Guitar Photograph Challenges - Dealing with Reflections
Shooting any object with a reflective surface can be challenging, but add multiple planes, textures, and different types of reflective materials and you've got the formula for some serious hair-pulling fun!
The first guitar company I worked with honestly couldn't have been more perfect for forcing one to develop creative lighting solutions for their design boasts zero flat surfaces. This means that regardless of where you put the light(s) - you're going to have a direct hot-spot where the light is reflected back to your camera.
Lighting flat surfaces is relatively easy. Think about the game of billiards (pool if you are from the Southern United States like I am). When using the cue ball to bank round another ball so you can hit the ball you want, you have to do some simple (or not so depending on your spacial perception abilities) geometry. It's all about angles.
Lighting in photography or cinematography works off the same set of principles. You have the angle of incidence for the light - or the angle you need to use to push the cue ball into the ball you are trying to hit. From a flat service the angle is exactly opposite the angle you have your camera placed. So if you are shooting your subject at a 45 degree angle and you don't want a reflection of the light on the surface of your subject - simply put the lights anywhere except 90 degrees from your position (or 45 degrees on the other side of the subject). Curved items are more difficult.
Which led me to a very frustrating conclusion... since guitars have all these knobs and switches, strings made of metal, chrome and/or brass parts, frets made of steel, wood surfaces with shiny gloss finishes - all of it caught an annoying reflection on one sort or another.
I finally decided that the only way I was going to control the reflections was to control the light completely... which meant shooting in a completely dark room with black walls, floor and ceiling.
I usually use 3 lights - two 1K studio lights (tungsten balanced bulbs) and 1 250w snoot (a small focused slight with little spread for highlights such as the chrome bridge). If shooting a flat guitar I'll put both lights (equal distance) around 20-30 degrees from the side of the guitar - using my meter to determine distance.
Moving lights - BTW - is a very easy way to do broad changes to exposure. There is a law of physics called the "Inverse Square Law" - it pertains to the way light behaves. Let's just say we are dealing with 1000 lumens of light falling on the subject at 5 feet away from the light source. If I move the light 5 feet further away what happens to the intensity?
Common sense would say, "you doubled the distance so you cut the light by 50%" - but that would be wrong. By doubling the distance I actually cut the light by the inverse square of the distance - meaning if I had 1000 lumens at 5ft, I'll have 250 lumens at 10ft. Likewise if I move the light closer by 1/2 - from 5ft to 2.5ft - I didn't double the amount of light - I quadrupled it - there is now 4000 lumens falling on the subject. It's easier to test this with an incident light meter - but try it... little moves of a light can have a really big affect.
Controlling the light is a major part of any successful photo. Being aware of where a light will create a flare on your outer lens element, knowing where shadows fall or where the light will start to wrap around an object and create edge light on the object's far side... all of this is vital to know, understand, and plan for.
The difference between a lucky shot and a truly inspired, professional shot is knowing everything you chose and why when you released the shutter.