Nikon SB-27 - Manual (Page 55)

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Notes on close-up flash operation
Take note of the following points when shooting close-ups with the
SB-27 mounted on the camera.
Usable lenses are limited.
Avoid using any Nikkor lens whose actual length is shorter than
that of the built-in bounce flash adapter ( line in the illustration).
If the lens is too short, the incident light falling on the lens reflects
on the built-in bounce flash adapter causing a flare effect on the
film. (See the illustration [top] on the seal affixed to the bounce
flash adapter.
Other notes
In close-up flash operation, we recommend taking trial shots first
because pictures may be slightly dim or a shadow may appear
around the edge of the frame depending on the lens in use or the
flash-to-subject distance.