Here's a small painting of my friends backyard I'm working on when I can get over to their house .
I decided to use this painting to demonstrate what tools I'm using to measure a scene as I set the drawing for a painting. For portraiture I use what's called a "proportional divider" to check my proportions and to double check as I work. A proportional divider lets you do a variation on the "sight size" method. The difference with a proportional divider is that your drawing doesn't need to be one-to-one as with sight size. I also sometimes use a standard Staedtler drafting compass because I can use the graphite tip to mark but then things must be one-to-one. A lot of artists use their paint brushes held out at arms length to check proportions. The problem I encountered in the past with all these methods is that they require you to keep your arm at a set distance from your body while you measure and then repeat that distance every time you measure. The standard way around this is to keep your arm locked straight out. Even locking your arm out, there is a little too much wiggle room for me. I now use a homemade tool to help. Recently I came across a video of Antonio Lopez Garcia painting outside on James Gurney's blog using a similar solution to this problem so I thought I would share how I use this method.
In the video Garcia is using what I think is a modified ruler to keep a compass at a set distance from his eye. He places one end of the ruler on his cheek and the compass on a block of wood glued to the ruler. This eliminates the one remaining variable you have when holding a measuring device at arms length... head and neck movement. When I made measurements just holding my arm out I found that I would sometimes catch myself leaning in with my neck towards my proportional divider and thus change the measurement by changing my angle of view in relation to the divider. Make this error enough times and your careful measurements don't really matter anymore. The "ruler" method gets around this problem incredibly well by locking the distance between your eye and the divider or compass. It's so accurate that if your support (canvas) is big enough you would start to get distortions as you moved towards the outer edges of the support (to get around the distortion you would have to construct a larger, more complicated device and then always measure when looking from a set point in space). To visualize this distortion imagine a photo taken with a wide angle camera lens. I noticed a small amount of this distortion on the red wall in my painting although I was far enough away and the panel is small enough that you don't really perceive it in the image. In the past I was always surprised to see this distortion effect in some of Antonio Lopez Garcia's cityscape work. Normally you only see this effect in paintings when the work is based on a photo. Once I started using this method myself I now understand where the effect comes from in his paintings even though he is painting from life. Below are some images of how I use this method and the current painting I started using it. I'm using a ruler to show how you can do this with tools most people have:
This painting is on an old panel that I had laying around the studio. I'm not the biggest fan of painting on canvas anymore. I've grown to prefer primed panel. This started off as an experiment so I thought I would use one of my old canvas panels. Now that I'm in the middle of the work I really like it and wish I had used one of my non-canvas panels.