Estimated reading time: 8 minutes
If you’ve ever spent time looking at lens specifications, you’ve come across an MTF chart or two.
These can be extremely confusing, even for more seasoned photographers.
So in this post, we’re going to do a deep dive into MTF charts.
And by the time you’re finished, you’ll be reading MTF charts like a pro.
Are you ready?
Let’s get started.
MTF Chart: Table of Contents
What Is the Point of MTF Charts?
Modular Transfer Function charts, also known as MTF charts, are a graphical way to display lens performance.
In other words:
MTF charts only come about after a lens has been developed, in the testing phase when the lens is being evaluated on its optical performance.
Scientists measure a lens’s ability to produce sharp, high-contrast images–and they display the results via MTF charts.
Specifically, MTF charts display the measurement of two lens characteristics:
Contrast and resolution.
While the technical meaning of these two features could fill several articles, you don’t need to understand it all.
Just know that contrast refers to the ability of a lens to render clear edges (that is, edges that transition rapidly from light to dark, rather than from light to gray to dark).
Whereas resolution refers to the ability of a lens to render small details.
These two features together go into a casual photographer’s evaluation of sharpness, which is why it’s important for lenses to offer both good contrast and good resolution.
The consequence of this:
The ideal MTF sharp indicates high resolution and high contrast.
Now let’s take a look at how these characteristics are measured for MTF charts:
Measuring Resolution and Contrast
Camera manufacturers measure a lens’s resolution and contrast in very similar ways:
By printing out images of lines, then photographing them with the lens.
Generally, both resolution and contrast are tested twice, using two sets of lines:
Sagittal lines and meridional lines.
Both types of lines move diagonal to the camera sensor, just in opposite directions.
Now, when contrast is being measured, the lens is used to photograph a set of lines spaced at 10 per millimeter.
If the lens is able to portray rapid transitions from line to white space to line to white space again, it is very contrasty.
While 10 lines per millimeter sounds very cramped, it’s not as cramped as the lines used to resolution, which are spaced at 30 lines per millimeter.
Here, if the lens is able to capture each of the tiny lines and the white space in between, it offers very high resolving capabilities.
To recap in simpler terms:
Contrast is measured by photographing bigger diagonal lines and looking at whether they have clear edges.
Resolution is measured by photographing smaller diagonal lines and looking at whether they come out with good detail.
And then the lens’s ability to render these lines is graphed on an MTF chart.
Now that you understand what is displayed on an MTF chart, let’s take a look at how they are read:
Reading an MTF Chart: The Axes
MTF charts have two axes:
An X-axis, which indicates a distance (generally in millimeters).
And a Y-axis, which indicates a decimal/percentage.
Now, the distance on the X-axis corresponds to the position on the camera’s sensor.
The zero marker corresponds to the exact center of the camera sensor.
While the 20 marker corresponds to 20 mm from the center of the camera sensor (which is the edge of a full-frame sensor).
Related Post: Full Frame vs Crop Sensor Cameras
The reason for this axis is that lenses perform differently in the center compared to the edges. You often see lower resolution and lower contrast along the edges and corners of an image compared to the center–and it’s important that this is displayed as part of a lens’s specifications.
The decimal on the Y-axis of a lens should simply be thought of as the lens’s resolution and contrast reading. So if an MTF chart indicates a value of 0.8 or above, you can assume the lens features good contrast and/or resolution, whereas if an MTF chart indicates a value of 0.3, the lens has some optical issues.
Reading an MTF Chart: The Lines
MTF charts offer four lines.
As you already know, MTF tests involve two sets of diagonal lines. This should explain the difference between the different line consistencies (that is, the dashed and solid lines).
The MTF chart key will indicate whether the sagittal or meridional lines are dashed or dotted.
But what about the differences in line color? What do they indicate?
They simply refer to the different tests, where one set of colored lines indicates the resolution test, and the other set of colored lines indicates the contrast test.
