We were discussing specular highlights in response to this thread and I had a bit of time so I thought I'd do a quick demo. I thought it might be useful as a general post.
(Note that lighting is not my strong point so please feel free to rebuff or add to this...)
Specular highlights are a direct reflection of the source. They are important as they give an indication of the type of surface (shiny, wet, etc) and the shape of the subject. The nature of a specular highlight will depend on the size and distance of the source. The more distant or smaller the source, the smaller and more intense the specular highlight will be. To soften the intensity of a specular highlight you need to use a relatively large source (compared to the subject) positioned as close as possible to the subject. This sounds a bit counter-intuitive, but of course you need to lower the power of your or adjust your accordingly as you bring the closer to the subject. (I went past a local restaurant a while ago where they were taking some food shots out on the footpath. The table was maybe 3-foot square but they had this giant octabox pushed right up against the table. I assumed at the time that the tog was just trying to justify his fee , but I understand it a little better now). Moving the closer can have other effects such as softening the shadows (as the 'wraps' around the subject more) and may affect how the drops off across the frame.
It is best to get specular highlights right in camera. While you can probably recover some highlights in post if shooting , the specular highlights can often be quite a few brighter than the rest of the image so will be limited in how much can be recovered.
Remembering high school physics, the angle-of-incidence = angle-of-reflection. What this means in practice is that hitting a curved object will have a narrower specular highlight than hitting a flat object (at the appropriate angle) which will have a wider specular highlight. This means that altering the angle of the , or the camera, or the subject can have an impact on the appearance of the specular highlight.
While it depends on the look you are going for, it often works well to get some of the object's colour showing through the specular highlight, particularly where that area is relatively large.
(I wonder if there was something Freudian in my selection of subject matter here? )
Shots are out-of-camera jpegs, cropped slightly and resized. Levels adjustment on #3. I didn't give any consideration to white balance, etc here.
1. 50cm square softbox with SB-800 at full power approx. 90cm straight above subject, f/11 1/60s -200
(Not actually as bad as it looked on the LCD - 90cm is not hugely distant here)
2. 50cm square softbox with SB-800 at half power approx. 30cm straight above subject, f/22 1/60s -200
(Maybe a little under-exposed). Highlights are now larger but softer, and there is colour showing through. Note some loss of shape info in that crumpled bit near top of chilli)
3. Shot #2 with quick levels adjustment
(I think my preference would be somewhere between 1. and 2.)
There are a number of other ways to reduce specular highlights. You can spray the subject with a matte spray to reduce how reflective it is (apparently hairspray works well, but probably won't do much for subsequent use of your food items). Cross-polarization is another method, but beyond the scope here.
Anyhoo, hope this helps some...