Pixiq

How Sharp is Sharp ?

When it comes to imaging and lens systems, two things often get confused: resolution and sharpness. The first you can measure if you set some ground rules but the second is subjective – literally in the ‘eye’ of the beholder.

There comes a time in a  young life where you may or may not just wonder about things…for some it passes and they get on with reality (what exactly is that?) for others it is a way of life: questions, questions…maybe I am old enough to know better but every time there is a new book to write, I tend to ponder the basics and stir over things I take for granted. Complacency is never a good thing and ideas change.

Did you ever wonder if you see ‘red’ the same as someone else with ‘normal’ vision…not the  friend you used to beat at pool who turned out to be color blind?  How would you know… well, you could measure the size of the electrical impulse generated by rods and cone cells on the retina, get into the brain (yeh). Well, no you can’t…you can use inductive logic to say it's likely but this has long been a fundamental perception puzzler ever since would-be Neanderthal philosophers sat outside the cave entrance, gazing up at the stars with a skull-full (in every sense) of fermented honey or chewed on certain botanical products of the land…herbs and fungi and said "Ugg!" 

And what about taste? Well we can certainly infer that not everyone experiences the same sensation just by looking at density of taste buds on the tongue where some people have a value 100 times more than others.

In photography, as in many other subjects, a simple question may lead to a discussion of concepts that are difficult to pin down and this confusion has been exacerbated with the advent of digital imaging because some ideas from the world of film photography do not translate directly due to the way an image file is produced by a digital camera’s electronic systems.

Resolution is the ability of any lens system (including our eyes) to separate two objects (e.g. points or lines) that are very close together. The higher the resolving power of a lens, the better it can separate such details clearly. I’ll delve into this in the next post when I feel brave enough to tackle ‘circles of confusion’ with the intention of making them circles of not-so-much confusion.

On a print, for example, you can relate resolution to the 'circle of least confusion' or separation of pairs of lines close together and use those bits of advanced mathematics (Bessel functions) that have been around a long time and used to deal with diffraction…come to think of it life might be too short to stir those brain cells back from the dead: necromancy as opposed to memory in my case.  However, when we talk about the resolving power of the sensor, it depends on more than separation of photosites and their sizes. A full analysis would have to take into account the shape of the pattern in which they are arranged, the diameter of each sensor site, and the distances between the sites—which could vary in different directions depending on the arrangement chosen (think of Fuji and hexagons of different size…). Resolution along the diagonal would differ from that calculated for the horizontal so it is no wonder that there is still quite a lot of confusion – in circles or otherwise.

To muddy matters further, the idea behind resolution in term of lens systems, is different than the use of the term to signify the quantity of pixels on sensor or in a digital image.

Resolving power is not everything even if lens testers try to make you believe it is - some of the sharpest macro lenses ever made came from Leitz whose expertise with binoculars and microscopes is second to none. Their macro lenses were not often top in magazine tests but the designers understood better than anyone the importance of other factor such as lens contrast and what is now called 'bokeh' in addition to resolving power for creating the impression of sharpness. This in the real world of image creating is far more important than any set of figures.

Sharpness depends on you the viewer and is thus much more subjective. The perception of sharpness depends on several things that are within our control to optimise as photographers.

1. Resolving power- the ability of the lens system to separate fine details.

2. Contrast - How the lens copes with changes from light to dark or from one shade of color to another, particularly at boundaries between objects. Sharpening algorithms are a great tool for increasing the perception of sharpness – though they will not work on a lousy image so it helps to do the best you can to start with. Think of how when drawing you can use a heavier pencil line to denote a boundary…in basic terms Unsharp mask and other sharpening methods increase contrast of pixels at boundaries… same basic idea but very nifty.

3. Depth of field - the greater depth of field you have (particularly in macro shots) the sharper an image seems to the viewer. Since depth of field increases (at a given magnification) as you stop down to smaller apertures it would see best to stop down and use f22 and smaller. However as the hole in the iris diaphragm gets smaller the effect of the diaphragm edges on the wavefront is proportionately greater and diffraction (the spreading of the light) becomes more apparent and softens the edges of details. Image stacking provides a way around this.

4. Absence of blur: the movement of camera, subject, shutter bounce…can all cause fuzziness of edges of objects – great when you are having a ‘creative’ day but a pain when not. Controls within sharpening programs enable a slight correction for this but first define your blur is it motion, lens or Gaussian. My advice is to click preview and try.

5. Lighting – in particular, the  direction of lighting.

In every workshop I have ever led an appreciation of Lighting is the single thing you can change to make a person feel their time and money has been well-spent. Photography is literally writing with light and so many forget that after all the money spent on gadgets the light can screw things up completely. In the macro realm light from the side can pick out small details by creating tiny shadows behind them ‘relief’ that turns something from appearing flat and lifeless into an image with depth’.

6. How you view the image. Whether an image appears sharp or not can depend on ambient illumination and how far away you are from it. Admit it we expect far too much from our images on a computer screens and then ruin it all. Why, because we typically sit much too close while checking images at 100%  (some of us…even 200% because we are fanatics) 

 Just consider that when viewing an image at 100%, each pixel in the file is represented by a pixel on your computer screen. A file with a width of 4200 pixels and screen resolution of 72 pixels per inch would be 58 inches (1.48 meters) wide. If that picture was on the wall, you would want to stand much farther back to look at it: in fact just over 4 feet (50in – 1.27m)  to maintain a ‘comfortable’ viewing angle of 60° along the diagonal. When we are too close, our eyes will resolve the individual pixels (dots) so it will not look as sharp as possible.

Be reasonable...but never stop pushing your boundaries.

So what to go for sharp or not... a battle between craft and art.

Don't buy into the idea that crap out of focus images are somehow more creative...especially if they are monochrome. Sorry if sensitive souls find that a bit cynical...perish the thought, but I have seen so many magazine shots that make me wonder.

To get the kind of sharpness that jumps from the screen or out of the page takes mastery of camera, use of aperture, lighting, keeping things free of vibration and all the rest. It means putting in the hours to learn the basics which in our quick-fix age might not appeal to some.

But don't run away with the idea that shallow depth of field/blurs are there for the taking - in many ways it is harder because this is what takes flair, the artists eye. And...you may never get it.

Musicians out there will know that if you put in long hours playing scales, arpeggios and so on that the mastery gained gives you freedom to experiment. It frees your creative soul not inhibits it and the palette you have to work with is that much richer.