Is there really any specialized knowledge that my photographer should have? Don't they just push buttons?
Photography is the science of writing with light. Isaac Newton wrote a book on optics to better understand light transmittance through glass. Through this research he discovered that white light contains all colors of the rainbow. He split the white light of the sun into the three primary colors, determined how aspherical elements focus light and engineered lenses for telescopes. So, yes, your photographer should have a deep understanding of optics and the technology he or she is using. Otherwise they may be just pushing buttons (and you don't want that).
There is a lot of math involved in photography. Although a photographer does not need to know the equations, per se, they should have a working understanding of optics and how cameras create images. A photographer needs to know about all facets of the equipment they are using and the science that makes it work. For example, when comparing lenses, would it be bettwe to use an 85mm Full frame f4 lens at it's widest aperture or a 42.5mm Micro 4/3 f2 lens at it's widest aperture?
Notwithstanding the sensor, in order to compare these two lenses you have to understand what the comparison actually means. To do this your photographer will have to understand what an "f" number is as it applies to aperture (since we are only discussing aperture and focal length here).
The answer is that these two lenses would render almost the exact same kind of image with regard to the defocused areas, the compression of the full frame lens would be more, but the cropped field of view from the Micro 4/3 would almost match the full frame, rendering a very similar compression. Light gathering capabilities of the Micro 4/3 lens would be four times that of the full frame lens, but the physical iris opening of each lens would be the same.
Therefore, for all intents and purposes, the senses will render similar results, and either one would be suitable for beautiful portraits. However, many photographers will disagree completely. This happens when the individual does not understand optics. I am happy to explain.
The equation for determining the widest maximum aperture of a lens is the lens focal length divided by the maximum physical opening. Lenses are rated in millimeters as a standard, and therefore apertures are as well.
Given the full frame example above we can work backwards. We know the focal length of the lens is 85mm. We know the aperture is 4. Therefore we can divide 85mm by 4 and get 21.25mm. The aprture at f4 on this lens is 21.25mm in diameter.
Onto the Micro 4/3 lens. Given that the focal length is 42.5 and the aperture is f2, we simply divide the focal length by the aperture to find the result: 42.5mm divided by 2 is 21.25mm.
The results are not interesting, they are unremarkable. An informed photographer would never have argued the merits of this. they would have dimply done the calculation in their head and would have concluded that the choice is not which lens to use (because the lenses here are almost identical), instead the photographer would have made a determination about which lens to use based on the shooting conditions required and the other equipment needed, such as which camera body to bring.
I personally find it laughable when photographers get into quarrels about gear. Most don't even know the equations or have very little idea about the technology being used. This is a situation which can turn a portrait session into a nightmare because knowledge is power. No knowledge, no power.
A working undersanding of image sensors is very important in photography. The reference standard when comparing sensors is to use 35mm film as the base. Everything is referenced off of the 35mm film specification. There are several film formats available form which digital sensors are generally compared.
Full frame is a term that references full frame or 35mm film. APSC is a specific term for Advanced Photographic System Classification C. There were other classifications of APSC film, such as H. This is a smaller film size than Full frame. Another film classification size still very important today is Medium Format. Medium Format films and larger are considered Professional Format and used by professionals mainly working in commercial photography.
It is interesting to note that up until the digital camera revolution in the early 2000’s, full frame film, or 35mm film, was the AMATEUR film format. In the late 80’s and 90’s, if a professional was photographing your wedding, that person was probably using Medium format film. People who used 35mm film were considered amateurs.
Times have changed since then. This is because of the difficulty of producing full frame and larger format digital cameras and due to the expense of creating such cameras. However, this cost is coming down and several manufacturers make medium format digital cameras for the professional photographer market. These cameras generally cost $10,000-30,000 each. So, until this price comes down, most wedding photographers you meet will probably use a digital camera system which includes APSC and Full Frame formats.
As long as your photographer has a working understanding of these things, and can talk about exposure and image composition, you are probably dealing with a professional. If they cant, or you just want to check, ask to see their file data for their images. It'll show you if they are shooting on manual or automatic, and tell you a lot about their style. As a general rule, true professionals use manual or a semi-automatic manual and converse very easily when talking about the mechanics of photography.