Comparisons - Bad vs Good
Here are some demonstrations some examples taken with two standard prime lenses that score quite differing levels of infrared hotspot - using the same scene. The two lenses were chosen using the results from these pages. Each two images were taken using the same settings (aperture, shutter & ISO) on the same camera (Sony A7) with the same filter (B+W 093 - 830nm).
False Colour / 590nm
This is not the best time of year to be shooting colour infrared examples. This is not the worst example by far. I just found a half decent composition and this is what came out. I would have liked foliage and/or blue skies & water for this to really shine: To show off why hotspots are so bad in colour IR. I'll get some better examples soon, but for now this will do. Since this is channel swapped it's the blue channel that originally has the hotspot. What it does show quite well is how far the hotspot destroys the colour separation throughout the frame. You'll notice that the sides of the canal on the bad example are yellow when they shouldn't be and this happens right into the corners.
Subtle Lighting & Exposure
Hotspot intensity is affected by the dynamic range of your scene, but that doesn't mean more subtle lighting won't cause one. Also note that lenses suffering from hotspots seem to produce darker exposures at the same settings. This has happened in 10/10 samples currently tested, perhaps confirming that hotspot is leaching light from the scene itself, rather than from outside the frame.
The Importance Of Detailed Testing
I like to shoot dramatic lighting in IR. Such as clear skies, or areas of shadow (which create a clean & dark canvas) surrounded by luminous sunlit foliage (see below). A hotspot here would easily damage this mix of bold and delicate tones. This is why such accurate ratings are so important to me when choosing a lens & I would like to share the information I have gathered to any other photographers who might share a passion for this imagery.
The following example was shot with the Sony/Zeiss FE 55mm f/1.8. It scored a very high rating (9.7) almost throughout the entire aperture range. Since this is also a very high quality lens it has been my main lens used for travelling and I have found it to be quite nice for landscape photography. I would of course love to have a zoom that was this sharp and free from hotspot, but the more I test lenses the more I feel that such a lens does not exist.
The test that I came up with to show (and rate) the effect had to be easily repeatable. As much as I would have liked to show real-world examples for this it would have proved too difficult to replicate the same lighting and background throughout the year and with various types of weather. To get accurate metrics I came up with an indoor scene that would show the hotspot as well as possible. I found a light that emitted a lot of light, something that was shaped in a way that I could fill the outer parts of the frame without covering the middle. I needed to be able to place it far enough away from the dark background that not much of the IR light reached it and I needed to be able to get far enough back from the subject (light) that I could test longer focal lengths (I can test up to 200mm reliably here).
Reasons For Poor Performance
There is no easy way to tell how a lens will perform in IR by it's price, size, complexity or normal (visible light) image quality. Assumptions based on age and speed can be used as a rough indication... Older and slower lenses are usually better, which could explain what's happening here, but unfortunately this cannot be relied upon.