Aspicus and the Art of Photography

From Snapper to Shooter (hopefully)

Project 3

Focus with different apertures

What: The aim of this project is to explore the effect of changing the aperture on an image. The project involves taking several images of the same scene at different apertures and commenting on the effects on the depth of field.
Where: In my garden.
When: Early evening in March. Sky was very overcast, and it was beginning to get dark.
Why: I found it quite difficult to find a suitable scene for this project.  I tried various walls and hedges and so on, and saw what the project was getting at, but found it difficult to determine the boundaries of the sharp zone (i.e. the depth of field) due to parts of the scene getting in the way.  In the end I constructed my own scene in my garden that you can see below.  The grass allows me to easily determine what is sharp or not, and the tennis ball something to zoom in on.  It’s not very pretty but does I think help me explore the ideas behind the project.
How: Camera settings were manual, focal length 130mm, ISO200, various aperture (see below), and various shutter speeds. I used a tripod as directed in the course notes.  The project asks the student to identify the limits of sharpness of each image.  I’ve included these in the images below as red lines.  I don’t have the know-how to do anything clever like make the image change on mouse-over (which a short google search suggests is a very thing difficult to do on wordpress.com).  Putting red lines on the image was the only software manipulation (bar conversion from RAW to jpg of course).

The images
Wide open aperture (f/5.6):

1/20 sec shutter speed

Medium aperture (f/11):

1/5 sec shutter speed

Smallest aperture (f/36):

2 sec shutter speed

Observations

There are a number of observations from the above images and limits of sharpness.

The first one is that when I looked closely at the image I was expecting the depth of field to be a band of sharpness in the middle of the image.  However looking closely at the images (particularly for f/5.6 and f/11) suggested that the sharpness declines at the edges of the image as well as at the front and back.  I therefore guessed that it was in fact an ellipse (which I’ve drawn) but have yet to confirm this.  Given the round nature of a lens this would not be surprising if it was the case (in fact on reflection anything else would be surprising).

Secondly, I was quite surprised that the entirety of the image was not in sharpness for the f/36 image, as for other shots I’ve taken f/22 is usually more than enough to get sharpness throughout the image (noting the caveat below).  This may be something to do with lens diffraction (see below), or the combination of focal length and scene.  I’ll need to search some more to answer this one I think.

Thirdly, it is worth noting that the degrees of sharpness changes throughout the image.  For example, although the circle of sharpness changes a bit for the f/11 shot compared to the f/5.6 version, the rest of the image is a lot sharper.  A similar effect is seen with the f/36 image.  Therefore, depth of field is not really a hard boundary (as shown in the images above), rather it decays at the edges.  The narrower the depth of field the slower this decay, meaning that more of the image will be sharper (not necessarily sharp) than an image with a shallower depth of field.

Finally, take a look at the composite below, which shows a blown-up image of the tennis ball for each shot.

The f/11 version appears marginally sharper than the f/5.6 version and a great deal sharper than the f/36 version.  For the f/5.6 version I can sort of understand this, as the tennis ball is a 3 dimensional object, and the depth of field is so narrow, that some of the tennis ball may be outside the area of sharpness.  However for the f/36 version I would expect that given more of the image is sharper, the tennis ball itself would maintain its sharpness. I must admit that I’ve come across this phenomenon before as I used to think I was having difficulty focusing with small apertures on my 18-200mm lens.  I came across the idea of lens diffraction (follow the link for an easy to follow explanation) which suggests that it isn’t really a question of focus. I include this issue here as I believe it is relevant to the project in terms of exploring depth of field. Lens diffraction basically means in the maximum sharpness of an image decreasing as the aperture becomes smaller.  Therefore, making an aperture smaller and smaller will lead to more of  an image being sharp, however the maximum sharpness will also change (it may decrease to begin with, then decrease).  Apparently the sweet-spot (i.e. the apertures for which maximum sharpness is greatest) for my 18-200mm lens is between f/6 to f/10 or so.  I’m not sure if this is generally true, or if this sweet-spot is the same for all lenses.  I assume that the really expensive lens are less prone to it.

Further Questions

1. Is depth of field really an ellipse as I assume it is above or is it in fact rectangular?
2. Is depth of field only dependent on aperture, or is it also dependent on focal length?

3. Why is the entire image at f/36 not sharp? If it to do with lens diffraction, or a combination of the scene and focal length?  A few experiments may do the job here.
4. Do all lenses have a sweet-spot between f/6 to f/10 or does it change for each lens?  Do more expensive lens result in greater maximum sharpness throughout the apertures?

What I’ve Learned

1. Depth of field appears to be a sphere (or a circle) rather than a strip across the frame.

2. Smaller apertures means that the depth of field increases.

3. However, maximum sharpness does not necessarily increase as the depth of field grows, and may actually decrease due to lens diffraction.  The sweet spot of maximum sharpeness  for my lens is f/6-f/10.


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7 March 2009 - Posted by | Projects

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