1800 by 1200 is a 1.5 to 1 ratio, and 400 by 300 is 1.33 ratio – changing ratios will always distort the image.
What you can do, is crop the original image to 1600 by 1200 (ie, cut off a bit of the sides), and then you can resample down to 400 by 300 without any problems.
cheers
Klaas
To add to Klaas’ excellent advice, when you crop, you can use the Marquee tool and in the options bar type in the values you want, using the units you desire (pixels, inches, etc.) then you simply click, move the selection around to position it where you want, and Image|Crop.
No need to crop to 1600×1200 just type in the 400px for Width and 300px for Hight in the option bar after you select the crop tool. making sure you use the same resolution and crop.
That assumes that the you only want a 400by300 piece of the original. The OP mentioned reducing the image, so I assumed that it would be resampled down.
If you enter values in the options bar with the crop tool, you will resample your image using the algorithm that is the default in the Image|Image Size… dialog. If you use the Marquee tool, and use Image|crop, your image will not be resampled.
Thanks Tony, I didn’t know that (don’t use the crop tool) …
