Megapixels and Actual Sharpness vs. Percieved sharpness

[manaheim]

I don't think the point has been made in this particular way yet so perhaps this will help understanding on the resolution/sharpness thing.

Of course, feel free to refute me.

Sharpness is essentially well-focused detail. If there is a fine grain of sand and we can see it and see it clearly, we have well focused detail, and thus sharpness. If it is out of focus, it isn't sharp... if we can't see it or see it as more of a slight gradient in color because we can't make out the edges... it isn't sharp.

In our eye/brain world we have a certain level of detail that we can see... our eyes and brain are effectively analog in their composition, so there is not really a "resolution", per se... but our eyes are very sensitive to a fairly significant level of detail. You can see more or less, depending upon the focusing strength of your eyes... some people can see specs of dust across a room... others cannot see oncoming cars. More and less sharp, respectively.

In our digital photography world we are creating a capture of an analog world using a digital mechanism... Dots. What's more is we have have a very finite number of these dots. We must, therefore, interpolate details between these dots to give us the final image.

The key here is that interpolation. Interpolation is essentially educated guessing, and it isn't always correct. The more interpolation you have to do, the more guesswork is in your image, the less sharp the image will be. If I had to render a scene using no more than 20x20 pixels, that would be ridiculously "unsharp"... kind of analagous to a person who can't see oncoming cars... 100x100 would be better... they would probably know something big was coming and they needed to move... 1000x1000 better! Can make out even what the make of the car is... 10,000x10,000... well, now we can see the expression on the driver's face and the whites of their eyes as they bear down on us.

In other words, as we get more pixels, the image becomes more sharp.

So, whatever the human eye resolution happens to be... 100,000x100,000... whatever... at some point you create the capability to create a digital image that is equally as sharp as what the human eye is capable of seeing... and in fact, you can theoretically eventually surpass that capability entirely. (in truth you can effectively do it now with zoom lenses, but that complicates this too much so we'll ignore it for now)

So yes... more megapixels gives you the capability to create an image that is more sharp.

 

[Antithesis]

This article seems to reinforce that: Sharpening 101

It makes the very valid point that any detail smaller than a pixel is totally lost. So, effectively a larger MP sensor will have more detail and will therefore be sharper. But, then you run into the problem of the anti-aliasing filter. From what I read and gather, a 20mp camera with a strong AA filter will end up with slightly soft images at the pixel depth and will require a lens that can come close to outresolving the sensor to even do that.

But, all else being equal, a 20mp image that is downsized to an 8x10, 300dpi print will either look the same or possibly worse than a 6mp image printed at the same size. I'm mostly concerned with an image that looks sharp at usable sizes (for me, under 11x14" or on a computer screen... maybe the occasional 16x20). I could care less about detail that I won't ever see on a 300dpi image, because it literally ceases to exist in the print.

The main reason I ask is this: The highest MP camera I've owned (Canon 5D) seemed to produce overall softer images than the lowest I've owned (Nikon d40, with I assume the same weak AA filter that was found in the d70 as they have the same sensor IIRC). What's even worse, is that I was shooting the 5D through some pretty nice glass (24-70 f2.8L, 70-200 F2.8L, 50mm 1.4, 85mm 1.8, etc.), and the images from the d40 appeared "crisper", even when shooting through budget glass (Sigma 10-20, 18-55, etc.).

I did manage to get some tack-sharp images from the 5D, particularly through the 70-200 and the 85mm, but for the most part I had to sharpen in post. Especially shooting in CR2 with no in-camera sharpening.

So, I see these 18mp and 24mp cameras coming out... and I don't have any desire for them. It just seems like you'll have to get some crazy good glass to even keep up with them, and therefore have to shell out sickly amounts of money. I feel content with 10mp, or possibly even less. Have camera sensors gotten big enough (and even surpassed necessity) for most intents and purposes? Especially considering the whole relative viewing distance thing?

 

[Derrel]

The way the camera manufacturer "sets up" the sensor and the camera's anti-aliasing filter determines how "sharp" or how high the acutance appears on images right out of the camera. Some cameras have a pretty strong anti-aliasing filter and produce images right out of the camera that appear rather soft, like the Canon 20D, whie other cameras have a pretty high acutance look to their images, like the Nikon D70. Those are two,specific cameras that Thom Hogan has compared.

My question to the OP is , "What Thom Hogan article are you referring to???"

Higher MP-count cameras produce larger images with higher resolving power than smaller sensors do. I find that a larger capture,like an EOS 5D 12.8 MP capture, sized down and sharpened well, looks quite good--better in fact than a smaller D70 6MP or EOS 20D 8.2MP image.

 

[manaheim]

Anti- I kind of left that antialiasing filter point out of my discussion and probably shouldn't have.

The antialiasing filter is basically the camera manufacturer's way of getting around the problem of having a finite number of pixels to capture what is effectively an infinte resolution image. If they didn't put the filter on, you would have moire patterns and stepping in the image. That would be bad, thus the filter to blur the colors a bit between the pixels to give it the appearance of cohesion and consistency.

This is also precisely why you have to sharpen in post-processing. ALL digital cameras (with the possible unique exception of the D70 and any other camera that has such a weak AA filter) need sharpening in post processing to bring back some of the edges and details that are lost as a result of the AA filter.

As far as not buying a 20MP camera because you'd have to buy glass to keep up with it... well, yes and no. If you're thinking in terms of "20mp is so close to my eye's resolution that I need super sharp glass to make sure to take advantage of it"... well... I doubt it. I don't know what the human eye equivelent resolution is, but it's very likely well above 20MP.

That being said, obviously sharper glass is always a good thing... on 3MP images and on 100MP images.