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What is the most correct way to delete all photos from a camera after they all have been copied to a computer:
Or no difference at all?
There's no "right" way to do this, it's what works best for you. In general, I just tend to do the "delete all" unless the card is quite full, then I may format. It's just a question of speed, I'll tend to use the fastest path to clearing it off.
Now, there are some that recommend regular formatting of the card for various reasons. I'm not in that camp, the basis for it isn't entirely sound. However, the upside is the detection of potential hardware flaws on the device. So, even if it isn't specifically regular, doing it every now and then may have some use.
Either way, I only do it after I've made two successful copies (primary and backup). I've lost images in the past by not being more careful about it, so now I take a little time to be sure.
I'm not sure that the effects would be different but I would say the safe bet is
'Format the memory card using the camera menu'
Just make sure you have all of the pictures saved elsewhere before you do this (accidentally wiping out photos sucks)
Read your camera's manual. They usually have recommendations.
Eliminate human error. Card errors of any type are much, much more more rare than you just messing up.
The format used on cards has been a standard for a long time, and modern sizes are such that what metadata or hidden files a computer might leave are pretty insignificant. If you can find a method that keeps you from screwing up and deleting stuff by accident, then the most you'll likely need to do is a periodic in-camera format, just to be on the safe side.
I'd also add another option to the list, which is letting some sort of image-library management software take care of clearing the card for you; sort of midway between the camera and treating it like a generic USB stick. I believe Lightroom has an option for this, as does Image Capture and iPhoto on the Mac.
If you use your cards in different cams you'll notice that quite a few cameras only show pictures took with this one camera.
So if you chose "delete all" it will only delete those images it knows, made by the cam itself and you'll end up with a partly filled memory card, not being able to take as much pictures as it should.
Same if you sometimes have other files on your memory cards, which eat up space.
Another thing, if the file system ever got corrupt for any reason, delete all won't fix it, while format will fix logical file system corruption.
I'd always use format. Except maybe if the format is extremely slow, taking several minutes (like it was in the Leica M9, not sure if they fixed it by now).
Of course, in case of format everything is gone which, for me, is what I want, but if you tend to put important data or pictures on a memory card without having a backup somewhere else, might not be what you want (but you would not do that, would you).
Flash cards generally are limited in the number of writes rather than the number of reads (as in the MTBF is a function of writes not reads). Formatting a card is a much "cheaper" operation (in terms of writes) than deleting all the images, so it's possible that formatting the card rather than deleting may help prolong the life of your card, although in the grand scheme of things, who can really tell.
Note that formatting the card as opposed to individual file-deletion, whilst the most efficient in terms of ‘how much data is written’ to achieve it, is almost negligible compared to its ‘day job’ when you’re writing gigabytes a day to it simply taking photos.
There is one additional downside to formatting - the computer will consider it to be a totally new device it has never seen before, so will ask the same old dozy questions as to how to deal with it.
If your workflow is dependant on the computer being aware of, for instance, data copy locations per card, erase after copy or not, which camera the card belongs to... etc, then format rather than simply getting the computer to file delete is far more hinderance to your workflow, for a minimal gain in potential card life.
Additionally, whilst backups are vital, I’ve never known a file read to not checksum-match without being notified immediately that an error occurred, so i would consider manual checksumming to be overkill. That being said, I’m on Mac. Windows may handle things differently.
Erasing rather than formatting will let you use a tool like photorec to recover photos off your memory card, should the need arise.
I've erased a card thinking I'd copied photos over, but actually hadn't, and photorec came through and recovered them for me. If I'd formatted, I'd be SOL.
Use a "verify and move" command or script of some kind, that deletes the files after they have been copied and re-checked for matching, and is fully automated so it deletes exactly those files that were just validated.
The typical move command does that one at a time, which I find less than ideal. I would rather not write to the card at all, until after all copying is done. That way there is no vulnerability to corrupt the card with a bad flush or badly-timed crash or hardware failure.
So,
Formatting is best, unless you're pressed for time.
Writing to a previously used location of flash memory requires that it be erased first. By formatting, you are pre-erasing the entire space, ready for immediate writing. It also resets any fragmentation. If you rarely or never format, fragmentation can develop, which can effect write performance:
1) read speed (nice to have): reviewing pictures, and transferring them off the device.
Unlike mechanical hard drives, fragmentation does not affect the read speed of flash memory.
2) write speed (critical): taking pictures.
Even though there are no mechanical latencies for flash memory, "write amplification", (page-level rewriting), can eventually cause significant slow downs: https://en.wikipedia.org/wiki/Write_amplification
This happens when existing files need to be moved while trying to store a new file. This involves erasing and rewriting, and that has to be done in large chunks. Imagine swapping houses with your neighbor that otherwise has the same floor-plan as you.
Note that if you keep the picture resolution setting constant, picture files tend to be nearly the same size, which goes a long way to minimizing fragmentation in the first place. However, the effects of fragmentation can eventually result in noticeable slow downs. Here are some interesting test results for SSD hard drives: http://www.lagom.nl/misc/flash_fragmentation.html
