Image Quality

Image Quality – Sweet spot

Sometimes, incredibly, you have to move back to see more.

For the multiple flash work, I prefer to shoot at high f-numbers to display the entire set of wings in focus and within the depth of field. For most lenses, image quality begins to degrade with increasing f# starting at f/5.6. For the long telephoto lenses, it gets particularly bad starting at about f/16. Note that the only image quality I am considering here is that which can be assessed by a resolution target in the center of the frame. This degradation is dependent of the type and quality of lens to some extent, but physical determines it. This degradation occurs because light passes through the lens aperture, an aperture that essentially becomes ever smaller, approaching a pinhole, at higher f#s. Light is diffracted, it curves as it passes by an edge or through a small opening. This leads to an overall fuzziness of the images.

Now here is a very interesting result I discovered (note that this highly dependent on your particular camera and optics). For the Canon 400mm f/2.8 L II on a Canon 5DS, these two configurations have the same image quality:

1. Object distance 20 feet, f/14

2. Object distance 17 feet, f/18

Because of diffraction the f/18 row has similar image quality even though the image is larger (more pixels) because the object is closer.

Keeping in mind that the flash-to-bird distance is the same for both rows, row 1 can use less flash energy because at f/14, (14/18)^2 or about 60 % more light is collected and therefore 60% less flash energy is needed to result in the same image quality. An alternative would be not to change the flash energy and use 60% lower ISO.