slipbyu2 was wondering if I use MTF resolution charts in deciding on a lens.
Let’s start with defining what the heck an MTF chart is.
If you don’t know, an MTF chart is a somewhat standard way to measure the optical performance of a lens.
It specifically measures the contrast and resolution performance of a lens.
Now, I’m simplifying this, but what an MTF test does is analyze a pattern of fine lines as seen by the lens at different points on the lens.
The pattern looks like this:
The lines are placed at different angles to measure how the lens performs along the different axes, Because a lens could have good performance along one axis, and poor performance along the other.
The lines that point towards the center of the lens are called the Sagittal lines, and the lines that point in the other direction are called the Meridonial lines.
Now a perfect lens would show line pairs with no blurring between the black and white lines, which would look like this.
However perfect lenses don’t exist, so there will always be some blurring or transition.
How much blurring there is is an indication of how good or bad the lens performs.
The results of that test are then graphed onto a chart that looks like this.
The X axis of this chart indicates how far from the center of the lens the measurement was taken at.
So zero indicates the center of the lens, and 20 indicates a distance of 20mm from the center of the lens.
The Y axis indicates the amount of light the lens can transmit, indicating its performance.
It starts at zero to indicate no light transmission and ends at 1 indicating perfect light transmission.
And the graph will show at least two lines, one for contrast, and one for resolution.
So for a perfect lens, the chart would look like this:
But because no lens is perfect, a typical chart looks more like this:
In this example, the orange line indicates the contrast performance, and the green line indicates the resolution performance.
And what this tells us is that at the center of the lens, the contrast performance is at .9, and the resolution is even better at .95
As you move out from the center of the lens to 5mm the contrast performance drops off to .8 and stays at .8 at 10 and 15mm, and then at 20mm from center it drops off dramatically to .4
The Resolution performance is similar, dropping to .7 at 5mm and staying there out to 15mm before dropping to .3 at 20mm.
So looking at this chart you could say that this lens generally has pretty great contrast and resolution performance in the center and outwards, but performs poorly at the edges.
So that’s the basics of how you read a chart, but things quickly start to get complicated for a couple of reasons.
First, remember the line pattern has the sagittal and meridonial lines arranged in two different directions, so a chart will typically show you a solid line to indicate the performance on the Sagittal lines,
and a dashed line to indicate performance on the Meridonial lines.
In which case the chart would actually look like this:
So each orange line indicates contrast performance, one for each axis, and each green line indicates resolution performance, again one for each axis.
Now remember I said this was a somewhat standard way to measure lenses, and that’s absolutely true, because each manufacturer uses their own testing methods and publishes their own unique MTF charts.
This means if you want to compare a Canon lens with a Nikon lens, the MTF charts are useless.
Now if you’re comparing two Canon lenses or two Nikon lenses, then you can compare the MTFs, but if you wanted to compare say the sigma seventy to two hundred mm lens to the Canon seventy to 200, the MTF charts would be useless.
Now, there are third parties that do standardized MTF tests on lenses, and those charts can be useful in comparing two different lenses from two manufacturers,
but to answer slipbyu2’s question, I’ve never once looked at an MTF charts when buying a lens.
First, I just don’t care what some chart says about the lens.
Generally speaking, if you’re buying a quality lens, it’s going to be sharp with good contrast and resolution.
And because no lens is perfect, pretty much every MTF chart is going to give you some version of, “this lens performs well at the center and loses quality at the edges”.
Second, the MTF chart tells you nothing of the subjective performance of the lens, which can only be evaluated by you with photos you took using your your camera.
So when I buy a lens, the first thing I do is get out there and take some damn photos with it, and then I spend a little time looking at those photos to see how I like the lens.
If I don’t like how they look, I send the lens back.
Third, MTF charts are a bit like navel gazing in the photography world.
I know some people love their MTF charts and DXO mark scores and all of that crap, but I just don’t care.
Put the camera and lens in my hand and because I’d rather be taking some damn photos.