Exporting in megapixels not accurate. Please help

[gavins33007671]

Hi all,

I want to export to an exact megapixel amount in Lightroom Classic, but when I select this option to 5.0 megapixels for example, as shown in my photo, it exports at 1.29mb. I have also tried this by first entering the megapixel amount, then unchecking the resize to fit box but it gives the same size once exported as if it was ticked. Hopefully there's an easy fix here as I dont want to hav to do it with guesswork.

Thanks in advance / G

 

[Per Berntsen]

Megapixels and megabytes are not the same thing.

The number of pixels in an image is measured in megapixels (millions of pixels).

A 6000 x 4000 image contains 24 million pixels – 24 megapixels.

So when you set Resize to fit to Megapixels and 5 MP, you get an image measuring 2778 x 1800 pixels, if image proportions are 3:2.

The space an image takes up on disk (file size) is measured in megabytes, which has no direct correlation with pixel count.

Several factors influence file size – pixel dimensions and bit depth, and for jpgs also compression ratio and subject matter.

I'm guessing that you want to export jpgs with a particular file size, which is not possible, except by trial and error.

You can use the Limit file size option to keep the file size below, but as close as possible to, a particular file size.

This may result in unacceptable image quality, because  LrC might have to use a high compression ratio.

Unless you really want to export in megapixels, set pixel dimensions instead under Image sizing.

I normally use Long Edge, and the short edge will be calculated automatically.

You may find this article useful:

What is a digital image?

[gavins33007671]

Thanks for the detailed explanation. Good to learn the difference. Its for 2 reasons I wanted to achieve this, 1 to save precious HD space and 2, sometimes on the Web when uploading images somewhere, it requires image size to be under a specific size. So from what you've said it's better for image quality to use the long edge pixels method and trial and error with that measurement. 

[Keith Reeder]

"Its for 2 reasons I wanted to achieve this, 1 to save precious HD space"

You'd save more by not exporting until/unless you need to. That's not being glib - Lr will save all your adjustments, so you don't have to permanently commit them to an exported file.

"and 2, sometimes on the Web when uploading images somewhere, it requires image size to be under a specific size. "

5 Mb for a web-presented image would be enormous: by the same token, a 5mp jpeg file would still be more than enough to fill 60" a 4k TV screen to very high quality: most web images are significantly less than 1mp in size.

Seriously: forget about mbs and mps, and export to say, long edge. It's not about image quality, it's about need: I've never come across a web image host that's particularly fussy about file size in mb (unless you're uploading to some stock sites), so why worry about it if you don't need to?

[dj_paige]

The option you are looking at can somewhat control megabytes and has nothing to do with megapixels, which are two entirely different things. You can't get an exact number of megabytes, REPEATING IN CAPITAL LETTERS CAN'T BE DONE. Furthermore, there is no point in requiring an exact number of megabytes, its wasted effort that solves nothing. So there's no fix, and no reason to perform a guessing process.

As pointed out by @Per Berntsen perhaps you want an exact number of megabytes, in which case he has also explained how to do this.

[gavins33007671]

As mentioned to Per, the reasons I want exact MB are for uploading to various places on the Web that require image sizes to be below a certain size, often stated in MB. I'll use his recommended which I know isn't an en exact result in MB but close enough for my needs. coincidentally it was the same I was doing before, albeit trial and error and I wondered if there was a more accurate way, which I now understand there isn't, so trial and error it is which is fine

[dj_paige]

As mentioned to Per, the reasons I want exact MB are for uploading to various places on the Web that require image sizes to be below a certain size, often stated in MB. I'll use his recommended which I know isn't an en exact result in MB but close enough for my needs. coincidentally it was the same I was doing before, albeit trial and error and I wondered if there was a more accurate way, which I now understand there isn't, so trial and error it is which is fine

By @gavins33007671

Yes, your original message stated that you want the export to be EXACTLY a certain number of megapixels, and even if we understand it to be megabytes, you can't get an EXACT number of megabytes (and its not even reasonable to try to do this).

Now what you are talking about is different — now you want the export to be less than a certain number of megabytes, which is different, reasonable, and certainly do-able.