Two MTF Charts? Four MTF Charts? Why Are There So Many?
Here’s one question that new readers of MTF charts often have:
Why are there sometimes two MTF charts or four MTF charts? Where is the second set (or third and fourth set) of data coming from?
Here’s the answer:
Because lenses perform differently at different apertures, lenses are frequently tested at their widest possible aperture (i.e., wide open). Then they’re also tested at another aperture, such as f/8.
So one MTF chart indicates lens contrast and resolution wide open, while the other indicates contrast and resolution stopped down.
Another reason for additional data has to do with focal length:
Because lenses don’t perform equally well across all focal lengths, they’re often tested at several points across the range–and each test receives its own MTF charts.
Reading an MTF Chart: Putting It All Together
You now have all the information you need to read an MTF chart.
What should you do with it?
Well, whenever a new lens is released, the manufacturer generally provides MTF charts. You can take a look at these charts to see whether the lens is sufficiently sharp.
Bear in mind that there’s no objective measurement of “too soft” or “sufficiently sharp.” Whether a lens is sharp enough depends on your personal preferences and your needs as a photographer.
But the higher the readings, the better.
By the way, it’s worth recognizing that different readings matter more or less depending on the type of photography you do.
If you’re frequently resolving very fine details (such as in a landscape), the resolution measurement will matter a lot.
Whereas a portrait photographer that doesn’t want to overemphasize wrinkles may not care so much about resolution measurements.
Also, if you frequently shoot at f/8 and above, the wide open MTF chart may be irrelevant to your photography. After all, who cares if a lens is soft wide open if it’s sharp where you need it to be?
So don’t get too hung up on overall MTF values.
And speaking of getting too hung up on MTF values:
They aren’t the only measure of a lens’s optical performance. You may find that a lens that performs poorly in terms of its MTF measurements has a particular character that you like about it; after all, there are some lenses that don’t even try to be sharp, such as Lensbaby’s Velvet lineup.
And lenses can also perform well in terms of their MTF measurements but do terribly in other ways–for instance, they might produce unacceptable levels of chromatic aberration.
Lens MTF Charts: Conclusion
Learning to read MTF charts isn’t the easiest topic out there–but now that you’ve finished this article, you should be able to use MTF charts with ease.
You can now evaluate lenses and their sharpness, without ever having actually used them!
Pretty nifty, right?
Why are there often two MTF charts?
The two MTF charts indicate that a lens has been tested at two different apertures. Generally, a lens is tested at its widest aperture (which depends on the lens, but is often around f/2.8 or f/4), as well as at another, medium-level aperture (such as f/8).
What do MTF charts measure?
MTF charts measure two things: resolution, and contrast. Resolution refers to the ability of a lens to resolve tiny details, whereas contrast refers to the ability of a lens to render rapid transitions from light to dark and back again.
Do MTF charts matter?
MTF charts matter to some degree; they do show whether a lens offers good contrast and resolution under controlled conditions. But there are factors that MTF charts don’t take into consideration. For instance, an MTF chart doesn’t show whether a lens performs well at alternative apertures. MTF charts also don’t indicate a lens’s struggles with chromatic aberration, nor does it indicate susceptibility to flare.
What is a good MTF chart value?
The higher, the better! In reality, lenses have optical problems, even the best ones. Which means that you’re never going to reach exceptionally high MTF numbers. If you find a lens with a value of 0.8 and above, then you can expect excellent performance, whereas a lens with a low MTF value of 0.3 will likely be soft.
Do MTF values change as focal length changes?
Yes! If a lens has multiple focal lengths, multiple MTF charts are required to indicate that lens’s optical prowess across its range. Lenses vary in terms of performance at different focal lengths, and this should be reflected in MTF charts. For instance, all-in-one zooms tend to perform poorly at both extremes, and supertelephoto zooms get worse as you reach the longer end of the range. Were you to measure either of these types of lenses, the MTF charts should get progressively worse as you go from, say, 200mm to 300mm.