Sensor and Pixel Basics

Question

The recent introduction of the 5Ds with its 50.6 megapixels leads me to ask about sensor size and pixels.  There is an on-line tome by W. H. Majoros that describes the situation well, I think.  You can reference it at "digitalbirdphotography.com".
Frequently when discussing full-frame vs APS-C cameras, the fact that the image appears larger on the APS-C photo is spoken of as its having an increased magnification.  But this is misleading.  The "real" size of a photo, when displayed on a computer monitor, is seen when looking at it at 100% magnification, in which case there is a one-to-one mapping of sensor pixels to monitor pixels.  The displayed photo size of a photo is due to the linear pixel density of the camera sensor, not the overall size of the sensor.
Regarding the ultimate sharpness of a photo, while it is limited by the pixel density (higher is better), it is also limited by the pixel size because of electronic noise (lower pixel density and thus larger pixel size is better).  So at some point higher linear pixel density becomes a negative feature.
Which brings me to a comparison of the new cameras, the full-frame 5Ds and the APS-C 7D Mark II.  I compute that the linear pixel densities of tese are both about 240 pixels/mm.  Until now the full-frame 5D cameras have had lower pixel densities, which seemed like an advantage due to their larger pixels, especially at higher ISO.
So except for the larger FOV provided by the 5Ds, why would anyone want this over the 7D Mark II?  I'd appreciate others' thoughts on this.
Thanks,
Edward1064

Answer 1

Edward1064 wrote:
The recent introduction of the 5Ds with its 50.6 megapixels leads me to ask about sensor size and pixels.  There is an on-line tome by W. H. Majoros that describes the situation well, I think.  You can reference it at "digitalbirdphotography.com".
Frequently when discussing full-frame vs APS-C cameras, the fact that the image appears larger on the APS-C photo is spoken of as its having an increased magnification.  But this is misleading.  The "real" size of a photo, when displayed on a computer monitor, is seen when looking at it at 100% magnification, in which case there is a one-to-one mapping of sensor pixels to monitor pixels.  The displayed photo size of a photo is due to the linear pixel density of the camera sensor, not the overall size of the sensor.
Regarding the ultimate sharpness of a photo, while it is limited by the pixel density (higher is better), it is also limited by the pixel size because of electronic noise (lower pixel density and thus larger pixel size is better).  So at some point higher linear pixel density becomes a negative feature.
Which brings me to a comparison of the new cameras, the full-frame 5Ds and the APS-C 7D Mark II.  I compute that the linear pixel densities of tese are both about 240 pixels/mm.  Until now the full-frame 5D cameras have had lower pixel densities, which seemed like an advantage due to their larger pixels, especially at higher ISO.
So except for the larger FOV provided by the 5Ds, why would anyone want this over the 7D Mark II?  I'd appreciate others' thoughts on this.
Thanks,
Edward1064
Canon's ISO ranges include, at the top end, a regime described as "ISO speed expansion" or something similar. One way to describe the transition point into that regime is "the point beyond which you may not want to go if you can't tolerate a lot of noise". If you look closely at the specs, you'll see that that point is two stops lower for the 5Ds than for the 5D Mk III.
So yes, larger pixels are better, and the greater FOV may be the only advantage of the 5Ds over the 7D2. But if you need it, that's how to get it.
And one is left to hope that the 5D3, with its evidently better low-light performance (and noticeably lower price), will remain in the product line for the foreseeable future.
Bob
Boston, Massachusetts USA