I have had the same question from 3 different members of the club in the last month. The question and answer go something like this:
Question: I’m a beginner with Lightroom 5 and I found how to resize images for email; File .>Export > with Preset for EMail.
I set up a custom edit at 500K max down the bottom right corner, but the image when it arrives in my INBOX for a test is only 157kb!
What am I doing wrong?
Answer: You are doing nothing wrong.
Your image size when stored as a JPG will vary in size depending on the amount of complexity in the image. (ie how similar are the colour etc.) So an image with only a single colour throughout, even though its 1024×768 72DPI will be much smaller in kb than the same size image with a complex image. Its down to the compression algorithm for JPG. Setting maximum size to 500kb means—”don’t exceed that amount”. If the amount of compression required is low, then the file can be small and still have minimal JPG compression.
I’ve attached some images (on the right) to show you what I mean. Each of these images were saved at the same size 1024×768 72DPI . However the file size varies from 64kb to 503 kb depending upon the complexity of the image. The solid colour is the smallest file (64kb), while the 7 colour gradient is larger (181kb) and the sunflower is considerably larger again (503kb). They are all exactly the same resolution & size. The jpeg file size varies with the complexity of the colour content.
What I do in LR4 is use my own preset which has the following settings:
- Export to: Choose folder later
- Existing Files: Ask what to do
- Rename to: Custom Name
- Custom text: Name_Title_XXX
- Image Format: Jpeg
- Colour Space: sRGB
- Limit file size to: 500k
- Resize to fit: Long edge
- 1024 pixels
- 72 pixels per inch
LR5 should have something like that – the important settings are
- the File Setting of Limit File Size to 500kb and JPG/sRGB. The Image Quality is automatically adjusted (the more compression the worse it is)
- the Image Sizing – note the 1024 pixels on the long side and the resolution of 72dpi
In Photoshop you are asked to select a JPG compression ratio. A higher compression will result in a smaller file size. If the compression is too great you will see banding and pixilation of the colours. When using Photoshop, I start at the highest image quality (largest file size) and gradually turn up the compression (turn down the quality) until I get the file size just under 500kb. Sometimes I will achieve 495kb, but at other times the best I can achieve is 250kb. Lightroom will do this juggling automatically when you set the “limit file size” box.