There are times when you can be to close. As people age the lenses of the eye tend to harden and the muscle control of the same gets a bit weaker. This makes it hard for some people to focus on the nearer by object. You will note in the illustration that 6" is mentioned. For an 18 year old, thats about where they can focus, and that will extend up to 12-14" with age even with corrected vision. On the other side of that, if you get past 24" your ability to resolve to a proper sensitivity level is impaired. While AWS does not specifically mention it, it is inferred. As for angle, the ASME portion is 30 degrees as judged from the surface to be examined. The ASME requirements are based on the 20/20 acuity standard developed in the late 1800's. The 20/20 value is the baseline for 100 percent visual acuity. That does not necessarily mean that is required physically to note the smallest rejectable discontinuity. For instance: simple line detection. (18 percent nuetral grey card) has a 1/32 black line (750 micro meters). a legally blind person (20/200) can reliably detect that line.
AWS has taken all that into consideration in their physical requirements for CWI's.
There are a host of factors that play into this (parallax, training, physiological, luminance/chromatic etc contrast, sharpness to name a few) but at the end of the day, the inspector will have been expected to detect the rejectable indications as found in the relevant code. If you are looking at something less than 30 degrees and more than 24" away without having qualified your ability to detect the required minimal elements, then you will find yourself trying to explain why you did that when there are industry standards out there that as a minimum, give relevant information in that regards. If for instance the crack opening dimention is < 25 micrometers/1mil and this is the probable flaw type, and your looking at it from 4 feet away and at an angle nearly parallel with the examination surface, your going to find it really hard to justify why you didn't see it should something go south on that component.
My opinion for what it's worth,
Gerald