A few nice photo cards images I found:
Greetings cards / notelets
Image by natalief
A sample of A6, blank inside, greetings cards (or notelets), made from my photos.
I am still in the process of pricing these and working out how I could sell them - Etsy, local shops or any other ideas.
I can make these with almost any of my photos. I have loads that are not on flickr as well.
atc with cabinet card photo
Image by Bellah
DIY Manfrotto tripod box gray card
Image by Claudio Matsuoka
Upon receiving my new tripod, I noticed the box color being suspiciously similar to a standard gray card. Would it work as a middle gray card, saving me a few bucks not having to buy the real thing? I cut a reasonably sized piece, and a few test shots later it seems that the Manfrotto tripod box indeed makes a usable approximation of a 12% gray card (for use with an ANSI/ISO standard calibrated photographic exposure meters, like the TTL meter found in current cameras).
The gray picture in the top left corner is the side of the tripod box. Assuming 90% white reflectance, 12% gray normalized to 0-255 range becomes 12 / 90 * 255 = 34. In a 2.2 gamma RGB color space, (34 / 255) ^ (1 / 2.2) * 255 = 102, which is the average pixel value in the center of the card photo. It doesn't say much about the card color accuracy, but it at least shows that the camera meter is indeed set to 12% gray.
The test pictures show the different results you can get from the automatic exposure modes in my XTi (evaluative, partial and center-weighted, no spot). A backlit subject frequently confuses the camera -- note how the pictures in aperture priority mode are all underexposed. The last picture was taken using the partial meter reading from the poor man's tripod box gray card, and shows a much better exposure than the previous three. The bad contrast is caused by the UV filter (B+W 010 UV-Haze MRC) and backlight. All pictures taken with the EF 50mm f/1.4.