Zoom to 100% not working properly

[bsomberg]

The Zoom feature in lightroom has started to behave strangely.  If I click on the 100% option, rather than zooming it reduces the image so that it fills approximately 50% of the workspace.  If I then select a lower zoom level (say, 50%), the image is reduced further.  I can only zoom by selecting a value such as 200% or higher, but it is hard to say what zoom level that actually corresponds to.

What's going on here?  Thanks.

[dj_paige]

What is the image size in pixels (width and height)?

If the image size is small, this sounds like correct behavior.

[JohanElzenga]

If you use a high resolution monitor like a 4K or 5K monitor, then you will normally set the resolution in MacOS System Settings to a lower one, because the menus and other interface items will be tiny if you don’t. So let’s assume that you set it to 2560 pixels wide. If MacOS would simply use this resolution, then all applications would draw their interface items for a monitor of 2560 pixels wide. The hardware resolution of you monitor obviously did not change however, so MacOS would have to upscale this to 3840 pixels and that means everything on screen would look soft. So what MacOS does is tell the apps that the resolution is 2 times 2560 pixels, so they will render their interface at 5120 pixels wide. Now MacOS can downscale this to 3840 pixels, which keeps it sharp. But that means that the apps think that your monitor is 5120 pixels wide!

Lightroom should obey this when drawing interface items, but ignore it when displaying images, but unfortunately it does not. This is a nuisance and fundamentally wrong. I have been in a fight with Adobe over this for many years, because it means that a “100%” view in Lightroom is not really 100% (one screen pixel is one image pixel) unless your monitor setting is either the native setting, or 50% of the native setting (so 1920 x 1080 pixels for a 4K monitor). In all other cases you think that you are viewing the image at 100%, but in reality you are still seeing a resampled image. Unfortunately, Adobe is unwilling (or unable) to solve this.

So depending on the size of your image, the "100%" view could indeed be smaller than 'Fit', even if your image is larger than the native screen resolution.

[Conrad C]

What are the width and height of the image, in pixels?

What are the width and height of the screen, in pixels?

100% really means 1:1, or one image pixel to one display pixel, so how large 100% actually looks depends on the difference between those two sets of dimensions. So we need to compare those two sets of dimensions to be able to tell you exactly how large 100% magnification should look on your display.

[JohanElzenga]

100% really means 1:1, or one image pixel to one display pixel

By @Conrad C

Yes, we need to know the size of the image. But as I just explained, 100% unfortunately does not mean 1:1 if you use a high res display on a Mac and set MacOS monitor resolution to anything other than the native resolution or 50% of the native resolution.