You sound like the US government when stating the new post - 9/11 guidelines for taking passport photos, no smiling
I have an idea. If you want to keep the result as isolated as possible from any sort of emotional influence you can simply take a piece of 8.5x11 paper, place it over the image of the patient for both the before and after, from the eyeballs down, leaving only the eyebrow on up visible for a true comparison.
However, I think you should consider something even more important and that is how the photos are taken. Are the before photos wet and the after photos dry? Are the before photos taken without a flash and the after photos taken with flash? Did you know that if a hairline pic is taken without flash before and with flash after it can literally DOUBLE the perceived density of the final result? These are tricks used by clinics that have a hell of a lot more influence on how the result looks than by someone smiling or not. Just something to consider but again, to alleviate the smiling issue, just cover it up from the eyes down. Problem solved.
Hasson & Wong NEVER use flash photography to improve our results. H&W is the ONLY clinic to standardize on HD video and high resolution images so you can see the real difference that matters, the details.
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All opinions are my own and may not be the same as Dr. Hasson and Dr. Wong