Quick Tutorial - The impact of diffraction...

I continue to see photos posted here on NSN taken at very small apertures like f/22 when they don't need to be.  Basically, unless you absolutely need f/22 for DOF or to slow down the shutter speed, you are much better off shooting at a more moderate aperture.  As the aperture diaphragm opening gets smaller and smaller, more light bends around the aperture blades causing a loss of sharpness due to this - this is called diffraction.  The smaller the pixel sites are, the more likely it is that you are recording the diffraction and reducing the ultimate detail that your camera and lens are capable of.  This is one of the reasons focus stacking techniques have become so popular among landscape photographers - it allows us to customize the DOF while maintaining apertures that do not show visible diffraction.

Here is just a quick test I did last week...  This is done with a Phase One XF camera and 100 megapixel sensor back using a state of the art Schneider Kreuznach 80mm f/2.8 Leaf shutter lens (50mm full frame DSLR equivalent).  The subject rock is at infinity for the lens.  The photo was taken with the XF camera's seismic shutter release where the camera will not fire until it is completely still based on it's own internal seismic sensor.  The mirror is locked up for this and the lens' leaf shutter is open prior to exposure and the photo is initiated using an electronic first curtain and when the shutter closes it is just the tiny leaf shutter in the lens.  The Focal Plane shutter and mirror are never used and therefore had no impact on the final sharpness of the photo.  The lens, camera, and data back were perfectly calibrated for focus using Phase One's Focus Trim (similar to AFFT on Nikon or AFMA on Canon). There simply is no way to use better technique and guarantee ultimate sharpness.

The image on top is taken at f/7.1 while the image on bottom is taken at f/22.  Both are shot at the sensor's native ISO of 50.  As you can see, there is dramatically more fine detail on this 100% crop from the center of the image on the f/7.1 sample.  Each crop is a mere 0.16% of the entire 101 megapixel image.  When downsized for WEB you wouldn't see a difference but when making large prints, the difference in acuity is very noticeable and the sharper image will appear much more 3 dimensional to the viewrer.

Please be sure to click on the image to see it in actual size.

Nice to see a helpful lesson at NSN.
TY

hi EJ,

Thanks for this and many other educational lessons.

Just a quick question that may ultimately have the dreaded, "It depends" answer but since I moved to a D800, I tend to not use f/22 nearly as much as I used to with my D2x.  However, I generally back off to f/16 mostly, sometimes to f/11.  I presume this is camera/lens specific, but are they are general guidelines you suggest for maximizing acuity?  I don't have focus stacking software at this point, fwiw.

Martin 095 wrote:are they are general guidelines you suggest for maximizing acuity? 

For a D800 and most lenses f/5.6 will max acuity.  You will see little difference at f/8.

hillrg wrote:

Martin 095 wrote:are they are general guidelines you suggest for maximizing acuity? 

For a D800 and most lenses f/5.6 will max acuity.  You will see little difference at f/8.

A D800/D810, For normal 20/20 vision doesn't enter diffraction until after f/7.1 and you will likely not see a difference out to f/11.  The other high res 35mm cameras are the 42mp Sony a7R Mk II which has a diffraction limit of about f/6.3 and the 50mp Canon EOS 5DSR which is around f/5.6.  You can typically add 1 to 2 stops before seeing a real visible difference on anything but test charts.

Personally if I have to go beyond f/11 on any of the high rez 35mm cameras, I will focus stack unless there is no way to do that (tall grass in blowing wind for example).

On the more common full frame cameras that are in the 20 to 24mp range (5D3, D600/610, D750, 1Dx, 1Dx2, D4, D5, etc) I wouldn't worry much out to f/16 but not go beyond that unless there is no other way around it.

Thanks as always for your helpful answers!

Thanks E.J. For posting that information. Using Canon full frame 1Dx, 1Dx2 and 5D3 try to keep it under f16. I bought a 5Dsr and did not think about diffraction, and starting at around f/5.6 ! Did not expect that. Need to be more selective on choosing apature in the future...

PV Hiker wrote:Thanks E.J. For posting that information.  Using Canon full frame 1Dx, 1Dx2 and 5D3 try to keep it under f16.  I bought a 5Dsr and did not think about diffraction, and starting at around f/5.6 !  Did not expect that. Need to be more selective on choosing apature in the future...

It depends on what you use the images for.  If you are always downrezzing then no problem using small apertures.  All you are doing when stooping down well into the diffraction limited regime, you are effectively reducing the resolution of the camera to a megapixel level commensurate with the diffraction limit at the aperture you are using.  So if you shoot at f/8 on the 5DSR you are basically reducing the resolving power of the system (lens and sensor) to something similar to the D810 or a 36mp sensor - not too bad.  But if you a re shooting that 5DsR at f/22, you are getting more pixels but not more resolving power than something like a 1Dx or 18mp.  My main point is that I still see some people shooting at f/22 just because...  not because they actually need to and as a result they are seriously reducing the resolving power of their gear.

Thanks E.J. For the explanation.

Martin 095 wrote:hi EJ,

Thanks for this and many other educational lessons.

Just a quick question that may ultimately have the dreaded, "It depends" answer but since I moved to a D800, I tend to not use f/22 nearly as much as I used to with my D2x.  However, I generally back off to f/16 mostly, sometimes to f/11.  I presume this is camera/lens specific, but are they are general guidelines you suggest for maximizing acuity?  I don't have focus stacking software at this point, fwiw.

You don't really need a specific stacking software, Photoshop does a good job of it. Load your images into stack, select all layers, align using "auto", then blend using "stack images". If you need more specific instructions, please email me at jfabian@redrockphoto.net

I hope this helped.
JF

Excellent info since I just went to the D810.