skin detail as a function of light intensity?

[stillhope]

I understand reciprocity in the sense that one can appropriately change any 2 of: the shutter speed, the amount of light, or the aperture size and maintain basically the same exposure, within limits.

However, I've seen some things that suggest that this may be true of the overall exposure, but that the appearance of human skin is different with different amounts of light, even though the exposure is the same.

In other words, if I take a picture of a person using a strong light, say F11 at 1/250, then I reduce the light considerably and take the same picture with F11 and the shutter speed longer to get the same exposure, they'll look different, particularly subsurface color details in the skin. I'm guessing it might have something to do with differences in the way the light penetrates the skin relative to the light's intensity. Is that possible? Is this something well known to portrait photographers, or am I imagining it?

 

[iolair]

It feels unlikely to me. Find a subject and do some tests to check it out - then share the details with us so we can learn.

On the other hand, it's quite possible strobes change their colour temperature very slightly with the intensity of the flash, so the slight change in colour could cause this affect. You could counteract or check this by putting a grey card in the shot.

The colour of the light definitely does make a difference to the amount of detail shown in the skin.

 

[tirediron]

I've never worried about light penetrating the skin; what can change drastically is the specularity of the light, such that in one situation, you may have very bright specular highlights, and in the other, you will have a more even, diffuse lighting with virtually no specularity.

 

[Big Mike]

It might also have to do with the different reflectance types that skin has. Just about all skin has properties of diffuse reflection (think white paper) and direct reflection (think polished metal).

Direct reflection is responsible for the glare that we often see, but it can also be a general sheen on the skin. This type of reflection can tend to accentuate texture but it can also tend to mask color.

So when you change your lighting or camera position or pose, I'm wondering if you aren't also changing the type of reflections that are showing as prominent.

 

[KmH]

In my experience, your supposition is not true for skin, but it is for some other types of materials.

Wedding photographers sometimes have issues with wedding dress materials that fluoresce under strobe lighting (flash).

If you use f/11 and 1/250 and then "reduce the light considerably" you'll likely have to slow a shutter speed to freeze slight subject movement and control camera shake blur.
One stop less light would require a shutter speed of 1/125
2 stops = 1/60
3 stops = 1/30
4 stops = 1/15