please explain pixel density

[yeux tortu]

D700 and D300 have same output of 12 megapixel. I understand that these pixels are on different size sensors so that per mm there are

more pixels on the D300. Does this theoretically mean that the detail is greater due to finer digital grain?

[Matt Laur]

If you use lenses on each camera that produce the same field of view, then the level of detail (in terms of the number of pixels used to record each object in the scene) will be the same. Roughly speaking, if that lens is a 30mm lens on the D300, it would be a 50mm on the D700.

<br><br>

By equipping each camera thus, you're projecting essentially the same image onto each camera's sensor. The the same number of pixels recording an image that now fills each sensor in the same way, then <i>for that image</i> you've got the same pixel density.

<br><br>

But, if you used the SAME lens on each camera, the D300 would record a smaller <i>portion</i> of the projected image... but using the same number of pixels. So, for that <i>portion</i> of the image, it has higher pixel density... but you're missing out on the rest of the image. That's why the DX format favors people who like to use longer lenses (say, for sports).

<Br><br>

All that being said, there are other optical issues here (say, performance at high ISOs in low light) that will contribute to differences in the quality of the recorded image. Hope this helps.

[carl_johnson3]

A 12mp DX sensor has the same pixel density as a 27mp FX sensor (which is 2.25 times the area).

[mike_davis]

In addition to the issue of noise associated with increased pixel (photosite) density, thanks to the effects of diffraction, there is a correlation between pixel density and the number of f/stops you can use, from among those available on a given lens, without inhibiting a desired print resolution at an anticipated enlargement factor.

 

The f-Number at which diffraction will begin to inhibit a desired print resolution for an anticipated enlargement factor can be calculated as follows:

 

N = 1 / desired print resolution in lp/mm / enlargement factor / 0.00135383

 

An increase in the desired print resolution requires a smaller f-Number.

 

An increase in enlargement factor requires a smaller f-Number.

 

Conversely...

 

Use of a larger f-Number (smaller aperture) requires either a decrease in your desired print resolution and/or a decrease in enlargement factor. If you insist on using f-Numbers that are greater than the value calculated with this formula, you'll have to make due with a lower print resolution and/or a smaller print.) Such compromises are frequently necessary with high-density sensors (>400 pixels/mm), but seldom necessary with low-density sensors (<200 pixels/mm). Unfortunately, the vast majority of people working with high-density sensors are completely unaware of the fact that they are suffering a compromise in print resolution when they choose to make prints as large as the pixel count encourages and fail to select the widest aperture (smallest f-Number) provided by the manufacturer.

 

As pixel density increases, the number of f/stops that will support a desired print resolution at an anticipated enlargement factor will decrease, as will the number of corresponding shutter speeds available for your use. For example, if diffraction forces you to shoot wide open to support your desired print resolution at an anticipated enlargement factor, you’ll also have a choice of only one shutter speed at any given ISO setting.

 

- Pixel densities at or below 110 pixels/mm, will permit you to use nearly every f-Number available on any lens that's purpose-built for a given format (this excludes using large format lenses on a 35mm-sized sensor, for example), without inducing diffraction that would inhibit a desired print resolution of 5 lp/mm in a print that's scaled to the dimensions had at an unresampled image resolution of 360 dpi (necessary to support 5 lp/mm in a CMOS sensor that suffers losses due to the Bayer algorithm and AA filter).

 

This is true, whether we’re talking about a 5 MP sensor or a 50 MP sensor, a 7mm diagonal sensor or a 70mm diagonal sensor. As the pixel density increases, the number of stops that will support a desired print resolution of 5 lp/mm in a print that's scaled to the dimensions had at an unresampled image resolution of 360 dpi will decrease.

 

- At about 200 pixels/mm, roughly half of all the f-Numbers offered on a lens (and half of all the shutter speeds that would otherwise be available at a given ISO setting) will be incapable of supporting a desired print resolution of 5 lp/mm at the enlargement factor had when the print is scaled to a corresponding image resolution of 360 dpi (required to compensate Bayer and AA losses).

 

- At about 400 pixels/mm, whether you are using a 5 MP sensor or a 50 MP sensor, a 7mm diagonal sensor or a 70mm diagonal sensor, if the combination of pixel count and sensor dimensions yields a pixel density that high (>400), you'll find yourself limited to shooting wide open or nearly so, with only one or two corresponding shutter speeds from which to choose, if you are to prevent diffraction from inhibiting a desired print resolution of 5 lp/mm in a print that's scaled to the dimensions had at an unresampled image resolution of 360 dpi (necessary to support 5 lp/mm in a CMOS sensor that suffers losses due to the Bayer algorithm and AA filter).

 

- At 600 pixels/mm and greater, (as with the extremely dense sensor found in the Pentax Optio A20 and A30, for example, which has a density of 636 pixels/mm), you will find yourself with absolutely NO f-Numbers capable of supporting a desired print resolution of 5 lp/mm at the enlargement factor had when the print is scaled to a corresponding image resolution of 360 dpi (required to compensate Bayer and AA losses). The lens simply won't be fast enough to prevent diffraction from inhibiting that resolution at that enlargement factor.

 

Even if your personal choice of a desired print resolution is only 1 lp/mm (equivalent to an image resolution of only 72 dpi, taking Bayer and AA losses into account for CMOS sensors) the fact remains that any increase in pixel density (whether we've reduced sensor dimensions or have increased pixel count) will have a corresponding decrease in the number of stops that will support your desired print resolution of 1 lp/mm. With high-density sensors, you are welcome to use all the stops made available on the lens, but you'll have to reduce either your desired print resolution or the enlargement factor.

 

Mike Davis

http://www.AccessZ.com

[ilkka_nissila]

In addition to diffraction, which primarily affects performance at small apertures, towards large apertures, the

MTF of the lens declines due to lens aberrations. The MTF at wide apertures is lower at the higher spatial

frequencies sampled by the DX sensor.

 

In practice, if you shoot an f/2.8 lens at f/5.6, things look roughly equal for the images captured with 12 MP FX

and DX sensors. But at either small or large apertures, the larger sensor gives a significantly better image.