How can I image a plane that shifts by about 25 cm onto a CCD without refocusing?
Asked 11/22/2021
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I have an optical system that forms an image plane at different distances over roughly a 25 cm range. I want to capture that image on a CCD camera, ideally without having to mechanically refocus each time the image distance changes. Is there an optical setup that can keep all of those image positions sharp on a fixed sensor, or is the practical solution to use focusing adjustment or depth of field?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
4y ago
2 Answers
1
I believe you are asking about the relationship between the focal length of the lens, and the distances of the object and image.
It is most common to use the thin lens formula as a good approximation for this relationship:
Originally by user49520. Source · Licensed CC BY-SA 4.0
user49520
4y ago
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In general, no simple optical system will keep multiple different image-plane distances perfectly sharp on one fixed CCD at the same time. A lens focuses only one plane exactly; if the image plane moves, the sensor or lens position usually has to move too.
From the answers, the practical options are:
- use a focusing mechanism, such as moving the camera/sensor or lens on a focus rail
- use optics/settings that give enough depth of field so the blur stays smaller than your acceptable limit or pixel size
- analyze the geometry with the thin-lens formula to choose focal length and distances appropriately
So if your image plane shifts by about 25 cm, the usual solution is adjustable focus. If you cannot refocus mechanically, then your only real workaround is to increase depth of field enough that the defocus is acceptable for your application. Whether that is possible depends on your magnification, aperture, sensor size, and required sharpness.
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