A print portfolio, as the name suggests, consists of photographic prints. If you are trying to impress, this is still the way to go.
Common sizes for a print portfolio are A4 (or 8x10) and A3. You can present your portfolio as a collection of individual prints in clear sleeves or in a book (a folder) with some sort of ring binder mechanism.
If you go for individual prints, you need a portfolio box (or case) to store and present them, not just a scruffy, old cardboard box. You need to buy specialist archival quality clear sleeves. Do not use the plastic pockets with punch holes down one side. They are the wrong quality and will damage your prints over time. Besides, they look cheap.
Using the type of ring binder file you would find in an office instead of a proper portfolio file looks cheap. Still, you do not have to spend hundreds of pounds on a good portfolio folder.
Manufacturers of good budget portfolio folders include Kenro and Secol. Spend just a little bit more and you can afford an up-market folder from Panodia or Prat, my personal favourite.
Aim to have between twelve and thirty images in your portfolio. Select only the strongest images - you want quality, not quantity. Nobody looks at quantity, but everyone is impressed by quality.
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If you have already been published, include your tear sheets (so called, because a page has been torn out of a magazine or brochure).
Use only professional quality prints in your portfolio. A photograph printed on your desktop printer, using cheap paper and budget inks is simply not good enough. Neither are enlargements from the chemists' or a snapshot taken with your mobile phone.
Replace damaged or worn photographs with fresh copies.