A depth allowance is incomplete until the reviewer names the plane that is allowed to move. Object placement and sensor placement are different geometry questions.
Identify the moving plane
Depth of field concerns the object-space range that remains acceptably focused. Depth of focus concerns image-plane displacement. Both depend on the relevant detail and optical criterion; the labels should not be exchanged. Primary reference: Edmund Optics: depth of field and depth of focus.
For a packaging line, mark whether the uncertainty is package height, surface shape or a camera/lens assembly interface. The desired evidence should follow that physical cause. A supplier's statement about one plane cannot fill a blank requirement about the other without a justified optical relationship.
Separate two invented requests
Request A says that the inspected top surface may occupy planes from 2 mm below to 3 mm above a reference plane. Request B asks about axial sensor-position variation inside an optical assembly. The first is an object-space interval of 5 mm; the second belongs on the image side. Copying the same 5 mm number into B changes the question.
The interval is fictional and provides no focus capability or HM tolerance. Specify the feature and acceptance criterion used to judge focus across A's interval. A large printed character and a fine edge need not provide the same conclusion from an apparently similar image.
Attach evidence to the correct requirement
Ask the reviewer to label the tested object positions on a drawing and retain images from the required bounds using the actual task. Keep any sensor-plane assessment with the optical assembly's own reference dimensions instead of attaching it to the package-height record.
When reading a quotation or a lens datasheet, verify the definition behind an abbreviated depth value before using it. The result should be a clear request about the permitted physical movement, rather than a reassuring number whose reference plane is unknown.
Customer Questions
Are depth of field and depth of focus interchangeable?
No. They refer to object-space and image-plane movement respectively.
Does a 5 mm example establish lens capability?
No. It defines a fictional requirement interval only.
What makes a focus criterion useful?
It identifies the required detail, position range and assessment method.
Primary References
These references support the technical principles discussed in this guide. The worked examples and review questions are educational.
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