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Determining  Print  Sizes  for  Prints  from  Digital  Cameras



Some conditions necessary for obtaining high quality large prints from your digital camera


1.  The images should be captured using a high quality camera.


2.  The camera should have a high quality lens.


3.  The images should look sharp, in focus, and of high quality when enlarged to 100% in an imaging program.

4.  The printer and photographic paper should be of high quality.


5.  The images should be printed at no less than 150 pixels per inch (as discussed below).


Note:  The distance from which most people will view the prints is also a factor in determining the maximum acceptable print sizes. The further the distance from which you view the prints, the less you will notice small defects in the prints.



Calculating the print size


The width of the print can be calculated using the following formula:


Width of print = number of pixels width of image   divided by   required pixels per inch (ppi)


The same formula is used to calculate the height of the print, but substitute the word "height" in place of the word "width" in the above formula!


For example, if the image size is 6000 pixels x 4000 pixels and the image is printed at 150 ppi, the print size is a large 40 inches x 26.67 inches (6000 / 150 and 4000 / 150). However, if the image is printed at 200 ppi, the print size is reduced to 30 inches x 20 inches.


Note that a very large print from a 20 megapixel camera that has a small sensor, is unlikely to be of the same quality as a print from a 20 megapixel DSLR camera that has a large sensor.



The controversy surrounding the number of pixels per inch needed to obtain high quality prints


In the above example, the pixels per inch were 150. However, it has often been suggested that, when printing an image from a digital camera, the resolution should be at least 300 pixels per inch (ppi), or even higher, say, 350 ppi or 360 ppi.


For example, if the image size is 6000 pixels x 4000 pixels and the image is printed at 300 ppi, then the print size is only 20 inches x 13.33 inches. However, it may be very difficult with the naked eye to notice much difference in print quality between an image printed at 300 ppi and the same image printed at, say, 150 ppi or 200 ppi.


This topic was discussed in some depth by contributors to the "Sony SLR Talk" forum on the web site of "Digital Photography Review (DPR". Click  here  to see this discussion thread. There was general agreement that, even when the pixels per inch of a print are as low as 150 ppi, a good quality print can be obtained. One contributor reported that he has obtained excellent quality 36 inch x 24 inch prints from images captured by the full frame 24.6 mp Sony A900 (click  here  to read this posting).

Note that the terms "pixels per inch" (ppi) and "dots per inch" (dpi), are often used interchangeably. However, click  here  and  here  to read good articles about the differences between "ppi" and "dpi". It seems desirable to use at least 150 ppi, and possibly as high as 200 ppi, to get good quality prints from digital images.


Practical tests for determining the maximum print size


In practice, before finalising an important large print, it is often useful, in the first instance, to make test prints of just small parts of the image. For example, click  here  to see examples of crops that have been made of just small areas of some images taken by Rob's Photography using the Sony A7RII full frame 42.4 megapixel digital camera and the excellent Sony Zeiss FE 55mm F/1.8 lens. If you print just the cropped area of an image that has been saved at high quality, this will give you a good indication of the maximum print size that can be achieved for that image.


For example, when testing whether an image that is 6000 pixels wide, will make a successful 40-inch wide print, it may pay in the first instance to print say, one quarter of the width of the image at a width of 10 inches, and then determine whether this print is acceptable. If this result is not satisfactory, then further tests with print widths of say, 8 inches or 6 inches can be made. For example, if one quarter of an image prints out successfully at a maximum width of 6 inches, then the maximum width of the whole print will be 24 inches.



Upscaling digital camera images


The purpose of upsizing digital camera images is to use interpolation software that can increase the number of pixels in an image so that larger and / or better quality prints can be obtained.  For example, if the dimensions of a digital image before upscaling, are 6000 pixels x 4000 pixels, and if this image is printed at 150 ppi, the print size is 40 inches x 26.67 inches.


Now, if interpolation software is used to upscale this image by 50%, its revised dimensions are 9000 pixels x 6000 pixels. If this image is printed at 150 ppi, then the print size is increased by 50% to 60 inches x 40 inches.


Click  here  to read an article about the application and limitations of interpolation in digital photography. 



Image stitching


Instead of upscaling a single image, an alternative way of producing a very large print is to use a computer program to "stitch" several different images together. When this technique is used, the photographer could, for example, capture each image using a 70mm lens, rather than using, say, a wide angle 24 mm lens.


For example, the photographer may need to use a 24 mm lens to capture a single image of a very wide panoramic landscape. However, stitching together several images captured with a 70 mm lens, would probably produce a large panoramic image that has much more fine detail than could be obtained by making an upscaled image of a single image of a panoramic landscape that was captured using a 24mm lens.


Click  here  to read an article that discusses in detail several of the issues related to making panoramic images using Photoshop to stitch several images together.





Click  here  to go to the web site of Ken Watson, which includes several informative articles about printing digital photos.


Click  here  to see examples of the amazing quality of images captured using the Sony Zeiss FE 55mm F/1.8 lens on the full frame 42.4 mp Sony A7RII, and  here  for images taken by the Sony A99 and Sony A900. To give you an appreciation of how these images will appear when greatly enlarged, crops of just small areas of the images are included.