Well, it seems like you already answered the basic question correctly: "I know that they're excited about their pictures mostly because of the memories from them and such, and I don't dare tell them that I don't think the pictures are high in quality"
Since they are not aspiring to be "serious" photographers, all you can do by pointing out flaws is hurt their feelings, or give them some negative emotions that will be attached to the pictures that would otherwise be happy memories. Remember that there are lots of genres of photography, with different intents and audiences. Casual Vacation Snapshots are in a category all their own, I think, and not worth scrutinizing so harshly. If asked, for example, "why didn't this one come out good?" you can offer constructive or instructive ("try this when you have backlight like that") comments.
I would not work on someone's image without their permission, even a snapshot. If you like, you can ooh and ah over a couple of the images as "best of the bunch" and you would like to play with them a little bit or something.
People have to be open to and welcoming of constructive criticism--and if they have not asked for such help, they likely won't even recognize it as constructive at all. So grin, bear it, and "focus" your discussions about the shots to the subject, location, experience they had there--those are the purposes of the photos :-)
Or ignore this completely! I won't be offended if you think this is lame advice. :-) |