Posts Tagged ‘f/9’

[Updated 4-4-12 with a short note on focusing options]

In our previous post I presented a lot of background information about the process I use to take my photos, and a problem that arose with a recent session where the stage I had constructed exceeded the limits available to my lens to achieve satisfactory focus. To circumvent the problem I decided to experiment with focus stacking, and see if it would be possible to create a single, seamless image from a series of separately focused photographs.

Going in, I had it in my head that I would take 9 separate photos—one at each of the nine auto-focus (AF) points supported by my camera, a Canon XTi. Each of these photos would, therefore, be focused on a different area of the target scene, and (in theory) I’d be able to somehow mask and layer these images in post production to create a single image where every object would be in sharp focus.

Since I shoot my images from a camera mounted on a tripod, and because the objects inside the light tent don’t move (ha! we’ll see about that…) capturing the images would be simple. I used an aperture of f/9 for each photo, set the camera to auto exposure, and carefully reselected nothing more than the AF point from one photo to the next. While I could have chosen a larger aperture to insure even greater sharpness at the point of focus, I decided to go with the much more conservative f/9, as my lens works very nicely at that setting, and—fearful of going too shallow—I wanted a reasonable amount of depth at each AF point, which I theorized would make the post production effort to seamlessly combine the images a little more forgiving.

The image below illustrates the nine AF points as viewed through the camera’s viewfinder. You can click on the image to see a larger view and better distinguish each point of focus.

AF points using a Canon XTi (click for a larger view)

As you can see, focus points fell on:

  • The train in the very back
  • The woman standing at the base of the log cabin
  • The dancing girl on the right
  • The “lizard woman” just below and to the left of the dancing girl
  • The “scorpion woman” emerging from the records at the bottom center
  • The “snake woman” at the left center
  • The middle devil playing the yellow-ish horn
  • The leg of the woman in blue (Tammy Faye!) riding the train
  • The right hand of the girl emerging from the record hole at the center of the composition

Note that two of the 9 AF points fell on the record album, while none fell on any of the objects that were closest to the camera. This was an unfortunate residual effect of the stage construction and the vantage point from which I chose to shoot the photos. The points falling on the record album are on the same plane, farthest from the camera, so I’d only need one when it came time to eventually create my focus stack. The objects in the foreground that fall below the lowest AF point would be slightly more problematic.

The photo above was taken with the AF point set to the top middle, falling on the people sitting inside the train. It is worth zooming in to see how the image is focused at this point relative to other areas of the photo:

Left: Detail at AF point
Right: Detail at foreground, far from AF point

At the point of focus everything is nice and sharp, while in the foreground 14 or 15 inches away from the AF point the image is unacceptably blurry. That’s okay, of course, since in post production the blurry part of the photo will be replaced by in-focus imagery from a completely different photo.

Recall that the objects in the foreground (like that very blurry “spider woman” above) did not have the good fortune of falling within any of the AF points supplied by my camera. We’ll pause momentarily while those of you with superior camera equipment snicker.

::: snicker! :::


Okay, back to my nine measly AF points. To bring the “spider woman” and other foreground objects into focus I took one extra photo with the AF point on the “scorpion woman” at the bottom of the photo, though for this photo I set the aperture to f/18 to get better near focus depth.

Focus on “scorpion woman” at f/18 to bring foreground into focus

Though I’d wanted to keep the aperture constant throughout the stack of photos, I really didn’t have any choice on the foreground image, as f/9 would provide only about an inch and a quarter of acceptable focus in front of the point of focus; roughly from the face of the “scorpion woman” to the right (hidden) side of the “spider woman.” At f/18 I was able to extend the near focus to almost two and a half inches, bringing nearly everything in the foreground into an acceptable range of focus. Yes, by deviating from the aperture used in all the other photos I’d face some other challenges during post production, but I found this to be an acceptable tradeoff.

It was my choice to use each of the nine AF points as the basis for my focus stack, as it suited the composition of this particular photo . You may do just as well choosing to use only two or three points of focus, depending on the conditions of the scene you are shooting. The techniques we’ll discuss in part three will work just as well with a stack of three photos as they will with a stack of 9 or 27. Also (and this did not occur to me at the time), I could have brought the extreme foreground imagery into focus my using the often ignored manual focusing controls of my camera. Tsk, tsk, tsk… I rely too much on auto this and auto that.

Stay tuned for Part Three of the series where we’ll dive headlong into the post production lunacy that is creating a single image from 9 separately shot and edited photos in Aperture.

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