Diffraction and IR Photography

A few years after I began my photography adventure I took a photography class that came free with my first SLR camera. I thought I knew a lot about photography.  After taking the class I thought I knew all there was to know about photography.  Funny thing is that all these years later I’m still learning, almost on a daily basis. One photography class take away for me were all the details about depth of field and aperture. When I went to any action event, I’d shoot at high f/numbers so everything was in focus (before the days of auto focus).  Some years later while working on my undergraduate degree, I became interested in optics. I was hired to work in an optics research lab (see my holography post).  It was then I started learning about the wave nature of light and how diffraction occurs.  I mostly worked with lasers.  So most of my diffraction experience was with monochromatic light (one color). Below is a shot of textbook linear laser diffraction through a small opening.  This is the effect on light as it passes through small openings.  This also occurs in photography with all the colors diffracting different amounts, but occurring simultaneously.

My interest in photography began to mesh with what I had learned about optics. I had always wondered about white light diffraction, especially in camera optics. It turns out that diffraction can be a fairly significant issue with camera lenses. You can shoot everything at f/22 and have great depth of field and focus. But there’s a trade-off.  As the aperture size decreases, diffraction increases and becomes more visible. There is diffraction at all f/numbers though its more pronounced at smaller apertures.

I like to understand where all my lenses are sharpest and where they are soft. So when I added another medium format lens to my fleet, I decided to do a little more testing. Interestingly, my 165mm f/4 LS medium format lens has a minimum aperture of f/32. At this aperture diffraction makes the image quite soft.  It is so evident that it’s even evident in the live-view display.

I shoot IR almost exclusively with medium format lenses. Check out my blog topic on medium format lenses. They are huge,  but I really like using them. Here’s a comparison to a 50mm f/1.4 Minolta manual focus lens that I bought with my 35mm SLR. These are the lenses I included in my testing for this blog (except for the Minolta lens).

Left to right in the rear: Pentax 67 55mm f/4, Pentax 67 75mm f/4.5, Pentax 67 165mm f/4.  Front row: Minolta MD 50mm f/1.4

I found some level of diffraction in all three of the lenses I tested.  On this particular test subject (my neighbor’s palm tree), diffraction is much less evident in the 55mm and 75mm lens, but very clear in the 165mm lens.  This is partially due to the minimum aperture size of f/32. The animations below are all 740nm IR photos, shot with a custom white balance.  No other processing was done.

Pentax 67 55mm at  f/4, f/11 and f/22

Pentax 67 75mm at f/4.5, f/11 and f/22

Pentax 67 165mm at f/4, f/11 and f/32

Diffraction is inversely proportional to aperture diameter. So diffraction is less visible as the aperture diameter increases. Unfortunately, diffraction is proportional to the wavelength of the light being shot.  This means more diffraction at longer IR wavelengths. For those of us shooting IR, we will likely have to use a slightly larger aperture vs. when shooting the same equipment in color. If you shoot IR around 740nm, diffraction will be about 30% worse than at the center of the visible spectrum (about 550nm). That seems significant but doesn’t seem to affect my images much. However, you should keep this in mind if you’re planning to shoot IR at small aperture diameters (large f/numbers).

You may have noticed something else that is evident when shooting at small apertures.  Any dust or streaks that happen to be on the lens, filter or sensor become much more evident at smaller apertures.  Take a look at the speck to the upper right of the bird in the 165mm image above.  The dust spec is invisible in the f/4 image, but very clear in the f/32 shot. Yet another reason to open up those apertures.

First test shot from my new 75mm Pentax 67 medium format lens (shot at f/11)

I find that most of my lenses are sharp around f/11 when shooting at 740nm. So that’s where I mostly shoot. It matters little to me if the exposures are longer. I always shoot IR with a tripod so I can take full advantage of the lens sharpness. If you’re interested in some light diffraction theory, there’s an interesting article I found discussing diffraction and photography.  One last detail to consider.  The onset of diffraction occurs earlier in crop sensor cameras vs. full size sensors.  So if you’re shooting with a smaller sensor, you may need to open your aperture more than would be necessary on a larger sensor.  Lots to keep in mind.

So what does this all mean? Well, if you typically shoot at large f/numbers, with a crop sensor camera and/or shoot IR photos, you can probably increase your image sharpness by opening the aperture slightly. Do your own tests on the lenses that you use most frequently. It’s important to know where your lenses perform best whether you shoot color or IR. Most importantly get out and shoot.  It’s the only way to learn.