Resizing a Passport Photo

Here in the UK, we've got new, strict rules on the sizes of passport photos.

The actual photo should be 35mm x 45mm

The face part of it should be between 29mm and 36mm (crown to chin-tip).

The question is, how can I do this in Photoshop? It would be nice to use the ruler tool to measure my face and let Photoshop resize using that as the criteria (i.e. resize the face, but let the rest of the image do what it wants).

Or do I simply trial and error it and keep using the ruler tool to check the size of the face after each resize?

Set up your document size as 35x45mm.

Pull guides in from the rulers to vertical center to align the nose on, and horizontal guides at 6mm and 39mm, leaving a 33mm space between for the face.

Save this document as a template.

When you get portraits to resize, copy them in as a layer and resize using the guides.