What is the minimum usable megapixel count ?

I have lots of prints - even quite large - made from a 5.2MP camera. The biggest disadvantage was that the photos couldn't be cropped as much, but most of them did have enough details to look splendid when printed.

A good exercise would be to print the same image at the same size but different dpi, write the dpi on the back, mix them up, then sort them visually by dpi. There may be a value at which you can tell a difference, and yet another where it matters visually. And as others have noted, it does matter the distance from which the image is viewed. I've seen a very large optical print from an 8x10 negative at the High Museum that is very detailed, a view of a beach scene looking down from a high hotel balcony in Hawaii. On close examination you can see the grain etc., but at normal viewing distance it's pretty impressive. People get a little too caught up with the detail, but perhaps it's a little more justified when you have little color squares instead of grain globs.

J Michael wrote in post #16242256
A good exercise would be to print the same image at the same size but different dpi,

Er, how?

If you have an image that is 3000x2000 pixels and you print it at 300 dpi then it will be 10"x6.67". If you want to print it a different size then you need to change the dpi, or resize the image so that the pixel dimensions are different.

Yes, resize it and set the output dimension and dpi. The point being to get the dot size variance to see the effect.

I look back to my film days...

• Most folks considered a 16x20 enlargement from 135 format to be the limit of enlargeability, in large part based upon the visibility of grain. Folks did not so much think about amount of detail, but more about the grain.
  
• Folks who shot on Medium Format did so because a 16x20 required less magnification to achieve 16x20" size print than 135 format...17x magnification for 135 format vs. about 9.5x enlargement for 645 format vs. 7.3x for 6x7 frame.
  The benefits of Medium Format was not only less apparent grain size, but also the inrease in film area to capture the same amount of subject. This provided better tonality gradations or improved color gradations; again, most folks didn't so much think about detail resolution as much as reduced grain and improved tonal quality.
  
• Typical lenses rated as 'very good' presented 64 line-pairs/mm of detail. At 17x magnification, that resulted in only 3.76 line-pairs per millimeter of detail on the final 16x20" print! significantly less detail than the human eye can detect at a 24" viewing distance!
  

• Camera manufacturers (and most folks) are obsessed with Pixel Count. In part, this is due to the fact that a 8Mpixel sensor cannot capture as much detail as a 15MPixel sensor-- lens tests by photozone.de prove this point -- so we have spent the last 10 years catching up with film.
  
• Discussions published about pixel count vs. theoretical diffraction limitations imposed by physics of optics have shown that about 25Mpixels or so will be what is needed in APS-C sensor resolution to hit diffraction limits when a perfect lens is at f/5.6, and we know that the typical lens -- even an 'L' -- is far from perfect! So the modern APS-C camera with 20-25Mpixel is probably maximizing what we can capture from a typical lens today. (Yes, this will probably be the subject of debate, but let's assume this to be true for a moment...)
  

1. If I try to make a 16x20" print from an 8Mpixel FF dSLR, I take 2336 pixels vertically and magnify the original image by 17x to make the 16x20" print...leaving me with 137 pixels per inch. If I try to make a 16x20" print from an 8MPixel APS-C dSLR, I take 2336 pixels and magnify the original image by 27x...leaving me with 87 pixels per inch!
   
2. We hear that the 20/20 acuity standard human eye has resolution which hits its limit at 0.5 minutes of arc (one can see two lines separated by a distance of one arc minute)...if we look at a 16x20" print from 24"/610mm viewing distance, the eye can perceive down to 0.08mm or distinguish two lines which are 0.16mm apart (at 24mm viewing distance), which computes to 318 pixels per inch (It takes three pixels to represent two lines separated by a space.) Aha!
   
3. We also hear about 300 PPI (Pixels Per Inch) is what we need for terrific print quality, and about 100 PPI is as low as we should go
   
4. Back when APS-C was the only thing that the average amateur could afford, printer makers made printers which could output 13x19" prints...107 pixels per inch from the APS-C 8Mpixel cameras!
   
5. Unfortunateky we also hear from non-understanding commercial photographic print makers that they want 300 DPI, when the DPI count merely pertains to the number of dots of ink (Epson printer drivers assume 360 dpi, but its nozzle array specs are much higher depending upon printer model! Canon says 300 dpi but its nozzle array specs are also much higher depending upon printer model!), NOT number of Pixels Per Inch!
   

1Tanker wrote in post #16240087
(i just can't afford a 500mm or larger lens).. like most of us are. I can crop a bird shot, much more with my 18MP, and 20+ would be a bonus.

Thus, would you rather have a 16mp with a 3x zoom, or a 6mp with a 20x zoom?
I guess a large zoom camera will be too big and heavy to carry, and take up too much space in luggage.
Thus, I'd rather have a 12mp with a reasonable 10x zoom.

BTW, 4x6 prints from a 6mp P&S come out just fine. In fact, I will make larger prints soon.

Gomar wrote in post #16248544
Thus, would you rather have a 16mp with a 3x zoom, or a 6mp with a 20x zoom?
I guess a large zoom camera will be too big and heavy to carry, and take up too much space in luggage.
Thus, I'd rather have a 12mp with a reasonable 10x zoom.

BTW, 4x6 prints from a 6mp P&S come out just fine. In fact, I will make larger prints soon.

