If you are the member of any photography message board you have probably seen people either post or ask for 100% crops of images to check focus or sharpness. 100% seems like a misnomer since what people are really asking for is "a crop as if you were viewing the image at 100%." Since a really 100% crop would just be one that includes the full image.
This post will show you 4 ways that I have used in the past to provide a 100% crop. All will be of this full image:
Shot at longwood gardens with my 5Dm2 and 100mm L macro.
1) Screen Shot - I usually do this in LR, but you could also do it in PS. First zoom into your image so that you are viewing it on your screen at 100%. In LR you do this by clicking on the image or above the preview image in the upper left select the 1:1 view. In PS select View then Actual Pixels.
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Then on a mac select CMD + Shft + 4 and cross hairs will show up, draw a box around the part of the image you want to share.
2) LR Crop - Create a virtual copy of the image (CMD + ') and then using the crop tool crop to just the part of the image you want to share.
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Then export the image with no size constraints.
3) PS Crop - Open the full sized image in PS. Select the Marguee tool (M). Set the Style to "Fixed Size" and enter the dimensions you would like to use in pixels. I entered 800 px for width and 600 px for height. A selection box will appear on the image, move it to the part of the image you would like to share.
Them select Image - /> Crop. Save this as a new image.
4) Full sized image - This isn't really a crop but it is a way for others to see your image at 100%. Upload the full image to a host that will not resize the image (such as flickr), and post a direct link to the full sized image.