I'm experiencing a problem about exporting pics from Lightroom, taking them to the photo lab to print (4x6's) and resulting size is never correct. A lot of the pic are cropped or is too small. I can see the whole picture but only a bit shows up.
I have a Canon 5DMkIII, which is a full frame sensor camera, so therefore I really am not understanding the 4/3 or 16/9 ratio that you mentioned.
I photo lab tech (at Costco) who uses Lightroom mentioned that it's the aspect ratio that I have to set in the Develop module of Lightroom. Then someone else in the lab mentioned it's the use of the Sync button. I've tried both, nothing works.
I'm ready to drop Lightroom and use a different photo developing application. I desperately need help and quickly please.
It's not a Lightroom issue.
If you print on 4x6 you need to make sure that you crop your image to the same aspect ratio before exporting. You should then have a minimal loss on all sides if the lab is working good.
Learn here abour crop: https://helpx.adobe.com/lightroom/how-to/lightroom-crop-image.html
4x6 is the same aspect ratio as your camera sensor, so if you do any cropping in LR, make sure that the padlock icon is locked. Click on it to toggle between locked and open.
When locked, it will always crop with the aspect ratio of the original.
The native aspect ratio of your camera is 2:3. If you have set it up to take pictures with a different aspect ratio like 3:4 or panorama, you will need to crop the image with a 2:3 ratio in Lightroom to make the image fill the paper. (click the arrows nect to the padlock)