Crop to A4 paper size

>What is the correct ratio for 20x15 and 30x20?

4x3 for 20x15 (this will have to be entered as a custom ratio) and either 2x3 or 4x6 (same ratio) for 30x20.

>Why are both 8x10 and 8.5x11 listed? (Only 8x10 is listed in PSE 6.)

8"x10" was a very common photo printing size in the U.S. in the days of silver-and-chemical printing. 8.5"x11" is standard U.S. letter size paper (instead of A4), which is also a common size for digital print paper. Few if any digital printing papers are made in 8"x10", but because frames and precut mattes are typically made for that size, it is common to make the prints on letter size paper, cropped for the older-size matte.

>how do I flip the crop between landscape and portrait?

Grab one of the corner crop handles and move it to where it would be in the other orientation. For example, if it's portrait, grab the upper right handle and move it down and to the right, and the orientation will become landscape.

(Jao made the same points while I was typing away.)

>What is the correct ratio for 20x15 and 30x20?

I am not sure about those sizes, but 20x15 has the ratio 4:3, so that is likely a digicam oriented format. The paper is probably really 8"x6", or 20.32x15.24 cm. 30x20 is simply a 6x4 ratio of course but larger. The real size is likely 12"x8" or 30.48x20.32cm, which is also a very standard size for the lab printers.

>Why are both 8x10 and 8.5x11 listed? (Only 8x10 is listed in PSE 6.)

8x10 is a traditional photography format (it's 200% enlarged 4x5) that has ratio 4x5. 8.5x11 is the aspect ratio of a letter-sized piece of paper. Letter is as you probably know what is used in the US in place of A4. It annoys me that Lightroom does not have A sizes as a standard ratio. The European A sizes all have square-root of 2 to 1 ratios (about 1.41:1) and nicely sits between 4:3 and 6:4.

>Lastly, how do I flip the crop between landscape and portrait?

Drag one of the corners of the crop below or above the 45 degree line with respect to the opposite corner and the crop will flip. It takes some getting used to. Not very intuitive indeed.

Thanks for the interesting information. Consequently, I should select:
1) 5x7 when I plan to make a 13x18 print.
2) 4x6 for a 10x15 print.

What is the correct ratio for 20x15 and 30x20?

Based on what you wrote, it seems to me that the ratio is 8x6 and 12x8, respectively. But neither are listed.

Why are both 8x10 and 8.5x11 listed? (Only 8x10 is listed in PSE 6.)

Lastly, how do I flip the crop between landscape and portrait?

>The standard (fixed) crop sizes seem to be in inches and not centimeters

No they are not, they are ratios. They do NOT have units. When you select 4:6, One dimension simply relaqtes to the other as 4 relates to 6. There is NO length scale involved. One thing you need to realize is that the machines in Europe very often are exactly the same machines used on this side of the pond. Silly enough, they will print on inch-sized papers. What this means is that when they tell you 10x15cm, it is actually 4x6 inches or 10.16x15.24 cm. For many sizes, this implies that the ratios in the rounded off centimeter sizes are not the actual aspect ratios. For example, a 13x18 cm print is really 5x7 inches. This is a slightly different ratio and cropping to 13x18 ratio will give you a wrong ratio for the actual paper size.

Lastly, be aware that most lab printers apply a slight bleed, which means that they slightly enlarge the image in order to be able to cut the edges so that you do not get white rims.

The standard (fixed) crop sizes seem to be in inches and not centimeters. When I crop and subsequently print a picture at the local shop in Germany, the sizes don't match exactly. The same is true in PS Elements 6. I have not found the preference where I can change this to the standard print sizes in Germany.

In the crop part of develop you crop to a ratio not to a size. You can define any ratio you want. Just go into the little popup. Alternatively, simply go into print, setup an A4 paper with no margins and make sure you set the image to scale. This will give you an instant crop at A4 when you print. You can move the image around to get a better crop if needed.