"Having enough pixels left" after cropping

Probably a question for "digital imaging for dummies," but what does it mean exactly to have enough pixels left? Does it just mean that you wouldn't need to enlarge an image to make it the largest size you'd want it to be? Or that it's at least a certain size that's left? Or something else? Thank you.

When you crop, you reduce the pixel dimensions of an image. If you start with a 4200 x 2800 pixel image and you crop heavily, you may end up with only 1800 x 1200 pixels (fewer megapixels). This would result in (for one example) not "having enough pixels left" to print very large.

Hi Barbara,

I'm wondering if this refers to a comment I made? I sometimes use that phraseology. All I mean by it is kinda what you're saying. For an example, let's say your total file size, number of pixels, whatever are enough to make an 11x14 print. If you crop 50% of the image file then without changing anything else, technically you could only print it at 5.5x7 (assuming the same print resolution.)

I sometimes say this because I have no idea what the file size is of the files folks are working from. So if your native file resolution allows you to make a 20x24" print without up-sizing, you could crop 20% and "still have enough pixels" to print a 16x20 without changing any pixes/resolution, etc. (other than the crop of course.)

Not sure if this makes sense.

Lon

Alan and Lon have covered the basic issue: the more pixels from the original capture you crop away the less pixels remain for making larger prints. Books/magazines typically want 300 ppi for reproduction, so an 8x10 in a magazine would need to be 2400x3000 pixels. You also need to keep in mind that photo editors who request your high res image may want to crop it to fit their editorial purposes and if you don't have a lot of pixels to send them they lose that freedom and may not be able to use your image at all.

But there's another reason to limit cropping that a simple count of pixels doesn't address: the effect cropping has on the lines per millimeter resolution delivered by the lens. The amount of detail that makes it to the final image is a function of the number of pixels you end up with plus the lens resolution. The more you crop the less resolution you get from the lens.

I can only speak for myself, my camera.
As long as the image quality & original sharpness is there, it's best if you stay at or above approx 3000x2000 on your crop..
I've made quality prints up to 16x24 from a 3000x2000 crop of the original. I just sold a 36x36 canvas wrap from an original square crop 2600x2600 Birds & Blooms winning dove image that looks great..
I use Genuine Fractals as my re-sampling software. After re-sampling the 8 bit file I save as JP. Open one time and save using a small amount of USM for print..

"The more you crop the less resolution you get from the lens."

Richard, I am very curious about this remark. How does cropping affect the native resolution (in line pairs/mm) of a lens? It seems to me that if a lens is capable of reproducing (say) 100 lp/mm and the sensor is capable of recording that amount of detail then the file would have that level of detail, cropped or not. If you then crop the image, the amount of detail has not changed. True?

Now, if one decides to resample the image, either up or down, then yes, the amount of detail (resolution in lp/mm) will change. This is a function of pixel count per mm and contrast between the line pairs.

I am not disputing your assertion, I am merely curious as to your rationale.

--P

Preston:

I've been sloppy in my statement, mostly because I'm having trouble expressing the point I was trying to make.

Yes, the native resolution of the lens stays the same - nothing done to the recoding medium can change that.

The point I was trying to articulate is that cropping effectively takes that lens resolution and spreads it over a smaller area, so the lens resolution get "diluted" and the image has less resolution.

I don't have any example images to prove this but I maintain that a 6MP uncropped original image will capture more fine detail than a 6MP crop from an 18MP original if both are taken with the same high quality lens.