“What are MTF charts?”
MTF charts, what are they and how do I read them + once I do, how to I take that information to the field? At some point as a photographer you’re going to want to know this information to pin point your specific photographic needs that will match your lens with your talents for maximum quality output from both you and the lens. Congratulations, you’ve reached the point in your photography career where there are more important things to know about your lenses other than the widest f-stop, zoom capability, price, size, closest focusing distances, AF speed and Image Stabilization. MTF charts can indicate to the knowledgable reviewer some of the optical characteristics they can expect from a particular lens.
MTF stands for “Modulation Transfer Function”. The simple idea of the MTF chart is to measure the contrast and sharpness of the given lens and its resolution or quality in performance. All MTF charts have an X and Y axis.
Y axis (vertical axis) is represented from 0 to 1 in %.
“.1” = 10% & “1” = 100% (the closer to 1 the closer you are to a lens with perfect sharpness and contrast)
X axis (horizontal axis) is graphed from 0 to 20 in millimeters from the center of the lens outward of a 35mm image to the corner of the frame, which is approximately 21.5mm away. The farther the line reaches the right side of the chart, the better the performance of the lens to the edge of the frame.
At a first look at the MTF chart, the X & Y axis will tell you how sharp the lens is and how much coverage will be provided at different quality levels. Now I’m guessing you might want to know what all the lines mean as well. First look we have blue and black lines, thick, thin and dotted lines. They all mean something important and something very different. Below is the general legend to have next to you when reading an MTF chart:
Thin lines = High res/high sharpness
Thick lines = Low res/high contrast
Blue lines = F-8 (the “sweet spot”)
Black lines =Lens tested at widest possible aperture of the lens.
Solid lines = meridional (corner to corner)
Dotted lines = Sagittal (90 degrees to meridional)
With what we know so far we can conclude the following:
- The higher up the vertical axis the thin lines, the sharper or higher the quality.
- The higher up the vertical axis the thick lines, the higher contrast the lens will produce.
- Generally speaking, a lens whose thick lines are above .8 would be considered professional or of the best quality. Below .6 would be considered below normal quality standards for a professional and amateur.
- As well you can conclude that the closer the black and blue lines are to each other, the higher quality of the lens.
- The lesser the lines fall from 0 point of the y axis, the better the image quality is towards the edge.
Notes about contrast
Contrast is the ability of the lens to differentiate and render similar tonal values of smaller details. Resolution and contrast tend to go hand-in-hand as you can’t distinguish one with out the other. A sharper lens will improve the visual contrast, not the actual resolution of the lens.
What were talking about is the ability of the lens to differentiate between smaller and smaller details of more and more nearly similar tonal value. This is also referred to as “micro-contrast.”
If you’re in the market for a 50mm lens with a wide aperture then the term “bokeh” (pronounced boaken) should mean something to you. Its a word the Japanese created to describe the blur that lenses produce in the out of focus areas of the frame. The Canon 50mm 1.2 is said to have the best bokeh for a SLR 50mm of Canon’s three 50mm’s. With that said, if you’re looking for lens with pleasing bokeh, you want to find a lens whose MTF charts render the dashed lines closer together.
For a 1.6X cropped sensor, you can ignore everything beyond 13.5 mm. Further, anything beyond about 18 mm with a full frame sensor will only be visible in the extreme corners of the photo.
Blue Lines. These are most relevant for landscape photography, or other situations where you need to maximize depth of field and sharpness. They are also more useful for comparisons because blue lines are always to be at the same aperture: f/8.0.
Bold vs. Thin Lines. Bold lines describe the amount of “pop” or small-scale contrast, whereas thin lines describe finer details or resolution. Bold lines are often a priority since high values can mean that your images will have a more three dimensional look, similar to what happens when performing local contrast enhancement.
While MTF charts are an extremely powerful tool for describing the quality of a lens, they still have many limitations. In fact, an MTF chart says nothing about:
- Color quality and chromatic aberrations
- Image distortion
- Vignetting (light fall-off toward the edges of an image)
- Susceptibility to camera lens flare
These are my personal notes on my TS-E 90mm after reading the MTF chart (on left). I will often refer to them for a quick refresher before heading out on a shoot.
This lens is perfect all the way to the edge of the frame for high contrast @f-2.8 to f-8 and even further to f-11 shooting panoramic shots with excellent edge quality.
Feel free to shift in any direction to stitch wide open to f-22. (f-32 is max)
This would also do great on macro shots or portrait shots with excellent bokeh all the way to the end of the frame.
Something to take note of… every lens company has their own version of an MTF chart. They all give the same readings, but each company may look and reference the data differently. For ease of leaning what MTF charts do, I kept with one lens manufacturer, Canon. Check the lens manufacturer website for more information on their MTF charts.