"Crop-ability" of images?

Started 3 months ago | Discussions thread
bobn2
bobn2 Forum Pro • Posts: 69,811
Re: "Crop-ability" of images?
6

dperez wrote:

More photographers who’s judgement I respect are talking about moving to Olympus. A question came up the other day and I’m trying to figure out the answer.

Nikon D850, 600mm f/4 versus M1X, 300mm f/4.

As I understand it - the lenses are the same equivalent - 600mm. The Nikon shoots at f/8 and ISO 3200 and 1/500. The M1X shoots at f/4 so the DOF is the same, and ISO 800 and 1/500, so the exposure is the same, right?

Wrong! Somewhere along the way you've picked up a wrong idea of what 'exposure' means. It does not mean how light or dark the image is. It means what is the light energy density at the sensor. It is determined by shutter speed, f-number and scene luminance. In your example, since the shutter speed and scene luminance are the same, but the f-number is two stops higher on the Nikon, it will have two stops less exposure. But it will make up for it by having four times the sensor area.

Go outside and shoot a backyard bird. Same spot, same distance, same bird, both cameras.

On EACH sensor does the bird take up approximately the same percentage of the sensor? On the D850 image the bird’s head covers about 20% of the width of the sensor. Does the bird’s head cover the approximately the SAME 20% of the sensor on the M1X – within reason since one is a 3:2 sensor and the other is a 4:3 sensor?

If SO, the bird’s head is 20% of 5184 = 1037 px on the M1X and 20% of 8256 = 1651 px on the D850. Based on that, the subject in the D850 image has 60% more pixels available for cropping. What am I missing?

The Nikon is a 45MP camera, the Olympus is a 20MP camera. That's about the long and short of it.

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