Depth of field
The size of a camera’s sensor has a big impact on the depth of field in your photos: the smaller the sensor, the greater the depth of field at a given lens aperture and equivalent focal length. This is a good thing if you’re photographing landscapes, still lifes, macro shots or any other kind of photography where you want as much depth of field as possible. It’s not so good if you want to make your subject stand out by blurring the background. As a rule of thumb, DX cameras will effectively give you one f-stop more depth of field than FX ones at a given aperture, assuming you’re using an equivalent focal length to allow for the sensor’s ‘crop factor’ and to get the same angle of view
The larger sensor in FX-format Nikons like the D750 means that you need to use longer focal length lenses to get the same angle of view, and this produces shallower depth of field. In the days of film, when focusing was relatively primitive and photographers relied heavily on ‘zone focusing’, extra depth of field would have been a bonus. However, with today’s super-sharp lenses and pin-point focusing, photographers can properly exploit their cameras’ shallow depth of field effects, creative background blur and beautiful ‘bokeh’ – and FX Nikons can do this better than DX models.
The differences are subtle. In our sample shots, you have to look quite closely to see that the background in the picture from the D750 is more blurred. The important thing to remember is that depth of field is not an ‘on-off’ thing. Sharpness falls away progressively, not all at once. The difference between the D500 and D750 is not the difference between sharp and blurred, but degrees of blur. After a while, though, you do start to notice that this tends to give full-frame shots a bit more spatial ‘depth’.
This picture was taken at a focal length of 35mm (50mm equivalent on the D500) at an aperture of f/5.6. The lens was focused on the flowers, but you can see the detail on the newspaper print in the background is still relatively clear. You’d need a lens with a wider maximum aperture to blur it more than this.
This was taken from the same position at a focal length of 50mm on the D750. The lens aperture was the same at f/5.6, but you should be able to see that the print in the background is more blurred. This shallower depth of field comes from the longer focal lengths used to get the same angle of view on a full-frame camera.