Help me understand ppi from digital camera

Posted By
Oct 27, 2003
When I open a new photo image from my camera and look at the specs in Image> Resize Image, the file size is 11.2MB, the pixel dimensions are 2288 x 1712, the document size is 31.778 x 23.778 and the resolution is 72ppi. I assume that the 72ppi is related to screen resolution and making the image a manageable size, right?

The math makes sense:
31.778 x 72 = 2288
23.778 x 72 = 1712

Ok, now I want to resize this image to 6 x 4.49 (4.49 enters automatically when I enter 6 in width). The pixel dimensions change to
432 x 323 and the resolution stays at 72.

If I do the math, it tells me that to have the equivalent number of pixels as my original 2288 x 1712 image in a 6 x 4.49 image, the resolution should be approximately 381.3ppi. To get that 381.3ppi I divided 2288 by 6 (same result if you divide 1712 by 4.49).

So…if I change the resolution now for my 6 x 4.49 image from 72 to 381.3 I have an image that’s pretty darn close to the original and the file size is identical (11.2 MB).

You are probably wondering where the question in this post is, right? It helped me lots just writing it out! What I want to know is "why?"

Why is it when you resize the document size to 6" the ppi stays at 72? Why does it not change to 381.3?? Inquiring minds want to know. 🙂

Looking forward to some discussion on this!


MacBook Pro 16” Mockups 🔥

– in 4 materials (clay versions included)

– 12 scenes

– 48 MacBook Pro 16″ mockups

– 6000 x 4500 px

Oct 27, 2003

You are leaving Resample checked when you change your dimensions… if you un-check it, and then enter 6 for your width, both the height and resolution will adjust automatically.

Oct 27, 2003
Red face. Duh…I KNEW it would be something simple! So much for a discussion, eh? At least I know my math was right. 🙂


Master Retouching Hair

Learn how to rescue details, remove flyaways, add volume, and enhance the definition of hair in any photo. We break down every tool and technique in Photoshop to get picture-perfect hair, every time.

Related Discussion Topics

Nice and short text about related topics in discussion sections