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PIXELS AND RESOLUTION
Note number two: I am not going to address resampling here because it degrades an image and has little application in fine art photography. (Resampling is when Photoshop adds pixels to an image.) First some definitions… Pixel: A contraction of the words picture and element. A pixel is one unit or element among millions that make up the image sensor in a digital camera. Pixels are the electronic equivalent of silver halide crystals embedded in the gelatin of black and white film in that both record the light presented to them during an exposure. Depending on the location of an individual pixel in an image sensor, that pixel will record its discrete part of an exposure. Resolution: The capability of a camera, lens, and image sensor to make the details of an exposed image clear and distinguishable. Resolution is one of the primary factors in achieving clear, sharp prints. All other things being equal, the more pixels contained in a camera’s image sensor, the larger the print that can be made with good resolution from an exposure made by that camera. Mega-pixel: A unit of measurement employed to quantify the total number of pixels in a digital image. One mega-pixel = one million pixels. Pixel dimensions: The size of a digital image expressed as the number of pixels along the image’s height and width. Pixels per inch (ppi): The resolution of a digital image is relative to the number of pixels per inch as measured along the image’s height or width after it is enlarged. The more pixels per inch, the greater the resolution. When we make a fine art print from a digital image, one of our goals is to achieve excellent clarity and sharpness in that print. In other words, we are striving for excellent resolution of the details of that image. If we are making a 5 X 7 print, the resolution in that print will almost certainly be quite good. The reason for this is that image sensors in the vast majority of digital cameras contain at least enough pixels to produce good resolution in a small 5 X 7 print. But what if you want to make large, fine art prints, say 20 X 24, from your camera or a camera you are thinking of purchasing? Does the image sensor in that camera contain enough pixels to provide good resolution in a print that large? Do this: Imagine a jigsaw puzzle. But imagine the individual pieces of the puzzle all being square and all being the same size. Imagine there are six million of these individual pieces tightly arranged into vertical columns of 2000 pieces and horizontal rows of 3000 pieces. Imagine that this picture puzzle measures only 1 inch high by 1½ inches wide. Now imagine that a picture is imposed on the puzzle and notice that each and every piece of this puzzle contains its own discrete part of the overall image. We now have an analogous representation of an image sensor, the millions of pixels it contains, and how they are arranged. In digital camera terms, our imaginary image sensor would be a 6-mega-pixel sensor. (2000 pixels X 3000 pixels = 6,000,000 pixels; mega = million; thus, 6-mega-pixels.) We now have enough information to determine if we can make a large print while maintaining good resolution/clarity/sharpness from the original image recorded on our imaginary 6-mega-pixel image sensor. Let’s say we want to make a print that measures 24 inches along its horizontal dimension. We know that the original exposure contained 3000 pixels along its horizontal dimension. Because we do not get any more pixels for this image (see Note 2 above), the existing pixels of the image must be distributed over our new enlarged size of 24 inches. So what will be the ppi count of our enlargement? If we increase the horizontal dimension (which contained 3000 pixels to begin with) to 24 inches, we will end up with only 125 pixels per inch. (3000 pixels divided by 24 inches = 125 ppi.) Do you see that? Now let’s say we’ve cropped the original image a bit (which I do all the time). In that case the ppi count is going to drop to maybe 100 ppi, maybe less. At 125 ppi the sharpness and clarity (resolution) of the 24-inch enlargement is not going to be very good. At 100 ppi or less, the print is going to be lousy. There are just not enough pixels to spread over 24 inches and still provide good resolution. It has been my experience that the very best resolution occurs between 200 and 300 ppi. However, when working with larger prints I can get good resolution down to 160 ppi and sometimes down to 150 ppi. One reason for this is that the larger the print, the greater the normal viewing distance. Think about it…if you look at a 5 X 7 print you get quite close to view it. But if you look at a 16 X 20 or a 20 X 24 print, you stand back a bit in order to take it all in. 150 ppi in a 5 X 7 or 8 X 10 print (this would assume severe cropping of an image) would almost certainly provide poor resolution because the image would be viewed from much closer than would a 16 X 20 or 20 X 24 print. Let’s call this practical resolution, meaning that good resolution is not only dependent on the ppi count of the enlarged image, but also on the size of the enlargement and its relative viewing distance. A good example of practical resolution relative to size of print relative to viewing distance is the painting technique of pointillism. Seurat’s “Sunday Afternoon on the Island of La Grande Jatte” is a huge painting, seven feet high by ten feet long. If you stand back at a proper viewing distance you see the people, trees, and lake quite clearly. The painting has good resolution. But if you move up close to it you discover that the painting consists of thousands of individual dots of pure color and it is difficult to distinguish people from trees from lake. You just see a lot of tiny dots of different colors. By now you should have a decent understanding of how the number of pixels in a camera’s image sensor relate to how large a print you can make with good resolution from an exposure made by that camera. This relationship of pixels to print size is of major importance if you are considering the purchase of a new camera. All other things being equal, the more pixels contained in a camera’s image sensor, the larger the print that can be made with good resolution from an exposure made by that camera. If you are thinking about buying a digital (DSLR) camera for creative/fine art photography, get one with a 12 to 14-mega-pixel image sensor. Bob Osborn P.S. Yes, yes, yes…I admit that I left out the color recording capabilities of pixels. My excuse is that the above is complex enough for the time being. Any more information and you guys would start falling asleep. PHOTOGRAPHIC PSYCHOANALYSIS Copyright (c) 2009 by Robert Osborn. In fine art photography workshops you repeatedly hear, "Don't take a picture, make a picture." So, let me ask you...Just how do you make a picture? What attracts you to a certain subject and what mental processes occur prior to and during recording the image, and then later when you print the image? A couple years ago, while preparing to teach a workshop, it occurred to me that I needed to answer that question for myself...exactly answer it. However, I found that to analyze and describe a mostly intuitive process that has evolved over many decades is a difficult thing. Call it Photographic Psychoanalysis. What I learned is why I do what I do when photographing. And gaining a clear understanding of what I had been doing intuitively made it possible for me to consciously improve how I visualize, compose, expose, and print photographs. Having articulated the process, here is how I now both intuitively and consciously make a picture: I search for a subject that to me is aesthetically and emotionally compelling. Imagination is very important to this search. J. L. Lowes, in The Road to Xanadu, described it perfectly: "Out of chaos the imagination frames a thing of beauty." Once I find a compelling subject my first consideration is the ambient light and how to work with it. Often it is the quality of light that makes a particular subject so compelling in the first place. I then visualize how my subject, proximate objects, light and shadow areas, repeated patterns, and background might be organized and simplified by camera position, lens choice, and exposure. My goals during this organizational process are threefold. First to enhance the literal, emotional, and aesthetic elements of my subject. Second to create a powerful, clean image with a strong abstract design that will translate well into the two-dimensionality of a final print. Third to eliminate distracting objects. During the editing process each succeeding step (choice of gray scale or color, tonal relationships, cropping options, early prints, etc.) provides more information about the subject and the design of the image, which helps me convey and often improve on the ideas intended during my original visualization and subsequent organization of the subject. By this right-brain left-brain cooperative process, and with an active awareness of how the image might be perceived by a viewer, I eventually arrive at a best final print. What I'm suggesting is that you try this exercise. It can improve your photography. Bob Osborn P.S. I acknowledge leaving out of these thoughts the creative excitement that consumes us when we discover and photograph a worthy subject. Maybe I'll try to put that into words in a future THOUGHTS ON PHOTOGRAPHY...if I can figure out HOW to put it into words.
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