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OK
the trouble I have with this orchid is not blowing out the detail in the white petals, but get enough light to show up the purple sepals.
Shots taken in situ.
Fower about 18mm high
Taken with canon 40d, 100mm marco, twin ight flash or natural light

Labellum

Blow out the whites, ouch

Flash at -1

Blow out the whites, ouch

location shot

The flash at -1 gives you some idea of where this can be taken.

Where were your lights located relative to the flowers? Their positioning may be significant in terms of helping you get this right.

The flash at -1 gives you some idea of where this can be taken.

I am wondering if using the camera's histogram could be of some use. The first photo is the best, but could it be slightly under exposed?