question on resolution

theoretically we have an 8 megapixel camera with a 400mm lens and a 16 megapixel camera with a 200mm lens. if they both took the same picture from the same placement then cropped the 16mp/200mm to have same view as 8mp/400, would thier resolution be equal? consider both full frame, just so its not asked.

I'm sure there would be some minute differences, but generally speaking, yes.

EDIT: Wait I take that back. I may have spoke too soon. Going to do the proper math now. On second thought, I'm pretty sure you would need 4 times the resolution for your statement to be true. 80 percent sure.....75-80.

Uncropped you have SQR 2 times more resolution, cropped you have the same resolution as with the 8MP picture.

you would need a 32mp camera @ 200mm to get the same crop as an 8mp camera @ 400mm

Bamamike wrote in post #2505192
Uncropped you have SQR 2 times more resolution, cropped you have the same resolution as with the 8MP picture.

I'm not sure about that. If the 200mm picture is half the height and half the width of the 400mm picture, cropping the 16MP picture would only leave you with 4MP.

I think 4x the resolution is required, because the 200mm shot is double the width and double the height, which is 4x the area of the 400mm shot.

To crop a picture taken with a 200 mm lens to be the same as one taken with a 400 mm lens you would only use 1/2 of the height and 1/2 of the width of the total frame. That means you are only using 1/4 of the total area of the frame. If you started our with 16 mp and only used 1/4 of the frame you would end up with a 4 mp image. You would expect the resolution of the 400 mm on the 8 mp camera to be better because it has 8 mp compared to the 4 mp of the cropped image.

Incidentally, going from 8 mp to 16 mp does not double the resolution. Resolution is measured in line pairs per mm in the vertical, horizontal or diagonal direction. To double resolution you have to double it in both horizontal and vertical directions, so you need 4 times as many pixels. It would take a 32 mp camera to have double the resolution of an 8 mp camera. That is why as more and more pixels get packed onto sensors we see less and less improvement in resolution with each increase.