[Fstoppers Original] What Is Lens Diffraction And When Does Diffraction Happen?

If you are like me then you might not always get caught up in some of the super technical aspects of photography. One aspect of photography I recently investigated was the loss of sharpness caused by Diffraction. Last night while playing with the new Nikon D800 camera I examined lens diffraction and how diffraction can seriously affect the sharpness of your photography.

One thing that always blew my mind in science class was when my professors told me light can act both as a particle and as a wave (quantum physics was never my strong suit). When described as a wave, light usually travels in a straight line but can also bend when passing through or around objects. When light travels through your camera's lens and through a large aperture, light doesn't bend or disperse all that much. But something strange happens when light passes through a tiny hole such as a small camera aperture: it bends and interferes with itself. This interference is called diffraction.


Diffraction in simple terms is a phenomena that occurs with light when it interacts with an obstacle. Most of us are familiar with light diffraction patterns found on the backs of CDs, in water molecules in the air, or on spider webs when looked at the right angle. Diffraction can also occur in your DSLR camera which can become a major problem and cause your images to lose their sharpness. Here is a simple diagram that shows how light particles hit your camera's digital sensor when going through large apertures and smaller apertures.

So what does this all mean for your photography, and how does diffraction make images soft? I recently did a few tests with a Nikon D800 and a Nikkor 60mm lens to see exactly what the real world effects of diffraction were on macro photography. The first test was my own eye. We setup an Alien Bee R800 Ringflash and took a few photographs at different apertures. The resulting 36 megapixel images revealed that stopping down the macro lens past f22 resulted in a lack of sharpness. I would have loved to have taken test shots with an even wider aperture like F8 but the ringflash was too powerful at the lowest setting for anything below f22 at ISO 100. Here are the resulting photos (click each image to view full res).

After reviewing the results and seeing first hand how super small apertures can decrease sharpness, I decided to test something more static and remove as many variables as possible such as camera and subject movement and lens focusing distances. Lee and I decided to see how much detail we could resolve in a $50 bill. We used the same Nikon D800 and Nikkor 60mm lens but instead of a ring flash we backlit the currency with a strip box fitted to a Profoto Air 1000. Now with the camera on a tripod we were able to see exactly how different apertures were affected by diffraction. Here are the results:

As you can see, the images from F8-F16 look the sharpest and then diffraction really starts showing up in the smaller apertures. However, with macro photography, sometimes you are more concerned with overall depth of field more than overall sharpness. You can clearly see how shallow the depth of field is on the first 3 images and how the entire bill becomes in focus as we decrease the aperture. Below are the full Nikon D800 files at the widest aperture and the smallest aperture so you can examine the effects of diffraction more carefully.


Obviously lens diffraction only becomes concerning at super small apertures. If you are shooting portraits then you will probably never stop your lens down far enough to see the effects of lens diffraction. However, when you are shooting with very powerful studio strobes, you might have to shoot at much smaller apertures if you cannot get your lights far enough away from your subject. Diffraction affects each lens differently and as pixel size decreases diffraction usually shows up earlier. So my best advice is to run a few tests with your different cameras and lenses and find the sweet spots where diffraction is tolerable. For you super scientific thinking photographers out there, feel free to leave your thoughts in the comments below as we would love to hear your explanations as well.

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34 Comments

Carlo Parducho's picture

One of the bests posts ever!

"Like"
^_^

My macro lens gets hit by diffraction around f13. Then again I'm getting an f26 effect at that aperture. 

Mark's picture

There seems to be more than diffraction affecting the sharpness in this test. I am no technical expert, but in my own diffraction tests the entire image looses sharpness as the f-stop gets smaller. This test shows a sharp center at f8 and soft towards the edges (bottom of flag pole) and then the edges of the image get sharper with smaller f-stops down to f32 or f42 before overall sharpness begins drops off. I think the lens is negatively affecting the test.

Patrick Hall's picture

are you sure you aren't looking at the shallow depth of field at f8?  At the angle we shot, the DOF at f8 is mm and falls off quickly.  

