Adobe Photoshop CS5 for Photographers - Martin Evening [35]
Yet, there are times when even these methods may fail and this points to one of two things. Either you have a corrupt file, in which case the damage is most likely permanent. Or, the file extension has been wrongly changed. It says .psd, but is it really a PSD? Is it possible that someone has accidentally renamed the file with an incorrect extension? In these situations, the only way to open it will be to rename the file using the correct file extension, or use the Photoshop File Open command and navigate to locate the mis-saved image (which once successfully opened should then be resaved to register it in the correct file format).
Save often
It goes without saying that you should always remember to save often while working in Photoshop. Thankfully you won't come across many crashes when working with the latest Macintosh and PC operating systems, but that doesn't mean you should relax too much, since there are still some pitfalls you need to be aware of.
Graphic Converter
If you are still having trouble trying to open a corrupted file, the Graphic Converter program can sometimes be quite effective at opening mildly corrupted image files.
Choosing File Save always creates a safe backup of your image, but as with everything else you do on a computer, do make sure you are not overwriting the original with an inferior modified version. There is always the danger that you might make permanent changes such as a drastic reduction in image size, accidentally hit ‘Save’ and lose the original in the process. If this happens, there is no need to worry so long as you don't close the image. You can always go back a step or two in the History panel and resave the image in the state it was in before it was modified.
History saves
Alas, it is not possible to save a history of everything you did to an image, but if you go to the Photoshop preferences you can choose to save the history log information of everything that was done to the image. This can record a log of everything that was done during a Photoshop session and be saved to a central log file or saved to the file's metadata.
The other thing you can do is go to the Actions panel and click to record an action of everything that is done to the image. The major downfall here is that Actions cannot record everything. For example, you cannot record brush strokes within an action.
When you save an image in Photoshop, you are either resaving the file (which overwrites the original) or are forced to save a new version using the Photoshop file format. The determining factor here will be the file format the image was in when you opened it and how it has been modified in Photoshop. Over the next few pages I'll be discussing some of the different file formats you can use, but the main thing to be aware of is that some file formats do restrict you from being able to save things like layers, pen paths or extra channels. For example, if you open a JPEG format file in Photoshop and modify it by adding a pen path, you can choose File Save and overwrite the original without any problem. However, if you open the same file and add a layer or an extra alpha channel, you won't be able to save it as a JPEG any more. This is because although a JPEG file can contain pen paths, it cannot contain layers or additional channels, so it has to be saved using a file format that is capable of containing these extra items.
I won't go into lengthy detail about what can and can't be saved using each format, but basically, if you modify a file and the modifications can be saved using the same file format that the original started out in, then Photoshop will have no problem saving and overwriting the original. If the modifications applied to an image mean that it can't be saved using the original file format, it will default to using the PSD (Photoshop document) format and save the image as a new document via the Save As dialog