BTW, Rienz...I'd be happy to explain how to use an MTF. It seems complicated, all those lines, but once you get the general idea, they are actually EXTREMELY informative, and it isn't all that difficult to understand.Hi Jon,
Please do ... I'd appreciate that very much ... and am sure others like me will also appreciate it very much.
Thanks in advance
Alright. Here goes. First, I'm just going to cover MTF charts. You don't actually need to know all the fundamental science that goes into resolving power to actually understand an MTF chart. All you really need to know is how to read the MTF chart, and that will tell you pretty much everything you need to know about the theoretical characteristics of a lens. Since most manufacturers publish MTF charts, and most use the same general standard (30lp/mm resolving power\sharpness & 10lp/mm contrast), they are pretty easy to compare brand to brand as well.
So, first off, an example MTF chart. This is the EF 24-70mm f/2.8 L II:
There are two charts here, because they represent the wide angle and narrow angle extremes of the zoom ratio. This one single chart tells you everything you need to know about a lens, and you can compare two MTF charts to each other to determine differences in each lenses performance (or, determine the differences between one extreme and the other of a zoom.)
So, first a breakdown of the chart itself. The MTF chart (absent any plot) represents the resolving power (sharpness and contrast) of a lens, from the center of the frame to the corner of the frame. The center of the frame is represented by the leftmost edge. The corner of the frame is represented by the rightmost edge. The vertical (y-axis) scale is an indication of how close to "ideal" resolving power gets. The vertical scale ranges from 0.0 to 1.0. In "historical" terms (and this really stems from the film days, so take it with the understanding that it definitely does not apply quite the same way today), MTF above 0.6 is "good/satisfactory" and MTF above 0.8 is "excellent/superior". Personally, I make the assumption, given how crisply modern digital sensors resolve detail, and the fact that people are increasingly publishing their photos online in full 1080p resolution, that MTF above 0.7 is "good/satisfactory" and above 0.9 is "excellent/superior".
Now for the plot. There are usually eight lines plotted on an MTF chart. These lines are blue and black (for Canon, other manufacturers may use other colors), thick and thin, solid and dashed. These lines represent how the lens reproduces sets of diagonal lines, the first set angled 45° and the second set 90° perpendicular to the first. These lines are called Sagittal and Meridional lines. They are angled at 45° and 135° because that is the ideal orientation to measure the behavior of optics from center to corner (vs. 0° and 90°, which would be more ideally suited to testing a lens center to edge...which is really insufficient.) The reason perpendicularly angled lines are used is because lenses do not behave the same when resolving detail at all angles...astigmatism in the lens design will often affect how lenses perform with fine detail angled differently across the lens. From the center to top right/bottom left corners, sagittal lines are angled parallel to the vector from the center to the corners of the lens. Meridional lines are angled perpendicular to the vector from center to top right/bottom left corners.
There are four sets of lines in total used in a standard MTF test. The first set are the 45° & perpendicular 10lp/mm lines. These lines are alternating white and black (technically speaking, the chart base is white, and there are thicker lines drawn at even spacing with thick white gaps between them, and thinner lines drawn at even spacing with thin white gaps between them.) The second set are 45° & perpendicular 30lp/mm lines. The 10lp/mm lines, which represent lower resolution detail, are used to measure lens contrast. The softer the transition between thick dark and thick white, the lower the contrast of the lens. The 30lp/mm lines, which represent higher resolution detail, are used to measure resolving power.
Can the black lines be resolved as fully separated with a white line in-between (resolved, high contrast)? If parallel black lines are separated, how quickly does the white line in-between become fully white (sharpness)?
So, we have an MTF chart that represents resolving power (y-axis) from center to corner of a lens (x-axis). This chart has eight curves plotted on it, that represent four sets of sagittal and meridional lines on a test chart. Why eight lines, rather than four? The MTF chart contains plots for both wide-open (max aperture) performance as well as f/8 performance. So, four sets of lines for max aperture:
- f/2.8 sagittal thick solid (10lp/mm)
- f/2.8 meridional thick dashed (10lp/mm)
- f/2.8 sagittal thin solid (30lp/mm)
- f/2.8 meridional thin dashed (30lp/mm)
- f/8 sagittal thick solid (10lp/mm)
- f/8 meridional thick dashed (10lp/mm)
- f/8 sagittal thin solid (30lp/mm)
- f/8 meridional thin dashed (30lp/mm)
Now, onto interpreting an MTF chart.
Using these eight curves plotted on an MTF chart, you can derive everything you need to know about the lens. The higher up the chart they are plotted, the better the contrast and resolution. You will normally see that as the curves move from the left edge to the right edge of the chart, they tend to "fall off", they drop lower. This is an indication of how the lens' performance chances from center to corner. You will also notice that one set of curves, either the solid curves or the dashed curves , tend to perform better than the other. This is an indication of astigmatism in the lens...sagittal curves (solid) may maintain higher performance than meridional curves (dashed).
This so happens to be exactly the case with the 24-70mm lens. At both wide (24mm) and narrow (70mm), the 24-70 resolves sagittal lines (45° angle) better than meridional lines (90° angle), and the meridional curves tend to fall off quicker in the midframe to the corners than sagittal curves do. Sometimes you may notice that some of these curves don't have a consistent falloff, they may turn "bump", resulting in slightly better resolving power just past midframe, then fall off again all the way into the corner. The nature of each curves falloff is an indication of how the various optical aberrations affect a lens' performance from center to corner. Depending on exactly what aberrations a lens may suffer from and to what degree, the nature of the curves' falloff will differ. Astigmatism and other imperfections in lens manufacture will result in falloff differing even from sagittal to meridional lines.
You will also find, once you start reading MTF charts, that wide angle lenses tend to have more chaotic late midframe and corner performance than normal and longer lenses. Telephoto/supertelephoto lenses will often have nearly flat MTF curves from center to corner (especially if they are higher end). It is not unusual to see crazy meridional performance in lenses as you approach the corners in wider angle lenses, especially ultrawide to wide angle zooms. There are simply certain compromises that must be made in order to produce a wide angle zoom lens that performs acceptably at as many focal lengths as possible.
That's pretty much it. A crash course in reading and understanding MTF charts. There is a lot more theory that builds up to WHY these kinds of tests are used to accurately measure lens resolving power, but you don't necessarily need to understand all that underlying grit in order to effectively use MTF charts themselves. All they really are is a plot of four types of detail from the center to the corner of the lens. Those plots measure contrast and resolution (sharpness & acutance) across the surface of the lens, and offer a fairly precise indication of how optical aberrations will affect your IQ in different regions of your frame.