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.