How
to read Sigma's MTF charts
By Bob Martin
MTF (Modulation Transfer Function) charts are the industry standard way of recording how each lens performs and are useful tools when you come to choose your next lens. Despite what you may think, it’s actually remarkably easy to read MTF charts, once you understand them. Do you remember the first time you tried to tell the time using a traditional analogue clock? To start with you’d need ages to tell the time but now you just glance at your watch and tell the time instantly, it can be just the same with MTF charts once you get a bit of basic knowledge about them.
How
does the chart work?
We have two sets of values to work with on the chart so let’s
start with the numbers running down the side, which are come under the
title of Contrast. On MTF charts they are displayed as decimals but
it will probably be easier if you imagine that it’s actually a
percentage instead. Think of the top as being 100% (best possible) and
the bottom as 0% (worst possible). So even before we know what the lines
mean you can probably guess that if the lines are at the top of the
chart, the lens offers good quality.
Along the bottom of the chart runs some measurements labelled as Image Height (mm). This is the height from the centre of the frame to the corner of the frame. If we draw a diagonal axis across an image from the camera, the two lines obviously meet in the middle of the frame. This is referred to as 0mm on the chart, the very centre of the frame where the lens is at its very best quality. The chart then continues diagonally from the centre of the frame until you come to the corners at the 21.5mm mark where lenses typically aren’t at their best quality.
Here we have the image circle produced by the lens with the camera’s image size (36x24mm) depicted by the black rectangle. Each coloured ring relates to the columns on the MTF chart and is a measurement from the centre of the frame.
What
do the lines represent?
When you come to buy a lens, the main thing you’ll want to know
is probably how sharp the images it produces will be. So why isn’t
there a value on the chart for sharpness instead of contrast? It seems
as though that would make more sense doesn’t it? The reason for
this is that contrast and lens sharpness are more closely linked than
you may think, so we can put them together on the same chart.
A test chart made up of tiny parallel lines is photographed with the lens at its maximum aperture. The lines are measured in lines per millimetre (lp/mm) and we use two basic types for MTF charts - bold lines (10lp/mm) and fine lines (30lp/mm).
The larger lines (10lp/mm) are used to test the contrast ability of the lens, in other words, how well the lens can distinguish between light and dark areas. In simple terms, the Red lines represent the contrast ability of the lens.
The smaller lines (30lp/mm) are used to test the ability of the lens to resolve fine details and can be translated as the effective sharpness of the lens. In simple terms, the Green lines represent the potential sharpness of the lens.
These two types of line are broken into two sub-sets, lines which run diagonally bottom-left to top-right are called Sattigal and those that go the opposite way are known as Medirional line sets. The results from both sets of these lines can reveal allsorts about the lens but most importantly you can discover how pleasing the out-of-focus areas will look. These areas of a picture are known as bokeh, which derives from a Japanese word meaning blurred or fuzzy. The closer the dotted line and thick lines are on the chart, the more pleasing the bokeh will be.
