MTF Chart Interpretation

Msmoto

Modulation Transfer Function.....yikes, how confusing. So, I have produced a couple of images which could be helpful in the clearing up the confusion of exactly what these mean. My understanding, the charts measure the resolution and contrast by looking at thin (30 lines per mm) and thick (10 lines per mm) in orientations which are lined up with a radius from the center of the lens outward (sagittal) and lines which are tangential to a circle of coverage, i.e, at 90° to the sagittal lines (meridional).

The measurements are taken at varying distances from the center of the image, and distance from the center vs. resolution of the images on the chart used. The higher the resolution, the higher on the vertical axis the point shows up. This is highly simplified as the mathematics are complex.

In general, the thick (10 lines/mm) are a measurement of contrast and the thin (30 lines/mm) are more a measure of sharpness.

One caveat is to avoid comparing MTF charts for lenses which vary widely in focal length as this has no meaning. The usefulness is looking at perhaps wide angle lenses or telephoto, but not to attempt to compare the two.

How do we remember all this. Unfortunately MTF charts vary in how they show the data. Here are two examples which I have drawn, hopefully to help anyone who has an interest in this.


A different style MTF Chart:


Please...if there are errors or omissions in this post, help me out and correct me.

Here is an MTF chart from Nikon to see as an example:

MightyY

So what exactly do the numbers along the axes represent? All I know of them (or think I know) is you want the Y axis to be close to 1 and that the X axis represents distance from center? Is that right?

Ade

Yes the X axis is the distance from the center, measured in mm.

The MTF chart tells us how much contrast is lost (or retained) as we go from the center of the lens to the edge of the lens. In the above example, the contrast is measured at two different frequencies (red line vs. blue line), and along the diagonal of the frame vs. perpendicular to it (solid vs. dotted lines).

Aside from contrast, the higher frequency measurement (30 lines/mm) gives us some notion about the lens's ability to resolve small details (resolution).

In general the higher and straighter the MTF lines, the better the lens is.

Caveat: one cannot compare measurements from one manufacturer vs. another's. E.g., Nikon MTF charts are based on actual measurements while Canon MTF charts are based on theoretical calculations. There are pros & cons to both approaches.

Msmoto

@ Ade

Thanks, good points, especially the comparison issue. As noted in the two charts, one uses dotted for the meridional and the other uses dotted for the sagittal lines on the target.

BabaGanoush

I'm curious to know how many data points/measurements there are in Nikon's MTF curves for a typical lens. As far as I know, they never say how far apart in millimeters the measurements are spaced in their bench tests. How many copies of a given lens do they test? These curves are presumably averages, but for how many samples? What's the likely variance in performance on these tests? Anyone know?

PB_PM

I thought Nikon's charts were based on the designs (on a computer) rather than a physical model.

Ade

This article from Cambridge in Colour mentions:

A big problem with the MTF concept is that it's not standardized. Comparing different MTF charts can therefore be quite difficult, and in some cases even impossible. For example, MTF charts by Canon and Nikon cannot be directly compared, because the Canon uses theoretical calculations while Nikon uses measurements.

Then again they could be wrong.

Msmoto

I think the general indicators...the closeness of the sagittal and meridional lines, how much the lines fall off towards the edge, an idea of the contrast and resolution, these are all shown in a general way. Actual objective comparisons is probably not a valid use of the charts.

SquamishPhoto

All I know is that the new 800mm from Nikon is supposed to have the best MTF chart of any lens. Ever.

Me wanty.

Msmoto

Here it is Squamish, the MTF from the new 800mm f/5.6 Nikkor



This is simply amazing...

PB_PM

It is beautiful isn't it? No wonder it costs more than a small car...

SquamishPhoto

Its the last piece of the Fat Cat Trinity: 200mm f2, 400mm f2.8, 800mm f5.6

Start saving!

Golf007sd

@PB_PM You will get more out of 800mm 5.6 and it will surly hold its value far more than the car you speak of.

PB_PM

@Golf007sd No doubt about that. No plans, or budget for the 800, or a new car for that matter. A wide angle is on the list though.

Msmoto

Fat Cat Trinity...I love this....But, my understanding is...to be a real "fat Cat" one must have one of these..Zeiss 70-200mm T2.9 Compact Zoom CZ.2 Lens. Full frame coverage for only 20 large. I want to see the MTF chart on this lens....