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Figure 2-12 (A) shows a situation whereby detection of lack-of-side wall fusion in a V-weld is not performed optimally. Angled radiation (B) is more likely to show up this type of weld defect.

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As one replaces one with another objective lens, typically while going from lower to higher magnification, a microscope with parfocal lenses will remain more or less in focus. In practical terms, this may not mean that no adjustment of focus will be necessary, just not an enormous amount of change in focus (ideally) should occur.

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On a radiograph, a three-dimensional object is presented in a two-dimensional plane (the film). The appearance of both the object and its defects depends on the orientation of radiation relative to the object.

Physically, what is happening with parfocal lens is that the focus of the lenses involved occurs in the same plane. That is, the sharpest perception of the magnified image can be found in the same place which in practical terms means that changing magnification by switching objective lenses does not change focus.

Similar or nearly identical plane of focus associated with the different objective lenses employed in a compound microscope.

As shown in figure 1-12, the image of a gas cavity in a casting may be circular or elongated depending on beam orientation. In general, the beam of radiation should be at right angles to the film and a specimen should whenever possible be laid flat on the film cassette. Special angle shots are, however, sometimes useful.