How much cropping with an image from a DX camera take

[BRuke1]

I know there are so many variables that there's no definitive answer.
I'm just looking for a rule of thumb for what one can reasonably expect.
Can an image (highest resolution available) from a mid-priced Nikon DX sensor camera (e.g. a D3400 or D5600) hold up is cropped by half? How about if only 25% of the picture is retained?
I'm thinking of counting petals on a flower, reading the small text on a store front. Not the point at which it becomes absolutely pixelated.
(This is just for purposes of viewing on a normal size screen, PC or tablet, or printing 8X10. No posters or anything big.)

[Manfred M]

As you have already noted, there is no clear answer here and it depends a lot on your final use of the image.

If you are viewing the image on a standard computer screen with a resolution of 1920 x 1080 pixels, you only need around 2MP to fill the screen and both cameras are 24MP, so you can in theory throw out 22MP and the image would still be fine. Make a large sized print, and you are already pushing sharpness limits with the sensor size.

There are other factors in play as well; the sharpness of the image you are working with. If you shot from a very sturdy tripod or used a stabilized lens (VR in Nikon speak) and got a very sharp image image, you could get away with an aggressive crop, but it there is a lack of sharpness from motion blur or the focus being off a bit, you could not crop as much.

A very sharp pro lens (unlikely to be found sold with the camera models you mentioned) might give you more leeway, but a lower quality lens would see a less sharp image. If you shoot the lens fairly wide open or stopped down to f/16 or smaller aperture, diffraction softening would come into play. The "sweet spot" of most lenses is when they are stopped down 2 - 3 stops from wide open

If the crop was from the centre portion of the image, you could get more sharp material to crop than you would find near the edge of the lens, as resolution tends to drop off as you get closer to the edges. Zoom lenses are often a bit less sharp at either the short or long end of the zoom range, and that tends to be specific to the lens model you are using.

If everything in the image lies on the focus plane, the image will be sharper than when you get away from that (depth of field / circle of confusion constraints).

Shooting at the base ISO (lowest ISO your camera has; which is probably ISO 200 on those models) will give you more leeway than higher values, as you lose colour bit depth, dynamic range and get more digital noise as you increase that setting.

Those are just the issues that I can think of off the top of my head. There are too many variables for a rule of thumb to work. Some trial and error on your part is probably going to give you an acceptable answer.

[BRuke1]

Thanks. That helps a lot.

[Geoff F]

A lot of my insect photos go to an internet recording site where all images are resized to 720 pixels high and that is for identification checking. So, no matter how many pixels I begin with I crop and resize to end up at that size.

But I retain larger images in my personal records for printing.