Describing infinity-corrected objectives

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Lou Jost
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Joined: Fri Sep 04, 2015 7:03 am
Location: Ecuador

Describing infinity-corrected objectives

Post by Lou Jost »

The terminology used to describe objectives was developed when there were only finite objectives. Infinity-corrected objectives are fundamentally different, and it seems to me that we need a better way of describing infinity-corrected objectives, given that we can achieve whatever magnification we desire, through our choice of tube lens focal length.

The numbers that really define the behavior of these objectives are their good field of view and their NA. Magnification is not an intrinsic property of infinity-corrected objectives. We can choose whatever magnification we want. From the NA, we can determine if the desired magnification on a particular sensor will be affected by diffraction (by calculating the effective aperture from our chosen m and the given NA, and comparing this with the effective apertures known to cause visible diffraction degradation for that sensor). Knowing the FOV, we can determine if there will be vignetting at that magnification (by checking FOV*m < sensor diagonal).

Sensor coverage is also not an intrinsic property of these objectives. For any size sensor there will always be tube lenses long enough that a given objective will cover that sensor.

We can figure out the FOV from the field number for the standard tube lens focal length and the standard magnification of the objective, but we shouldn't be misled into thinking that those latter two numbers are individually special.

That's my rant of the day....

The fly in the ointment is that we know the quality of the edges of the image circle depends on both the objective and the choice of tube lens design. But the limiting value for a given objective should be independent of tube lens design, so the corresponding FOV on the subject should be well-defined.

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