LR6 Problem with getting smaller File size when Exporting

[rollsman44]

 I can never seem to figure out why this happens .  I import Raw files to LR make my adjustments and select all the ones to be Exported
   I select folder on my desktop to where its going and I select Megapixle size( say 18Meg)
   After the images are exported to my desktop I open the image and see its NOT 18 Meg but say 10
  Happens all the time as long as I have been using LR several years. I just accept it as is.  Thank you

[pegelli]

Are you talking about "image size" or "file size"

I've never noticed any differences in image size to what I have specified, but you set this by pixels per side and not by total megapixels.

If you mean file size (and then it's Megabytes, not megapixels) it's only an upper limit ("limit filesize to"). If your settings don't have enough pixels or too low quality and the Megabytes stay below your setting the file gets smaller, only when it's too big it lowers quality to meet the maximum you specified.

[rdonson]

What does your EXPORT dialog box in Lightroom look like?? 

[rollsman44]

See attached image .  Thank you

 I entered 14 Megpix for size. Is this correct? I am trying to get a BIG File size to make large Prints ( 17x20, 20x24)
   If I have been doing it wrong can you tell me the correct way to size my images.  Thank you
   I put 14Meg, and my file size when its exported to me desktop comes out to be about 6 -8 Meg  and not the 14 I put in.

[pegelli]

Uncheck the box labeled "don't enlarge" and see if that does the trick.

[rollsman44]

  Ok I will try that later and get back to you

[Wayne Fox]

See attached image .  Thank you

 I entered 14 Megpix for size. Is this correct? I am trying to get a BIG File size to make large Prints ( 17x20, 20x24)
   If I have been doing it wrong can you tell me the correct way to size my images.  Thank you
   I put 14Meg, and my file size when its exported to me desktop comes out to be about 6 -8 Meg  and not the 14 I put in.

Why are you using the “megapixel” option at all?  If  you are trying to get a big file, just output a full size tiff file at the native dimensions.  If you want to make it bigger than the size, then use pixel dimensions.

As far as best way to size your images, let us know your workflow.  Are you sending these to be printed somewhere?  In all of the years I’ve used LR I”ve never once used the resize file option set to use megapixels as the size criteria.

[rollsman44]

I export them to a file on Desktop and I have them printed in an outside lab. I am not sure about how to do it by dimension. For ex: I need various size prints
  I would appreciate if I can learn the Best way . Some labs don't accept Tiff files so I use JPEGS, Thank you,  Dennis

[Slobodan Blagojevic]

Forget about "resize to fit megapixels" option, that simply doesn't work in LR. Probably a bug.

The only way to resize your pictures is to determine the required ppi (pixel per inch) for the media (paper, canvas, etc.) and multiply that by the desired size of the print. Say you want the best quality 20x24 from a paper, you'd work with something like 300 ppi. The resulting file dimensions would be 6000x7200 pixels. If you are printing on canvas, you could get away with 150 ppi or even 100 ppi, in other words, file dimensions might go as low as 2000x2400 pixels.

If your original file does not have enough pixels for the 7200 pixels on the long side, just let Lightroom interpolate (enlarge) it to those dimensions. Or you can use something as 180 ppi (instead of 300 ppi) and still get reasonable quality.

[pegelli]

I agree using the "Megapixel" option isn't the way I would do it, but if it's what the OP wants it does work (no bug).

Just tested by exporting a native 6 MP picture to 20 MP (and the box "don't enlarge" unchecked) and it produced a jpg of 3647 x 5485 pixels (= 20.0038 MP, so close enough)

Checking the "don't enlarge" box kept the picture in it's original dimensions (6 MP)

[rdonson]

If this is to be sent out to be printed professionally is there really a need to resize in Lightroom?  Most professional printers I know have RIPs that would likely do a far better job of resizing than Lightroom. 

[rollsman44]

  Thank you everyone

[Slobodan Blagojevic]

Forget about "resize to fit megapixels" option, that simply doesn't work in LR. Probably a bug...

I spoke too soon. What I meant is that it doesn't work as most people expect. People mostly use it hoping to arrive to a specific file size in MBs - Megabytes. As in: "do not send files larger than 50 MB," for instance.

Lightroom's "Resize to fit" function is, however, designed to deliver specified Megapixels, not Megabytes. Whether the output file is jpeg or tiff, 8 bit or 16 bit, Megapixels are always going to be the same, as they represent file's dimensions. What is going to change are Megabytes, depending on subject matter, compression algorithm and desired bits.

For instance, in the attached example, an originally 8-Mpx file from Canon 20D (3504 x 2336 pixels) is enlarged to 20 Megapixels (5477 x 3651 px).

The jpeg Megabytes size is only 5 MB, due to 8-bit file and compression, while tiff files are either 120 MB (16-bit file) or 60 MB (8-bit file).

How does a 20-Megapixel file become a 60 Megabytes one? Color files have 3 RGB layers, red, green & blue, and each layer is 20 MB, as requested. Having a 16-bit file doubles the output in Megabytes to 120.