Some time ago I wrote about taking art images for my mother in law. Since I don't have my dream lens yet, I had to compromise on the lens and use the great (but not ideal for this task) Nikon 18-70 lens. (The image to the lest if one of the original paintings)

I got a few mails and comments about the issue of getting closer to the pictures to make the picture fill a wider part of the frame.

Sample Comment (by 'Anon'):

Kind of a newb, but why would you have used a zoom lens? And at what
distance/mm? I would think 50-70mm would be ideal, or would getting any
closer affect the "family of angles" thing?

As Norm replied, the main issue of getting further from the image was the Family of Angles constraint. Let me explain:

What is The Family of Angles?

The easiest way to describe the family of angles is to imagine the following setup: Say you are shooting a sheet of bright metal posted on a wall - much light the art that my mother in law makes. Now, Place a torch (or a flashlight) where the camera is placed to take the picture.

If you turn this torch on, some light will be reflected off the metal to create a hard border between light and dark. For our imaginary setup, this border will kinda look like a pyramid with its head at the painting and the base of it on the wall behind the camera torch.

The rule about the Family of Angles is as follows: what ever light source that is found with in this family of angle will be reflected through your subject into the final image.

But Paintings Are Not Made of Metal

True. The description above is good for shiny-mirror like object and is not accurate when lighting non shiny subjects. However, the painting has some shininess and placing a light source inside it will create a reflection of this light source in the final image.

Getting Closer and Further From the Painting

Now after explaining that why I can have the flashes inside the family of angles, I can explain the issue with getting further from the painting.

In the image to the left you'll see two possible positions for the camera. The first is the original location, as I described it in the original post. (This is how I created the diagram)

The second location is an imaginary location that will contribute to a frame filled with the painting.

Look what happens to the Family of Angles as the camera gets closer to the painting, the place forbidden for flashed is getting bigger. In this illustration the location of the flashes is still OK. In the location of the shoot, moving closer to the image would have meant getting the flashes within the family of angles and getting a bright annoying reflection on the paintings.

Lastly, let me recommend a great book (I have mentioned it a few times before) Light, Science and Magic by Fil Hunter, Steven Biver and Paul Fuqua. It helped me to get this setup right, but holds tons of great advice to the novice and experienced photographer.

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