Is lens testing at chart distance really an accurate measure?

Started 2 months ago | Discussions thread
Bernard Delley Senior Member • Posts: 1,490
accuracy, distance, continuity
1

techjedi wrote:

Erik Kaffehr wrote:

Hi,

Lenses are generally designed for infinity, except 'macro lenses' that are designed for close up work. Many modern lenses use 'floating elements' to keep sharpness within the whole focusing range.

This makes sense to me in terms of design intent, but when I think of floating elements, I go back to the idea that zoom lenses are tested across their zoom range. I feel like if lens elements are moving, there are going to be spots that are sweeter than others. Perhaps its more reliable than I think and modern lenses really can be tested at one distance and extrapolate that performance to all other focus distances.

MTF tests on the optical bench are normally done at infinity.

So if you have a 12mm prime, how do you place a MTF chart at infinity?

When testing with charts it is usually regarded to be satisfactory to test at around 50 times the focal length distance. That needs a huge test chart, like 1.2x1.8 m for 24x36 mm. Also the test target needs to be near absolutely flat and the camera needs to be very well aligned with the chart.

Okay, so are you saying that a valid "infinity" focus test for a 12mm lens is having the chart 600mm away? That is less than the length of my arm and not what I would consider infinity focus? Forgive me if I misinterpreted the math there of "50 times focal length distance".

If you think of the point spread function and the MTF as a function of distance, it is clear that these functions are also functions of distance, which stirred up the OP question. These functions of distance must be continuous functions. It makes engineering sense to optimize lens performance such that performance stays reasonably high in the range of intended focus distances. The peak performance would probably happen at a finite distance.

The performance variations versus distance may be seen in comparison with performance variations across the image field. Such variations across the image field may easily amount to +- 0.1 [MTF] , as was shown in a previous thread, range of MTF variation . The image field MTF for a real lens is only within a range of what the radial MTF plots suggest.

One way to test with a small target is to have it at distance and move the camera so it is positioned in different places in the image, but that needs the camera being refocused, so that test ignores field curvature.

This makes sense. I was wondering if smaller charts were used like this, so thank you for confirming that.

In essence:

  • Test charts need to be large.
  • Small test charts are OK, if you plan to shoot objects of similar size.
  • The smaller the distance, the higher the need for precision.

I don't normally see MTF for two distances, are these from MFR or a testing site?

What a modern lens looks like?

Is a calculated MTF trustworthy?

Calculated MTF represent a design expectation. How the real lens with its manufacturing tolerances turns out, may be on a different page. It is certainly illuminating to measure, if you are interested in photographic science and technology.

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