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In my second assignment I focused on sharpness; what it is and how it effects your photo’s. I set myself the ask to experiment with sharpness.

What causes unsharpness:
Unsharpness is when light from a single point arrives on multiple spots on the sensor (see also the segment on theory at the end. There are 3 causes for unsharpness:

1. Focus. Any object not at the focus distance (S1 in the picture in the theory below) is out of focus and causes unsharpness
2. Motion. If either you or the object moves during the time the shutter is open, the object will be projected on different places on the sensor.
3. Lens deviation. The above theory applies to perfect lenses. Deviations in the exact shape of the lenses can cause light to travel on slightly different paths.. This is why expensive lenses are sharper. Much more effort goes into making the lenses as perfectly shaped as possible.

Effect of unsharpness on your image
First a very important point. We stated above that unsharpness comes from light from a single point arriving on different points on the sensor. This is only perceived as unsharp if your eye can see the difference. If those different points on the sensor are seen as one point by your eye, your image will be sharp. This relates sharpness to how your images are displayed. If you print or display them large, sharpness is a much bigger issue than if you print them on 4″x6″ or have a small version shared on facebook.
Now back to the effects of unsharpness on the image. Let’s look at the effect cause by cause

1. Focus. unsharpness by objects which are out of focus give a very natural blur. Your eyes are used to this kind of blur. it is used to separate the thing which has your attention from the rest. This is why it is a very useful tool when you want to draw your public’s attention to something in your image, simply place that into focus. An image which has noting correctly in focus will seem odd, you are easily distracted. In the two images below the focus is slightly different one is on the face and the other slightly behind the face. (click on the image for a larger version)
   
   Focus on the face (click to enlarge)
   
   Focus behind the face (click to enlarge)
2. Motion. The one kind of sharpness you always want to avoid is camera shake. Motion blur by camera shake feel like dizzy. The motion blur induced by you being not completely steady. Whether you see camera shake in your images depends on the shutter speed and the focal length of you lens. The basic rule of thumb is to take the focal length of your lens (say 50mm). The longest time your shutter can be safely open is 1/(focal length). So for 50mm it would be 1/50th of a second, for 200mm 1/200th of a second. Some factors modify that safe shutter speed. Image stabilization will allow lower shutter speeds (for 50mm it might go as low as 1/13th).
   
   Without VR (image stabilization), (click to enlarge)
   
   With VR, (click to enlarge)
   Motion blur can be a tool when you want to relay motion in the image.
3. Lens (optics) This one is the least noticeable, but when sufficiently enlarged you can see it. Below I used my prime (50mm F/1.8) and kit lens (18-55 F/3.6-5.6) both at 50mm and F/5.6. The idea is that both the prime itself is a sharper lens and that at F/5.6 the prime lens is closer to its sharpest F/stop than the zoom is. (Lenses are less sharp at their maximum and minimum F number). The image is a small detail. When you look at the whole image is is not noticeable.
   
   Prime lens (click to enlarge)
   
   Kit Lens (click to enlarge)

Well this turned out a lengthy post I hope it was useful to you as it was to me learning all about sharpness.

Happy shooting!

Some theory: what is (un)sharpness?

Your lens is, eh well, a lens (actually a whole array of lenses but that is beside the point). A lens projects an image on a surface.

Lens workings (Creative Commons image)

The red lines in the picture are the paths light is traveling upon towards your camera. (note that the paths shown are not all paths there are many more going in between the red lines) Your camera sensor would be the green arrow at S2. That is if your image is in focus. So if the image is in focus (sharp!) all the light paths from the object arrive on your camera sensor on the exact same spot. Now imagine the sensor moving a bit closer to the lens (so a bit to the left of the green line). You see now that the light paths arrive at different places on the sensor. The object is now smeared over a small area on the sensor. In your image the object is blurred or not sharp.
Note that when you focus effectively what your camera/lens is doing is changing the distance between the lens and the sensor