Quote:
Originally Posted by Amaranthus
(i) In the above example with the TV FR back focus of 56 +/- 4mm and the camera with a 40 mm BF, you would need a spacer of 12 to 20 mm in order to achieve focus - correct?
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A spacer of 12 to 20mm will give you the correct distance from the FR to the camera sensor assuming that the 40mm distance specified for the camera is the
optical distance. The optical distance and the physical distance won't be the same if there are filters or other optical windows in the light path, e.g. a 3mm thick Astrodon filter reduces the optical distance by about 1mm.
Note also that getting the correct spacing between a field flattener or focal reducer and the camera sensor is to guarantee that the FF or FR operates correctly. It doesn't necessarily guarantee that you can reach focus and you may need to make adjustments in the spacing in front of the FF/FR to make this happen.
Quote:
Originally Posted by Amaranthus
(ii) If you had a FF with a BF of <40mm, it could not be used with the above camera - right?
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Correct modified by comments above.
Quote:
Originally Posted by Amaranthus
A related question - some equipment specifies extremely precise BF distances (e.g. 53.2 mm, whereas others give an integer and a tolerance range [e.g. the TV FF noted above]). Is it always the camera that requires the precise distance, and if you have a FF with a range, then as long as the range includes the camera BF distance you will be okay?
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The spacing specifies where the sensor needs to be in relation to the FF or FR for it to perform best. If the specs include a range then hopefully anywhere inside that range will give good performance. FRs tend to perform OK if the spacing isn't perfect but the size of the image circle and the amount of focal reduction may vary. FFs are typically more fussy and if you don't get the spacing right then you'll probably get a less flat field and some visible aberrations at the edges of the image circle.
Cheers,
Rick.