Since our last project of focus stacking, the next logical question was why you don’t shoot at the smallest aperture to have the greatest depth of field, consequently, not needing multiple captures or even getting by with only one. Good question, with a one word answer – diffraction. The tradeoff with a small aperture to get the maximum depth of field, apart from slower shutter speed is loss of image quality caused by diffraction.
Diffraction affects the entire frame equally, most noticeable, in the loss of fine detail. In simple layman’s terms, light travels in rays. As it enters and leaves the lens, with a large aperture opening, the rays pass fairly unmolested, with only a small percentage of the rays touching the edges of the aperture, compared to the total amount of light rays, on the journey to the capture plane (sensor or film). As the aperture is closed down the light rays pass through the smaller opening and a higher percentage are disrupted by the edges of the smaller aperture, this disrupts the path of a higher percentage of light rays and degrades the image at the capture plane. So knowing this begs several questions which I have listed below.
- Q – Are the edges of the image more affected than the center? A– No the entire image circle is uniformly affected.
- Q– Is diffraction affected by subject distance to the capture plane? A– No, it makes no difference if you are shooting at infinity or close up. Lens itself might
- Q – So why are lens “offering” an f22 or f32 (in 35mm format)? A– Well because they can, and with film you can and could stop down further than with digital. And today’s lenses are children of our film lenses. More seriously, depending on what you are shooting, fine detail might need to be sacrificed for depth of field, think clouds, the sea, reportage, small print, or screen reproduction, to name a few examples. The more you close down your aperture the smaller your best quality maximum final print size will be.
- Q– Is the inverse true, a large aperture giving you the sharpest image, most fine detail? A – The short answer is no. Most lens makers have their own proprietary philosophy on lens design, but it is safe to say that usually about 2 – 3 stops from maximum aperture you are hitting the “sweet spot” for finest detail and “normal” depth of field, depending on the distance to the subject. Should a lens be optimized for sharp detail at maximum opening of the aperture, it surely is advertised that way, that is generally not the norm in common lenses.
- Q – Is a five blade aperture more disruptive than an eleven blade aperture? A – Yes the 5 blade will cause more issues. The five blade aperture will yield a pentagonal aperture with ’corners’. These corners where the blades join cause distortion, when stopped way down, so F16 or F22, can actually produce a star effect.
It should be kept in mind that light rays are bent whenever they hit any object. Also, once diffraction starts to have a negative affect after leaving the “sweet spot”, as the aperture gets smaller, it basically falls off a cliff, the percentage of light getting though the smaller aperture and amount that is being “disrupted” goes up exponentially. There other factors which can mitigate or factor into the amount of diffraction, such as the pixel type and size, film grain size, size of capture plane, which present themselves in the Airy Disc formula, but diffraction cannot be avoided and should always be considered.
So what I did with time on my hands and also curiosity, was test two lenses (a 40mm Rodenstock and Schneider 60XL) on a Cambo technical camera – both stellar lenses. I used two Phase One IQ4150 backs, the regular color back and the Achromatic. I was looking for the sweet spot on each lens. Also I was curious because one, the Rodenstock, is a retro focus design lens, and the Schneider is a traditional design where the nodal point is close to the middle of the lens. Note also, one back was a normal color back with a hot filter, and color filter of Bayer array design, while the achromatic back had no Bayer array. I was wondering if the Bayer array gets more confused by the scattering of the light, compared to not having one. Also the IQ 4150 is a backlit sensor, meaning the front side which captures the light is less cluttered with circuitry, this is a good thing and a vast improvement over previous Phase One sensors, not just in resolution, but in color cast, and light fall off. It is a fantastic Medium Format back and has the ability to go on a SLR, a technical camera, a studio camera or a field camera.
So anyway these are the results, both back backs were shot at their base iso and on a tripod.
If you are after all the nuances afforded using a fantastic back like the Phase One IQ4150 and these fantastic lenses, with a tech camera, I would definitely tell you to keep your aperture within the range noted in the table. As noted if you are using any camera where you can adjust the aperture, diffraction is always lurking.
I personally find that thinking about the physics and chemistry behind photography; stimulates the creative side of image making.
