I ordered a number of standard-size mattes with an advertised opening of 20x28 cm, but found when I got them that the actual opening was 18.5 x 26.5 cm, which cost me quite a bit of photo real estate (not enough to ruin the images, as I'm not yet accustomed to cropping as tightly as I could). Is it normal that the advertised "inner size" is larger than the actual opening? Are there any standards for how much?
Thanks,
-Lars