UK passport photo: 45×35 rules, both routes.

Printed photos must be 45 mm high by 35 mm wide (the standard UK photo booth size), with your head measuring 29 to 34 mm from crown to chin, and must not be a cut-down version of a larger picture. Online applications use a digital photo instead: at least 600 x 750 pixels and between 50 KB and 10 MB.

Taken in the last month. That is stricter than most countries (the US and Ireland allow 6 months), and you must supply a new photo with every new passport application even if your appearance has not changed.

For online applications, yes, officially: HM Passport Office lets you take the photo with your own device during the application, with someone helping you. Two honest caveats from the official guidance: booth or shop photos are 'more likely to be approved', and if you use your own device you upload the photo uncropped (head, shoulders, and upper body) because the system does the cropping for you.

Printed photos: plain cream or light grey, in clear contrast with you. Digital photos: any plain light-coloured background. Note the difference from the US, which wants white or off-white; a pure white background can lack contrast for very fair subjects under UK rules.

Plain expression with your mouth closed, eyes open and visible, facing forward, no hair in front of your eyes. Glasses should not be worn unless you have to; if you must, no sunglasses or tinted lenses, and no frames or glare covering your eyes. Head coverings are only allowed for religious or medical reasons.

Children must be alone in the picture, with no toys or dummies. Under 6, they do not need to look at the camera or hold a plain expression. Under 1, eyes may be closed, and you can support the head with your hand as long as the hand is not visible; lying the baby on a plain light-coloured sheet and shooting from above is the endorsed method.

Two ways. For the paper route, it crops your photo to the exact 45x35 spec with the 29 to 34 mm head guide overlaid and gives you a print-ready file for any photo counter. For the online route, use the guide to check your framing, expression, and background compliance before you upload the uncropped original in the application.

The usual causes are shadows on the face or background, poor contrast against the background, hair over the eyes, glare on glasses, and photos older than a month. Fixing the lighting and backdrop before you shoot beats any amount of editing, which is not allowed anyway.

Photos must be unmarked on both sides, except when your application requires countersigning; in that case, the countersignatory writes on the back of one photo as instructed in the application. Send the two photos loose, not attached to the form.