When smartphones are video recording stuff, pretty much everything is in focus, where as "big" cameras don't do this? If I wanted to record something with my camera I will have to constantly adjust the focus. I know cameras like the Canon 4000D aren't built for video recording, but why is this?

  • \$\begingroup\$ Funny, that's the thing that made me buy a real camera in the first place, I hate that long depth of field in every shot, with no proper control over it. It's because they have a tiny sensor & aperture, but I'll let someone smarter than me put it into a real answer. btw, we don't deal in video on here, only stills photography, but I think the aspects in your question are a fit for both. \$\endgroup\$
    – Tetsujin
    Dec 26, 2023 at 15:43
  • \$\begingroup\$ I have 3 immediate thoughts after reading this question... The difference between things being in focus and out of focus is related to focus, aperture and depth of field. It is not related to whether the device is a smartphone or a dedicated camera. You are making false connections here. You need to research aperture and depth of field. Second, what about autofocus? You don't have to continually focus the camera, it could be able to autofocus. Thirdly, as Tetsujin mentions, often the photographer doesn't want "infinite" depth of field and everything appearing in focus. \$\endgroup\$
    – osullic
    Dec 26, 2023 at 16:24
  • 1
    \$\begingroup\$ Moderator note: while video is off-topic here, good answers to this question will be equally appropriate for still photography so I'm of the view it's on-topic. \$\endgroup\$
    – Philip Kendall
    Dec 26, 2023 at 16:30

3 Answers 3


This is a problem/feature known as "depth of field".

Without entering into details too much, for the same framing on the sensor and equivalent light gathering (ie, aperture) there is a lot more depth of field on a small sensor than on a big one (and of course the sensor on, a smartphone camera is a lot smaller than the one on a DSLR), and you can very often just set the focus to the hyperfocal distance so that everything is in focus.

Otherwise the shallow depth of field of cameras is also a useful feature:

  • In photography this is sought after ("bokeh"), and smartphone cameras go to great lengths to emulate it by software.
  • In videography this is also used. You will for instance find plenty of movies where something/someone "enters the frame" just by changing the focus (it is very common in horror movies), but even outside of horror movies this is used to isolate the subject from the background.

This said, on a camera you have a way to control it: aperture. If you have enough light (or ISO sensitivity) you can close the diaphragm (f/8 or higher) to get more depth of field, and you can also use the hyperfocal distance.


It's primarily due to the difference in entrance pupils; which is the size of the aperture as viewed through the objective element. Smaller diameter lenses have inherently smaller diameter objective elements, and inherently smaller apertures/entrance pupils.

That means that the light rays entering the lens have not diverged/spread as far, which therefore means the rays do not need to be converged as much in order to be in focus.

I have this drawing which describes the difference due to varying the aperture opening on a given lens; but it also holds true for a large diameter lens versus a small diameter lens (both at maximum aperture).

enter image description here

The drawing shows it as depth of focus (relative focus/focus tolerance) at the image plane. Which then correlates to the relative sharpness (depth of field) of the images/video when viewed.


Simple answer.

  • Cellphones have a wide-angle lens.
  • Have relatively small apertures.

If you put a wider-angle lens on your camera and use a smaller aperture "everything" will be in focus.

Offtopic: Arguably, most photographers want a bigger camera with different lenses and wider apertures so that not everything is in focus. Even cellphones need to fake that "out-of-focus" thing.


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