Connect with me on Google+

Thursday, 18 February 2010

Focus at different apertures


This exercise was similar to the ‘Popeye’ exercise in that it required a scene with some depth; in that exercise I kept a fixed aperture and focal length, demonstrating how depth of vision (depth of field) can be affected by focusing at different distances. I also learnt that using depth of field can direct a person’s vision to a specific aspect of an image; ‘drawing their eye’ if you like.

This exercise is about how the aperture affects the limits of focus and sharpness. The idea is to take 3 images of the same scene with different aperture settings, then comparing the 3 images to see how those apertures changed the resulting images and mark off the limits of sharpness on the prints. I’ve used a blue outline to demonstrate this.

I wanted to use an outside scene for this exercise, however a cold (as in I’m feeling rough!) and the weather has given me the excuse of staying in again. I feel a bit guilty though as I really ought to be pushing myself out of this comfort zone and look for more opportunities away from home (Although I suspect it wont be the last time!)

That aside my scene was made up by placing a number of soft drink cans in a triangle formation on a white background. This was located close to a large window, with the ‘point’ of the triangle facing the window. The camera was facing the cans with the light from the window behind it. All lighting was natural from the window (no flash or indoor lights were used).

I kept a fixed 50mm focal length on the lens and my focus point was on the front drinks can. To maintain the same focus point and position I also mounted the camera onto a tripod. ISO was manually set to 100 throughout the exercise. The range of aperture size ranged from f/1.8 at the largest to f/22 at the smallest, incrementing in ‘half-stops’. See my earlier post of ‘getting to know your camera' for more information.

The only setting that I changed through the experiment was the aperture settings; I left the camera to set the shutter speed for the correct exposure.

Image1. This was with the larges aperture available on the camera/lens configuration. This had the view with the smallest level of sharpness and the most shallow depth of field. with only the surface of the can in focus, even the ring-pull on top the can is out of focus. Although not clear so much from the size of image here, it is much more clear on the printed version, despite this I found it fairly easy to identify where the limit of sharpness was with only a small part of the image in focus, I’ve marked this off roughly on the image below; Large aperture = Shallow depth of field. And my eye is drawn to a small part of the image.

50mm f/1.8 1/8 sec

Image2. In this image I selected an aperture around the middle of my scale of f-stops. As expected the depth of field, level of sharpness increased with the smaller aperture. I left the camera automate the shutter speed during the exercise and interestingly for image 2 the shutter speed was 1.5 seconds compared to the first shot of only 1/8th of a second. (More about this at the end of this exercise). The results of this image was a bit harder to tell where the limits of sharpness was, however from the image it I could see that the depth of the image that was in focus had increased and it now encompassed the surfaces of the second layer of cans, with the sharpness training off from that.

50mm f/6.70 1.5 sec

Image3. For the last image I used the smallest aperture that my camera and lens setup would allow, which was f/22. It was just was well that I was using a tripod for this, as the camera automatically selected a long exposure of 15 seconds for the exposure! As the first to images showed the smaller the aperture, the larger the limit of sharpness in the image, however for the sensor to get enough light to maintain the exposure with a very small aperture the shutter had to be open for a long period. That said, pretty much the whole image was now in focus with the largest depth of field.

50mm f/22 15 sec

Notes on shutter speed

As well as looking at how the aperture size effects depth of field, I also reaffirmed knowledge about the link between the shutter speed and size of the aperture. From my earlier section of ‘getting to know your camera’ I noted that each standard increment of aperture f-stop = half or double the amount of light reaching the sensor. What I didn’t note at the time (no pun intended) was that the shutter speed standard (full stop) scale = the same. i.e. that each increment of shutter speed = half or double the amount of light reaching the sensor. 

In the exercise I had the camera in AV (aperture control) mode leaving the shutter speeds up to the camera, even so I noticed that the shutter speeds that the camera picked were the same amount of increments (half stops in my case) as the aperture value e.g.

aperture f/22 f/19 f/16 f/13 f/11 f/9.5 f/8 f/6.7
shutter 15 sec 10 sec 8 sec 6 sec 4 sec 3 sec 2 sec 1.5 sec

As you can see, Image3 I set the aperture at f/22 and the camera set the shutter speed at 15 seconds. Then as each aperture increase (more light in) the shutter speed compensates the exposure by reducing the shutter speed (less light in) and thereby maintaining the constant exposure through the steps!

Summary – Another good exercise with this one showing that when aperture gets bigger then the depth of field gets less, and vice versa. Also another example of how the focus can draw attention to specific parts of an image.

No comments:

Post a Comment