Image size and reolution

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Photofan

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Hi
I have to produce jpeg images which are exactly 4000 pixels on longest side with an aspect ration of 3:2

I understand how to do this on export so that is fine.

However the final image has to be between 2 and 4 MB in size.

Is the size taken to be what it says in Explorer/Finder (mac) - ie 664KB? or is it 30.5MB as displayed when I look at the image size in Photoshop?

attachment.php
(image size as in Finder (mac)
attachment.php
(image size when opened in Photoshop)

Thanks - I will have further questions relating to LR when I have got this one clear in my head? (I still dont understand why the resolution doesn't make any diffeerence to file size - but maybe that is another topic!)

Joy
 

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Pixels per inch isn't relevant to the size of the final image: Quality is what makes the difference. If you need more info, suggest you buy Victoria's Missing FAQ's :)

I don't run a Mac, but I assume that Finder is correct. 30Mb sounds more like the original file size.

Dave
 
JPEGs are always compressed. Finder is telling you the size of the compressed image. When you open a compressed image in PS, you decompress it. JPEGs are 8 bit color images or 24-bit color per pixel. If you take the 4000X2667 pixels you get something over 10.6 megapixels. Each pixels requires 3 bytes to express it uncompressed so this is where your 30.5M is coming from. A JPEG file consists of a data block (the pixels) compressed using an algorithm that will provide a little compression or a lot. It also includes a Header block for all of the metadata and Thumbnail images. The more compression, the smaller the over all file. The more compression, the lower the quality of the image. So, JPEG compression is a trade off if file size is critical.

If you export a JPEG in LR, you can choose the output dimensions in pixels (4000X2667) and either a quality setting (a 0-100 scale representing 12 levels of compression) or you can tell LR to "Limit the File size" to 4000K (~4MB) and LR will choose the compression level to fit the exported file into that ~4MB requirement.
 
When exporting to JPEG there is a setting under File Settings called Quality. If you set it to 72, the JPEG will be produced as a 72DPI (dots per inch file) according to the Preview. This will produce a smaller file.

I don't know if there is a relationship between Quality settings and the Resolution setting under Image Sizing, for some reason I have forgotten when I choose 72 under quality I select 240 ppi under resolution.
 
When exporting to JPEG there is a setting under File Settings called Quality. If you set it to 72, the JPEG will be produced as a 72DPI (dots per inch file) according to the Preview. This will produce a smaller file.

I don't know if there is a relationship between Quality settings and the Resolution setting under Image Sizing, for some reason I have forgotten when I choose 72 under quality I select 240 ppi under resolution.
The Quality field is where to instruct LR to set the compression. The 0-100 scale in LR corresponds to the 0-12 integer scale field in PS. Any LR value above 92 will correspond to the PS setting of 12. Any LR value between 70 and 77 will produce a JPEG with the same compression and the same resulting size. The Quality Field on the Export dialog has nothing to do with resolution
In the "Image Sizing" section of the Export dialog, the filed labeled "Resolution" does not affect the image data or the size of the resulting JPEG file. It only places a value in the EXIF fields called "XResolution" and "YResolution". There is an additional EXIF field that describes the units (inch, cm or None) The default value for "XResolution" and "YResolution" if not value is supplied in the EXIF is 72. Most printers ignore the Resolution fields in the file header and so do most programs. You can safely leave this LR Resolution field blank.

Everything I know, I learned from Jeffrey Freidl
http://regex.info/blog/lightroom-goodies/jpeg-quality
 
Everything I know, I learned from Jeffrey Freidl
http://regex.info/blog/lightroom-goodies/jpeg-quality
Great article, thanks, I will do a deep dive on it over the weekend. By the way when I brought up 72 DPI, I didn't have prints in mind, I had a projector in mind. I can get my head around PPI, for prints, relatively well but it is not the same when a projector gets thrown in the mix.
 
By the way when I brought up 72 DPI, I didn't have prints in mind, I had a projector in mind. I can get my head around PPI, for prints, relatively well but it is not the same when a projector gets thrown in the mix.
For a projector, DPI doesn't apply. Sizing your image for a projector is the same as sizing your image for a TV, video, or digital device: What you need are the pixel dimensions of the screen. What are the pixel dimensions of the projector screen? 1920 x 1080 pixels? 1280 x 800? 1024 x 768? 1600 x 1200? Whatever that number is, that's the number you will want to enter into the Export dialog box. The Resolution can be anything so you can leave it at 72 dpi, it won't matter.

(For a projected image, actual dpi is variable. For example, if the image is 1920 pixels across and you project that 6 feet across, the resolution at the screen is 1920 pixels divided by 6 feet divided by 12 inches per foot, or about 26.6 pixels per inch. But if you move the projector in so that the image is only 3 feet across on the screen, the dpi goes up to 53.3 ppi. Then there's viewing distance; audience members closer to the screen perceive lower resolution than those sitting in back.)
 
For a projector, DPI doesn't apply. Sizing your image for a projector is the same as sizing your image for a TV, video, or digital device: What you need are the pixel dimensions of the screen. What are the pixel dimensions of the projector screen? 1920 x 1080 pixels? 1280 x 800? 1024 x 768? 1600 x 1200? Whatever that number is, that's the number you will want to enter into the Export dialog box. The Resolution can be anything so you can leave it at 72 dpi, it won't matter.
Typically 1400 x 1050.

(For a projected image, actual dpi is variable. For example, if the image is 1920 pixels across and you project that 6 feet across, the resolution at the screen is 1920 pixels divided by 6 feet divided by 12 inches per foot, or about 26.6 pixels per inch. But if you move the projector in so that the image is only 3 feet across on the screen, the dpi goes up to 53.3 ppi. Then there's viewing distance; audience members closer to the screen perceive lower resolution than those sitting in back.)
If I am reading this correctly, it follows that it is best to export a JPEG for projection with the highest quality possible because the projector itself reduces the resolution of the file projected image depending on the distance to the projected screen.
 
Typically 1400 x 1050.
If I am reading this correctly, it follows that it is best to export a JPEG for projection with the highest quality possible because the projector itself reduces the resolution of the file projected image depending on the distance to the projected screen.
OK, so 1400 x 1050 should be the export size for that projector. If many of the images will be magnified during the presentation you can increase those pixel dimensions; for example a 2x zoom on that projector would look best if the image is 2800 x 2100 pixels.

The projector itself doesn't reduce the resolution, it always puts out 1400 x 1050. The final perceived resolution is affected by the combination of projector distance to screen and the viewing distance of each audience member.

Setting the JPEG quality level is a separate issue from resolution. When exporting, I am not sure how much more quality the audience will notice above about JPEG 70 unless it will be a very nice projector, calibrated, displaying on a high quality screen that doesn't lose too much contrast. If you have the disk space you can export at maximum quality, but run a test to make sure the larger file size doesn't make it take longer to read and display each image, with no noticeable increase in quality.
 
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