Hi,
Theres was a recent question about how large we can print. Bart van der Wolf has a very nice tool on the net to calculate that:
http://bvdwolf.home.xs4all.nl/main/foto/dofplan/dofplan.htmlEyesight and viewing distance matters a lot. I have a few 70x100 cm size prints but I normally print just A2 (16x23"). I like large prints but suffer from acute wallspace limitation syndrome.
One of the experiments I have done was to make a large print from a 10 MP APS-C camera. The image was carefully processed and optimised for large size printing and than printed at 70x100 cm (27.5 x 39.4"). That print actually is quite good, if you view it at something like 80-90 distance. I have since shot the same subject, with 12, 24, 39 MP cameras in APS-C, 24x36 and 39x48mm format, but did not need to replace it. I have 20/20 vision with corrective glasses, AFAIK, so normal quality in Bart's should suit my vision.
Filling in 0.9 m viewing distance and 10 MP gives 1003x674 print size, very close to my viewing experience.
Next experience was when I stated using an 24 MP full frame camera. I compared my 24 MP APS-C camera to my 12 MP APS-C camera, printing at A2 size. I made three comparisons. In two cases the 24 MP camera was slightly better and in case the two images were indistinguishable (by any tested observer). Let me make clear that the image quality on screen was significantly better with the 24 MP camera.
Bart's tool gives 41x62 cm size for the 12 MP camera when viewed at 50 cm distance. So, Bart's tool says that there would be little visible difference at 50cm viewing distance, and that was the case.
The third experiment was a shot made at the same time with 24MP APS-C and a P45+. In a sense this shot was slightly biased in favor of the 24MP camera, as it was used with a zoom lens giving the wanted framing, on the P45+ I used the longest lens I have 150/4 and I would have needed an 180 mm lens to match. That cropping essentially dropped the MP advantage of the P45+.
I made 1/3 crops of the images and printed at A4 size. Viewed on screen the P45+ image had an obvious advantage. The A4 prints corresponded to 57x86 cm (22x34"). At that size the prints were difficult to tell apart. Once you found a detail and compared details side by side you could see the difference. Just flipping around the copies and have a good look without referring to know details I guess my hit rate was about 30%.
In this case Bart's tool gives 87x58 cm print size for normal vision and 50 cm viewing distance. We also failed to see significant difference at shorter viewing distances, too.
To sum up:
Although what you see on screen are the pixels that go to the printer, the printing process is a great equaliser. The gain in image quality is much less in large prints is less than what I would have expected after viewing the image on screen.
Bart's tool gives print sizes that seems relevant to my experience.
As a side note, I don't think we view prints as close as we may believe, 25 cm is quite short, we need good illumination. For large prints I would guess that 50 cm is pretty close. That is about half an arms length.
Best regards
Erik