There is a lot of discussion going on if the Xiaomi Yi has a focus issue.
In this blog post is a tutorial on how to fix it:
Updated: July 14th
At first I will describe how Matthias and I checked the focus of the camera and then I’ll give you some tips on how to check your camera and try to give an explanation in terms of optics.
We worked on a Z221 hardware model (the hardware version can be determined by the first 4 digits of the serial number). Find more info in this post here.
How to Fix it
At first you have to open your camera. Make sure you take out the battery of your camera first! We used a scalpel and a hard piece plastic similar to a pick to open up the camera.
Start at the lower left corner of your camera and peel it open like a fruit. Be careful not to break the connectors which tie the frontside to the body of the camera.
The lens is fixed with hot glue at 3 spots and removed them with the scalpel. Be careful that the loosened parts of the glue don’t fall into the camera. We used a piece of adhesive to protect the camera and additionally it helps as the glue sticked to it.
Once you removed the glue you are able to turn the lens which is mounted on a thread. On the piece of adhesive we marked a spot as well as on the lens itself to check the position we have changed.
Then we hooked up the camera on a TV set via the micro HDMI port to get the live view of the camera und turned the lens to see the results.
We set up a test picture with a moire pattern and some letters at some distances an checked the sharpness of the picture on the TV screen. More on which distance to focus later on.
Once we were satisfied with the result we glued the lens so it can’t move anymore. We used superglue and let it run down on a small plastic bar to hit the right spot. We didn’t have a hot glue with a fine nozzle, but you can use that as well.
Now you are done and as you can see in the picture below, we changed the lenses position by less than a mm.
Focus & Distance
I try to give you a short overview of some terms used in photography and optics. At the bottom you will find some wikipedia links if you want to get more information.
Focus & Terms
Precise focus is possible at only one distance; at that distance, a point object will produce a point image. – wikipedia
In optics, particularly as it relates to film and photography, depth of field (DOF), also called focus range or effective focus range, is the distance between the nearest and farthest objects in a scene that appear acceptably sharp in an image. – wikipedia
With your lens you can precisely focus at only one distance, but an area (DOF) before and after the distance will be acceptably sharp within the sharpness gradually decreases.
So what is acceptable sharp?
The criterion for the desired acceptable sharpness is specified through the circle of confusion (CoC) diameter limit. This criterion is the largest acceptable spot size diameter that an infinitesimal point is allowed to spread out to on the imaging medium (film, digital sensor, etc.). – wikipedia
Let’s also take a look at the term “Hyperfocal Distance”
In optics and photography, hyperfocal distance is a distance beyond which all objects can be brought into an “acceptable” focus. – wikipedia
Which means that from a certain point everything appears acceptably sharp.
So how can this point be defined and on which factors is it depending?
- focal length of the lens (for cameras usually specified in mm)
- lens aperture (specified in f-number)
- image sensor (shape and size)
The data for the Xiaomi Yi:
- focal length: 2.73 +-5%мм
- lens aperture: 2.8
- image sensor: 1/1.28″ – 1.12 μm × 1.12 μm
This is a guess as I only know it is a Sony Exmor R sensor, but don’t know the type. This is the closest found on the wikipedia list.
I don’t want to write pages about this topic, as there is much to discover on wikipedia for those who are interested. (links below)
To sum it up:
- From a certain point everything appears to be sharp in your photo/video
- This point is set because of the camera parts: lens & sensor.
To get the maximum depth of field you would set the focus to a point shortly before the hyperfocal distance so the DOF and the hyperfocal distance add up. That is most desired for a fixed lens camera.
I calculated this point (or tried to according to the info I got).
My result: When you focus your camera on a point approx. 50 cm away from the cam, your picture is sharp from about 25 cm to infinity.
If you set the focus within the hyperfocal distance you loose an (in this case rather short) area closer to the camera which will be a little more unsharp.
Which Distance to Check Focus?
It is very hard to adjust the lens precisely on this camera. You have to glue and fix it again and you have no distance references on the lens, just a live picture to check.
I would suggest choosing a distance you typically place objects in when shooting. I chose an area between 1 and 2m. As mentioned above precise focus is only possible at one point, everything else will be acceptably sharp within this hyperfocal distance.
So when I’m shooting I think this is a distance where I want to highlight objects.
Also to give you some relations: When you look at the distance marks of this 14mm Canon lens you can see that the area between 1m and infinity is very small und very little turns on the lens are necessary to get the right spot:
So precisely defining a spot within that area is also very hard to achieve and I prefer objects in 1-2m being sharper than objects around 50cm.
Also I can only estimate the hyperfocal point as I don’t have all the information needed.
How to check your camera?
If seen hundreds of photos on forums and Facebook where people ask if their camera is out of focus. Sure everyone want’s to get the best results.
But in most of the cases it can’t be determined by these photos provided. You’ll need very good lighting conditions and weather conditions. If the air is moist and hot the background will be blurry. Also there have to be objects in the picture which are positioned from very close up to very far.
Additionally when you upload a photo on an internet platform they are compressed in most cases which makes it even harder.
If you feel the camera is very slightly out of focus you should also consider: is it worth opening the camera and und unloosen the lens?
Furthermore when you are correcting the lens you need to check it live on your TV, so you have to check and decide if the result is getting better or not!
Of course it also depends on the quality of this screen!
So I hope you understand that it is very hard to give a remote diagnosis as most circumstances are unknown. As written above the hyperfocal distance of the camera covers a very large distance (about 25cm to infinity), you can fine-tune your camera, but you are doing it at your own risk.
I also don’t want to rule out another possible error. The camera has an aspheric glass lens. So I think if the center of the aspheric lens is not on the same axis of the center of the sensor you might get a distortion in your picture which would be interpreted as out of focus.
Also the Xiaomi Yi has a very wide angle lens (170°) so the edges might be distorted as well which could also lead to an out of focus conclusion.
I’m not an expert in optics, I use photo and video gear on a daily basis. So if some is more experienced in this field, I’m very happy to discuss this topic in the comment section below, as I’m very interested.
Special thanks to Matthias for taking the photos!
If you perform this on your camera, you’re doing it at your own risk!
Links from wikipedia: