For a unique ID most people go with a time stamp of when the photo was taken: YYYYMMDD-HHMMSS(+optional description).
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I find there is a very useful advantage of using a sequence number (YYYYMMDD-SEQN) rather than a timestamp in my filenames. When discussing images from a shoot with a client, we can refer to images by simply the sequence number. My client can tell me that they want prints from images -023, -049, and -050.
I find that being able to refer to images by a short simple number makes communication easier. Sure, we could use 6 digit time stamps, but I think it's easier to use smaller numbers.
Of course, this is simply my personal preference.
This sounds like an additional and -as you say- useful way to relate to a client from a photoshoot. It requires an agreement and understanding of that procedure in your images taken (before the photoshoot?). The question I have, is, is this common for professional photographers to do or do most clients just want images that they have in their mind catogorised differently? And is this sequence number an automatic feature in the EXIF data? Or where?
I use Photo Mechanic to ingest the images. For location shoots, it ingests from the card. For studio shoots, the camera sends every image to my computer via FTP, and Photo Mechanic ingests it from the FTP folder.
As part of the ingest, Photo Mechanic renames the images to YYYYMMDD-SEQN, and adds any metadata that is common to the whole shoot. Photo Mechanic keeps track of the next sequence number to be used. Photo Mechanic can also take a folder of existing images, sort them by capture time, and rename them to my standard.
Once I have the image, I can do a first pass through the images, and mark the ones the client shouldn't see (For instance shots of grey cards). Photo Mechanic then produces a website of watermarked images, which I upload for the client to peruse.
If I didn't want the client to know that some images had been removed. I can renumber the images I want to show the client.
For simple studio shoots (like a headshot), the client can immediately review the images with me using Photo Mechanic and choose the one they want.
I specifically use time instead of sequence because I'm often shooting with more than one camera and would like the images in chronological order regardless of which camera I took them with.
When I am shooting with multiple cameras I ingest all the files into Photo Mechanic, click on "sort by Capture time", and then rename them as YYYYMMDD-SEQN.
However, if your tools don't have that functionality, I can understand why you may prefer to name according to time rather than a sequence number.
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I also take a photo of a clock with each camera. The clock needs to have a second hand. Often I use my smart watch or smart phone. Both of those are synced to atomic time, and will be correct. By comparing the capture time in the EXIF data to the time shown on the clock, I can tell how far off the camera's clock was.
I pull up all the images from each camera, and go to that camera's picture of the clock. If the capture time in the file doesn't match the time shown on the clock, I type in the time shown on the clock, and Photo Mechanic will compute the appropriate offset, and correct the capture time for those images.
If I do this for all the cameras, I know that the capture time in every file is within a second of matching my reference clock.
Once corrected, I have Photo Mechanic sort all of the images by capture time. I can be confident they are now in chronological order, even if the clocks in the various cameras were wildly wrong.