I get a lot of similar questions in email that I thought I’d periodically summarize and add here to hopefully help others.
Question: For a show submission, they require 1800 pixels on the longest side and 72 dpi. My image is 1400 x 1800 and 300 pixels per image. If I cange the 300 to 72, the pixels on the sides shrink to very little.
Answer: In the Image Size dialog box, uncheck “resample.” Resampling takes away or adds pixels. Thus, when you change 300 to 72, resampling tosses out the extra pixels and the resolution of the image shrinks. When you uncheck the resample box, the number of pixels do not change. Rather the size of the pixel changes. Bottom line: When going from a high ppi to a lower ppi, but keeping the resolution (or number of pixels in the document) the same, uncheck the Resample box.