White flowers are the hardest to photograph. Iris especially are difficult because they reflect so much light but are thin enough to also refract a fair amount of light and light therefore transmits through the leaves.
To compensate for this, you must use a tripod so that you can use a low ISO setting and achieve enough depth of field to capture the transmitted light as well as the reflected light. During editing in post production it will be necessary to reduce the highlights by at least one and a half f-stops to be able to see the texture of the bloom in the white area.
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