Exercise: Focus at different apertures

As one varies the aperture used to capture an image there is a discernible variation in the area of the image that lies within the acceptable limits of sharpness.  In this exercise I explored this relationship by taking a photograph of a scene multiple times and varying the aperture each time.

In order to make comparisons on the effect of aperture upon the area of acceptable sharpness within a photograph it was important to capture the same scene multiple times using a variety of apertures.  To achieve ensure that I would be capturing the same scene each time I fixed my camera to a tripod and did not move the camera throughout the exercise.  Variations in the aperture used would require adjustments in the shutter speed to be made in order to achieve the same exposure in each photograph.  I decided to allow the camera to adjust the shutter speed for me and set the camera to aperture priority mode.

I decided to use the trusty wall that I used in the previous exercise.  My reasons for this were in part because I was attracted to the colours and texture within that wall and in part because the wall very clearly demonstrated differences in sharpness across the scene.

Results
In each of the photographs below I have marked boundary of the limits of acceptable sharpness.

In order to capture the above photograph the lens was set to it's widest aperture, f/2.8.  The focus point was set to the middle of the frame and a sample picture was taken.  Upon looking at the histogram on my camera I decided to apply a full stop of exposure compensation and set the camera to record the image in RAW format.  This allowed me to capture a picture without any blown out highlights and which I could confidently underexpose in post processing without introducing noise.  The area of the photograph within the acceptable limits of sharpness is relatively narrow and surrounds the focus point.

In the photograph above I set the camera to a middle of the range aperture, f/8.  The area of the photograph within the acceptable limits of sharpness has grown significantly and is approximately double the size of the first photograph taken.

In the final of the three photographs taken in this series I set the aperture to the smallest available to me, f/22.  One can see that the entire scene from front to back has been rendered within the acceptable limits of sharpness.

Summary
This exercise clearly demonstrated the effect of aperture upon the area of the photograph that is within the acceptable limits of sharpness.  The creative opportunities that this offers the photographer are great.  One can choose to render an entire scene in focus by using a small aperture or one can choose to isolate a subject from other elements within the scene by using a large aperture.  While this is fine for static objects one must consider the effect of changing aperture upon shutter speed.  For example, in order to render the entire scene within the acceptable limits of sharpness the shutter speed had to be increased to 2 seconds whereas at a wide open aperture setting of f/2.8 the shutter speed was only 1/30s.  If the subject had been moving this would have meant that at a wide open aperture one may still have captured a sharp subject.  However at a narrow aperture the subject would have been extremely blurred due to the effect of motion.  In other words, while aperture can be used to capture a photograph with the required area within the acceptable limits of sharpness one must also consider the impact this will have on shutter speed if the camera is hand held or if the subject is moving.

Exercise: Focus with a set aperture

Depth within a picture, from foreground to background, represents an opportunity for the photographer to make creative choices. It is possible for the photographer to draw attention to specific elements within the scene via the use of differential focus. Alternatively the photographer may choose to bring more elements in to focus in order to tell a story for example. A combination of aperture, focal length and focus point can be used to make these creative choices.

In this exercise I explore the effect of choosing different focus points within a scene in combination with a large aperture. A large aperture produces a photograph in which a shallow area is within the limits of acceptable sharpness. The area of the photograph within the limits of acceptable sharpness surrounds the focus point chosen or the element within the scene which was used to focus the lens.

I decided to carry out this exercise in the same location as the previous exercise. This exercise required a scene with depth. I noticed a row of headstones which were aesthetically pleasing and which satisfied this requirement for depth. Choosing a viewpoint at an acute angle to the row of headstones I fixed the camera to a tripod and attached a 17-55mm zoom lens to the camera. I chose this particular lens due to its ability to work at a reltively large aperture of f/2.8. This exercise required me to compose a scene and then to take three photographs varying only the focal point between the photographs. The camera was set to aperture priority mode so that i could manipulate the aperture as required and allow the camera to determine the shutter speed required to create a correct expoaure. I found the exposure the camera created to be a little dull and so I adjusted this by increasing exposure compensation. Upon setting the lens to its largest aperture, f/2.8, I composed the scene and shot the photograph using a shutter release cable in order to minimize the chance of camera shake. The photographs I captured are shown below.

