Assuming for example a lens has a range of f/1.8 - f/22, for an ideal picture which apertures should I avoid, and for what specific reasons? Now if the answer is "do not use the first two and the last two stops", can I assume the same for all lenses? Or is this information very much dependent on the quality and type of lens in question such that a rule-of-thumb cannot be created.
The two "ends" of the question can be answered with
- How to stop a high F number from appearing blurry?
- What's the base f-stop when counting an ideal f-stop for lens?
In short, at the stopped-down end, you become diffraction-limited at around f/11 with current APS-C DSLRs; maybe a little sooner. On the faster side, generally lenses perform better about two stops down from the maximum, but that varies by lens. And, often in both cases you might choose to do it anyway for perfectly valid reasons.
Every stop has a use, otherwise they wouldn't exit.
Typically at the widest end (f/1.8) the lens will suffer from aberrations and produce a slightly softer image. But there are times you need the light gathering ability. The degree to which this is true depends on the quality of the lens and other parameters such as focal length.
Likewise at the narrow end (f/22) the image will be soft due to diffraction. But there are times when you need a slower shutter speed, or to maximise depth of field. In this case lens quality is irrelevant, but pixel/sensor size comes into play when determining at which point diffraction becomes visible.
What I do is to go to: http://www.the-digital-picture.com/Reviews/ISO-12233-Sample-Crops.aspx
and see for yourself how your lens performs at varying mm and f. Then learn how your lens operates from this.
It will depend on many factors.
Several of these are:
- How you wish the image to appear
- The characteristics of the specific lens
- The characteristics of the camera being used
There is no rule of thumb for how you wish the image to appear. If you wish to use a narrow depth of field you will need to use a wide aperture. If you wish for almost everything in the scene to be in focus you will need to use a narrower aperture. If you want the plane of focus to be as sharp as the lens is capable of you will most likely need to use an aperture in the middle of the lens' range.
Each lens is a compromise between many factors that affect lens design. A theoretically "perfect" lens is not possible but the closest we could hope to make would be a single focal length and single aperture shaped in a perfect circle. As soon as we begin to add additional capabilities to the lens, we also begin to add design compromises. How many blades do you need to make the adjustable aperture "circular enough"? How much distortion is acceptable in exchange for a wide focal length range? How much more are we willing to spend for that next minute increase in performance? There are some tendencies that many lenses share that might be useful. Most lenses are sharper at the mid frame and edges when stopped down one or two stops from their widest aperture. This is especially true of lower quality lenses, but not so much the case with manyof the highest quality lenses. Most lenses tend to project images that are noticeably darker on the edges than in the middle of the frame when set to the largest apertures. Sometimes these "defects" are desired by the photographer, but many times they are not.
The media used to record the image will affect the amount of diffraction on an image using a particular lens at a particular aperture. On digital cameras the Diffraction Limited Aperture (DLA) is determined by the size of the sensor's pixels. With film it was the size of the grains in the emulsion, thus the DLA of the same camera/lens combination would vary based on the film being used. This is because it is related to the size of the circle of confusion for a given aperture. With a digital sensor the DLA is the aperture at which the size of the circle of confusion becomes larger than the sensor pixels and begins to visibly affect image sharpness at the pixel level. Diffraction at the DLA is barely visible when viewed at 100% (1 pixel = 1 pixel) on a display. As sensor pixel density increases, each pixel gets smaller and the DLA gets wider. DLA does not mean that narrower apertures should not be used. It is where image sharpness begins to be compromised for increased DOF. For more about diffraction, read this question. Current Canon DLSRs may have a DLA as low as f/6.8 (Current Digital Rebels) and as high as f/11 (EOS 1D X). Most other manufacturers offerings fall somewhere along the same lines.
Ultimately you must consider all of the factors involved to decide what is the best aperture to use for a particular photograph. Many times it will be a compromise between several factors such as more depth of field (narrow aperture) and usable shutter speed and ISO (wide aperture).