How to tell if a photo has been resized?

Question

I'm curious if photographers that upload to various photo service sites such as www.pictage.com (no longer in business apparently) or www.gradimages.com can unwittingly have their content resized/resampled. I'm trying to make sense of how these images I bought are found with such a low dpi/small size. It's hard to imagine a pro would go cheap on the resolution. I know one was an aspiring amateur but the other I would assume was a professional taking grad school photos.

To share some data

WeddingPhoto1.jpg
Width 3744 pixels
Height 5616 pixels
Horizontal resolution 72 dpi
Vertical resolution 72 dpi
Camera model Canon EOS 5D Mark II

GraduationPhoto1.jpg
Width 1488 pixels
Height 2240 pixels
Horizontal resolution 300 dpi
Vertical resolution 300 dpi
Camera model NIKON D70s

The first is a poor DPI resolution, the second is on the small side. I'm wondering is there a table or some way of knowing that the details found in an image's metadata correspond to anything the used camera is capable of natively generating?

I would also welcome any commentary or insights into the particular values seen for these photos, whether they'd be up to the task of printing say an 8x11 or poster size, etc.

Answer 1

The DPI is insignificant. It means nothing until the image is printed. Cameras almost always just put 72 DPI in the metadata, although on a handful of models it is configurable. Still, no matter what is set there, it will be wrong every time a different print size is used.

The first image is exactly as the camera shoot it. The Canon 5D Mark II is a 21 MP camera and makes images of 5616x3744 pixels at its maximum resolution.

The Nikon D70 is a 6 megapixel camera and is much older. That one produces images of 3000x2000 pixels, so the image has been changed from its original. It is possible it was scaled down but it is even more likely that it simply was cropped. When shooting people, particularly with movement, it is safer to shoot a bit wider and crop after to produce a more compelling image.

Answer 2

There is a huge problem with units out there.

The resolution in dpi does matter only when comparing two images from the same distance. What actually matters is points-per-degree.

if you are looking at billboard placed far on a building you can see an image of acceptable quality even if it has resolution of 1dpi. On the other hand, if you are looking at miniature using magnifying glass, the 300dpi wouldn't be sufficient.

When taking image, the image size in pixels is the only measure that matters. Image size in cenimetres is useless. if you print it with 1:1 aspect ratio and 10x larger and put nest to each other so they would seem same size, there won't be siginficant change.

If you want to chech whether the image was resampled, you have to check the pixel counts with respect to the datasheet value.

In many situations, there are unwanted details near the edge of the frame and it is just cropped off. This results in decreasing of the size of the image and may alter the width/height ratio. The standard image size for DSLS is 3:2; when printing anything else the image was cropped.