Camera vs. Scanner -- Point and Counterpoint

In my last post received an interesting comment:

Unfortunately you have made an elementary error.
Resolution though quite important when comparing like for like cannot be used when comparing different types of camera or scanners.
Sensor size is far more important when comparing cameras.
You have also missed the effects of focus points on the output when comparing cameras and scanners.
A flatbed scanner retains the focus (multiple focusing) as it advances down the page whilst cameras may only have a single focus point or perhaps five focus points.
This causes the image to be vary in focus especially when large areas have to be imaged.
Cheers
Guy

Hmm. Did I "make an elementary error?" I reread my article carefully. I don't believe I had gotten to the point of using resolution to compare cameras and scanners, in fact, I said that measuring resolution was independent of the device's claimed sensor size or resolution. As I said, "the proof is in the pudding" which means you have to make the physical comparison and decide the difference in resolution between any two products. Bear in mind what I said in my first post, "But you can't beat physics. There are limitations to any optical system such as the wave length of the light involved and more importantly, the resolution power of the human eye."

Here are some of the factors that determine the ultimate resolution of any optically related digital device:

1. The quality of the optics whether a lens or some other equivalent.
I can have a Canon EOS 1D-X, a $6700 camera body, and use a cheap, poorly constructed lens and get bad results. Another example is increasing the megapixel rating on smartphones, without increasing the quality of the lenses, there may be no apparent increase in image quality.

2. The overall quality of the camera, scanner or other device.
A cheap camera is a cheap camera. It may make acceptable images for your purposes, but under a variety of circumstances, will not perform as well as a well designed and engineered camera. With scanners, the differences are a little more difficult to determine, but they are still there.  However, in some cases price is not an indicator of quality.

3. The number and density of the sensor elements.
This is usually translated into a term called "megapixels," meaning the number of distinct sensors in the sensor array. For this issue I refer you to an old Ken Rockwell post from 2008 that still, unfortunately, applies to this day. Here is one statement made in the post that stands out: "Camera makers use the number of megapixels a camera has to hoodwink you into thinking it has something to do with camera quality."

4. Image clarity, i.e. how the image was produced.
Camera movement and focusing are critical elements in producing a clear, high resolution image. I will address the issue of focusing as criticized in the comment below in this post.

There are other factors that include light conditions, the ambient temperature of the air and a lot more technical problems involving the introduction of "noise" in the electronic and engineering sense, into any system.

Oh, and I almost forgot. YOU CAN NEVER GET ANY BETTER RESOLUTION THAN THAT WHICH EXISTS IN THE ORIGINAL. If you photograph or scan a 300 dpi laser copy, that is what you get, not matter how sophisticated your reproduction equipment. In another example, an out-of-focus original photo cannot be "cured" by scanning.

Oh, and I almost forgot. YOU CAN'T AVOID THE PHYSICAL (I.E. THE SMALLEST POINT AT WHICH LIGHT CAN BE FOCUSED) LIMITATIONS OF ANY OPTICAL SYSTEM.

Now back to the comment. What role does the size of the sensor play in resolution?
There are dozens, perhaps hundreds, of articles online that discuss the issue of sensor size vs. resolution. Here is one from SnapSort.com that says "Learn about True Resolution." Now, if you are really interested in learning about how sensor size affects resolution or whatever, then consider the following statement from an article entitled "Digital Camera Sensor Sizes" from Cambridge in Colour.
"If you wish to maintain the same depth of field, larger sensor sizes do not necessarily have a resolution advantage." So, it turns out that sensor size is only one factor in determining resolution.
The commentator quoted above confuses depth of field with focus. The subject is much more complicated than it appears. I haven't gotten to the issue of focus at all. I could go on, and I probably will but in another post. Here is the point I am trying to make:

If you are deciding whether to use a camera or a flatbed scanner or whatever, you absolutely need to compare the images resulting from you own equipment to see if one is better for you than the other. When I get the opportunity. I will show the differences in detailed images. The commet makes the assumption that the focus of a scanner is the same across the entire document and that a camera cannot duplicate this focus. Hmm. Let's see if that is really the case.

Thanks for the comments, they keep me honest and help me explain my subject better.