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Specifying the correct Zoom for your application
Every zoom optical application has a set of interrelated performance
constraints. These constrained variables include:
- Working Distance
- Field-of-View
- Camera Format
- Resolution (N.A.)
With a zoom optical system, the working distance remains constant as
magnification changes. The resolution improves as magnification increases
and the depth of field increases at lower magnifications. Once you have defined
your imaging requirements, go to the
Optical
Performance Specifications Chart for the Zoom system you have chosen to identify the performance parameters
and equipment options that yield the desired specifications for your
application. In this example, refer to the NIR Zoom 70XL Optical Performance Specifications Chart (pdf) that you have just opened with the link above.
One characteristic that is placed on the optical performance specification
charts of zoom systems that operate in the visible is the depth of field.
This parameter was not included in the NIR Zoom 70XL chart due to the
extremely broad spectral band that the zoom can cover. Refer to the Optical Glossary of this website to better understand
how wavelength and depth of field are related. By using the formula and
knowing the wavelength being used, the depth of field can be calculated.
Specification Example: Assume you have identified these imaging requirments for your application.
- 1/2" Camera Format
- 3.0 x 3.0mm, requried field-of-view (FOV)
- Working Distance is flexible
Objective: Your goal is to image the total field-of-view, while maintaining the maximum ability to zoom in to higher resolution.
Execution: Because camera/monitor manufacturers never want to have dark corners on
monitors, they overfill the monitor by approximately 10%. This means that
visually you are missing 10% of the field sent to the camera, and
therefore, need to compensate (NOTE: typical imaging software will still be
able to work on the entire image even though your screen may not display
all of it).
You should select combinations of equipment that will cover
a 3x3mm F.O.V. within the smallest dimension of the 1/2" Camera Format and
still provide you with the most room to zoom up. Referring to the excerpts below from the NIR Zoom 70XL Optical Performance Chart, a
number of options that yield your desired F.O.V. with a 1/2"
format camera become apparent.
2.0X
Aux Lens 0.5X Threaded TV Tube
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N.A.
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0.048 -
0.16
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F.O.V.
(Low Mag.)
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4.7 x
6.3
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F.O.V.
(High Mag.)
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0.68 x
0.91
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W.D.
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32mm
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1.5X
Aux Lens 1X Threaded TV Tube
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N.A.
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0.036 -
0.13
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F.O.V.
(Low Mag.)
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4.2 x
5.6
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F.O.V.
(High Mag.)
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0.61 -
0.81
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W.D.
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52mm
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1X Objective Lens 1.5X Threaded TV Tube
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N.A.
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0.024 -
0.08
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F.O.V.
(Low Mag.)
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4.2 x
5.6
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F.O.V.
(High Mag.)
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0.61 x
0.81
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W.D.
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89mm |
Conclusion: You should make your selection based upon the relative priority of Working
Distance over Resolution. If depth-of-field is most important, select the
lowest N.A.; if resolution is the driving factor, select the option which
yields the highest N.A.
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