Aperture Settings and Image Quality

CRR_3158_001_big_mouthIn previous posts I wrote about the implications of the size of the iris of the lens with the image quality. I’m not saying the value of the image, but the features of an image. The enumeration below must be taken into account:

  • Depth of field (DoF)
  • Chromatic aberration (CA)
  • Vignetting (V)
  • Diffraction (D)

About the DoF, there is a lot of information everywhere. You know the principle: higher f-stop number (minimum apertures), long DoF; lower f-stop number (maximum apertures), shallow DoF. You can see this post to get a review of the subject.

The chromatic aberration is a lens defect, when the light rays passing through it, they seem to split, deflected, creating double edges or fringes of different colors. It’s very common over the edges of the images with low quality lenses, but any lens can suffer it. The lens is not capable to focus sharply all the colors,  because of the dispersion of the light. Post-processing software can correct CA without any apparent trace. That’s is the main reason of almost nobody seems to care about CA during a shooting. For the most part, the DoF requirements seem more important. You can reduce the CA closing one or two f-stops the aperture of the iris, but not too close to produce diffraction (see below).

Another implication of the size of the iris you choose, is the effect of vignetting. These are the dark edges of an image, now occasionally used for an old «vintage» effect. This is another lens aberration or distortion that can be corrected in post-processing or by closing several stops the aperture of the lens. Also many camera already have embedded software settings to avoid it.

In photography, the diffraction is a light phenomenon produced with the most little apertures of a lens. The image became softer at the closest apertures (f/22 or more), when by the DoF principles you probably expect to be sharper and in focus. The case is solved easily with choosing of one or two f-stops above those apertures.

The consequences of the maximum or minimum aperture selections are very important for the image qualities. Too open means shallow DoF, but too close will also means softness by diffraction. Too open means—again for the shallow DoF—that you also might have trouble with the sharpness or focus of your subject. In short distances, any little movement forward or backward from you or your subject would loss the focus over the previous zone. Although by the rule of simplification, you want to distinguish, pop out, your subject from background, and foreground, you have to deal with the others factors that affects the DoF: subject distance and focal length of your lens. Remember: short distances tend to shallow DoF, short focal lengths tend to long DoF.

You have to deal with all these settings and choices to get the effects you want to convey in the images, and use this knowledge to your benefit. The sweet spots of f/5.6 up to f/8 are the most recommended aperture setting, when you only care about taking a sharp image most of the time. But if you have  some distance from your subject, it will probably good enough to get a sharp image from lower apertures (f/2 or f/1.8). Combine that with a wide angle lens (35mm, e.g.) and you complete assure the sharpness. Or, on the contrary, you can use a telephoto lens, zoom in, and get a sharp subject, but a nice backgrounds blur, almost with a great range of apertures (up to f/8 including).

In any case, the main consideration is the problem of subject focusing. What it will be in focus and how? The distribution of the image content determines the answer of this question. Is the subject in the front, the middle or in the back of the whole picture elements? Do you want your main subject in focus? Remember that not always the focus will determine the main subject. Another way to distinguish a subject is the size of it along the frame. A blurred face, that occupies 50% of the image, is probably more prominent that any sharp little object that fills only 5%.

In other posts I will share some ideas regarding the selection of shutter speed. The main issue there is that you can show or freeze motion by selecting the proper shutter speed. The mark of 1/250th of a second seems to be the frontier of both sides. But there are other considerations that include the distance of the subject that is moving and your actions to maintain the focus over the subject in motion.


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