MTF (Modulation Transfer Function) charts are painful to look at, but they’re really easy to read. In this article I show you how to approach reading your first MTF chart, and what good it will do you.
What is an MTF chart?
Each lens comes with one ore more MTF charts. In short, the chart gives you clues about how a lens might behave in terms of resolution, edge contrast and even bokeh!
What are the benefits of knowing how to read an MTF chart?
By learning how to read MTF charts (which you can do in a few minutes, it’s not that hard) you can find the following:
- The resolution of a lens, or how sharp it is
- How well it can resolve edge contrast or acutance – or how sharp it ‘looks’
- How good the bokeh is (you read that right), and
- At which aperture the lens delivers the best results (only to a certain extent, see below)
Who would have thought these simple droopy lines actually give us that much info? If you don’t have access to lenses (how many of us have?) then the next best thing to an objective test is the MTF chart. Let’s learn how to read it.
How to read an MTF chart
Here’s a video that explains how to read MTF charts easily:
Notes:
- The parallel lines are called Sagittal lines, and the perpendicular lines are called Meridonial or Tangential lines. Now you know why I didn’t include the names in the video. It’s not important to remember their names.
- Lenses referenced are Zeiss Planar T* 85mm f/1.4 ZF.2, Zeiss Otus 85mm f/1.4 ZF.2, and Zeiss Distagon T* 15mm f/2.8 ZF.2.
- If possible, check out the published MTF charts for these lenses.
Summary
If you have no time for the video, here’s a quick summary:
- Lenses are tested at two resolutions – lower (usually 10 line pairs per mm, or lppmm) and higher (usually 30, 40 or 50 lppmm). The first is for acutance, the second is for resolution.
- With 8K and modern sensors, we really need to start having tests for 100 lppmm, but most of our current crop of lenses will look really bad on it.
- Each resolution test is done two ways – with lines parallel to the diagonal and perpendicular to the diagonal. It’s like lifting dumbbells with both hands, one hand is always stronger than the other. A man with two strong hands is considered stronger than the man with one strong hand and one weaker hand.
- The higher the lines the better. Usually they peak at the center (zero point) and drop off at the edge of the sensor.
- The x-axis represents half the length of the diagonal (of the sensor). A full frame lens will be about 21.6mm and for APS-C it’s smaller (changes slightly depending on sensor size).
- The y-axis shows transmission of light. 1.0 or 100% is full transmission, which glass will never achieve. 0 or 0% is no transmission, which is a lens cap, not a lens.
- Lens designers try to find a good balance between the resolution and acutance lines. This is because our perception of sharpness is split somewhere between resolution and edge contrast/acutance. That’s why sharpening filters work.
- The closer the Sagittal and Tangential lines, the better the bokeh, supposedly.
- Each lens is normally tested wide open and stopped down.
- Each lens is tested at infinity focus, and some publish results at close focus.
- It is theoretically possible to find the best aperture for a lens, but it would involve hundreds of tedious MTF charts and tests, in all possible combinations of distance to chart, focus distance, aperture, etc.
What does the name ‘Modulation Transfer Function’ mean?
Function means formula, for lack of a better word. You throw in x, you get y.
Modulation means modulating, like moving in waves. There are no waves in an image, or are there? The line-space pattern represents a square wave but in reality optics resolves it like a sine wave. It’s impossible for optics to reproduce edges 100%. In short, your lens sees the zebra lines in a blurry way, like a drunkard.
What about Transfer? In short, transfer is transfer – move from one to the other.
Putting it all together, the modulation transfer function is a way (formula) for engineers to get numbers out of pointing lenses at thin lines so they can be plotted on a graph.
What you shouldn’t expect from MTF charts
MTF charts are great for general ideas regarding lenses, but shouldn’t be trusted 100%.
- Don’t trust the numbers! 0.8 on a Canon MTF chart is not the same as 0.8 (or 80%) on a Zeiss chart, and so on. Just study the general shape and fall-off.
- You can’t compare lenses from different manufacturers. It only works if you compare charts where all aspects match – focus distance (usually infinity), aperture (usually wide open or stopped down), same distance to test chart, etc.
- If you change the focus point the MTF chart changes. Even the simplest prime lens behaves in complicated ways.
- If you change the aperture by a third of a stop, the results change. This is why we can’t have MTF charts for every aperture, it’s too tedious.
- It’s specifically hard to test zoom lenses. Usually, they only test at the widest and longest focal length. But this doesn’t tell you how well a lens performs at other focal lengths.
- The MTF chart can’t tell you anything about flare, lens aberrations, filters, etc.
- You can’t directly compare a wide angle lens to a telephoto lens.
- In the real world, a sensor also has a say in the final image quality. MTF charts don’t take into account how a sensor behaves. Noise, resolution, color, etc. all play a massive role in the final result.
I hope this article has helped you overcome your fear of MTF charts and how to read them. Once you practice reading a few, there’s no going back.
I personally think it’s an important tool in any cinematographer’s arsenal. However, the best way to study lenses is to film with them and to use a projection system to study its characteristics. The combined knowledge will then be complete.
