Image Clarity 101: What makes an image sharp?

A common problem with photos is lack of sharpness. The image is either soft, or out of focus, or the wrong part of the image is in focus, or ... the list of possible issues is a long one.

Just like exposure, focus is one of the parameters that the photographer must learn to control in order to create images that show what he or she visualized before pressing the shutter release.

This is the first of a series of posts that I will call "Image Clarity 101". The name is a tip of the hat to John B. Williams who wrote the excellent "Image Clarity: High-Resolution Photography", which - alas - seems to be out of print.

I am going to look at what causes an image to be "sharp". In later postings, I will explain what equipment and techniques to use to actually get an image to be "sharp".

Focus

If we imagine that a subject is made up of an infinite number of minuscule points, each of these points emits an infinite number of light rays that hit the entire area of the camera lens. As the light rays pass through the lens, they are focussed on some point (the focal point) behind the lens, creating a cone of light.

Parallel to the lens plane is the focal plane, which holds an infinite number of focal points for all the visible points of the subject.

All of this is shown in the illustration below.

Illustration 1: Focus Explained (simplified)

The image plane is the surface the image is projected onto. This is where the film or image sensor is located. For an image to be in focus, the image plane must be the focal plane.

When you twist the focus ring on a lens, you are effectively moving the focal plane. When the image is sharp, the focal plane is located on the image plane.

Are you with me so far? Good!

Circle of Confusion

Now it gets a bit messier: since we are dealing with the real world, the light rays do not actually all converge on a single point, but form a small area, the circle of confusion. 

In the illustration above, you can see that in the focal plane most of the light originating from a single point of the subject hits the small circle. As the image plane moves away from the focal plane, the light is spread over a larger area.

If this is too circle of confusion business to too confusing, have a look at the following, simplified illustration:

Illustration 2: Cone of Light

The subject is projected by the lens as a cone of light rays which strike the image plane. The tighter the cone is at the image plane, the close the image plane is to the focal plane.

If the circle of confusion is small enough, the human eye will register it as a single point. How small the circle needs to be is perhaps a topic for another post. For the time being, we can assume that if the circle of confusion is somewhat smaller than a single photo site on the sensor, it will not be a limiting factor for the overall sharpness of the image.

Illustration 3: As the circle of confusion increases, the image becomes less sharp.

So we reach a simple conclusion:

If a single point of the subject is rendered as a small enough circle of confusion, it will appear sharp in the image.


There is one important point to consider: all of what has been discussed above is only true for flat subjects and ideal lenses. In the next installment, we will look at sharpness for a non-flat subject.

You may find a used copy of the excellent Image Clarity here:
Amazon USA Amazon UK Amazon DE

If you don't feel like getting an old book, you might try perusing the good old Wikipedia which contains much more on the subject.

Bee on a Tattered Heliopsis Flower

I have been doing a lot of macro work lately, mostly studying my favorite summer garden flowers: oxeye (aka heliopsis).

The oxeye blossom has an interesting life-cycle, that may be the subject of a future post. In the following image, I found a blossom near the end of its lifespan: the petals are coming apart and the whole appearance of the flower is ... tattered.

Still, there was apparently enough pollen and nectar left in the center to make it interesting enough for bees to come visiting.



Because there was a slight wind causing the blossom to sway on its long stalk and because the bees where buzzing about, I decided to shoot hand-held, not from a tripod. Despite the bright afternoon sun, I needed some additional flash in order to get the short exposure time required to eliminate motion blur.

So there I was holding the camera in my left hand and a speedlite attached via a short cord in the other hand ... No idea what the neighbors where thinking, but then, they should be used to me by now. :-)

I think I will get a flash bracket real soon now.
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Pink Anemone XXV

About three quarters of a year after Anemone against the Light I have created another image of a pink anemone flower, this time lit from the front.

Anemone XXV

The flower was growing in the shade of a hedge, so a speedlight to camera left provided additional light. I used a reflector to camera right to soften up the shadows.

Decaying Beauty and Lavender Rose

I've been quite busy lately, fortunately not too busy to test the wonderful Canon EF 100/2.8L USM Macro lens. We've had a lot of rain so the garden is not offering quite its usual abundance of flowers, but I've managed to get a few passable pictures:

Decaying Beauty

and

Lavender Rose

Both are roses, of course, the first being well past its prime, with the petals all wilted and gone. It was taken in available light using just a tripod for support.

The second is a fresh rose with lavender. I place both in a small vase, hung up a black cloth for the background, and used two speedlights for lighting. The first is to camera right, the second is to camera left, behind the flower.

Fireshow

Sanja Vatra asked me to take some pictures of her fire show during the Uferlos Festival last week. I was more than happy to oblige.

The weather was horrible, it was raining cats and dogs. Despite this, a nice crowd gathered to see a great show.



In order to get the fire swirling and some of the background, I dragged the shutter at 1/3s. The auto-focus of the Canon 5D decidedly did not feel up to the task, so I used f/8 for a good depth-of field and focused manually. These choices left me to use ISO 400 in order to expose the fire right.

I then fired the flash on ETTL with -1EV compensation to Sanja properly. Judging by the recycle time, the flash was firing full pop.

Looking at the pictures on the camera LCD after the show, I thought the whole effort was wasted. After I got home, warm, and dry, I had another look on the computer and felt much better. :)

Have a look at my Flickr! photostream for some more pictures.

Bleeding Heart Flowers

The windflowers in the last post are wilting away, unfortunately before I had a chance to re-visit them. Instead, the bleeding heart flowers are in full bloom - such is the cycle of life.

The bleeding heart, also known as Venus's car, Dutchman's trousers, lyre flower or by their latin name lamprocapnos spectabilis is originally from Korea and north and western China, but it seems to do very well in almost any temperate climate, so it is a very popular garden plant.

We simply let it grow and cut the wilted stalks short in the fall and they come back year after year.



In order to take the photo, I mounted the camera on a tripod and used an insane amount of extension (over 50 mm for a 70-200 mm lens). Despite an aperture of f/16, the depth of field is extremely shallow, about 2-3 mm in my estimate. That, combined with the light wind that moves the flowers like swings on a playground, made it challenging to get sharp images.

What really amazes me is how the lower white parts of the blossoms are textured. I had never noticed this with the real flowers before taking these pictures. Which just goes to show how photography can expand your awareness of the world. :-)

White Windflowers

The weather has finally turned sunny and warm, spring is definitely here. In fact, it's almost warm enough to be early summer.

These beautiful white flowers grow in my garden under a rose bush. They are windflowers, also known as wood anemone, windflower, (European) thimbleweed, smell fox, or by their latin name anemone nemorosa.



The flower is quite small (about 2 cm across), so I had to get very close using an extension ring between the lens and the camera. A tripod was required, of course. ;-)