Originally posted by Canuck_west A good explanation of why legacy lenses may not be as good on digital:
sensors
The pertinent comment, from that article, is:
"if photosites are above 8 micron then high quality legacy 35mm lenses are likely to be adequately matched in resolution terms.
BUT if photosites are less than 7-8 micron, then the lenses may be the limiting factor in resolution, especially zoom lenses and dedicated lenses designed for digital may be required to get the most out of the sensor (hence Olympus ZD lenses)."
https://www.pentaxforums.com/forums/10-pentax-slr-lens-discussion/303658-k-m-...ml#post3368817
The 24MP Pentax K3 has a pixel pitch of 3.88 microns
The 50MP Canon 5DS has a pixel pitch of 4.13 microns
The 16MP Pentax K50 has a pixel pitch of 4.78 microns
The 36MP Nikon D810 has a pixel pitch of 4.87 microns
The 10MP Pentax K10D has a pixel pitch of 6.07 microns
The K10 clearly doesn't resolve as well as the K3 even with legacy lenses yet is already using pixels far smaller than the 7-8 micro value specified in that article. The other thing to note is that if a legacy lens performs well on the K3 today - there is no reason to expect that the center performance will suffer on FF since the pixels are already smaller and more densely packed on even the K50 than a Nikon D810. The only real question as stated by another is will a given lens have poor border/corner performance and vignetting compared to expectations. CA and other issues should be similar to what A7 and Pentax users already see today - which is well within reasonable for many if not most legacy lenses.
This isn't to say there is no advantage to the newer lenses - it is just to calm Fear Uncertainty and Doubt before people do dumb things like dump their legacy glass anticipating a bonfire sale.
Lastly just to make a clear and concise point:
Smaller sensors require much better lenses than larger one's. Imperfections and minor aberrations are easier to ignore and have less impact on large sensors than small one's This was true in the film era and it is true today. I am not talking about the outliers like corner performance of poor lenses being revealed as the lens is asked to deliver a larger image circle - I'm talking in general about lenses designed for each sensor.