(Craft of photography: part 2)
With ever increasing resolution in digital cameras, diffraction or spreading of light rays, impairs a landscape photographer’s ability to render an image as sharply as we might expect, given the resolution of the sensor. What is diffraction? How does it manifest itself in our images? What can we do to minimize its effects?
Diffraction occurs when light passes over the edges of the iris in our lens. As light starts to spread out, photons that should have been captured by a single photoreceptor in the camera’s sensor strike the intended receptor and strike adjacent receptors. Bending or diffracting of light increases as the diameter of the iris decreases (as we stop the lens down from fully open, e.g., at f/1.4, to smaller apertures such as f/16). The impact of diffraction becomes more readily observable as we shrink the spacing of the photoreceptors (i.e., we increase the number of pixels for a fixed size sensor).
As light spreads out, we start to notice the impairment through the loss of detail in finely textured subjects. Fine texture that might have been crisply rendered at f/4 starts to become fuzzy as we shoot at smaller apertures or shoot with higher pixel-count sensors. Sharply defined and focused subject edges start to blur. We start to lose detail, such as grasses in the landscape or leaves on the trees.
I first noticed the effects of diffraction when looking at a series of landscape images taken at f/8, f/11 and f/16 on my then new 10 megapixel Nikon D200 camera. I expected to see increased depth of field, more of the subject in apparent sharp focus, as I stopped down. I did notice an increase as I moved from f/8 to f/11 but the image at f/16 did not appear to have any added depth of field. Actually it did but gains in apparent focus depth of field were being offset by losses to diffraction.
I found a very accessible explanation on the superb Cambridge in Colour website from Sean McHugh. I urge you to read all the tutorials on Sean’s website including the discussion of diffraction. Sean provides tools that predict the combinations of sensor size, pixel density and aperture selection that will lead to the onset of visible diffraction in our images.
For a given sensor size and pixel density, I have found the extinction limit as reported by Sean’s calculator to be the aperture at which I first notice diffraction’s effects in my images. Stop the lens down beyond the extinction limit and diffraction becomes increasingly visible. Open the lens wider than the extinction limit and diffraction effects fall away.
Using the advanced calculator provided on Cambridge in Colour, I estimated the extinction limit for a series of Nikon cameras with the following results:
Full-frame (35.9mm x 24mm sensor) cameras (“FX” in Nikon marketing terms):
- 12 MPx: Extinction Limit = f/15.9: e.g. D700
- 16 MPx: Extinction Limit = f/13.7: e.g. D4
- 24 MPx: Extinction Limit = f/11.1: e.g. D3X, D750, D610 similar
- 36 MPx: Extinction Limit = f/9.2.: e.g. D800, D810 similar
Similar means extinction limit will be at approximately the quoted f/stop across the range of cameras with the same sensor size and approximately the same pixel count.
APS-C (23.6mm x 15.8mm) cameras (“DX” in Nikon terms):
- 10 MPx: Extinction Limit = f/11.4: e.g. D200, D80
- 12 MPx: Extinction Limit = f/10.4: e.g. D90, D300, D300S
- 16 MPx: Extinction Limit = f/9.0: e.g. D7000
- 24 MPx: Extinction Limit = f/7.4: e.g. D7100, D7200, D3200, etc.
As we increase the pixel count in our sensors, the onset of diffraction becomes visible at wider and wider apertures.
What does this practically mean? I currently shoot with two Nikon bodies: D750 (full frame) and D300 (cropped sensor).
- When using my D750, formula says diffraction starts to become visible at f/11.1. This means that I try to limit my selected aperture to f/11. If the subject has a lot of fine detail or texture I want to capture, I practically limit the aperture to f/8 or wider.
- When using my D300, formula says diffraction starts to become visible at f/10.4. If the subject has a lot of fine detail, I again will limit aperture to f/8 or wider. If the subject has only modest fine detail, I will sometimes shoot at f/11 if I believe the gain in depth of field is important for the composition.
More generally, I observe:
- As a landscape photographer, I usually want to have great depth of field in my images. Yet, to capture as much fine detail and image sharpness as I can, I choose to trade depth of field for improved resolution. (I will cover my approach to recovering depth of field through focus stacking in a future post.)
- Given my concern with diffraction and need for depth of field, I chose to move to a larger image sensor (“FX”) rather than staying with my familiar crop-sensor format (“DX”) when I wanted to move from a 12 MPx to a 24 MPx camera. Similarly, I chose the 24 MPx D750 rather than the 36 MPx D800/810 series.
- Not all landscape photographers share my choices or judgments. Many have opted for a 36 MPx sensor camera. Others shoot at f/16 and f/22.
- Some will argue that losses to diffraction can be recovered by down-sampling an image. For example, shoot with a 24 MPx sensor and then down-sample to a 12 MPx image file using Photoshop or similar tools. I agree this is correct. For my own shooting, if I want a 12 MPx image, I will shoot with a 12 MPx body.
- Many genres of photography routinely use moderate (f/4 to f/5.6) and wide apertures (f/2.8 and wider) (fashion, portrait, street, wedding, etc.). My concerns for diffraction are not issues for those photographers who benefit from the added resolution of even 24 MPx APS-C sensors without concern for diffraction.
Next post in this series will be: Image Sharpness and Lens Selection.