Circle of Confusion Explained, With Galaxy Phone Values

The circle of confusion is the largest a blurred point of light can be and still be seen as a point by the human eye. It is important in understanding depth of field, hyperfocal distance and other photographic concepts.

It also helps explain why even shots taken wide open on a Galaxy phone can still keep so much of the scene sharp. The following table compares the circle of confusion for cameras found on current Galaxy phones (with sensor sizes taken from GSMArena) with the commonly used value for full-frame cameras.

What the circle of confusion means

When a point source of light is in focus its image on the sensor will be a point. When it’s out of focus its image will be a disc and the further out of focus it is the larger that disc will be. There is a size of disc so small that it will appear to our eyes to be a point. This maximum size for this disc to still appear as a point is known as the circle of confusion.

The actual value we use for the circle of confusion is arbitrary and depends on how large the final image is and how close we view it. A typical starting point as described in the Wikipedia article on circle of confusion is that the final image is around 25cm wide and we view it from around 25cm away and the maximum size of the disc on this image is 0.2mm (this equates to a disc 1/1,250 of the width of the image). When scaled down to the sensor this leads to the commonly quoted value of the circle of confusion being the diagonal of the sensor divided by 1,500.

Full-frame camera and Galaxy sensors compared

The following table shows the rule applied to a full-frame camera and the sizes of sensors currently found in Galaxy phones. All sensor diagonals were calculated from pixel size and number of pixels information given by GSMArena, hence they are rounded numbers.

The smaller CoC (about 3.5 times smaller for the Galaxy main camera) means the phone camera must be more precisely focused to achieve sharp images. The smaller actual focal length needed to achieve the same framing increases DoF, and that effect is stronger.

From Wikipedia’s article on DoF, it turns out that for the same framing and f-stop, DoF is inversely proportional to CoC. For the same framing, DoF is proportional to f-stop.

Combining the two, we get that the equivalent full-frame aperture to get the same DoF as a phone camera is the phone camera’s f-stop multiplied by the crop factor (ratio of full-frame’s diagonal to the phone camera sensor’s diagonal).

That’s why Galaxy phones can keep a scene sharp from a few metres all the way to infinity, which means that landscapes will usually be in sharp focus with no effort on your part. This is also the reason why optical background blur on phones requires something in the foreground to focus on, and why Portrait mode applies the blur digitally. Our bokeh explainer covers that side of it.

Why cropping and 200MP change the answer

The diagonal-over-1,500 rule presupposes viewing the entire frame at a normal size. If you crop to half width and view at the same size, that’s equivalent to having made an enlargement twice as big, and blur circles that were within the permissible CoC before cropping will be magnified into visibility. In effect, your permissible CoC has been halved.

That’s because CoC is defined for images viewed at a “normal” size. It’s the same idea in pixel terms. The main camera sensor on the S26 Ultra is about 9.8mm across, so in 12MP mode there are 4,000 pixels in that direction, and each one is about 2.4 microns across. Your 0.008mm CoC is thus about three pixels across, which is why 12MP images appear acceptably sharp in line with expectations set by the definition of CoC. In 200MP mode each pixel is about 0.6 microns across, so that same 0.008mm CoC is about 14 pixels across, which you’ll find looks soft if you view the image at 100%. The standard isn’t a promise that 200MP images will be sharp at 100%, though!

Samsung’s own advice about blur in some areas of 200MP images is basically that focus behaviour is a normal optical characteristic, and depends on aperture, shooting distance and position of the subject. Our 12MP vs 50MP vs 200MP guide covers when the extra pixels are worth it.

You can demonstrate this yourself in a couple of minutes. Find a scene that includes something around a metre away, with a skyline in the background. Take a 12MP photo and another in the highest resolution offered by your Galaxy. Viewed full screen both will appear acceptably sharp front to back. Zoom to 100% and examine the distant end of the scene and the higher-resolution image will show the blur first, even though focus behaviour hasn’t changed.

Picking a CoC for your own sums

Apart from working out things like hyperfocal distance or depth of field manually you're unlikely to need to know this number. If you’re going to use it, follow these steps so your use of it is consistent.

1. 1

   Start from the sensor

   As each Galaxy phone camera features different sensor sizes, please refer to the table above for those on the S26 Ultra, S25 Ultra and S26.

2. 2

   Use diagonal / 1,500 for normal viewing

   This is great for pictures that are primarily going to be printed up to around A4 size or looked at on a computer or phone screen. In the case of the S26 Ultra’s main camera, it’s 0.008mm.

3. 3

   Halve it for crops and big prints

   Diagonal / 3,000 is probably best if you’re going to be viewing at 100%, making large prints or heavily cropping, and examining the results critically! It will give you 0.004mm circles of confusion for the same camera and double your hyperfocal distance.

4. 4

   Keep the units consistent

   Also bear in mind that, as DOFMaster's equations page cautions, both CoC and focal length should be in millimetres, and it's the lens' actual focal length that's of interest here, not its 35mm-equivalent.

Sharpness is also related to saliency (where people look first). A sharp area against softer surroundings can help define a focal point, and the CoC can tell you how large that area is.