Does unit focusing change depth of field compared with internal/element focusing?
Asked 6/5/2023
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If two lenses of essentially the same design and nominal focal length are focused on the same subject at the same distance, but one focuses by moving the whole lens (unit/bellows focusing) and the other by moving internal elements, will the unit-focusing lens have shallower depth of field because close focus involves more extension, higher magnification, and bellows exposure compensation?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
3y ago
2 Answers
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As you know, the f-number expresses the speed of the lens (image brightness). We calculate this value by dividing the focal length of the lens by its working diameter. Thus, a lens. 50mm focal length with aperture set to f/8 had a working diameter of 6.25mm.
The dimension we call focal length is only valid if the object being imaged is at infinity. If the object is closer than infinity, the distance lens to image increases with the focus which is a repositioning. For most picture tanking setups, this projection distance increase only minimally effects image brightness (it is moot). However, when doing close-up work, the projection distance greatly increases. In other words, the lens must be of the macro design, or we must dismount the lens and remount using a bellows or extension tubes.
As an example - When imaging at unity (life-size often called 1:1) the lens must be repositioned 2x focal lengths forward of its infinity focus position. If the lens is set to f/8, this increased lens to image distance invalidates the engraved f-number. We must now compensate 4x (2 f-stops). This is one example of the bellows factor impact.
Bottom line – It makes no difference if the lens is repositioned for close focus by changing the spacing of lens elements or moving the whole lens as a unit. It is the increased lens to image distance that must be compensated for. We calculate this by the formula M+1^2 (M + 1 squared when M = magnification). We work this problem to find out a revised shutter speed or f-number or combination of both.
Now a maco lens has a trick up its sleeve. The front element group acts as a magnifier that causes the apparent diameter of the iris (aperture) to appear to change as we focus. In other words, as we focus the distance front group to the iris changes causing the iris to appear larger or smaller. If the outside world sees an enlarged iris, its revised diameter allows more light to enter. In this way the macro applies the bellows factor compensation.
No significant difference in depth-of-field either way.
Originally by user44949. Source · Licensed CC BY-SA 4.0
user44949
3y ago
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Not inherently. Depth of field is mainly determined by aperture and magnification at the subject distance. If two lenses are framed the same and focused on the same subject, unit focusing by itself does not automatically give less depth of field than internal/element focusing.
What changes at close focus is that a unit-focusing lens needs extension, which increases effective f-number and requires bellows exposure compensation. That affects exposure and diffraction, but it does not mean unit focusing has uniquely shallower DOF. The key question is the actual magnification and effective aperture at that focus setting.
Internal/element focusing can also change focal length and magnification as focus gets closer; many lenses are designed to reduce extension by shortening their effective focal length. So two lenses focused at the same distance may not have exactly the same magnification or DOF, but that difference comes from their optical design and resulting effective focal length/magnification, not from “unit focusing” versus “element focusing” alone.
So the short answer is: no, unit focusing does not inherently reduce depth of field; any DOF difference comes from magnification and effective aperture at the chosen focus distance.
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