Pixel density - can the playing field be leveled???

Started Jun 6, 2009 | Discussions thread
Jay Turberville Forum Pro • Posts: 12,917
Re: The answer of your question is right here

I said that resizing an image doesn’t change the noise level. It
simply makes the noise smaller, as with everything else in the image.

And I showed measurements that show that the noise level does change.

And I've made prints (and so have a couple others in the forum in
past discussions) showing that downsampled images, when printed,
display the same apparent noise as prints made from the full-res
image. The only image quality comparisons that matter to me are
prints at the same size.

All well and good, but that wasn't the OP's question.

This goes with my primary message to the OP in my first response,
which basically said that the noise in the images produced by the
15MP sensor and the 10MP sensor area already the same. No resizing
is necessary...printed samples will exhibit the same noise.

This may or may not be true. It all depends on the scale at which you print and the scale at which you observe. A number of things could occur.

1) The printing process itself can act as a low pass filter and noise and detail are lost compared to the original image. The result may or may not be equal apparent image quality.

2) The printing process and scale may preserve all of the image detail and noise of both comparison images, but:

a) at the viewing distance, the eye acts as a low pass filter and the noise effectively disappears to the viewer

b) The eye does not act as a low pass filter and the viewer sees slightly more noise (though at a smaller scale) in the higher resolution image along with slightly higher detail levels and experiences a general impression of quality that is about the same as the lower resoltuion image - or maybe an impression of slightly higher quality. It probably depends a lot on the image.

3) The printing scale and process (very large print), not only preserves all of the image detail, but shows it at a very large scale. The viewer might more clearly see more apparent noise as well as more image detail in the image from the higher resolution sensor.

If you’re saying that the downsampled 15MP image will have less noise
than the original 10MP image. I say produce the prints that
demonstrate it. If you don’t care about prints, then I guess we’re
at an impasse.

First, there are two original images. One from a 15Mp sensor and one from a 10Mp sensor.

Second, and for the last time, the question was never about printing.

As I think back about all the posts, I don't think you have been describing resolution reduction - which is what the OP asked about. You seem to be mostly talking about image scaling. These are not necessarily the same things.

If we imagine a crop of our two images from two different density sensors on two display devices that show the images at 100% pixels, and then move the higher resolution image further away until it has the same apparent image detail size as the lower resolution image (same viewing scale), we will observe that the size of the noise in the higher resolution image gets smaller and becomes less obtrusive. But, of course we know that the noise did not actually go away or change in any way. It obviously is still there. Depending on the viewing distance and our visual acuity, all of the noise and detail in both images may be preserved and still be viewable. I believe this is what your are attributing to your printing process.

But if we view the images such that the lower resolution image is being viewed where its finest detail is right at the limit of visual acuity, we can be assured that in the viewer's eye that some noise and some image detail in the higher resolution image hasn't simply become smaller and less obtrusive. It is actually no longer visible. The viewer's eye (lens and/or retina) is acting like a low pass filter. The eye limits the resolution. The image the brain sees is a version with less resolution. This is more akin to what I've been describing. When real resolution is reduced, some real noise is also reduced. This is the answer to the question that the OP posed.

BTW, this goes directly to the point that the more scientifically educated among us have been saying - that noise is also scalar. This also goes directly to a basic error on DPReview's part. They did the riight thing with resolution measurement by presenting it normalized to image height by using Line Widths per Image Height (LWPH). This method appropriately accounts for the different sensor sizes and gives the reader a good measure of the real differences in image detail that can be delivered to a final image - all in one number. It would be misleading and confusing to simply give the resolution as line pairs per millimeter (lp/mm). All of the super high density and super small sensors would have better numbers - which would be misleading and/or create a need for the reader to do futher calculations to do apples to apples comparisons.

When DPReview deals with noise they should do some similar sort of normalization for image resolution or pixel count. But they don't. They give the noise figures in values that amount to "per pixel" values when they should be "per image" values. While technically correct (just as lp/mm are correct), this can be misleading to those that don't understand the scaling/pixel count issues - just as lp/mm can be misleading to people who don't understand or fail to consider the image sensor size factor. The result is confusion, misunderstanding, and incorrect conclusions.

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