When I want to turn a 4:3 photo to a wide picture I just randomly crop some part of the picture to make its height less. I don't have any idea what is the definition of wide pictures. What is its aspect ratio?
Is there a concrete rule for it? How do you take standard wide picture which is compatible with wide TVs, monitors, etc.
There is no specific standard, but there are a few aspect ratios that turn up a lot.
There is no "standard" picture aspect ratio in photography, only some common ones. 35 mm frames are 3:2. Old TV was 4:3. HDTV is 16:9.
The way to pick the aspect ratio is to consider what the picture will be displayed on. If it will mostly be seen on a TV, then 16:9 is a obvious choice. If you will make a poster and hang it on a wall, then you can chose pretty much whatever you think is best for the picture.
Also, when converting pictures between aspect ratios, "randomly" cropping some height or width to get to the new aspect ratio is not a good strategy. Each picture is unique.
Some simply won't work as intended when cropped. In those cases, adding black (or whatever is background in your context) is better than cutting away the other dimension. Most of the time you can crop, but you should consider carefully what to crop out. Equal amounts top and bottom to make a wider aspect ratio, for example, is rarely the best answer.
No, there is no rule for that. Do whatever suits your image.
Like everyone else has said there is no concrete rule for this.
However I will attempt to answer the 'take standard wide picture which is compatible with wide TVs, monitors, etc.'
I find when taking photos, you shoot to fill the frame, and may not necessarily consider changing the aspect ratio until after. An example of this is your picture is 2:3. Instagram (as an example) used to only allow images which were 1:1. This will result in a heavy crop to upload it on there and part of the image could be lost.
A resolution to this is upload the image with an outer border or frame, so keep the image as is, but pad it to the specified ratio of length/height to fit the output dimension required. If you're more graphically inclined, you could do it with borders etc to make it look better then two black bars either side for example.
Create these various 'frames' for common dimensions you wish to use, so 16:9 for widescreen television or 16:10 for monitors.
it may not be the most glamourous approach but does result in the same image 'compatible' without distortion/crop/widened/shortened on various devices.
Concrete rules: no, none, zilch, never, unless someone specifies them to you for a certain project.
My advice is to take the biggest and best picture you can with your camera and make sure there is enough uninteresting stuff around the subject so that you can crop it to fit your target medium.
Long story short: It depends on how you want to publish and how you want it to look like. This is the rule.
Are all answers Yes? Then you have found The Dimensions for This Particular picture.
If you want to publish on screen try to be reasonably close to standard screen ratios - superwide and supernarrow pictures will be hard to display.
If you want to print, it depends on where the print will be placed.
