Tags
aperture, camera, conventional wisdom, depth of field, diffraction, lens, light, photography, pixel density, real world, theory
Photography is full of myths and conventional wisdom. Astonishingly, they occupy the attention of far too many for far too long.
Not a new phenomenon by any means, reams and reams of verbiage were devoted to the intricacies of film photography, it, nonetheless, seems to have exploded in the age of digital cameras and the internet. Anyone with a computer and an image processing program can examine an image right down to the pixel level and share his or her opinions with the online community. Never has it been a better time for the technically obsessed.
One of the common pieces of accepted wisdom in this digital age is the destructive influence of diffraction on your photograph. As long as there have been lenses, the physics of very small apertures leading to highly diffracted and thus soft images has been understood. These days, the pixel density of sensors also plays into this softness; as pixel density increases and pixel area decreases, the ability of each pixel to differentiate – and thus resolve – the light from a lens falls relative to the diffraction level produced by the lens. Images from high density sensors will, given the same lens and f-stop position, be softer.
Thus has been born the diffraction limited aperture (DLA) specification that indicates the minimum aperture size that any given sensor can resolve – a typical set of values can be found in this review of the Canon 5D Mark IV.
Once upon a time I paid attention to this. I even convinced myself the images from my old Canon 40D were somehow softer at an equivalent f-stop setting than those from my older 30D with its larger pixel size.
That was nonsense as far as the real world is concerned.
The simple truth is that any softness in an image due to diffraction issues of either the lens or the sensor or both is insignificant compared to the softness brought about by lack of focus, camera shake, subject shake, other limitations in the lens and falling outside the depth of field.
And of all those factors, camera and subject shake are by far the most important. Focus too, although most modern autofocus systems work well provided the correct focal point is selected. Depth of field – well, that works in opposition to the effect of diffraction within a lens. A smaller f-stop (more diffraction) provides a deeper depth of field (more elements before and behind the point of focus that can resolve as being distinct and not blurred).
This was brought home to me today by the above image of a columbine plant. The aperture used on the Nikkor 55m macro lens is incredibly small for a 35mm camera lens – f/32. It was necessary to get the depth of field you see. But the diffraction involved is largely imperceptible. The image is sharp and crisp. I didn’t expect this – everything I understood about both the lens and high density sensor on the Nikon D750 camera convinced me that I should be getting an image that looked as if it had passed through a softening filter.
Once again I was reminded that the only true element in photography is the photograph you actually end up with. The more time spent experimenting the better. That is how you really learn.
