How useful is a variable aperture on the new iPhone anyway?

The iPhone 18 Pro has been announced and the new big feature is an adjustable iris on the main camera. The range of apertures available runs from f/1.48 to f/4. I have seen content from a number of creators reacting to this product, the most favoured line being that the variable aperture is a gimmick, because it’s meaningless with such a small sensor. In this article I will compare its capabilities with larger format cameras as a route to deciding how sensible this feature is.

First, the effect of the aperture on depth of field. A while ago I wrote an article on the object field method of determining depth of field. It produces the same results as the more common image field calculation, but relies only on the geometry in front of the lens. The variables in play are aperture diameter (size of the entrance pupil), subject distance, angle of view and ‘disc of confusion’, expressed as a fraction of the angle of view. The focal length of the lens on the phone’s main camera is 7mm, giving at entrance pupil size at f/1.48 of 4.73mm, and 1.75mm at f/4. The angle of view is 84%. The sensor is a 1.28” type, with an image area of 10mm by 7.6 mm – not so small, almost exactly the same size as a 16mm cine frame.

The same angle of view would be given by a 24mm lens on a full-frame camera. In this case the 4.73mm entrance pupil would lead to an f-number of 5, and 14 for the 1.75mm closed aperture. The corresponding figures for a Micro Four Thirds system are 12mm focal length, f/2.5, and f/7.0.

So, this phone’s camera has the same depth of field characteristics as a 24mm lens on full-frame with an aperture range of f/5-14, or a 12mm lens on Micro Four Thirds with an f/2.5-7 range.

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