Photo sizes and adobe software

Question

Hi
I'm attempting to make my first web banner in illustrator.
I have a photo that I bought of Istockphoto, it is 851 x 546 px
I created a new document in Illustrator 800 x 400 px.
I then opened up the photo in illustrator and it opened in a new document in illustrator. It didn't look as large as the artboard in my banner document.
I then copy and pasted the photo onto the banner document. I would have though that because the photo is 800 pixels across that it would be the same width as my artboard in the banner document, but it's much smaller.
can anyone explain this to me. I'm new to digital design.
David.

Answer 1

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AI works based on a document resolution setting which defaults to 300 DPI and will resample any other artwork that has a different DPI.
Illustrator does not resample a raster image to the Document Raster Effects setting when you "open" it.
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can anyone explain this to me. I'm new to digital design.
David,
"Pixels" as a unit of measure in an Illustrator document is completely bogus. It does not really mean pixels. It means points, or 1/72 inch. Illustrator is an object-based program. It can contain any number or combination of text objects, vector objects, or raster objects in the same document. Each raster image in an Illustrator document has its own PPI.
So, for example, you can have a 300 ppi image that measures 300 "pixels" (actually points) in width according to the rulers and a 72 ppi image that measrues 300 "pixels" (actually points) in width according to the rulers in the same document. In that case, the rulers will indicate that both of those two images "measure 300 pixels" in width. That's ridiculous. One has many more pixels within the supposed "300 pixels width" than the other.
If you then export that document as a single raster image, then everything on the page will be rasterized to whatever ppi you specify in your export settings. Thus, any raster images contained in the document would be re-rasterized (resampled) in the exported result.
So you can't think of the "pixels" indicated on Illustrator's rulers as indicators of the actual number of pixels contained in a raster image on the page. A pixel is not a unit of linear measure because a pixel can be any size you scale it to. The inclusion of "pixels" as a unit of ruler measure in Illustrator is just a confusing "convenience" for those who want to think in terms of the pixels in the final exported raster image.
JET