Is lens testing at chart distance really an accurate measure?

Started 2 months ago | Discussions thread
techjedi
OP techjedi Veteran Member • Posts: 3,961
Re: Is lens testing at chart distance really an accurate measure?

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".

What probably varies most focusing distance is probably field curvature and astigmatism. Just as an example, Hasselblad used to have three 'Planar' lenses for then 'V' series:

  • The 120/4 Macro Planar that was designed for close up work.
  • The 100/3 Planar that was designed for demanding distant work, like aerial photography.
  • The 80/2.8 Planar that may have been a compromise.

The 120/4 macro has a lot of field curvature at distance. That is something I have actually observed.

The 100/3.5 Planar has quite horrible astigmatism at close range, which I also have verified.

Zeiss designed a new 120/4 Apo Macro Planar for the Contax 645. That lens had a more complex design with a floating group.

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.

Hasselblad has an interesting article on evolution of lenses: https://static.hasselblad.com/2015/02/the_evolution_of_lenses.pdf

That article posts a comparison of MTF for the Planar 100/3.5 and the Hasselblad HC 100/2.2 at infinity:

Both these lenses are good, but probably not excellent by today's standard.

Looking at the same lenses at 1.2 m, a different picture arises:

Here the Zeiss CF 3.5/100 has astigmatism while the 2.2/100 HC maintains sharpness.

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

What a modern lens looks like?

This is Hasselblad's data for their new 90 mm lens. Almost certainly at infinity. These data are calculated, taking diffraction into account. The data is not measured from a len sample.

Best regards

Erik

Is a calculated MTF trustworthy?

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