If you’re like most photographers, sharpness is probably near the top of your list of desirable attributes in a lens. A lens that is sharp from corner to corner is often worth its weight in gold (or so the manufacturers tell us). How do we measure sharpness, though?
The last time you purchased a lens, you might have noticed a chart on the box that looked like a graph from math class. It likely had a series of dashed and solid black and blue lines (red and blue if you’re a Nikon shooter) meandering their way toward the bottom right corner. The graph is called the “MTF Chart,” which is short for “Modulation Transmission Function,” a measure of a lens’ ability to transmit light (keep in mind that glass is not totally transparent). These charts carry an incredible amount of data, including information on the sharpness and resolution of a lens at maximum aperture, at f/8 (if you’re a Canon shooter), contrast, astigmatism, chromatic aberrations, and even bokeh.
Normally though, photographers are interested in an MTF chart for its ability to convey the relative sharpness and resolution of a lens. It’s important to understand how the information is displayed. A perfect lens would be perfectly transparent; every photon of light that entered it would reach the sensor. Unfortunately, no lens is perfect, although many, including myself, would argue the unique way in which a lens loses light gives it its character. The loss of light is referred to as contrast modulation, since less light reaching the sensor means less of a distinction between light and dark, or less contrast. Contrast is fundamental to resolution; good contrast means well-defined edges, which means good resolution. Think of contrast’s effect on resolution this way: would you rather read a book with small black text on a white background, or small dark gray text on a light gray background?
Resolution is usually measured in lines per millimeter (l/mm) or line pairs per millimeter (lp/mm). Contrast modulation is typically measured at two resolutions: 10 lp/mm (low resolution) and 30 lp/mm (high resolution). Note that this is equivalent to 20 l/mm and 60 l/mm. Take a look at the MTF chart for the Canon 50mm f/1.8 II, shown below:
The thick lines represent modulation of 10 lp/mm pairs, while the thin lines represent modulation of 30 lp/mm pairs. Remember: fine lines represent fine resolution. Lens manufacturers include both resolutions because they represent different characteristics to our eyes; a lens with excellent 10 lp/mm performance will have excellent contrast reproduction, while a lens with stellar 30 lp/mm performance will have top-notch sharpness (keep in mind that this is a subjective term; it just so happens that 30 lp/mm represents a good level at which the human eye perceives sharpness). Additionally, the solid lines represent meridional transmission, while the dashed lines represent sagittal transmission. Meridional lines lie perpendicular to radial lines from the center of the frame, while sagittal lines lie parallel to them. The distance between corresponding sagittal and meridional lines can inform the quality of the bokeh of a lens. The closer the lines, the smoother the bokeh will generally be at a given aperture.
In the MTF chart above, you’ll also notice the numbers along the x and y axes. The numbers along the y-axis are decimal representations of the percentage of light transmission, while the numbers along the x-axis represent the distance from the center of the frame in millimeters (the corner of a 35 millimeter sensor is about 21.6 millimeters from the center). Lastly, the black lines represent performance at this lens’ maximum aperture of f/1.8, while the blue lines represent performance at f/8. An ideal lens would have 8 straight lines across the very top, representing 100% transmission of all light in all tested scenarios and thus, no loss of information.
Understanding a Lens
Let’s put this all together now to reaffirm what we expect of a typical lens. Lenses are typically sharpest, and have the best contrast, in the center of the frame, so we should expect all 8 lines in the graph to slope downward as we move left to right (outward from the center of the sensor), which they generally do. Next, we should expect lens performance to improve by stopping down, so all the blue lines (representing f/8) should be above the corresponding black lines (representing f/1.8), which they are.
Where MTF charts really become useful, however, is in comparing lenses. Let’s compare the telephoto end of Canon’s new 100-400mm f/4.5-5.6L IS II with the legendary 400mm f/2.8L IS II. Note that because telephoto lenses are generally better performers than wide-angle lenses, you should compare similar focal lengths.
The 100-400 f/4.5-5.6L IS II is no slouch of a lens; it has been met with many positive reviews. As can be seen by the thick black and blue lines (both solid and dashed) at the top, the lens is close to top-notch in contrast. In general, anything above .6 is considered passable, while above .8 is considered excellent. Thus, the 100-400mm is quite good across the entire frame in contrast. On the other hand, the fine lines (resolution) begin well in the center of the frame, but sagittal resolution drops off to a less stellar (but still respectable) level near the corners. As expected, stopping down helps (blue lines above black), but not as drastically as in the 50 f/1.8 II case. Remember that the difference between the blue and black lines in that case is over three stops, but only one stop in this case. On the other hand, the differences between solid lines and their corresponding dashed lines are more pronounced, particularly when compared to the 400 f/2.8L II, so we can expect bokeh to be a bit less smooth with the zoom lens.
As you can see, while the 100-400mm is a formidable lens, the 400mm prime is in a class of its own. Contrast and resolution are stellar across the frame wide open and only get better when stopped down. Bokeh can be expected to be stellar as well, which it is. Of course, we would expect such remarkable performance from one of Canon’s top super telephoto primes and its MTF chart confirms that.
MTF charts are a great way to quantify some of the most important attributes photographers seek in lenses. Learning to read them can give one the ability quickly and objectively compare lenses.