Hi,
The term megapixel race is often used. But are small pixels good or bad?
What I would state is small pixels are mostly advantageous as long as they are within optimum of the presently used pixel designs.
It could be argued that we don't need a lot of megapixels, and in many cases it is quite true. Full screen HD is just two megapixels. So, we don't need high MP count cameras for web size images. At least not until 8K TV is here…
On the other hand, pixels scale nicely down. So there is very little disadvantage to high MP images.
Let's look at some examples, the first one is one of my own. I made two similar shots with a Hasselblad Planar 100/3.5, using the same camera position (or so I believe). The first image is with the P45+ back and it produces around 39 MP. The second one was a shift stitch using the same lens with the Sony A7rII. The Sony A7rII is a 42 MP camera, but the stitched image was around 82 MP and the surface are was about the same as on the P45+. The link below goes to the full size image, the downscaling of the image in the forum software creates some artefacts of it's own.
http://echophoto.dnsalias.net/ekr/Articles/MPRace/Planar_100.jpgThis a crop from the original screendump, just showing the most interesting part of the original
screen dumpThis the full image, but downscaled by forum software:
The images are all actual pixels, the first one is the P45+, the second one is the stitched image from the A7rII, downscaled to 39 MP. The third one is the original stitched image.
Check out the sign "Centralstationen" on the P45+ it is almost unreadable. The back resolves the text, but demosaic really struggles. Colour aliasing error are quite abundant, with the moiré on the blinder perhaps being most typical. The image at the center is downsampled to 39 MP. So, resolution is exactly the same, but the downsampled image is much cleaner. The image on the right shows the full resolution of the 80 MP image and it has no aliasing. Why? I don't know. Some possible explanations:
- Sensor outresolves the lens, that is the lens can deliver very little contrast at the pixel level.
- Sensor outresolves the subject, that is the subject lacks fine detail that would not be resolved by the sensor
- Microlenses on the sensor reduce aliasing as sampling is done over a larger area (area sampling vs, point sampling
So a high resolution sensor will deliver a cleaner image than a lower resolution sensor of similar size.
The next point is that it is often said that small pixels are noisy. Let's compare some high ISO low MP cameras and some of the high MP champs. In the four corners: The Sony A7sII is a 12 MP camera mostly intended for motion and high ISO shooting, we can pair it with the Sony A7rII, the resolution champ within the A7 family.
The Canon 1DXII is Canon's low light offering and the 5DsR is the resolution king from Canon.
We can compare the four using a very nifty feature from DPReview that offers different kinds of comparisons. Let's compare the four at 12800ISO and actual pixels:
http://echophoto.dnsalias.net/ekr/Articles/MPRace/12800ISO_actual_pixels.jpgThe Sony A7sII is arguably the cleanest while the 5DsR may be the most noisy. So, high resolution goes hand in hand with high noise at actual pixels.
The same tool at DPReview can compare images at normalised size, so all images are sized to same resolution:
http://echophoto.dnsalias.net/ekr/Articles/MPRace/12800ISO_normalised.jpgVoilá, all images are pretty much the same! So, once we compare the images at the same size the high MP images will be comparable to the low MP images.
Now, we switch to 100 ISO and compare gray text on black background at actual pixels.
http://echophoto.dnsalias.net/ekr/Articles/MPRace/ActualPixels.jpgHere the Sony A7sII fully falls apart, it has far to weak AA-filtering to handle pixel level colour aliasing. The higher resolution of Canon 1DXII paired with proper AA-filter does a much better job. The Canon 5DsR delivers the cleanest image.
I would probably argue that even the Sony A7sII can make very good A2 (16"x23") prints as I have been shooting a lot with 12MP cameras and made a log of good A2-size prints. The 39-50 MP cameras tested here allow for really large print sizes.
A clear disadvantage with high MP cameras is the increase in image size. But, raw images are often quite effectively compressed. The 39 MP images from the P45+ are around 50 MP in size and so are the Sony A7rII images using DNG-compression. On the other hand, full size TIFFs will be large.
Best regards
Erik