Cropping images

Frequently we see images posted in this thread, e.g., of a bird, with a note - cropped about 50 (80, 40, whatever) percent. As I am trying to evaluate the quality of a lens, it would be interesting to understand the meaning of such a statement. Does it mean that you throw away about 50 percent of the area, e.g., from 20 mp you leave 10 mp size? Or does it mean that each dimension is cut in half, and that would mean that the remaining area of the shot is only a quarter of the original?

What is your understanding?

i'm curious as well...i usually only bring up 100% crops, and i think many are confused on what that is as well.

Lbsimon wrote in post #17495462
Frequently we see images posted in this thread, e.g., of a bird, with a note - cropped about 50 (80, 40, whatever) percent. As I am trying to evaluate the quality of a lens, it would be interesting to understand the meaning of such a statement. Does it mean that you throw away about 50 percent of the area, e.g., from 20 mp you leave 10 mp size? Or does it mean that each dimension is cut in half, and that would mean that the remaining area of the shot is only a quarter of the original?

What is your understanding?

There's absolutely no way to know unless the individual stating the crop value defines what he/she means. Different folks use different terminology to describe how much they are cropping an image. There was one fellow who I recall kept stating that he was cropping some percentage but when all was said and done, he had no clue why he'd been stating what he had been for numbers.

SkipD wrote in post #17495517
There's absolutely no way to know unless the individual stating the crop value defines what he/she means. Different folks use different terminology to describe how much they are cropping an image. There was one fellow who I recall kept stating that he was cropping some percentage but when all was said and done, he had no clue why he'd been stating what he had been for numbers.

Of course there is no way of knowing, and that's why I asked this question in the first place.

Lbsimon wrote in post #17495462
Frequently we see images posted in this thread, e.g., of a bird, with a note - cropped about 50 (80, 40, whatever) percent. As I am trying to evaluate the quality of a lens, it would be interesting to understand the meaning of such a statement. Does it mean that you throw away about 50 percent of the area, e.g., from 20 mp you leave 10 mp size? Or does it mean that each dimension is cut in half, and that would mean that the remaining area of the shot is only a quarter of the original?

What is your understanding?

Heya,

So the image is made up of pixels that create a surface area. Even if you said you cropped X% surface area, you're still able to convert that to number of pixels removed. Vise verse if talking about MP (a pixel count) removed, resulting in whatever surface area is left over. It's interchangeable because surface area is a term resulting from the calculation of the length & width of units, in our case, pixels are used for this. The same is true for resolution, since resolution is a result of number of pixels. Some people may say that they reduced their 5000 x 4000 (pixels) resolution image to 2500 x 2000 (pixels) and removed 50% of the pixels and could say that's a 50% crop in that context.

I like to use surface area in general as the context for my crops. If I say 50% crop, I'm saying I reduced the surface area by 50% roughly. This can be translated to a number of pixels which can then be reduced to a simple megapixel number for someone that needed to know.

Context is all that matters. But even in the context, it's all interchangeable because the bottom line is the number of pixels makes the megapixel count and defines the resolution (number of pixels in an area) which can also be called surface area. It all calculates from the pixel count.

Very best,

MalVeauX wrote in post #17495559
So the image is made up of pixels that create a surface area. Even if you said you cropped X% surface area, you're still able to convert that to number of pixels removed. Vise verse if talking about MP (a pixel count) removed, resulting in whatever surface area is left over. It's interchangeable because surface area is a term resulting from the calculation of the length & width of units, in our case, pixels are used for this. The same is true for resolution, since resolution is a result of number of pixels. Some people may say that they reduced their 5000 x 4000 (pixels) resolution image to 2500 x 2000 (pixels) and removed 50% of the pixels and could say that's a 50% crop in that context.

I like to use surface area in general as the context for my crops. If I say 50% crop, I'm saying I reduced the surface area by 50% roughly. This can be translated to a number of pixels which can then be reduced to a simple megapixel number for someone that needed to know.

Your first example would reduce the pixel count from 20,000,000 to 5,000,000, or 20MP to 5MP. That's not 50% is it? It in fact results in 25% of the original area and of course 25% of the original pixel count.

If you crop one image plane by 50% - a factor of 2 - the area is reduced by the square of that factor, in this case by 4. Providing aspect ratio is maintained, both planes have to be reduced by the same factor, hence the squaring of whatever that factor is. To reduce to 50% would be achieved by a reduction of approximately 1.4x in both image planes.

Most people would understand cropping an image to be the same as the calculation used for the crop factor of a sensor, i.e. the physical size, not the pixel count.