I've read up on entry level DSLR quite a bit since I'm intending to get one. I've narrowed down to 2 choices.D70s and 350D. I know the pros and cons of both, but what i dun understand is the link btw dpi and resolution(MP).350d has 8MP but when i look at the EXIF of the file,it shows 72dpi.D70s has 6.1MP but 300 dpi. Will dpi affect print quality?
The link between DPI (dots per inch) and resolution is that if you have a picture with a resolution of 1800 x 1200,
@300 DPI, your printed picture size will be 6" x 4"
@72 DPI, your printed picture size will be 25" x 16.7"
Therefore at lower DPI, the same picture will be larger in print when compared to higher DPI.
At higher DPI, more dots are squeezed into each Inch and so more is squeezed into the same picture size.
As for the D70s vs 350D, you don't have to worry about the difference in DPI embedded in the EXIF because the DPI can be changed easily (such as in editing software) to a number you want, depending on the picture size you want to print physically.
The important thing is having enough resolution in the picture so that when you use a resonable DPI (such as at least 150 DPI), you are still able to get the picture size you want without losing too much details.
For e.g. if you want a 4R (4" x 6") size picture at 300 DPI, then your picture must have a resolution of 1200 x 1800.
If you want a 8R (8" x 10") picture at 150 DPI, then your picture must have a resolution of 1200 x 1500.
Of course, given a particular photo size, more DPI means more details because of higher resolution picture used but it also means bigger file and sometimes your cropped picture may not have enough resolution to print the size you want at higher DPI.
For e.g. if you want to print S12R (12" x 18") @300 DPI, you would need a picture with resolution of 3,600 x 5,400 = 19.440 Mega Pixels which is beyond most digi cameras currently.
In any case, big photo size are meant to be viewed from much further than arm's length and so a lower DPI is still acceptable. A S12R @150 DPI would need a picture with resolution of 1,800 x 2,700 = 4.86 MP only (1/4 that @300 DPI, half the DPI for length and breadth means 1/2 x 1/2 = 1/4).
For printng at shops, the usual requirement is at least 150 DPI. Try to use higher DPI (200-300) when the picture print size wanted is small (such as 4R) since the required resolution for such a size is not demanding : 4" x 6" @300 DPI = 1200 x 1800 = 2.16 MP only.
E.g.
http://www.so-net.com.hk/imagestation/faq/Faq05.php
In any case, beyond a certain DPI for a given photo size meant to be normally viewed from a certain distance, there is a decreasing marginal gains in picture quality. For e.g. most people won't notice significant difference in a 4R picture viewed at the usual distance when printed at 240 DPI or 300 DPI. Almost all people won't notice any difference beyond 300 DPI for 4R when viewed at the usual distance.
In summary :
1) You don't have to bother about the DPI embedded in the pictures of D70s vs 350D because you can change it to any number you want. You only need to take note of the resolution difference in the image sensor.
2) Yes, DPI affects picture quality. Higher = better because the printing dots are much closer to one another. However, higher DPI printing requires higher resolution picture. If the photo size you want is very huge, you may not have enough resolution to print at very high DPI.