> ... But how big can I go before it loses definition?
Hard to be definitive, but you need to consider things like viewing distance (i.e., you don't need 300dpi if you're looking at a picture on a wall from >1m distance) and what the picture contains (a foggy landscape will look ok big when a highly detailed city-scape might not). I have a cropped 3Mpixel picture on my wall (https://www.ukclimbing.com/photos/dbpage.php?id=16796) ~30" wide and I've never considered binning it 'cos it's too blurry.
I used to print up to A1 with a 20mp sensor at 300dpi with no loss of quality. Get in touch with Julian at the Pixel Printer and he'll tell you how big you can go.
> I wouldn't get over worried about the 300 dpi hype....
Agreed. 300dpi is only relevant for a print that viewed at ~30cm distance (e.g., book reading distance) by someone with perfect("20/20") vision. For a wall mounted picture you're probably viewing at ~1m, so 100dpi is plenty. I've had a somewhat cropped 3 Mpx image printed ~70cm wide and it still looked fine on a wall. If it's a good image, people will look at the picture not the pixels.
150dpi should suffice and you should be able to go up to A1 - from the viewing distance because of visual acuity it should look great. Going smaller will increase resolution.
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