Ok, so MTF is not an acronym for ‘Meet The Family’, ‘Mothers Thank Fathers’ or ‘Marry The Fucker’. Nope. None of those. It’s short for Modulation Transfer Function. Defining each word individually doesn’t even come close to the definition of the three words combined.
When I first took a look at a MTF chart, my left brown went up and I was completely clueless. Everywhere I looked, photographers were citing MTF charts as a way of grading lens capabilities. Ok, the idea made sense until I actually looked at one. The chart looked like a square with squiggly lines. There was an obvious X and Y axis points and 4 lines, two solid and two segmented.
As I read the ‘oohs’ and ‘aahs’ of posters, I thought, “What are they looking at? I don’t get it.”
Ok, so I asked Kayne (yet another question) to make sense of it for me. Here’s what he wrote:
What are MTF Charts… and how the heck do you read them?
First of all… MTF is an abbreviation for Modulation Transfer Function.
Light is… of course… altered or modified as it passes through a series of elements in a lens. An MTF Chart is a visual representation of this modification or altering of light as it passes through a lens.
A lenses quality is typically judged by its ability to sharply resolve fine details or its resolving power. This resolving power is dependent on a lenses abilty to accurately transmit contrast.
For example. Lets say we take a strip of white paper 1mm wide. And on that strip of paper we draw 50 black lines. The white spaces in between the black lines now become white lines. So in essence we now have 50 white lines and 50 black lines for a sum of 100 lines. Each of these lines is only .01mm thick.
Following me so far?
Now… is a certain lens going to be able to accurately capture these lines as seperate black and white lines? Or is the lens going to turn them into a fuzzy mess?
A lenses ability to accurately resolve the seperate black and white lines depends on its ability to transmit the contrast between the lines through its elements. The less the modulation(modification/alteration) of the contrast between the black and white lines will result in a sharper image. More modulation of the contrast between the black and white lines will… of course… result in a more fuzzy looking image.
Simply… the less modulation the better the resulting image is going to be.
Now… lets take a look at an MTF Chart…
For this we will use the MTF Chart for Dee’s new 50mm f/1.8 AF-D lens
The Y-axis or vertical axis of the chart goes from 0 to 1. These numbers represent a percentage from 0% to 100%. So a 0.7 value on the Y-axis means the lens can transfer 70% of the contrast in a scene through it.
The X-axis or horizontal axis is measured in millimeters and represents distance from the center of the frame/sensor. With 35mm film… the maximum distance from the center of the frame is about 21mm.
So knowing this you can now look at the chart and see that the lens is better/sharper in its center as the modulation is less(higher percentage value).
But what do those four lines on the graph represent?
Remember our little strip of paper only 1mm wide for this next part…
The top two red lines represent a test done with 10 lines per millimeter. This is a test of a lenses abilty to accurately reproduce contrast. The solid red line being Sagittal lines result and the dotted red line being the Merdional lines result.
The bottom two lines represent a test done with 30 lines per millimeter. This is a test of a lenses resolving power or resolution. The solid blue line being the Sagittal lines result and the dotted blue line being the Meridional lines result.
Sagittal and Meridional… there are those two words again…
This next image will show you what is meant by Sagittal and Meridional lines in a frame and how those results are measured…
So from that image we can see that Sagittal lines run parallel to a diagonal line that splits the frame from corner to corner. Meridional lines are at a 90 degree angle to the Sagittal lines.
Still with me?
I hope so because we’re almost done!
So we’ve got four lines…
One line representing Sagittal contrast.
One line representing Meridional contrast.
One line representing Sagittal resolution.
One line representing Meridional resolution.
With this new understanding take a look at the MTF Chart again.
The higher up the chart the solid and dotted red lines are… the better the contrast reproduction of the lens will be.
The higher up the chart the solid and dotted blue lines are… the better the resolution/resolving power and overall sharpness of the lens will be.
And finally an interesting note about these lines…
The closer together the lines are across the entire range of the chart… the better the bokeh of a lens will be.
And then he wrote:
How can you use these to compare them to other lenses…
Lets do that for a moment… please keep in mind that these MTF Charts are a representation of a test done with the aperture at f/1.4.
We’ll compare the AF 50mm f/1.4 D to the new AF-S 50mm f/1.4 G
The two MTF Charts… the old AF-D is on the left and the new AF-S G is on the right…
Right off the top with a quick glance one can see that the new AF-S f/1.4 is generally better in both contrast(the red solid and dotted lines) and resolution(the blue solid and dotted lines)than the old version of the lens… especially in the center of the frame/sensor(value 0 along the X-axis).
Note: The center of the frame/sensor will coincide with the center of the lens. As you move to the right along the X-axis you move out from the center of the frame and out towards the outer edge of the lens.
5mm out from the center of the frame/sensor the new 50mm is still about 10% better in transmitting both Sagittal and Meridional contrast. When it comes to resolution though… the old 50mm starts to catch up at this point. Meridional resolution is pretty much the same but Sagittal resolution in the old one is about 5% behind.
10mm out from the center of the frame/sensor the new 50mm is still better at transferring both Sagittal and Meridional contrast through it. At this point though the old 50mm beats the new by about 5% at transferring Meridional resolution but lags behind the new lens by about 7-8% when transferring Sagittal resolution.
15mm out from the center of the frame the new 50mm is better at transferring Meridional contrast by over 10%. Its also better with Sagittal contrast but only by a few percent. With Meridional resolution the new lens is almost a full 20% better than the old lens. But only better by a few percent with Sagittal resolution.
20mm out from the center of the frame the new 50mm is about 15% better at transferring Meridional contrast. But is actually a few percent worse with Sagittal contrast. The new 50mm is a few percent better with both Meridional and Sagittal resolution than the old lens.
Also with looking at the overall plotting of the lines… one can probably deduce that the new 50mm has a more pleasing bokeh… but it would be pretty close between the two.
Overall when comparing the charts one can see that the new 50mm is sharper and has better contrast than the old 50mm when shot wide open.
If I didn’t get it after this incredible explanation, I am never going to get it. And pretty pictures to boot!