best downsizing for superior IQ

I'm still pondering a Canon 5DsR, the 50 MP beast, in search of the ultimate IQ from the camera.

It is communally accepted that downsizing a high MP image to one of a lower one will produce a
better IQ than one would achieve by using a lower MP camera.

Question is, are you better off doing it in-camera as the Canon has the ability to drop down to a
1.3 or 1.6 crop in camera or, accomplish the whole thing in Photoshop by increasing the ppi?

BTW, what happens to noise levels with either technique?

First, cropping may make for a smaller image and a smaller file, but it doesn't affect IQ at all. Take a print, get out your scissors and cut out a section from the middle. Is that photo "better" now, or just smaller? Cropping is the same thing, whether it is done by using less of the sensor or by cropping in PS.

Second, what is IQ? Is it less noise and consequently greater DR - DR is a function of the depth of the noise floor and the ability to open shadows without making the noise objectionable. You will get this from downsasampling, but, as always, there is a trade-off; loss of resolution, I.e. fine detail. It all depends what you want to do. For a large print you will need lots of pixels. A highest quality 20x30 needs 54 Mp, a negligible upsize for the 5Ds, and a 24x36 needs 77.7 Mp - also a not very taxing upsize (20% on each side). But if you first downsize to 25 Mp to get less noise and then upsize to a 24x36 print, it will be 170% on each side - cleaner but less detailed and with the losses inherent in more extreme upsampling.

Of course, if you don't need 50Mp - prints smaller than 20x30 or display on a 5K or smaller screen - there will be some downsampling done somewhere along the line. How much benefit it provides will depend on the algorithms.

I wouldn't buy a 5Ds now unless you will be doing a lot of really huge printing. The 5D4 looks to be superior in almost every way. Better sensor module design will give around 2 stops more low ISO DR and bigger pixels will give better high ISO performance.
I

Chauncey - In your post you are talking about 'downsizing' and 'cropping' - These are two different things. As Elie says, cropping will not affect the image quality, but downsizing could.

chauncey wrote in post #18117710
I'm still pondering a Canon 5DsR, the 50 MP beast, in search of the ultimate IQ from the camera.
...

Ultimate IQ? If that is truly what you are after, you would not be considering a Canon DSLR at all. Look at some medium format digital, or maybe someone makes an 8x10 digital. Larger sensors gather more light. Light is IQ.

I suspect you are NOT looking for ultimate IQ, but the best image quality you can afford. These most likely are two entirely different animals. For most of what most folks do with a DSLR, you'll never see the difference between an XSi and a kit lens and a 1DX with a 24-70 LII. Granted, there are exceptions. Two of the biggest exceptions are when you are REALLY pushing print size, and/or ISO. Anything less than 24"x36" and/or ISO 3200 is very unlikely to show up in prints from either system described above.

There are much better ways to improve your work than buying a high pickle-density camera. Have you mastered composition, lighting, subject choice, framing, lighting (natural and/or artificial), lens selection, exposure parameter selection, or any of the other things that go into making a photo worth looking at or sharing?

A 5DS(R) will cost you a lot of money, and do nothing to improve YOUR ability to make a nice photograph. Practicing with what you have, and trying new things (UWA/astro/sports/BIF.​..) is a lot of fun, and costs nothing. Chasing ultimate IQ is a unicorn hunt; you'll be buying every new doodad that comes down the pike. Improving your repertoire is real, fun, and attainable.

Don't buy a camera for IQ. There isn't a hill of beans difference between any two current bodies. Buy the features you need. Use them. Need 50MP? Buy a 5DS. Need 14fps? Buy a 1DX II. Need 1.6X reach in an inexpensive body? Buy a used T3i. Nobody will ever see the difference in your results.

Due to the magic of Photoshop's photomerge, image size is not a priority.
I am however, beginning to think that I'm taking my anal attitude to the extreme.

chauncey wrote in post #18120810
Due to the magic of Photoshop's photomerge, image size is not a priority.
I am however, beginning to think that I'm taking my anal attitude to the extreme.

I can't quite decide, is that simply a statement of fact, or is it redundant?

most likely

chauncey wrote in post #18120810
I am however, beginning to think that I'm taking my anal attitude to the extreme.

I think you should entertain this notion. Unless you have the visual acuity of a bald eagle. Or you're a dedicated pixel-peeper. You don't need 50mp for a 20x30 print. I've printed that large from a 10 mp 40D. In fact I had a 3x5 foot print on display at the Long Island Aquarium for years. That one came from an 8mp 20D. Human visual acuity is the controlling factor. Large prints are meant to be viewed from large distances. Unless you want to walk up to your 20x30 print and see a bug on a blade of grass that was 40 feet from your camera position. But in that case, you should have taken a shot of the bug with a macro lens.

joedlh wrote in post #18120858
I think you should entertain this notion. Unless you have the visual acuity of a bald eagle. Or you're a dedicated pixel-peeper. You don't need 50mp for a 20x30 print. I've printed that large from a 10 mp 40D. In fact I had a 3x5 foot print on display at the Long Island Aquarium for years. That one came from an 8mp 20D. Human visual acuity is the controlling factor. Large prints are meant to be viewed from large distances. Unless you want to walk up to your 20x30 print and see a bug on a blade of grass that was 40 feet from your camera position. But in that case, you should have taken a shot of the bug with a macro lens.

True. But then the OP would want to see the lint on the dust on the pollen on the bugs antennae.

I once had an instructor that said if his prints weren't capable of being viewed at nose length, the weren't print material.

chauncey wrote in post #18120871
I once had an instructor that said if his prints weren't capable of being viewed at nose length, the weren't print material.

He must have been fairly young. I can't see anything within about 3' of my face. I think my arms are shrinking.

chauncey wrote in post #18120871
I once had an instructor that said if his prints weren't capable of being viewed at nose length, the weren't print material.

Knowing what I know now, I would have dropped the class. Right or wrong, I would have had him pegged as a pretentious d-bag. This was back in the film days? Maybe he was using hyperbole to emphasize the importance of getting things in focus.

We're drifting a bit here, but focus is only loosely related to IQ. Nobody (in their right mind) shoots an 85L II or a 135L at f/11 trying to get everything in focus. That can be done with $200 Tamron 70-300. Lots more to photography than focus.