Understanding "megapixels" and "DPI"

Fred wrote:

Why,in the media Photo section, can some fotos be enlarged and others can not?
Fred

Fred.. thank you for keeping us on our toes. And, before I get too deep into this.. remember "you asked". Warning: Technical Description(s) below.

The enlarging option has directly to do with how big the photo was when it was uploaded. To confuse the subject slightly.. is there are two size measurements... FILE size (in megabytes) and IMAGE size (in pixels).

If you had a picture that was 1024x1024 pixels, then that would be 1 megapixel, and at three bytes per pixel, the uncompressed picture woudld be 3 megabytes. Because JPG (jpeg) format is compressed, you will find that such images are usually much smaller than 3mb, depending upon the level of compression. Often, such pictures at 1024x1024 compress nicely down to about 0.5 megabytes with very little loss of clarity.

At ZCS, we have decided upon a current limit of ONE megabyte per (original) picture. That is not (directly) a limit on the number of pixels.. it's a limit of the number of bytes of disk space that have to be paid for each month. [note: on that subject.. we're doing fine on space.. we could currently handle somewhere around another 30,000 pictures, so please.. share!]

Now, to answer your question about enlarging:

In the gallery, there are three copies of every photo. There is the original, untouched image, the "normal view", and the thumbnail. When you upload a photo, it is stored without alteration in a folder of original photos. If the photo is over 800 pixels wide or tall, it is then shrunk down to a maximum of 800 pixels for the "normal" view. If the original photo is not that large, then it is just copied over as the "normal" view. A third copy of the picture is shrunk down to a maximum of 96 pixels wide (or tall) for the thumbnail photo.. that's why some thumbnails have blank space either above or beside them.

In the Gallery View, the "Normal" copy of the photo is shown, so it is at a maximum of 800 pixels wide (or tall). If you click on the magnifying glass, the webpage tries to display the original photo. If there is no magnifying glass shown, then you're seeing the photo at it's full size already.

Another issue with the "zoomed photo viewer" is.. it tries to display the photo to fit wihin your web browser window. Unfortunately, in certain cases that actually causes it to SHRINK the photo.

Here's a tip: right click the magnifying glass, and select "View link in new window". You'll get the full-size original image.

So.. to really answer your question "Why no enlargement" ... it 100% has to do with how large the photo was when it was uploaded to us.

Small tip for Fred: Your images would look a bit crisper if you could upload them at about double the size you are currently using. I don't know what software you're using, but the "large web" size is actually a bit small for what we can handle. if you can PICK a size.. shoot for that 1024pix size in max width or height.. those should look nice.