I guess it depends on the intended use. For what I do (portraits / landscape / cityscape) even the shots from an old 5.1 mp camera I used to have seem large enough, I actually have a couple 12x18 framed.

Amamba wrote in post #16239856
What, in your opinion, is the minimum MP count that still makes a camera useful for most purposes ? I ask because the megapixel wars of the last few years seem pretty pointless and I believe camera makers should instead concentrate on improving sensor performance. (Yes I understand that MP count is easily marketable).

I would say 5-6 MP is probably the lowest limit for me, where photos are still large enough for prints or large screens. At 10-12 MP, you have room for reasonable cropping. Anything above 16MP is a luxury, and perhaps an unnecessary excess - if it comes at a price of reduced sensor performance.

In the now obsolete era when people actually purchased printed magazines at newsstands - around five or six years ago. I submitted to a magazine an image that was created with a Fujifilm S7000 compact camera. The image had dimensions of 2848x2136 pixels. That file looked just fine in an oversized magazine when reproduced "full bleed" (from edge to edge on the page) in a publication that was obsessed with image quality. That magazine also accepted images from a 300D Digital Rebel and a Digital Rebel XTi without a single complaint.

DC Fan wrote in post #16250064
In the now obsolete era when people actually purchased printed magazines at newsstands - around five or six years ago. I submitted to a magazine an image that was created with a Fujifilm S7000 compact camera. The image had dimensions of 2848x2136 pixels. That file looked just fine in an oversized magazine when reproduced "full bleed" (from edge to edge on the page) in a publication that was obsessed with image quality. That magazine also accepted images from a 300D Digital Rebel and a Digital Rebel XTi without a single complaint.

Well!!! If you're gonna start posting real-world, logical examples, this conversation is over!!!

Obviously, since megapixels are the single determining factor, we all need to trade in our Canons and Nikons of whatever models for a Nokia Lumia 1020 camera phone (or is that a camera with phone functions), what with it's 41 megapixel sensor!!! It's sensor is a whopping 1/1.5"—8.8 × 6.6mm for those of you not used to this system of measurement—or approximately ¼ the area of a micro four thirds sensor!

And, as a side benefit, we wont have to lug around all those cumbersome lenses, either!

I mean, who could possibly want anything more?

I looked at one of Rembrandt's painting in the local museum. It really sucks on pixel level.

20droger wrote in post #16250116
we all need to trade in our Canons and Nikons of whatever models for a Nokia Lumia 1020 camera phone (or is that a camera with phone functions), what with it's 41 megapixel sensor!!!...And, as a side benefit, we wont have to lug around all those cumbersome lenses, either!

Whaddya mean?!
https://www.google.com …jojo_lens.php%3​B550%3B367
http://blog.gsmarena.c​om …enses-with-your-iphone-4/
https://www.google.com …-a-dsl-154895%3B540%3B313
https://www.google.com …rojector%252F%3​B600%3B450

Amamba wrote in post #16250298
I looked at one of Rembrandt's painting in the local museum. It really sucks on pixel level.

Funny then, that extremely high-resolution cameras are used to make reproductions of artworks, anywhere from 40 to 200mp+.

Wilt wrote in post #16243050
I look back to my film days...

Most folks considered a 16x20 enlargement from 135 format to be the limit of enlargeability, in large part based upon the visibility of grain. Folks did not so much think about amount of detail, but more about the grain.

The biggest difference between film grain and digital noise, is that grain was the physical limit of detail for film, while digital noise does not limit detail, unless there is an excessive amount of it.

No matter how many pixels you cram into how small a sensor, the only thing that will limit the sensor's capacity for detail is the resolution and diffraction limit of the lens...

Even the 40mp camera in the Nokia 808/1020 is very cleverly designed, as the aperture and resolution of the lens are precisely matched to the diffraction limit of the sensor, where COC = 2 pixels wide. In practice it can resolve as much detail as a 20mp DSLR.

This is why the best 35mm sensors of today can equal the quality of medium format film, and why medium format digital can equal large format film, digital follows different rules.

cdifoto wrote in post #16240732

tzalman wrote in post #16240649
Bill Gates said "640K ought to be enough for anybody," and to this day he keeps smacking his head and muttering, "When will I learn to keep my mouth shut?"

I'm not sure he's really that bothered by having made that statement, if he said it at all. Other than the flak he's caught over the years.

It's real easy to look back on that and laugh, but when the IBM PC-1 was introduced in 1981, it came equipped with 64K of memory. Those of us who upgraded our systems to 128K thought we had all the memory we’d ever need. At first, there were very few programs available that needed as much as 128K. But as more and more software became available, the complexity of the programs increased and so did their memory requirements.. but that took a relatively long time.

Does anyone remember what the computer market looked like back then (prior to DOS and the 640k limit -- which was imposed by IBM, not MS, because that high memory, starting at 640k, was where they loaded device drivers)?

http://file.vintageadb​rowser.com/l-gh5wsgao7jttws.jpg

"You read that right" takes on a new meaning now.

Back then, if they had engineered the system to work with, let's say 8MB of addressable memory, that would have been like "building a bridge to nowhere."

Kolor-Pikker wrote in post #16250423
This is why the best 35mm sensors of today can equal the quality of medium format film, and why medium format digital can equal large format film, digital follows different rules.

Yet that is not inconsistent with my limit of 16x20 from the tinier APS-C frame, where I would have used 16x20" from FF film previously, right?!