Duke wrote: ↑Sat Mar 20, 2021 2:11 pm
the various lenses from the different manufacturers each have their own characteristic bokeh, despite having the same focal length and aperture, because each has its own optical design and order of telecentricity.
If I understand your words correctly, then I disagree with you regarding bokeh and telecentricity. The issue that different lenses have different bokeh is well documented as being due to other aberrations, primarily over- or under-corrected spherical aberration. This is what causes the sort of asymmetric blurs, with bright center versus bright edge on opposite sides of focus, that are described for example at
https://www.kenrockwell.com/tech/bokeh.htm and
https://luminous-landscape.com/bokeh/ . This sort of asymmetry has nothing to do with telecentricity.
The
Edmund Optics article is talking about a different sort of asymmetry. Here I have highlighted some parts of the relevant paragraph:
Edmund Optics wrote:It is a common misconception that telecentric lenses inherently have a larger DOF than conventional lenses. While DOF is still ultimately governed by the wavelength and f/# of the lens, it is true that telecentric lenses can have a larger usable DOF than conventional lenses due to the symmetrical blurring on either side of best focus. As the part under inspection shifts toward or away from the lens, it will follow the AFOV (or the chief ray) that is associated with it. In a non-telecentric lens, when an object is moved in and out of focus, the part blurs asymmetrically due to parallax and the magnification change that is associated with its AFOV. Telecentric lenses, however, blur symmetrically since there is no angular component to the FOV. In practice, this means that features such as edges retain their center of mass location; an accurate measurement can still be made when the object is beyond best focus as long as the contrast remains high enough for the algorithm being used by the machine vision system to function properly.
In other words, EO is using the phrase "symmetrical blurring" to mean that the edge only blurs, rather than blurring and shifting position. With a non-telecentric lens, away from image center the blur will move laterally as focus is changed, moving one direction in front of focus and the other direction behind it. With a telecentric lens, it does not. This is not a matter of bokeh. When Edmund Optics says "larger usable DOF", they are describing exactly the same effect that I was referring to when I wrote that
rjlittlefield wrote:When making measurements, a telecentric lens gives more leeway for positioning errors without messing up the measurement.
So, if we equate "depth of field" with "allowable range of subject positions", then telecentrics give more.
--Rik