Results

In this first image the headstone nearest to the camera and to the left of the other headstones is in focus. I chose the nearest headstone as the focus point for this photograph by selecting a focus point in the cameras live view mode. Live view mode allows for extremely granular focus point selection. Normal operation of the camera requires one to choose from one of only 9 focus points on my camera, the Canon 50d. In live view I was able to specifically choose which headstone i wanted to have in focus without having to focus and recompose the scene which would have risked changing the scene between the different photographs. In the above photograph one can see that while the nearest headstone is in focus the other headstones, which are increasingly further away from the camera, rapidly become out of focus.

In the photograph above I moved the focus point within the camera to the second headstone along. The selected headstone is clearly within the acceptable limits of sharpness while the headstone closer to the camera and everything further from the camera are out of focus. However it should be noted that the elements within the scene that are further away from the lens than the focus point are much sharper than in the first photograph.

In the final photograph in this series the third headstone is seleced as the focus point. Once again the nearest headstone is out of focus. The second headstone is slightly out of focus and the third headstone is in focus. The other headtones further away from the lens than the focus point as well as the church in the background are far clearer than any of the previous photographs,

Summary
It is apparent that the depth of field that is within the limits of acceptable sharpness increases as the focus point is located further from the lens. While i was aware of this behaviour I had not realised how dramatic this effect is. Comparing the first and third photographs one can see that for a relatively small change of distance of the focus point from the lens the church which is far in the background becomes dramatically more in focus. My favourite photograph of these three is the second one. The reason for this is due to the fact that the other headstones and the background in general are out of focus and this draws ones eye to the headstone that has been selected as the focus point. This allows the viewer to concentrate on the chosen point without distraction.

Had I captured the first headstone in it's entirety the first photograph would have been my favourite as in that capture the other headstones and the background are the most out of focus. This really draws ones eye to the intended subject, the first headstone. In retrospect were I to take these photographs again I would make two changes.  I would compose the scene such that the nearest subject is within the frame rather than a small part of it. Secondly I would choose a composition where the third element was more in view. In the scene above the third headstone is at such an acute angle to the lens that it is not properly visible and therefore does not demonstrate the effects required by this exercise as effectively as possible.

Addendum
As I walked round the graveyard I was thinking about the relationship between the depth of the area of the photographs with an acceptable level of sharpness and the distance of the focus point from the lens. I realised that if more distant focus points result in a larger depth of acceptable sharpness then closer focus points would have a much shallower depth of acceptable sharpness demonstrating the effect of a large aperture on depth of field more clearly. I also considered the fact that a continuous subject extending in to the scene away from the lens would more clearly demonstrate what I had observed during this exercise so I set about looking for a suitable subject.

As I stopped to think about where I might find a suitable subject I realised that I was leaning against it! The wall behind me had wonderful colours and an interesting texture which I found aesthetically pleasing and interesting. Furthermore, the continuous nature of the wall meant that it would provide a great subject to demonstrate the points within this exercise upon. Finally, I was sure to position myself close to the wall to decrease the depth of field within the acceptable limits of sharpness, thereby demonstrating the effects of a large aperture more effectively and also producing a pleasing photograph full of beautiful bokeh.

In this first of three photographs I positioned the focus point to be as far to the left of the scene as possible and therefore at the nearest part of the wall. One can see the closest part of the wall is in sharp focus while other parts of the wall which are further away rapidly fall out focus.

In the photograph above I moved the focus point to the centre of the scene. Much more of the scene is in focus than the first photograph. However, the closest parts of the capture are out of focus as are the most distant parts. The continuous nature of the wall clearly shows how the depth of field within the acceptable limits of sharpness has increased as the focus point is moved further away from the lens.

For this final photograph in this series I set the focus point to be as far to the right as possible. One can see that much of the wall is in focus while the areas of the wall furthest from the focus point and closest to the lens are out of focus, as expected.

My favourite photograph within this series, and overall in fact, is the first photograph of the wall, with the focus point to the left of the scene and close to the camera. I enjoyed this photograph greatly because of the wonderful colour and texture in the wall. I also found the fact that the further parts of the wall were out of focus pulled my eyes back to the in focus area where I could concentrate on those colours and textures. The second and third pictures of the wall were not aesthetically pleasing for me. This was primarily because the areas of the wall that were in focus were far away and the textures could not be easily seen as they were small. Furthermore I found the large out of focus area to the left rather distracting as the in focus elements were unable to hold my attention.