Cropping and image quality

[oldgreygary]

I would be interested to hear from anyone on how cropping affects the quality of the image? I am uncertain as to what is happening to the image when it gets cropped. Or does adjusting the ppi amount have more of a detremental affect? Is there a tipping point were you crop and the image quality begins to drop off?
The camera I am using is a Canon G2. I shoot in RAW and the image size in Lightroom is shown as 2272x1704.
I know that some of the responses will say that I should get the composition right in camera which is fine. But the problem I have with the Canon G2 is limited focal length. So, for some, not all shots I have to do a fair amount of cropping to get the image that I want.

Cheers for now

Gary

[IanCD]

Hi Gary,
When you crop you reduce the number of pixels in the image. For example, if you take one of your images and decide to make a square (1:1) crop, leaving the short side as shot, the new image will be 1704 x 1704.
It sounds, though, as if you're cropping to get a Field of View equivalent to longer focal lengths - which is a bit like using digital (rather than optical) zoom in the camera.
In either case, you're reducing the resolution of the total image, but you're doing this by reducing the amount of image you're using: the amount of detail in the cropped (portion of the) image will be the same as it was originally. Whether this has a noticeable impact when you look at the picture depends on how much you crop it and how big the final image you're viewing is - if you magnify the final viewed image x 2 compared to the uncropped image to get the same size on screen or print, then the image may well appear more blurred. How far away you're looking at it from also affects this, but on screen and for anything other than large print sizes I'd guess this is likely to remain fairly constant.
I'm a little less clear on ppi and dpi, and the difference between them, so I'll leave someone else to tackle that bit in more detail - though reducing ppi also reduces resolution, so I think the same factors apply regarding noticeable differences in the final viewed image.

[herbert]

If you are showing your images online then you can see the effect of cropping using the 100% view. This will show every pixel in the image using a single pixel on the screen. So when you crop, view at 100% and that is what your image will look like without resizing.

If you are printing then you need to be aware of the PPI (pixels per inch). A good benchmark for fine printing is 300 PPI. This produces a resolution that is at the limit of human vision for a viewing distance of 12-18 inches. Less than 300ppi and you can start to see pixels in the print when holding it at reading distance (i.e. the image will not look as nice). Of course you can see some image problems if you look through a magnifying glass but no reasonable person does that so 300ppi is a good standard.

Original = 2272 pixels = 7.57 inches @ 300 PPI
2x crop = 1136 pixels = 3.79 inches @ 300 PPI
4x crop = 568 pixels = 1.89 inches @ 300 PPI

As you can see from the simple numbers you would not expect cropping to yield very good prints. Your original image will print very nicely at postcard sizes. You could use a lower PPI to make the image bigger but then you will start to be able to see the pixels on the print. However if using the images online then you will be able to crop a bit and still have a reasonable image since most online images are less than 1000 pixels on the long edge.

Alex

[Geoff F]

It tends to depend on the reasons for a crop and the actual software used, Gary.

Probably difficult to explain; I'm sure someone else can do it better and shorter.

If you are cropping and reducing the number of pixels proportionally your quality should remain the same although viewing the image may look different, as Ian said.

Some software settings may auto resize the image by adding extra pixels which can cause problems.

If your original image is greater than the intended print or screen size you can safely reduce your pixel numbers until you reach that size. You are only throwing away what you didn't need anyway. But you are now viewing the real full size image so any flaws will be more apparent. Just like zooming in on your screen.

One thing which I've never understood is, when you crop to reduce an image size, where do those 'thrown away' pixels go?

Choosing or changing the ppi can make a considerable difference on the end result. For example, printing at too low a ppi, say 100 ppi or less, produces a poor result. But I often find that having 'too many pixels' on a screen image image makes that appear soft and it actually looks sharper if I reduce the ppi to screen resolution (96 ppi in Europe) or zoom in a bit.

PPI and DPI are often used to mean the same thing although in reality Dots Per Inch is a printing term which actually refers to the placing of ink dots on paper.

When I was originally attempting to understand image resolution I changed the word pixels to pixies. Your pixies are standing in nice orderly rows but have a choice of hat colours. Which hat they wear (red, green or blue plus black) creates an image when viewed from a distance.

Like a crowd of people holding coloured cards in front of them at some displays etc.

When the pixies are standing close together their hats form a good tight image but as they move apart gaps start to appear between them and the scene looks a bit 'blocky'.

If you increase your image size and add extra pixies they look at what colour hats their neighbours are wearing and use that to decide their own hat colour.

But if you are doing a substantial downsize you are removing pixies and forcing those remaining to move closer together so the result can appear a little on the soft side. In which case, a little bit of extra sharpening can be needed to keep things crisp.

Does any of this make sense?

ps. And as I said in another post about those who recommend 'zooming with your feet' to get closer to the subject. Try it when standing on a cliff top; 'After you, Sir'!

[oldgreygary]

Thanks for your replies. Yes, it does make sense. I think Ian hit the nail on the head when he rightly says that by cropping I am trying to get a field of view equivalent to a longer focal length. I guess that I am trying to squeeze as much as I can out of the G2 while not compromising the quality too much. I have also just started to think about printing. So that was another reason that the quality issue was in my thoughts.

Dumb question, but am I right in assuming from the posts that more megapixels equals larger print sizes? But, if I was only ever producing images for the screen then something like 4MP would be more than adequate. In fact if I had the ability to have a longer focal length on the G2 then the images would be adequate for both screen and print (at approx. postcard size?) That leads to another question why do cameras have 18MP+ as if you are only ever going to display on screen then a huge amount of pixels must be wasted? Even for printing this is not required?

Cheers for now

Gary

[IanCD]

Dumb question, but am I right in assuming from the posts that more megapixels equals larger print sizes? But, if I was only ever producing images for the screen then something like 4MP would be more than adequate. In fact if I had the ability to have a longer focal length on the G2 then the images would be adequate for both screen and print (at approx. postcard size?) That leads to another question why do cameras have 18MP+ as if you are only ever going to display on screen then a huge amount of pixels must be wasted? Even for printing this is not required?
Gary

More pixels doesn't necessarily equal larger print sizes, but does give you that option. Colin (Southern) is very fond of the mantra that most pixels are 'wasted' if all you're ever going to do is display on the web at, for example, 1280 x 860 pixels... and in a sense that's right... But then not everyone wants to view their images at only the resolution that will fit on a 1280 wide screen... We might want to print bigger than A3 (without images becoming blurred), or view detail on screen at much higher resolution, and that's when you do need more resolution (= higher megapixel count).
Ian

[DanK]

YOu might find it helpful to look at the minimum file sizes that Smugmug recommends for the three labs they use:
http://help.smugmug.com/customer/por...n-for-printing