Crop & resolution

[whited3]

Help, I'm confused

ie I have an image taken with the 7D of 17.28” x 11.52” @ 300 dpi which I want to crop using a 1.8:1 (W/H) ratio.

If I crop this image free hand (no width/height set in the crop tool) the 300 dpi is maintained, regardless of the W/H dimensions of the crop.

If I create a crop preset of say 17.28" x 9.6" (1.8:1) and apply it the resolution is 300 dpi. If I apply the same preset but resize it smaller using the crop handles, the dpi is decreased.

Why is this so?

[Dave Humphries]

Are you intending to print it Mark?

If not, why worry about dpi?

I suspect you have a checkbox ticked you shouldn't, or vice versa - I work only in pixels (not inches), thus neatly avoiding this 'problem'.

I'll leave an accurate answer to someone that does this more than I do.

[whited3]

Are you intending to print it Mark?

If not, why worry about dpi?

Thanks Dave. Eventually yes, I will print. The print shop works in inches hence me working in inches and dpi.

[Colin Southern]

Hi Mark,

For the most part, DPI is irrelivant - it's simply the number of pixels along a given dimension divided by the length of that dimension, and in reality, "it is what it is". So if for example you have an image that's 3000 pixels wide - and you want to print it 10 inches wide - then 3000 / 10 = 300dpi. If on the other hand the image is only 1500 pixels wide - and you still want to print it 10 inches wide - then 1500 / 10 = 150dpi so you either have to accept the lower resolution (which would still look fine by the way), or only print it 5 inches wide if you're hell-bent on maintaining 300dpi.

It's irrelivant because at the end of the day, the image is only "X" number of pixels wide - you want to print at "Y" inches wide - and thus X divided by Y "is what it is". At the end of the day, you can't add information that's no longer there, so all you can do is print whatever you have at either the size or resolution you want (but not both). For what it's worth, I print all of my work at 180 dpi, and it looks just fine; I hear resolution like 300 dpi - 360 dpi - 720 dpi being bandied about all the time, but you really won't see much difference about about 180 dpi.

When you thing about it - if there are 25mm in an inch, then 300dpi is 12 tone changes per mm - or 144 potential tone changes per square mm; could your eyes resolve 144 tone changes per square mm at normal viewing distances? (mine can't, even with reading glasses on!).

So the short answer is "so long as you're not down-sampling the image when you crop, just don't worry about dpi".

[whited3]

Thanks Colin.

Something you've said got me thinking and I figured out what CS5 is doing.

In CS5 a preset crop has fixed dimensions of width and height. Let's say the width of the preset crop is 10". If I apply this preset crop unchnaged to an image that has a width of say 15" @ 300 dpi the new cropped image of 10" will be at 300 dpi. All is good.

Because the preset crop has fixed dimensions; if I apply the preset crop and then make it smaller using the crop handles I am actually enlarging the image to fit the preset's 10" width and therefore reducing the dpi. If I use the crop handles and make the crop bigger I’m capturing more pixels in the 10” crop thus increasing the dpi.

I think!

[Colin Southern]

In CS5 a preset crop has fixed dimensions of width and height. Let's say the width of the preset crop is 10". If I apply this preset crop unchnaged to an image that has a width of say 15" @ 300 dpi the new cropped image of 10" will be at 300 dpi. All is good.

No - all is very very bad. In this situation you've not only redimensioned the image (which should be non-destructive), you've also re-sampled it to keep the same dpi, but at a smaller size (in essence you've gone from an image that was 4500 pixels to one that is now only 3000. So in essence, you've thrown away a LOT of information. If you resize an image that's 15 x 10 @ 300 dpi to 10 x 6.67 inches, the dpi should increase to 450 dpi. The only time you'd keep the resolution the same when reducing the potential size of an image would be if you physically wanted a smaller file size because you needed to (eg) email the file(s).

In a nutshell, if you just want to RESIZE an image (in Photoshop anyway), pop into Image -> Image size -- make sure resample image isn't ticked -- and just plug in the dimensions you want. If you want to change the aspect ratio & resize at the same time then by all means use the cropping tool - BUT - make sure you leave the resolution field blank or it WILL throw away information.

[Colin Southern]

Just thought of a better way to put this ...

When you've resized an image - unless you're trying to make the file smaller - then the pixel dimensions of the image should still be the same after the resize as they were before. If they're NOT - then you're down-sampling and throwing away valuable info.

[whited3]

Thanks Colin. Just to clarify; I'm cropping a part of the image, not resizing the whole image. Are you and I talking about the same thing?

[Colin Southern]

Thanks Colin. Just to clarify; I'm cropping a part of the image, not resizing the whole image. Are you and I talking about the same thing?

Hi Mark,

That's the $64,000 question

Photoshop does it both ways ...

1. If the resolution field is blank (when using the crop tool) then it'll just cut off the excess pixels, but ...

2. If the resolution field is NOT empty then it'll cut off the excess pixels AND THEN RESAMPLE THE ENTIRE IMAGE TO MAKE THE RESOLUTION WHAT YOU ASKED FOR.

The 2nd option WILL throw away information if the specified resolution is less than what the resolution would have been after the resize.

I think what confuses a lot of people is that resizing an image (without any resampling) doesn't actually do anything (other than tell the image "you are now this size"). If I posed two image here - the first 1 inch long @ 300 dpi, and the 2nd 10 inches long @ 30 dpi, they'd both appear exactly the same (in fact I actually did it, but can't find the post anymore!). It's only when you printed the image would they physically appear different. And in terms of printing - for a given size - more is always better (even though you'll get to a point where you can't see the benefit of the extra resolution). Put another way, if 300 dpi is the ideal - and you only have 180 ... what are you going to do about it? and if 300 is the ideal - and you have 600 - there's no real downside.

So in summary - the golden rule is ALWAYS leave the resolution field clear when cropping, unless you really really really need to re-sample the image at the same time (which is almost never).

[whited3]

Thanks Colin, much appreciated

[rosapearl]

Excuse me for butting in, but if you have a "low resolution" photo upping the resolution in PS really doesn't help does it?

[Colin Southern]

Excuse me for butting in, but if you have a "low resolution" photo upping the resolution in PS really doesn't help does it?

Hi Jeannie,

It's debateable - it can't really add an information that's not already there, but it can help smooth things out a bit. For printing though (which is what Mark was originally asking about) it usually doesn't make any difference because the printer driver does this anyway (eg my photos are 180 ppi when I send them to the printer, but the printer always preint at 1220 DPI).

There are programs like Genune Fractals that claim to do a better job, but in my opinion, it's best to simply capture as much information as you can in the first place.

[rosapearl]

"...but in my opinion, it's best to simply capture as much information as you can in the first place."

I totally agree with that. I am trying to learn enough about RAW so I can take photos that way being as it is the best.

[Colin Southern]

"...but in my opinion, it's best to simply capture as much information as you can in the first place."

I totally agree with that. I am trying to learn enough about RAW so I can take photos that way being as it is the best.

Hi Jeannie,

Whooting RAW won't actually help capture images at any higher resolution, but there's more information available to (potentially) use in the pixels of information that are captured.