Output sharpening for projection

[Paul Bench]

I am using the Lightroom export function to create jpegs which are intended for use through a high quality projector and for this I resize (from larger originals) to 1400x1050 pixels.

The use of a projector is new to me so I am trying different amounts of output sharpening to see if there is an optimum set of parameters for this setup. I select Sharpen for Screen and then have the option to select Low, Standard or High.

My question is does the Resolution PPI setting affect the amount of sharpening as well?

I believe that I have seen references that it does, including on a thread on this forum (http://www.luminous-landscape.com/forum/index.php?topic=66916.0 ). However when discussing this with other members of my camera club they are adamant that Resolution PPI does not affect the level of sharpening for screen. Which is it?

I currently use a Resolution value of 72 PPI, if this does affect the amount of sharpening is this a good value to use for a projector?

Paul

[digitaldog]

My question is does the Resolution PPI setting affect the amount of sharpening as well?
I currently use a Resolution value of 72 PPI, if this does affect the amount of sharpening is this a good value to use for a projector?

It's the total number of pixels over each axis (WxH) that's key in terms of an output size, and what will be examined. Now that said, if you have 1000 pixels, the 'size' will differ if you divide 72 or 101, so set that such you get the size you desire. Then the sharpening will be applied based on that information.

In terms of the best settings, all are rather subtle and you'll likely want to run a test. I suspect in any case, you'll visually prefer some output sharpening to none, assuming capture sharpening is appropriate.

[Bart_van_der_Wolf]

I am using the Lightroom export function to create jpegs which are intended for use through a high quality projector and for this I resize (from larger originals) to 1400x1050 pixels.

The use of a projector is new to me so I am trying different amounts of output sharpening to see if there is an optimum set of parameters for this setup. I select Sharpen for Screen and then have the option to select Low, Standard or High.

My question is does the Resolution PPI setting affect the amount of sharpening as well?

I believe that I have seen references that it does, including on a thread on this forum (http://www.luminous-landscape.com/forum/index.php?topic=66916.0 ). However when discussing this with other members of my camera club they are adamant that Resolution PPI does not affect the level of sharpening for screen. Which is it?

Hi Paul,

Apparently, as it is implemented in Lightroom, the PPI setting in Export does change the sharpening that is applied. It is a bit of a black box, and it is not immediately obvious how the sharpening for a display is different from the sharpening for projection. The difference is related to the viewing distance for the projection compared to the viewing distance for a display.

I assume that you could get the best results by calculating how the PPI of the projection at a larger viewing distance translates to a PPI for a normal reading distance or a viewing distance of a display, as follows. If we assume that the projector uses a resized to 1024 pixels wide image, and projects that on a screen size of say 6 feet wide, that would produce a 14.22 PPI image (1024px / 72in = 14.22 PPI). When you view that screen from a 6 feet distance, that is between 3 - 6 times the normal viewing distance of a display, so you use a sharpening PPI of between 3 to 6 x 14.22 PPI, i.e. something between 43 to 85 PPI. Just need to adjust the parameters for your actual projector resolution and viewing distances, and resize the images on output to the projector specifications with the proper calculated PPI setting.

Cheers,
Bart

[digitaldog]

Apparently, as it is implemented in Lightroom, the PPI setting in Export does change the sharpening that is applied.
Bart

I just exported the same image using these settings:

1000x1000@72
1000x1000@300

Subtracted the two in Photoshop: result is they are identical. And that makes sense to me, the resolution tag (72, 300) is meaningless, the number of pixels is key.

and it is not immediately obvious how the sharpening for a display is different from the sharpening for projection

LR only supports output sharpening "to screen" and to print with two options based on paper type.

[Bart_van_der_Wolf]

I just exported the same image using these settings:

1000x1000@72
1000x1000@300

Subtracted the two in Photoshop: result is they are identical. And that makes sense to me, the resolution tag (72, 300) is meaningless, the number of pixels is key.

Hi Andrew,

Well, then Jeff Schewe was apparently wrong when he said:

The output sharpening is based on the final PPI of the exported image, so 360PPI will have different sharpening than the 72PPI exported image even if the image has the same pixel dimensions...

Cheers,
Bart

[digitaldog]

Well, then Jeff Schewe was apparently wrong when he said:

Hum... all I can say is, the two exported images were subtracted and were pixel per pixel identical. I'll try the same test again to verify.

Sure he's not referring perhaps to the Print module and the ability to alter the output there?

[digitaldog]

Tried the test again, different image, same results.

Testing with Lightroom 5 FWIW.

[JRSmit]

In the export form there is also a setting sharpening for print or for screen. That does make a difference. Whether the ppi does, i did not test, but always set it at 100. As i recall Jeff mentioned that sharpening for screen was made to work in the range of 70 - 120 ppi.

[Paul Bench]

I have repeated Andrew's test with the same results. I also included a test for Sharpening Screen High and Low, and found that the differences were more noticeable when using Blend Mode Difference rather than Subtract.

My tests covered both the extreme ends of the range (1 ppi vs. 65000 ppi), as well as tests within the 70-120 ppi range mentioned by Jan. To ensure image content wasn't implicated in the null results I also ran the test with Martin Evening's test image (Lightroom 4 Book, page 358, Figure 6.7).

Cheers,

Paul