Empirical observation #1: Cameras with higher resolutions, for a given sensor size, are harder to hand-hold and get a good result than those with a lower resolution.
And I define a good result as one which the image is critically sharp at 100% actual-pixels view, with no motion blurring or other artefacts such as slightly missed focus (we’ll get to this later). Motion blur is when the camera moves by a greater amount than a pixel during the time when the shutter is open; if it’s less than this, then you see either hard edges on your subject as a perfect sharp edge, or with slightly lower contrast than expected due to the image projected by the lens on the sensor plane ‘spilling over’ into the next adjacent pixel slightly when the camera moves.
The simple reason why higher resolution cameras are less forgiving is that for a given angle of view, the camera with the high resolution will put the greater number of pixels per degree of field of view on the subject. This means, that less angular movement is needed before it becomes visible as blur on the image – i.e. you need to be more static than with a lower resolution camera to avoid seeing the effects of motion blur at the pixel level.
But what about the image when viewed as a whole at the same reproduction size? If we started out with a huge number of pixels with both – say 24MP and 36MP and resized both them down to say, 4MP or so – we would not see any difference between two cameras assuming one assuming both were equally downsized. This is because the downsampling process has enough information for each pixel to accurately determine its color and luminance value and therefore appears as true detail. If we downsize 36MP to 24MP, it might or might not look better: a lot of the outcome depends on the interpolation process. It isn’t a ‘clean’ situation where each new, downsized pixel has multiple pieces of information (the original pixels) to form an average that would be the same whether you used 8 or 12 pixels originally. Since 36 doesn’t divide cleanly into 24 – you get 1.5 old pixels per new one – there’s always going to be some guesswork as to precisely how that half pixel is allocated. And depending on the algorithm, any one of the following might happen – blur edges; stairstep artefacts; haloes or abrupt transitions; odd discontinuities in diagonal lines.
Let’s take an example: if you have a scene shot with 12 (8.45um pixel pitch) and 36 MP (4.88um pixel pitch) cameras, and both are equally motion-blurred. The degree of camera shake may be less than a pixel on the 12MP camera – let’s say 5um for argument’s sake, which will still produce a critically sharp image. However, the 36MP camera is now blurred by more than one pixel, which definitely will not look critically sharp at 100%. Yes, we can downsize – but you are downsizing not to 1/2 of the original linear dimension (which would yield 8MP, and bin the two blurred pixels into one sharp one that looks perceptually okay), but to 1.7x of it – and once again, we run into potential issues in trying to fit discrete, nonlinear data into a linear continuum.
This is one of the reasons why less pixels might actually produce a perceptually sharper/ crisper image for a given reproduction size, providing that this size is reasonable for the amount of resolution you’ve got in the smaller image.
The upshot of all of this in practical shooting is that you’ll need to use a higher shutter speed than you expect, which lowers your shooting envelope by a corresponding number of stops as noise increases as you ramp up the sensitivity to keep shutter speeds high. Perceptually, if you’re going to be shooting under already borderline conditions to get a critically sharp image with a 12MP camera, you’re not going to get one with a 36MP one – even if you print them at the same size.
Empirical observation #2: Cameras with higher resolution require more precise autofocus.
Comparatively, this one is very simple to explain: if the camera misses focus, the resolving power of the lens is diminished. You need to have the ability to resolve slightly smaller structures than the pixel pitch of the camera in order to create a perfectly sharp image at the sensor plane. If you miss focus by an arbitrary amount – let’s say 1% of your subject to camera distance – this may result in a loss of resolution corresponding to the inability to resolve structures below 5um in size. For our 36MP, 4.88um pitch camera, this will appear to be a blurry shot. For our 12MP, 8.45um one, things still look critically sharp.
A lot of what we’re seeing isn’t necessarily worse autofocus; what’s happening is that we are now visibly reaching the limits of tolerances and precision since sensor quality is now high enough to resolve at or close to the previously acceptable tolerances.
A lot of the reasons why an image appears sharp or not has to do with the reproduction medium, as well as how our eyes interpret information: an image with hard edge is seen to be ‘sharper’ than an image without one, even if there is less fine detail or edge definition. The fact that digital capture splits out our images into distinct zones – the pixels – doesn’t help this at all, either. It’s important to remember that a slightly blurred image (depending on the degree of blur, of course) from a higher resolution camera may technically still have more detail, our eyes don’t necessarily pick up on this; it’s a perceptual thing. Bottom line: don’t use more pixels than you are capable of handling properly for a given situation. It’s one of the reasons why I don’t shoot the D800E for casual work…but it’s also the reason it’s my camera of choice when I have full control of the execution. MT
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