Getting a photo printed framed and displayed in an exhibition is very rewarding; but there is one misconception that seems to pervade the on-line world. The suggestion that the resolution of a photo (in dpi) is crucial. Setting pixels per inch (or dots per inch) will very likely be Ignored when you order any external print, of a given size so don’t worry about it.
It was once a useful measure when number of pixels in an image where smaller and older dot matrix printers (hence dpi dots per inch), or computer screens of lower resolution (more correctly PPI pixel per inch). Now most cameras capture way more pixels than needed and clever software in the printer drivers combine the pixel luminance and colour to given a smooth not jaggy (not pixelated) surface that match the printers capabilities. Don’t make the mistake of resizing your photos to smaller number of pixels wide and high.
The numbers are useful in estimating minimum pixel required for various size prints. At normal viewing distance (closer than arm’s length) a person cannot resolve individual dots or pixel when the resolution is between 240ppi (normal vision) to 300ppi (extremely good vision that can focus up close to the print). So these numbers are often used by graphic artist and printer manufactures to set the minimum size of an image in pixels when they know the output size required. Most printers are capable of much higher PPI densities.
Thus always give the printer the original resolution and ignore the dpi or ppi resolution setting. The software rendering your image to the printer will take care of that and give you the best possible print. If you are having the photo processed at a commercial service it will normally warn you if the image size in pixels is not sufficient to get a good print.
Just to confuse you further many older photo editors insist that you set dpi (200 or even 120 will work) before you can print, but it is probably time to update that program. Also many word processors and desktop publishers will actually do the resize to fit the size of the output (as a space saving measure). If you want to use those programs just use 200 dpi.
The following table given the minimum and preferred output size for given pixel densities.
This table is just based on the maximum dimension of the photo.
The minimum number of pixels is surprising low, lower than you might have guessed after looking at photo and particularly gear reviews on the net but they are real, and most cameras are capable of much larger images.