Differential Focus

Differential Focus

The effect is used to isolate a sharply focused subject against an out of focus background (or foreground).It's a good reason to buy a wide aperture lens.

Following on from my experiments with depth of field, the natural extension to that is differential focus. What is it, why is it useful why it's harder to achieve with smaller format sensors and phones than larger formats such as full frame and medium format, and easier with wide aperture lenses than kit lenses.


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So you may think you don't need a wide aperture lens what with the good to excellent high ISO performance of sensors these days. That's a fair point, and with a lot of photography done using mid to smaller apertures the point of using f/2.8 or wider may be questionable. The size and price may be off-putting factors too as wide aperture lenses tend not to be a bank balance's best friend.

Smaller maximum aperture lenses (such as kit zooms that are f/5.6 wide open at the long end) are also less of an issue with mirrorless cameras where the viewfinder compensates for the less amount flight entering the camera. Conventional systems in DSLRs do go darker – not massively so but some may find difficulty. If shutter speeds become slow, then where you have image stabilisation in the lens and/or camera body you may feel safe. There is a caveat ad it's in the name of the feature – it's just the image that's stabilised, not the subject. There are creative opportunities there of course, but it's sometimes hard to explain to a novice that although they've got a perfectly hand-holdable set-up at 1/15 s their subject is blurred beyond reasonable recognition. Wider apertures will let you go on shooting as conditions get more challenging.




That's it from a practical point of view. As a creative tool though, if wide apertures give the effect you want then you need to get one of these lenses. Software manipulation doesn't come close, I'm afraid. You do need to be accurate with your focus, as there's little or no room for error. An image that isn't focussed strongly on the main subject just fails.

Forget the maths, experience shows it boils down to the size (diameter) of the glass, so I you don't have an 85 f/1.2 then a 200 mm lens at f/2.8 would do (the diameter is similar). But for an even stronger effect try a 300 mm f/2.8 for portraits (the diameter is half as big again). Unconventional but give it a go.




Differential focus is used for creative effect in TV and cinema. Indeed, some sequences have been shot on cameras like the Canon 5D Mk III purely to make use of the effect. That's why it's useful to use full frame cameras for video.




There are many more exciting fast aperture lenses being produced for mirrorless systems, which is good to see. It's true they can be expensive though some of the manual focus options are more wallet friendly. There is some good news hoverer, and that's because of the plethora of adaptors available that many older lenses can be used. Get in there now for a bargain before everyone susses this one.


All text and images © Keith Rowley 2021