â˜…Photography questions about ppi/image sizeâ˜…PLZ HELP?

*** T i f f a n y *** · 04-21-2009, 07:48 PM

Hello.

My image should be...
1. Image should have a resolution of 72ppi.
2. Each image file must be 100KB no file sizes larger than 150KB.
3. Image physical dimension should be no larger than 4x6 inches and no smaller than 3x5 inches.
(These are the requirements in order to submit my image to my online photography teacher)

BUT when I made my image resolution to 72ppi and image physical dimension in to 4X5 inches, my image files becomes really small like about 20 KB !!! What should I do ???

I have both photoshop and XnView
Please explain in detail

THANK YOU

04-21-2009, 07:48 PM

This is one of the most confusing parts of digital photography.

1) the PPI setting doesn't matter. it really doesn't. Just ignore it.

2) the PIXEL DIMENSION of the photo is what matters.

You have been giving some confusing instructions
- 72 dpi/ppi
- 100k to 150k
- 3x5 to 4x6

Here is what it should have said:

"Size your image so that the short dimension is no smaller than 220 pixels, and the large dimension is no larger than about 430 pixels.

"When you save it, see what size file it makes. If it is smaller than 100k or larger then 150k, either change the "save" settings, or make the image smaller."

explanation: 3" x 5" at 72 ppi SHOULD BE STATED: 216 pixels by 360 pixels!!! The ppi DOES NOT MATTER - unless your have a printer with bad software.

Example: I can give you an image at 72ppi that is sharp enough for a billboard, and I can give you an image at 72ppi that is only 10pixels x 10 pixels - totally useless except for a dot on a page.

**mn8335** · 04-21-2009, 07:49 PM

There's no way to make a 4x6 photo in 72 ppi larger than 20 or 25KB. You're teacher is either ignorant or there's a typo in the instructions.

In Photoshop, set your crop tool to either 3x5 or 4x6 (whichever you choose) at 72 ppi. Then go to File --> Save for Web and Devices and save it that way. It gives you better clarity for web viewing than doing a straight "Save As."

Saving over and over as a JPG doesn't diminish the quality unless you are constantly re-cropping (like making a copy of a copy of a copy).

Your camera is already shooting in JPG (unless you are using a DSLR and on the RAW setting) so saving it as a PSD does nothing. (And you can't save it as a PSD and Save for Web and Devices. It automatically converts it into a JPG file.)

04-21-2009, 07:49 PM

Another way to look at the math is this.

At 72 ppi, one single square inch of image is about 5 k. (72 times 72 equals 5,184 pixels.

To make the image as LARGE as 150 k, you need about 30 square inches. This could be something like a five by six inch image, at 72 ppi. But since this exceeds your required dimensions, if you constrain your image to 4 by 6 inches, you will have a 24 square inch image. This will create a 124 kb image size.

Any combination of dimensions will work as long as you don't exceed about 28 square inches.

04-21-2009, 07:50 PM

try this;

forget about the ppi 72 setting. Isn't funny how stuff in school is always presented in a way that confuses something that is straight froward and simple. Go to photoshop. Open your picture under F: drive or however you access your saved pics.
Open. Select crop tool. Look at space bar a top six selections. You can set the crop tool dimensions, leave resolution at 300 for now. Save as. Find an image quality that correlates with the KB size you want. What I mean by this is as you move the curser (from 1-10) at the bottom beside "size" it will display the size at that selected setting. Adios.