a “fatter” 7.3u vs. a D810’s 4.3u pixel size, or am I splitting hairs here…

    Using “Infinite” wisdom? Is a shot which is focusing at infinity, less likely to encounter either aberration or diffraction, if you pick the sweet spot? And, having said that, still focusing at infinity, is a wider angle less prone, at the sweet spot, to picking up either effect vs. a narrower angle lens – does that come into play??

    Thoughts?

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    • Eric Sharpe

      After watching that video, I had a pretty good grasp of it all. Then I read your post, and in progress that was made has been reversed! :D

      I hope someone answers your questions, so I can re-understand again.

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    • robert garfinkle

      The thinking was, that a 24mp camera prolongs diffraction a bit more than the D810 (or D800(e) – take your pick), yet the resolution is finer than a D4 is, right – providing you a nice vehicle which is very resolute (resolute enough) and holds-off diffraction by a stop or so, thus paving the way for better depth of field with sharper images – in theory, yes?

      as far as the other, my points being, 1. does pixel size play a role on that grid (does the grid change with physically larger / smaller pixels in play – the video suggests not, indirectly), and 2. when focused at infinity, removing depth of field out of the equation, do you stand a better chance of eliminating the “unwanted” effects because you can pick the sweet aperture with out the depth of field issue, and I am asking if wide angle lenses are better, implying that infinity is hit quicker…

      or is my brain diffused… it’s possible…

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    • robert garfinkle

      uh, would there start to be a diffraction issue earlier using the upcoming 50mp cams – and, in theory, at what megapixel size do we overrun the sweet spot…

      let’s do some quick math

      16mp @ f11 = Diffraction
      36mp @ f8 = Diffraction
      50mp @ f6.3 = Diffraction?
      56mp @ f5.6 = Diffraction?
      76mp @ f3.2 = Diffraction? (& Some Aberration? too)

      is there a trend here – at what point do we jump ship

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    • Stan Rogers

      A lot of this is about looking at pixels instead of pictures. For a given output (a certain print size, say, or a certain final screen display resolution), the larger pixels will mask the effect of diffraction slightly longer. But the pixel-level “detail” you’re seeing is the result of lower-frequency sampling. Speaking of which, if the sampling frequency is low enough, the camera’s sensor will have an antialiasing (optical low-pass) filter, which will smear detail the same way that diffraction does — except that it will do it at *all* apertures.

      As for wide-angle versus long, the major aberrations you’ll encounter are about equal in magnitude but different in character. There is a “sweet spot” there as well, which is why a 50mm f/1.8 can be so good yet cost so little. Significantly wider, and the light needs to be bent more abruptly (at some point in the optical stack), so it becomes much more sensitive to lateral chromatic aberration, astigmatism and coma; significantly longer and longitudinal chromatic aberration and spherical aberration come to the fore. Any faster, even at that “sweet spot” focal length, and the contributions from the outer portion of the lens will make whatever minor problems that may exist in the basic design much more apparent.

      What it all SHOULD boil down to is this: when DoF isn’t critical (as in, you can get enough for your subject, and losing or retaining the background/foreground isn’t an issue, as in a studio setting), then use your lens’s best aperture; when focus stacking will get you where you want to go (in terms of attaining a deep field), then use your lens’s best aperture; in all other cases, use the aperture that gets you the shot you want (but try to avoid anything narrower than f/16 on a small-format camera *when that’s possible*) — and stop zooming in to 100%. Something that is ever so slightly soft under a magnifying glass but appears to be in focus will be better than something that has razor-sharp parts but appears to have missed getting the intended DoF.

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    • Holger Foysi

      Where is the problem? Downsize the D810 to 24MP and you have the same diffraction limit and improved sharpness (you use more information for interpolation and lack an AA-filter) as your D750. You can do the same when downsizing to 16MP, to get the same limit as your D4/D4s. But as soon as you open up your aperture you profit from the high number of megapixels, s.th. you don’t immediately get from your smaller MP camera. Do landscapes by combining two f5.6 or f6.3 or whatever shots in serene stacker to benefit from the high number of MP. You have more choices with such a camera.

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  11. Chuck Eggen

    Excellent explanation! Well worth the watch even if you think you know about refraction.

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