another dumb question

[Witch]

when i use my image editor, i always go into the scale image option and scale my images down, because if i dont, my computer locks up. since im no good with pixels and stuff, i use the inches option to scale. originally, my files are about 30X40 inches and i scale them down to about half that. my question is, after i get through editing, can i scale my image back up without losing any quality, if i havent saved it throughout my editing.

 

[Dwig]

...since im no good with pixels and stuff, i use the inches option to scale.

You need to get familiar with Pixels as your unit of measurement. This is the only image dimension that is "real". The other dimensions (inches, mm, ...) are "virtual" measurments.

...originally, my files are about 30X40 inches...

By itself, the Inch measurement is meaningless since there are no actual inches in a digital file. To have meaning you must also specify the pixels per inch. Again, the real dimensions of a digital image are measured in pixels, and only in pixels. To express the equivalent of the pixel size in inches you must provide the scaling factor, the number of pixels per inch (PPI). BTW, the PPI value is very often incorrectly labeled DPI (dots per inch), even in otherwise well written software and documentation. DPI is never correct when refering to digital images.

. my question is, after i get through editing, can i scale my image back up without losing any quality, if i havent saved it throughout my editing.

Your answer is "no". At least assuming that when you resize your image in inches you are keeping the same PPI value.

If you are starting with a 30"x45"x100ppi image and downsampling to 10"x15"x100ppi you are permanently lowering the image quality. A future upsample back to 30x45x100ppi will not recover the lost detail.

On the other hand, if you are changing the inch size and making a complimentary change in the PPI there would be no actual change in the pixels and no loss of detail (and your computer would probably still come to grief trying to work with the image). This type of change would be something like 30x45x100ppi changed to 10x15x300ppi; the two versions of the file would be the same size in megabytes and have the same number of pixels.

 

[KmH]

Witch,
Dwig is correct. Once you scale down photo detail information is lost. Scale up will either make the photo pixelated or blurry.

No, that isn't what Dwig said at all.

Dwig said, " On the other hand, if you are changing the inch size and making a complimentary change in the PPI there would be no actual change in the pixels and no loss of detail (and your computer would probably still come to grief trying to work with the image). This type of change would be something like 30x45x100ppi changed to 10x15x300ppi; the two versions of the file would be the same size in megabytes and have the same number of pixels. " emphisis added.