01 What is an MTF chart?

MTF stands for Modulation Transfer Function, or modulation transfer function. This function describes:The lens's ability to transfer the contrast of the actual object to the imaging plane..

Simply put, we can roughly understand it as the discrepancy between how a lens captures an image and how the actual object appears.

In the ideal scenario, the lens would transmit the contrast of the actual object onto the imaging plane with absolutely no distortion whatsoever. In this case, its transfer function curve would be a perfectly straight line at 100%.

Of course, in reality, lenses cannot achieve perfect performance due to various practical factors (such as chromatic aberration). As a result, this curve will always be less than 1, and...The farther away from the center, the faster the descent.. Even inThe MTF curves differ in different directions..

Therefore, typical MTF curves consist of line pairs in two directions—Radial S（Sagittal—spreading outward from the center and parallel to the radius—andTangential M（Meridional, which is perpendicular to the radius), with the two directions shown in the figure below:

The MTF chart is...Use line pairs at the same resolution to measure how contrast varies at different locations.And plot the relationship between contrast variation and distance, as shown in the figure below:

Let’s briefly review the MTF chart for now, and now let’s talk about how this data is obtained.

02 The origin of the MTF value

an MTF value measurement tool

In fact, the core definition of the MTF value has not yet been mentioned above.

The core definition of this thing is:The lens's ability to transmit contrast for a single spatial-frequency sinusoidal wave..

To measure this ability, a specialized tool is required.Sine Grating Test ChartPerform the test, as shown in the figure below:

It’s called a sine-grating test chart because its black-and-white variations follow the pattern of a sine wave, as shown in the figure below. Here are two sine gratings with different spatial frequencies:

However, in fact, the current mainstream approach for testing MTF is to useISO12233 Slanted Edge Test ChartThen, through mathematical transformation, an equivalent sinusoidal grating contrast curve is obtained.

The test card looks like this—just take a quick look, everyone.

How is the MTF value calculated?

Since this is just a brief introduction, we’ll continue to use the sine grating test chart as an example for testing.

During testing, the test card is placed at a specific distance and photographed using the lens (of course, the camera’s sensor capability must also meet the requirements). Then, the maximum and minimum brightness levels of the area to be tested are analyzed.

As we mentioned earlier, MTF measures the ability to transmit contrast, and calculating contrast requires the use of both maximum and minimum brightness levels:

MTF, on the other hand, is the ratio of the contrast before imaging (on the object side) to the contrast after imaging (on the image side):

Generally, for ease of testing, we set the object contrast to 1.

For example, in a sinusoidal grating test chart with a certain density, if we set the maximum brightness on the chart to 100 and the minimum brightness to 0, according to the formula above, we can calculate that the object’s contrast is 1.

In the image we captured, the brightest area has a value of 85, while the darkest area has a value of 15. According to the formula, we can calculate that the imaging contrast is 0.7.

Therefore, at this point, the MTF = 0.7/1 = 0.7 = 70%.

However, nowadays, the measurement method commonly used is primarily based on the diagonal test chart. This topic is rather complex to explain in detail, so I won't go into it further here.

03 MTF and Lens Selection

Finally, let’s talk about how to select lenses based on MTF charts.

As we’ve mentioned before, the MTF charts provided by lens manufacturers show the horizontal axis representing the distance from the center, and the vertical axis indicating how resolution varies. The solid and dashed lines represent the radial and tangential MTF, respectively.

To evaluate a lens using MTF, simply put, there are three criteria:

• The higher the curve, the better.:10Line-by-line comparison, 30-line resolution.The maximum aperture of an excellent lens starts at 10 lines with an MTF of 0.8, and can approach 0.9 at f/8; for a 30-line lens, the maximum aperture starts at 0.6.（Of course, the price is bound to skyrocket as well!

• The flatter the curve declines, the better.The horizontal axis of the MTF chart represents the distance from the center. A characteristic common to most lenses is...High image quality in the center, low image quality at the edges.. As reflected on the curve, it isThe curve is higher on the left and lower on the right.. CurveThe smoother and less steep the decline, the higher the consistency of edge image quality..

• The closer the S-line and M-line are to each other, the better.: The two lines refer to contrast in two directions,The closer the two lines are, the smaller the astigmatism and the more natural the blur.The more distinct the separation between the two lines, the easier it is for the edge blurring to appear harsh or even give rise to a “dual-line” effect.

In addition, when looking at MTF charts, you should also pay attention to a few points:

• A zoom lens may provide MTF plots at multiple focal lengths/apertures.It is important to note the curve patterns for different focal lengths and different apertures..Generally speaking, the MTF at the telephoto end is relatively poorer.The MTF at apertures around F4 to F8 will be better than the MTF at the maximum aperture (this is known as the "optimal aperture").If your budget is limited, you can focus primarily on the curve performance around apertures of F4 to F8.

• Analyze the MTF chart based on your actual needs and the camera body.With a large-format lens paired with a small-format camera body, the horizontal axis only covers half of the diagonal of the image format; when shooting portraits, you can focus on the center for higher image quality.

• MTF comparisons across brands may be difficult to directly compare due to differences in testing conditions.

  Overall, the MTF chart is not the sole criterion for determining a good lens, but it can serve as an objective starting point.

04 Summary

To sum it up briefly.

Looking at the MTF chart for comparison, an ideal lens would have a value of 1 across the board.

There are differences in the actual footage; consider it based on the curve.

Ten lines reflect contrast, while thirty reflect resolution.

The higher the curve, the more powerful it is—but of course, the price isn't low either.

High starting point means a good center; gentle descent leads to exquisite edges.

The S-line and M-line are not separated, with a soft, gentle blur that feels natural and not harsh.

Different needs call for different priorities—distinguish based on the actual situation.

Curves aren't the only option; they can also be used for reference.