more pixels are better!

Started Dec 14, 2008 | Discussions thread
bobn2 Forum Pro • Posts: 70,271
Re: more pixels are better!

Dear I Forget,

There is so much disinformation about this, it is not surprising people get confused. Unfortunately, one of the main disinformers is DPR. So, here goes:

All else being equal, more MP does indeed improve IQ, or at least, give the potential for improved IQ. Other factors also affect IQ, in different ways. In the end, in a digital camera, IQ depends of just four things:

i) The ability of the lens to direct photons reflected by the lens to the right place in the image plane.

ii) The ability of the sensor (plus it's optical low pass filter) to accurately record the number and location of those photons.
iii) The number of photons that go up to make an image.

iv) The ability of the sensor to record accurately the number of photons of different wavelengths such that an image can be reconstructed that stimulates the colour vision of the eye in the same way as the original scene.

Number (iv) you will notice is rather complex, and seems to be becoming a more discussed part of the IQ equation. Nonetheless, I'm going to ignore it here, since it is not so dependent on pixel count.

Looking now at number (ii), the more pixels a sensor has, the more accurately it can record the location of the photons landing on it. By itself, changing the number of pixels does not affect a sensors ability to record the total number of photons, so to that extent, more pixels increases IQ, absolutely.

The other issue in the IQ equation is the size of the sensor. At first sight, a larger sensor captures more photons, so should result in higher IQ. However, it's not that simple. If you aim to take exactly equivalent photos (that is with the same field of view, depth of field and shutter speed) with two cameras with different sensor sizes, it turns out that the number of photons going into the image is exactly the same, so from that point of view, all sensor sizes produce the same IQ. However, while this is theoretically true, generally the bigger the sensor you have the higher IQ you can produce. There are two reasons for this. Firstly, people don't always shoot equivalent images. Professional and enthusiast cameras tend to have better focussing systems, which mean people shoot with less depth of field which results in larger apertures and therefore more photons in the image. Secondly the resolution of the lens is exercised less by larger sensors, and in any case, lenses for professional/enthusiast systems tend to produce more resolution in realtion to sensor size than the smaller sensored 'point and shoots'.

Issues such as diffraction and per-pixel noise, which are often quoted as a determinant of IQ, are frankly a complete red herring. they only affect things if you compare cameras in non-equivalent situations, like different output image sizes or different dept of field. Keep things equivalent, and all else being equal the more pixels the better the IQ. Of course, all things aren't always equal, some sensors are more efficient at converting photons to signal than others. If a big, high pixel count sensor is very inefficient, it may be beaten by a more efficient sensor which is smaller, or has less pixels.

I know that a whole load of people are going to say what i have written is rubbish. They may say that, but none will be able to post a concrete example illustrating that it is.

Iforgetwhat8was4 wrote:

or are they?

Could someone try to give a clear but comprehensive explanation of
why more Mp doesn't improve IQ?

I understand that more pixels means more noisy pixels, and i've seen
that in my own cameras.

But should twice as many pixels give twice the dynamic range, simply
because you could sample two pixels for a value with twice the range
of a single pixel?

Isn't it a question of whether NR is losing detail that wouldn't be
there in the first place with less pixels?

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