Tam Nguyen's picture

Almost there. The colorful effect that you see on the back of a CD is technically called diffraction grating (http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/grating.html and http://en.wikipedia.org/wiki/TFT_LCD). And yes, you guys nailed it about light wave (or more like waves) interfering with itself, thus creating different wavelengths. Same deal with oily water. 

Oh, by the way, LCDs were invented based on this principle.

I guess majorring in computer engineering does come in handy at times. But when I wanna see how well my lens performance and at what aperture, I just go on slrgear.com or dxomark.com and read :)

Patrick Hall's picture

Don't mention DXO around here Tam, people might get outraged :)

Tam Nguyen's picture

Okay okay, how about Ken Rockwell???

 Hahaha ... your getting colder

Tam Nguyen's picture

You're*

Nicholas's picture

Very informative post, told very simply and effectively. And congrats on your new D800. 

アイザック (Isaac Medina)'s picture

Awesome vid. I can't wait for my D800 to arrive.

Great info, thanks

What perfect timing for this! I just did a bunch of macro shots this morning from about F2.8 up to F40. Now I can't wait to compare them and see what the diffraction does to the images.

your link to the nikkor 60mm macro seems to link to a canon 60mm. Also, prob a dumb question but how are you reaching f/40+ with a lens that maxes out at f/32?

Patrick Hall's picture

Ooops, WP didn't update the fixed link last night.  It's good now.

The Nikkor's min aperture is f/32 at infinity but when you focus close enough for 1:1 magnification it goes to f/57

good to know, thanks. i don't normally do macro shooting.

Apparently spelling wasn't your strong SUIT either. "strong suite" sounds like some kind of hotel room for body builders.

Patrick Hall's picture

just a typo, it's fixed now

 Great article, examples and video. Thanks for the hard work. One more minor quibble (it gave me a chuckle though.) Near the end of the article you mentioned finding the sweat spot of your lens. I might perspire like a pig, but my lenses haven't developed any permanent sweat spots yet.

Just checked my 17-40 for 17mm I am getting the best sharpness for f/8 f/9 f/10 f/11 F/13 f/14
and for 40mm the best sharpness I am getting for f/10 f/11 f/13 f/14 so it means I should choose between f/10 - f/14 to get the best sharpness in range 17mm to 40mm. Very good post.

Colin Ranger's picture

You also here problems at the extreme ends of either aperture or zoom range. Is this also diffraction or a different problem? People often say don't shoot at he max or min aperture or the max and min zoom.

Diffraction has nothing to do with the angle the light is hitting the sensor.  When light hits a hard edge, it is diffracted away from the original propagation direction.  See right side of attached image.  This diffracted wave will interfere with itself, but is not the root cause of the decrease in resolution.  Notice how it causes the red laser spot to appear larger than it would without the extra rings?  

Patrick Hall's picture

if the diffracted wave is not the root cause of the decrease in resoution, what do you propose is the direct cause?

I think Hwang misstated it in the last two sentences. Diffraction is the root cause, but the angle the light hitting the sensor has nothing to do with it.  If the angle the light hits the sensor had anything to do with it, then you wouldn't see the same strength of diffraction effects in the center of the sensor as on the edges.

... and diffraction would not been an issue in film photography.

 Without a digital sensor, diffraction will be reduced, but the light hitting the edges of the aperture blades cause diffraction. I suppose the big difference is that film might not resolve that level of detail.

Sorry -- I guess my last message was convoluted.  The diffracted wave is the root cause of the problem.  The interference is not the root cause of the problem (although it doesn't help).  My point is that diffraction has nothing to do with the angle the light is hitting the sensor.  

very good article Pat

Daniel's picture

Thank you so much.  I heard so much about diffraction but I didn't know what it really was until I saw this video on Youtube.  I also didn't know that this site existed until now, but I will visit more often from now on. 

I have a question though.  How does high megapixel count affect diffraction?  If I understood correctly from the video, the CAUSE of diffraction has nothing to do with the megapixel count, but will a 12 megapixel sensor SHOW the same amount of diffraction compared to a 36 megapixel camera (when both are viewed in same size)?  Or does megapixel count have nothing to do with diffraction?

great post

Great post, once again something else to throw in the mix, but essential to get that pic tac sharp,,

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