Finland-based commercial photographer Antti Karppinen shares what he thinks are the "7 Most Common Studio Portrait Light Mistakes" and how to fix them. Barring any creative differences, these are pretty good rules of thumb.
1. Main Light Too Low
If your studio light is too low the shadow cast from the nose can be broad and unflattering. The fix: bring your light up and try to maintain 45° angles.
2. Main Light Too High
If your studio light is too high you will get harsh shadows cast from your subject's brow. This puts the eyes in shadow and also creates a harsh shadow under the cheek. The fix: bring your light down and again try to observe angles of 45°.
3. The Fill Light Is In The Wrong Position or Power
Placing your fill light in the wrong position or at the wrong power can create a myriad of issues. Double catchlights in the eyes and a lack of distinction between the main light and the fill light are just two examples. The fix: place your fill light at center or just off center on the main light side and shift the power just enough to fill the shadows without overpowering your main light.
4. Unwanted Nose Light
When you're using a separation light, to separate your subject from the background, and you push it too far towards the front, the light will illuminate the nose, killing your desired shadows. The fix: make sure this light is only hitting the side of the cheek, and subject's edge by pulling it further back behind your subject.
5. Overexposed Separation
When you use too much power in your separation light you lose all of the texture and detail in your subject. The fix: lower the power.
6. Flare From Your Separation Light
Because the optimal position for your separation light is usually just behind your subject, the light is often pointed towards your lens. When this happens the light can hit your glass causing a flare, reducing your contrast. The fix: try using a lens hood, flagging the light, or reposition the light just enough to redirect the light.
7. Overexposing the Background
When attempting to create a blown out background you can overdo it causing light spills, and lens flares by using too much power. This destroys your contrast. The fix: pull your subject away from the background and/or reduce your light's power.
I hope you found these tips useful. If you liked this video, you should check out Karppinen's YouTube Channel for more of his stories, tips, and tutorials. I did a quick search on Fstoppers, just see how many times Karppinen had his work published here, and got more returns than I cared to count. Ok ok, six. It was six times, and now it's seven. Thanks for sharing Antti.