In a perfect world, all camera lenses are perfect. They would allow for 100% of all light passing through it to reach the sensor, without any imperfections or loss in resolution. Sadly, this is not the case, as all-glass elements present limitations.
This leads us to ‘What is an MTF chart?’. Read on to find out what MTF charts are, and how you can read them to understand the optimal performance of your glass.
Common MTF Lens Chart Questions
What Does MTF Mean in Photography?
You might be asking ‘what does MTF stand for?’. MTF stands for Modulation Transfer Function. At its most basic, an MTF lens chart shows you the measurements of the optical performance of any given lens.
An MTF chart is a very handy way of seeing where your lens performs well or doesn’t perform well. What we are looking for is optimal focuses, contrast, or the resolution of the lens we are using or wanting to use.
The information comes in the form of a chart where each lens compares to what we would call a ‘perfect’ lens. A perfect lens is one that allows 100% of entering light to pass through it to the sensor. Here, contrast is essential, as it relates to the resolution of the lens.
Most photographers believe that everything comes down to sharpness. It is the most crucial thing they look for in a lens. However, there is more to lens quality than just sharpness.
Contrast and resolution are also important, yet not the same as a lens’ sharpness.
What Is Lens Contrast?
Contrast is a lens’ ability to distinguish between light and dark areas of a scene or subject. A perfect lens would be able to show a dark subject in front of a white background without any fuzziness.
What Is Lens Resolution?
The resolution of a lens is the measurement of a lens’ ability to reproduce and render the fine details perfectly. A high-resolution lens with low levels of contrast can produce low-quality images.
This is due to the difference between light and dark areas not rendering correctly.
A good quality lens has a balance of both contrast and resolution.
What Is MTF Chart for a Lens?
The MTF chart is the favored way of looking at lens performance. This scientific review looks at equations rather than public opinion. Even camera features or computer software can change the perceived performance of a lens.
This MTF chart or graph uses two axes.
The horizontal axis (x-axis) shows the distance from the edges of an image to the center. The ‘0’ in the bottom left-hand corner is the center of the lens. The other numbers represent the distance, given in millimeters.
For full-frame (36x24mm) cameras, the distance from the center to the extreme edge is 21.6mm. This is one-half of the full diagonal frame area. So, the bottom of the MTF chart for a full-frame camera runs from ‘0’ to ‘21.6’, otherwise known as image height.
For APS-C Canon (1.6x crop sensor), the distance from the center to the extreme edge is 22.4mm. APS-C Nikon (1.5x crop sensor), the distance is 23.7mm.
The vertical axis (y-axis) shows the transmission of light that travels through the lens. The largest value is 100%, shown as ‘1.0’. It is important to know that no lens can reach this, as glass is not 100% transparent.
If the point of the graph is closer to the value of ‘1’, it performs better.
When we look at contrast and sharpness/resolution, the center of the image is where these values are at their highest. It goes without saying that the edges are where the performance drops.
One of the biggest benefits of using an MTF chart is that all companies use it. Thus, making it easier to compare two similar lenses from different manufacturers.
What Is MTF Testing?
Camera manufacturers use MTF charts to see how their lenses capture and reproduce patterns, such as repeating fine lines. The better the performance of a lens, the better it can reproduce these fine lines.
The contrast level of a lens is how well the lens performs with broad, thick lines. The lines used are set at 10 lines per millimeter, which is big enough for optical testing. We are looking to have a clean break between these lines and the background.
The resolution of a lens is how well the lens looks at fine details. The lines used here are much thinner at 30 lines per millimeter. Here, we want a clear distinction between the lines.
If they start to blur together, it means the lens has reached its resolution limitation.
We look at lines because they represent and render differently depending on their orientation. You might think that a scene with diagonal lines will capture the same as one with parallel lines. You’d be wrong.
Diagonal lines, running from the center of an image to the corner are easier for a lens to reproduce. This is when compared to parallel ones. We use two different groups of diagonal lines on the chart. They are called Meridional and Sagittal lines.
Meridional lines (dotted) are the lines that run from the center to the left-hand corner of the frame.
Sagittal lines (full) are the lines that run from the center to the right-hand upper-most corner of the frame.
MTF chart’s present us with a lot of information in the form of lines. Both contrast and resolution show themselves through sagittal and meridional lines.
This information comes from a test of any given lens wide open. This means that the above data shows the performance of the lens at it’s widest aperture.
We can see that the contrast of this lens is very strong in the centre. It reaches 0.92 or thereabouts, for sagittal lines. It drops to 0.82 or thereabouts at the edge of the frame.
For meridional lines, it starts around 0.9 in the centre but ends with 0.69 at the edge of the frame.
The resolution of the same lens has a lower performance. They start at 0.85 and 0.79 at the centre and end at 0.55 and 0.41 at the edge of the frame respectively. It shows how the performance drops the further you get from the centre.
When we look at the performance of a lens, the closer to the top the data is, the better. While 0.6 is acceptable, lenses with high-performance will present around 0.8.
Bokeh is a very valuable feature in some lenses. Especially portrait or standard lenses with very wide apertures. It is the blurred effect you get from your background when shooting.
You can tell the quality of blur or ‘bokeh’ from an MTF chart. To see if a lens has a more natural-looking background blur, look at the distance between the sagittal and meridional lines. The closer, the better.
Differences in MFT Charts
Currently, some companies show the data from two different apertures – both at its widest and at f/8. Canon, however, revamped their data and now only shows the widest aperture.
Canon also uses black and blue lines to represent their data, where others use blue and red.
Benefits and Drawbacks of Using MFT Charts
Manufacturers use MFT charts to show how their lenses perform. However, they might use different methods, meaning you won’t get an accurate, comparable performance.
The same goes for when you compare two lenses with different focal lengths. This is especially true for a prime lens and zoom lens that present the same widest aperture. These lenses are built very differently, and there isn’t a great way to compare them.
These charts are useful as they show contrast and lens resolution, but they don’t show all the important aspects of the lens. Information concerning distortions or aberrations are not included. Plus, the charts look at one or two focus distances, and not the entire range.
There is something you need to think about when it comes to using a full-frame lens on a crop-sensor camera body. The x-axis of the lens will go to 21.6mm, but due to the crop factors, there is no point in looking at data beyond 14.2mm (13.5mm for Canon).
This is actually a benefit, as you’ll see, the extreme edges present the lowest performance. By cropping them out, the edges of your image will retain a higher contrast and resolution.
You can’t cross-reference lenses to others from other camera manufacturers. It might be even difficult to compare lenses from the same manufacturer as the aperture they test at is different.
Some MFT charts use the widest aperture, and others use a different f/stop.