The earliest known photo dates from 1826, but the process was complicated, lengthy and costly. It took at least eight hours to several days for an image to emerge on a polished sheet of pewter, coated with bitumen dissolved in lavender oil that was the camera's "film". A practical photo process didn't come into being until Louis Daguerre gave us--you guessed it--the daguerreotype. The photographic process was steadily refined throughout the 1800's, evolving from the daguerreotype to the ambrotype to the tintype to the carte de visite to cabinet cards. The first step in identifying a time frame for your old photo is to identify which type it is, and, therefore, when the photo was taken. Each photo type had certain characteristics, and those are detailed at PhotoTree.com.
Once you've identified the type of photo, you can narrow the photo's date down further by looking closely at the details of clothing, footwear, hats, and hairstyles. Sometimes the background features, even those of a studio, can be helpful. The clothes and hair, though, are your best resources for zeroing in on a photo's likely date, thereby increasing the possibility of you putting a name from your family tree to the face in that photo.
|This not great photo is made worse|
by a handler's fingerprints.
There are plenty of books and online sites to guide you. Your job will be to match as many clothing and hairstyle features as possible in the family photo with the exemplars provided in books and at the online sites. To do this, you need to be able to see details clearly. You can use a strong magnifying glass, of course, but your best bet is to scan the photo at a high dpi (600 is best; no less than 300) so you can magnify the photo on your pc screen. If you don't have access to a flatbed scanner, use your smartphone (or have your favorite nephew or grand-daughter use theirs) to take a picture of the picture. It can be magnified on the smartphone or that brilliant younger person can upload it to your pc. Word of warning, many old photos taken when personal cameras started coming into use, do no magnify well. Most old studio photos magnify brilliantly, though. I have an 1876 studio photo of my great-grandmother that can be so highly and clearly magnified, I can literally stare into her eyes. (Which is pretty awesome!)
Also, when you're handling very old photos, it's best to use gloves. Even if you wash your hands thoroughly, you still have oil on your skin. You've probably seen an old photo that has a clear fingerprint impression somewhere on it's surface, often right on someone's face. That's why you want to wear gloves. Cotton gloves are best, but you can use latex. Even with gloves, try to touch the surface of the photo as little as is humanly possible.
When you've made an educated guess as to the date the photo was taken, resist the temptation to write on the photo, and NEVER use an ink pen or a marker pen. Old-fashioned pencil is best, only write on the back (verso) of the photo, and don't press into the photo back. The best way to label a precious old photo is to put the photo into its own clear sleeve or into its own paper envelope. You can then stick a label on the front of the clear sleeve or the paper envelope.
Whichever you use, paper or clear sleeve, be sure the product's package contains words like "archival safe", "acid-free", and/or "lignin free". You can usually find these kinds of products in good craft stores, and you can also buy online from archival supply sites like Gaylord's, BCW, and from we-sell-everything Amazon. By the way, you can buy pens that are archival safe and can be used to write on the back of photos if you really, really need to do that.
For online sources, this link is a good place to begin "Top 10 Resources for Dating Old Photographs"
In addition to online sources, I strongly recommend you consider investing in one or some of Maureen Taylor's resource books (see number 10 at the above site).
UNKNOWNS FROM MY OWN FAMILY ALBUM