Why is a pdf image so small?

[dogbone]

I was just doing a pdf test and I used a 25Mb psd image at 8 inches square at 300 ppi. I also had a jpeg version.

I opened two new InDesign document and imported both images (in separate docs) and saved them out as Press preset default pdf's.

Both were 1.6Mb. Then I realised I hadn't embedded the images so I embedded them and saved them out again and they were still 1.6Mb,

I opened one up in Photoshop and it asked me if I wanted to rasterise the file.

A couple of questions here: When Photoshop asks if I want to rasterise they file they aren't of course referring to the image are they because it is a bitmapped photo.

Secondly why is the file only 1.6Mb. How can this be.

 

[floriflee]

Are you sure it's saving the file as a bitmap inside the PDF?

 

[dogbone]

Well this is what I'm trying to understand.

If a normal photo is not bitmapped then what is it?

 

[balamw]

Well this is what I'm trying to understand.

I thought that by default PDF stored bitmaps in JPEG format. This would explain why the PSD and JPEG versions would be the same size.

B

 

[zarathustra]

Ok, to resurrect a dead horse:

The culprit is the Compression optioin of the PDF export. You have options to downsample in different ways, or not downsample, and you can also set compression, which is automatic by default (can be ZIP or JPEG). When you select the "Press" preset, it is bicubic downsample to 300 dpi, with automatic compression.

Now try to export the same document without downsampling, and without compressioin. I think you will se the difference quite clearly.

 

[dogbone]

Great, thanks for that, zarathustra. I'll check it out.