What does 100% crop mean

I've read explanations of what 100% crop means before and am always left unsure. The first picture is of a screen grab from Photoshop. The second is of the finished picture. How much of a crop is it?

That is a 100% crop.
When you crop from an original size image, without resizing either one before or after, it is a 100% crop.

100% means in this context: "preserve original pixels". So you can indeed do a 33% crop to get a 100% crop

Ok, I have and EOS R,30 meg and like many things about it. Speed is not one of them. Upgrading to an EOS R6 at 20 meg seems to be a bitter pill. By this definition using "crop" to compare the two camera's seems meaningless. If I were to "zoom in" the same amount with the picture taken from an EOS R6 would the picture degrade by 1/3--20meg vs. 30 meg, assuming that both finished pictures were displayed at the same size?

JamesEP wrote in post #19169449
Ok, I have and EOS R,30 meg and like many things about it. Speed is not one of them. Upgrading to an EOS R6 at 20 meg seems to be a bitter pill. By this definition using "crop" to compare the two camera's seems meaningless. If I were to "zoom in" the same amount with the picture taken from an EOS R6 would the picture degrade by 1/3--20meg vs. 30 meg, assuming that both finished pictures were displayed at the same size?

100% crop is a widely accepted term and method for comparing output of any digital camera, 10 to 100 megapixels, too. You do not zoom the 100% crop images when comparing quality. It reveals pixel quality, after a zoomed crop you essentially compare resize quality.

In the context of web display, '100% crop' often means to display a section of the photo which easily fits within anyone's monitor (no matter how low in total resolution they have...like 1300 x 768!) where there is NO SOFTWARE decimation being done in their PC/phone to 'fit' your pixels within the resolution limitations of their monitor...that is, 100% of your image's pixels are displayed 1:1 on the viewer's monitor pixels.

To explain further, an R6 sensor has 3684 pixels vertically, but a 4k monitor only has 2160 pixels, so it is impossible to display the R6 image 'at 100%', at best (and ignoring any pixels being used by a photo application for user control display) the R6 image is on the monitor at 58% of actual size and 42% of them are 'decimated' by the display software. If you wanted to show every pixel in the photo (and not have software decimation of pixels), you would need to take a section of the image which occupies (for example) 1500H x 1000V pixels and no one's PC or tablet or phone would use software decimation in order to display the entire image with all its pixels on the device's monitor.

When taking a 100% crop from two cameras and comparing photo quality, one has to be very careful to understand that cropping is NOT simply removal of a section of pixels, one is also removing some of the resolution of the original lens as well!

Let us assume two cameras with same overall size sensor, but very different (mythical cameras) resolution... 2000 pixels vertical in Camera 1, and 20000 pixels in Camera 2. Let us begin by assuming the lens itself delivers 2000 line-pairs of detail resolution vertically.
By taking a 2000 pixel section from Camera 1, and comparing a 2000 pixel section from Camera 2, it seems to be comparing apples to apples, but it is NOT.
I am taking a 2000 line-pair image from Camera 1, but comparing only 1/10 of the 2000 line-pairs from Camera 2...and Camera 2 looks far worse than Camera 1 even though its pixels better resolve what the lens delivers.
Ergo, comparing 2000 pxiels vertically from both cameras is an unfair comparison that makes Camera 1 seem better.

It is the most misleading term. It actually has no literal meaning. Confused me for a year when I started reading this forum. In a nutshell, it means you've cropped a photo to a small enough size that when viewed on a monitor you are seeing it at actual size - 100 percent, get it. If, whenever you see that phrase, you replace "100 percent crop" with "a small enough chunk of the photo was cropped out of the full photo that when you view it on your monitor, you see that part of the photo at the full resolution of the original photo", then you'll have it. Somewhat less pithy that way I guess.

Very simply means the image you see has every one of its pixels mapped to an individual pixel on your display.

Depending on the resolution of your monitor, that final crop might fill your display or only occupy a part of your display, depending on its resolution.

If your display was 5472 horizontally, the R6 full image would be a 100% “crop” already, but wouldn’t be for another display that was only 1080 for example, that display would map each 5x5 grid from the image to a single display pixel.