Ever heard of parfocal lenses?

I'm not talking about reading glasses although the opposite term with camera lenses is varifocal. I got a tip from a friend about zooming in to focus, then if you zoom out to get more of the scene in the photo you know your subject is still in focus. I had never even thought of doing that before. Mostly I rely on live view and a tripod with manual focus for that kind of thing but even so I would never have touched the zoom ring after focusing. So to me, it just felt wrong. However, it is a good tip but it's only possible on certain lenses.

Parfocal is just a term that means the focus doesn't change when you zoom in our out. With varifocal lenses, the focus moves when zooming.

How do you know if your lens is a parfocal? It doesn't seem to be a key marketing factor in selling DSLR lenses, I guess because there are more lenses that don't have the facility than do (apparently it's standard on cinema camera lenses but fairly rare on consummate optics). If you can't find out online then you can test it yourself. To be honest there are not many trustworthy online resources for this and sometimes people disagree about a particular lens model. So it's best to see for yourself. Just put your lens in manual focus, zoom out and use live view. Use 10x view to make sure an object is focused the best you can get. Take a photo. Then zoom in and focus on the same object again.  Zoom all the way back out and take another photo of the same scene as before. Compare both photos. If the focus has shifted and the object in the latter photo is blurry, you will know your lens is not parfocal. It might be best to try the test with objects that are near and far away and repeat it to account for any human error.

Apparently, my Canon f4 24-105 lens is parfocal (or not, depending what you read). However, I still wouldn't automatically use the zooming in to focus technique with it myself. I always double check the focus if i ever accidentally touch the zoom ring. But if you have an older camera which doesn't have live view (i.e. film camera users) or your camera and the object you wish to photograph is stationary, it's perhaps a useful trick to remember.