Evaluating the sharpness of your photographs seems to be an easy task at first glance. Simply put them into the slide projector, focus carefully and you can tell pretty quickly if they are sharp or not, right? ...Wrong! Using a slide projector to evaluate an image is unreliable, as the sharpness is dependent upon the quality of the projector's lens, the screen and the accuracy of the focus.

What about taking your film to a reputable film lab to process your prints? Nope, that won't work either as you are looking at a second-generation image subject to lab printing variations. So how do you evaluate your work for sharpness? Evaluating sharpness is the topic of discussion this month. It is a seemingly easy task that turns out to be quite difficult without the proper tools. Learning how to tell when your images are sharp pays big dividends in the end. As well as being able to make impressive enlargements, improving the quality of your photography, it will also make your images saleable to discriminating editors.

To properly evaluate sharpness you must eliminate as many complicating factors as possible. Slides are a better medium simply because they represent the image as it was initially exposed. The problem is that it is too small for the unaided eye to properly assess. To properly view a slide you require a little device called a "loupe". This is essentially a small lens that is placed over top of the slide. Just like camera lenses, they come in various degrees of power and quality. There are advantages and trade-offs depending upon how you balance the two variables of power and quality. We suggest a minimum of a 6 times power loupe for proper evaluation of a slide. Quality loupes are available in various power ranges and some even "zoom" from low magnification to high. Typically they are available from 4x to 10x.

As the power of the loupe increases, your total viewing area decreases and it becomes harder to see the entire slide without moving the loupe around. Additionally, unless you invest a lot of money to get a premium loupe, only the central part of the loupe will be sharp and distortion-free. Pictured here you will see a Pentax 5.5x loupe and a Nikon 10x loupe.

The lower magnification Pentax loupe allows for easier viewing of the entire slide whereas the Nikon loupe gives greater magnification. In terms of cost, expect to pay over $100 Canadian to get a premium loupe. The best way to pick a loupe is to go to a "professional" camera store that will have several premium loupes in stock, and try them out. Carefully evaluate how "crisp" they look and whether or not you can see the entire slide. Watch for distortions in straight lines as they travel to the edges of the viewable area, as well as for color fringing around objects. Color fringing is also called chromatic aberration and is caused by the lens not focusing all colors of the spectrum at the same focus plane. It is easily detected by looking at a junction of bright and dark areas. Chromatic aberration will appear almost like a colored "ghost" image along the edge.

Once you have a proper loupe, you will need a "light box". This is just a box with opaque Plexiglas on top. A color-corrected light source is contained inside and serves to illuminate your slide. A color-corrected light is not necessary to judge sharpness, but you will need it to correctly evaluate the color balance.

In field conditions there are many factors that construe to prevent us from getting the sharpest possible image. These include factors such as subject movement, camera mirror/shutter vibration, handholding versus tripod use, and the inherent sharpness of the lens. In a future tips article, we will discuss a test for judging the sharpness of a lens.

If you have never critically looked at sharpness before, it can be difficult to tell when a slide is truly sharp. When using long telephotos, it is helpful to remember that the depth of field (zone of acceptable focus) is very small. Therefore, you must examine the zone of focus and not the rest of the image. Look for intricate detail such as feather detail around a bird's eye. Where edges exist between areas of different tone, examine how well defined the edges appear. Consider the following analogy; try looking through your binoculars out a windowpane at an object. Next, compare this to when you are not looking through the windowpane. Even though you can see pretty well through the glass window, you cannot appreciate the detail as well. When an image is sharp, it will seem to "pop" out at you.

In summary, to properly judge the sharpness of your images you will need a quality loupe and light box. Using a sharp fine-grained slide film such as Fuji Velvia will give the sharpest images given your equipment and technique. Critical examination of your work will benefit in the long term by allowing you to make quality enlargements and opening up the possibility of selling your work. Additionally, an overall improvement in the quality of your work occurs as you begin to appreciate the impact of proper field techniques such as tripods and maintaining high shutter speeds where possible.