why does preset size cropping change resolution?

[Giles]

I have images taken on my not so new digital camera.
The default image size is 1600 x 1200 pixels @ 180 dpi.

I transfer the images to my computer and save each one as a tiff.

I adjust levels and save again (in a different folder).

I then crop to 4x6 and save again in a different folder, burn to cd and take that to the photo lab for printing.

It is my understanding that re-sampling should be avoided as it reduces picture quality.

I thought that since I was cropping the images, that all I would be doing is discarding pixels. However, when cropping in preset size mode re-sampling is taking place (from 180 to ~266). I checked the help file and noticed this;

"By default, when you crop a photo, the resolution remains the same as the original photo. If you use a preset size, the resolution changes to fit the preset."

What I dont understand though is if I didnt use a preset size I still could have cropped to 4x6 "manually" to keep the DPI the same.

I also read here;
http://www.arraich.com/ref/aatool_crop6.htm

That I could use the rectangular marquee tool in fixed size mode and could select 4x6 there to avoid re-sampling.

Why does the crop tool function differently?

Would resampling take place if I used the crop tool in the preset size mode AND entered 180 for the DPI?

Is there an easier way to avoid resampling / maintain quality when cropping large images to size prior to printing?

Thanks!
-Giles

 

[Sean]

It is my understanding that re-sampling should be avoided as it reduces picture quality.

That is precisely what the crop tool does, but it has to change the dpi to avoid re-sampling. There are 3 separate ways to control image size in PS: Pixel dimensions, document size, and resolution. However, many people get resolution and pixel dimensions confused. The difference, and how they correspond to one another is difficult to explain to people who don't understand it.
When you crop an image, you're simply cutting off extra pixels. But, if the dpi (resolution) stayed the same, the image would have to be re-sampled, either becoming larger or smaller in pixel dimensions.

Would resampling take place if I used the crop tool in the preset size mode AND entered 180 for the DPI?

Yes. It's better to think of dpi as ppi (pixels per inch). You're original document is larger than 4x6, so when you crop it to 4x6, Photoshop changes the ppi accordingly to fit each inch without re-sampling the image. So, setting the preset to 180 dpi would decrease the resolution of your image.
I don't know if I'm explaining this very well. If you don't understand, maybe some other members could help you better.

 

[Giles]

That is precisely what the crop tool does, but it has to change the dpi to avoid re-sampling.

Thanks for your reply. I thought changing DPI / resolution is re-sampling?

 

[ronmatt]

http://www.luminous-landscape.com/tutorials/understanding-series/und_resolution.shtml

My 2 C, Resolution is one of the most misunderstood things there in in digital photographt as well as design. Just when you think you've got it all figured out, your images do something wierd. This link may give you some insight.

 

[Sean]

Great link, ron! That explained resolution much better.

 

[Gaussian]

Welcome Giles.
I don't understand why you just don't crop the image without entering any in your option bar. The only thing that often matters is the height/width ratio, especially if you plan to put photos in an album, because it looks sloppy if all photos have a different ratio, unless you have a compositional reason for a different ratio, for example to really express the height, width or length of something.

With snap on it's easy to have the crop tool snap to the edges of the photo. You then continue by resizing the crop by moving a corner while holdig down the shift key (to lock the ratio). Then use the move tool to position the crop, maybe adjust the size again and press enter.

Personally I would avoid resizing with crop if quality is important. It's better to use the resize tool, it also offers 5 different interpolation methods.

 

[Giles]

Welcome Giles.
I don't understand why you just don't crop the image without entering any in your option bar.

Thanks for the welcome Gaussian.
All I really want to do is crop out a 4x6 area of each image for printing. As the image is natively 8x6 I thought I could crop out a 4x6 swath without having to resize, and therefor not have to interpolate / resample. Is this not the case?

Am I mistakenly thinking that the image was resampled when the resolution automatically increased after doing a pre-set 4x6 crop?

Thanks,
-Giles

 

[sPECtre]

You've got resampling if the number of pixels inside the cropped area change after cropping .
Resampling means that new pixels are created, usually by interpolation.

BUT, say that you keep the same number of pixels, but extract a very small area out of a huge image, you have no resampling, but might end with unsufficient resolution for quality printing.