What is the aspect ratio of my camera?
The aspect ratio of any camera is the proportion between the horizontal size and the vertical size of the photos that camera can take.
A camera that makes photos that are 2560 pixel wide and 1920 pixel high has an aspect ratio of "4:3" because 2560 divided by 1920 is the same as 4 divided by 3 (i.e. 4/3). The same aspect ratio will apply to cameras that take photos with the following sizes: 2048x1536, 1600x1200, etc.
A camera that takes photos whose size is 3008x2000 will have an aspect ratio of "3:2" because 3008 divided by 2000 is equal to 3 divided by 2 (i.e. 3/2). The same aspect ratio will apply also to cameras that take photos with the following sizes: 2240x1488, 1504x1000, etc.
This means that photos taken with a "4:3" aspect ratio camera will be wider than photos taken with a "3:2" aspect ratio camera having the same height.
To compute the aspect ratio of your camera, take a photo you have shot with that camera, measure its width and its height in pixel, and then compute their proportion (width / height).

Why does the grid depend on the aspect ratio of the camera?
The grid to be used depends on the aspect ratio of the camera because the calibration process is based on the behavior of the camera and its lens on the entire photo. The biggest distortion takes place near the edges of the photo, and that is where the program has to make the most significant corrections. The calibration grid must, therefore, have the same aspect ratio as the camera, so it can fit in neatly and cover the whole photo. It is important that the grid points reach the edges of the picture to guarantee the best calibration process.

What grid do I have to use to calibrate my camera?
There are two different calibration grids available for printing in the calibration wizard. They differ in their aspect ratio, and you have to choose the one that has the same aspect ratio as the camera you are using.
One grid has an aspect ratio of "4:3", it is based on a grid of 13x10 points, which means 12 horizontal intervals and 9 vertical ones. The other available grid has an aspect ratio of "3:2", it is based on a 16x11 grid with 15 horizontal intervals and 10 vertical intervals between the points.

The grid inside the photo is not at focus, what can I do?
Some camera may have difficulty in focusing an image at a short distance and with scarce light, as is often the case of the calibration grid.
You can try to print a larger image of the grid (e.g. if you have printed it onto an A4 page, you could try with an A3 page), so you can place it further from the camera.
If the problem is the illumination, try to place some additional light sources or, if the weather is good enough, take the photo outside. If you are using artificial light source, remember to set the white balance of your camera accordingly.
Another reason why the photo of the grid may not be at focus is the movement of the camera during the shot. You could try to use a tripod to avoid any movement.

The calibration process stops because LensTweaker detects a wrong number of points, the calibration grid does not match the photo aspect ratio.
According to the photo you are processing, LensTweaker will expect to receive the appropriate grid. If you are working on a photo with a "4:3" aspect ratio, the program will assume to find the 13x10 grid, therefore it will look for 130 different points. If you provide the 16x11 grid, LensTweaker will detect too many points and this will result in a stop of the calibration process.
In a similar way, if you are processing a photo with an aspect ratio of "3:2" and you provide LensTweaker with the 13x10 grid, the program will detect only 130 points when it needs 176 and the calibration process will be stopped.

LensTweaker does not detect some of the grid points.
There are several reasons why a point of the grid may not be detected by LensTweaker:
• it may be too close to the edge of the photo, or even overlapping it;
• it may not have enough contrast with the background;
• there may be shadows or dark spots near it;
• that the grid was too far away when you photographed it.
First check that the paper on which the grid is printed is clean enough, this means that any shadow or dark spot has to be avoided. If that is not the case you may need to print the grid again. Then try to take another photo of the grid, taking care that all points come in the area of the photo and that the distance is not too big. Be also careful to avoid any shadow on the paper grid when you take the photo of it.

LensTweaker detects some points that do not belong to the grid.
If there are some dark spots on the grid photo, the point detection algorithm may recognize them as grid points. First check that the paper on which the grid is printed is clean enough, this means that any shadow or dark spot has to be avoided. If that is not the case you may need to print the grid again. Then try to take another photo of the grid, taking care that all points come in the area of the photo and that the distance is not too big. Be also careful to avoid any shadow on the paper grid when you take the photo of it.

What happens if some of the grid points are not in the grid photo?
If some of the grid points are not inside the photo, LensTweaker can not detect the grid correctly and the calibration process cannot take place because of the wrong number of recognized grid points.

What happens if in the grid photo there are objects that are not part of the calibration grid?
In some cases background objects may appear in the grid photo. LensTweaker could recognize these objects as grid points. This will cause the calibration process to stop because of the wrong number of recognized points.
Try to take another photo of the paper grid placing the camera at a shorter distance, so that any object different from the grid will be left out of the photo.

I can't calibrate my traditional camera.
When you try to calibrate a traditional camera, you have to process a photo imported from a scanner, in this case LensTweaker says that no EXIF parameters are available for the selected photo.
This is because when a photo is taken with a traditional camera, must be printed and scanned to have a digital image. These two processes could change the center of the image in an unpredictable way, specific for the single photo. This makes it impossible to remove lens distortion automatically.

Can I remove automatically the distortion from a photo taken with a traditional camera?
It is not possible because LensTweaker does not allow to automatically calibrate a traditional camera.
This is because when a photo is taken with a traditional camera, must be printed and scanned to have a digital image. These two processes could change the center of the image in an unpredictable way, specific for the single photo, making it impossible to remove lens distortion automatically.

Can I remove manually the distortion from a photo taken with a traditional camera?
It is possible to remove manually the distortion from a photo taken with a traditional camera only if the center of the distortion is the center of the photo. The process needs particular attention. It often happens that the center of distortion in a photo taken with a traditional camera does not match with the center of the picture. When distortion is removed from a photo in which the center of distortion does not match with the center of the photo, some parts of the image can improve, but others can get worse.

I cannot view EXIF parameters in LensTweaker
The EXIF parameters, which are available only for photos taken with a digital camera, define the parameters with which the photo has been taken. If you are processing a photo that has not been taken with a digital camera, the EXIF parameters do not exist.
Some of the first generation digital cameras did not add EXIF parameters to the photo file. In this case it is not possible to view the EXIF parameters associated to that digital camera.