Focus and Recompose? Think it Through to Avoid Potential Pitfalls

Meadow in Fog, Yellowstone National Park

My normal working procedure is to use the single focusing rectangle located in the middle of my viewfinder frame, place that over the object in my image that needs critical focus, then hold that focus by either depressing the shutter button halfway (how most people do it) or by decoupling the AF function from the shutter button and using the AF ON button (Nikon) or Fn1 button (Olympus), recompose the the image, finally pressing the shutter button all the way.  Sounds good. Typical focus and recompose procedure, and in the old days, the technique one used when photographing with rangefinder cameras.

It works most of the time, but there are a couple of pitfalls that you have to understand and keep in mind in order to maximize the technical quality of your image.  The first is focus and the second is exposure settings. Let me explain.

There are two kinds of lenses: Curved-field and flat-field.  Most lenses are of a curved field design.  Macro lenses are normally of a flat-field design.  Both are designed to maximize quality for their particular usages.

Let's take flat field lenses.  Imagine attaching your camera to a copy stand with the sensor plane parallel to your copy stand's baseboard (your camera facing straight down).  You lay a photograph that you want to copy on the baseboard, ensuring it is flat.  You then position your camera so everything is perfectly within the viewfinder, focus on the center focusing rectangle, lock up your mirror, then make your exposure.  A flat field lens will be in focus in the center as well as on the edges across the "flat" field.  Great for copy work, macro copy work, etc.

A curved field lens is one that does not focus equally across a flat surface.  Same scenario as above.  You focus in the middle, but the edges of your copied photograph are not sharp!  What gives?  Imagine stretching a string from the camera's sensor out to touch the middle of the photograph (right where your focusing rectangle would project) you want to copy.  In our little example, the length of the string is just stretched tight at 15 inches.  Now, only moving the bottom of the string, like a pendulum, so that touches the work you are copying at the right edge and left edge. It no longer touches the photograph but is a couple of inches short. In a curved field lens, that would be your plane of focus.  Your center is in focus, but the edges no longer are in the optimum focus.  Focus is a radius from the sensor.

How does this affect your general photography?  Think of photographing an artist standing on the side of his or her display of paintings.  You are standing about 10 ft. away, swing your camera to the right and place your focusing rectangle on the artist's face and focus.  You now swing your camera back to the left to recompose the scene properly.  What you will have is the artist in sharp focus but the artwork will be closer than sharp focus.  As in the copy example above, you are working with a radius.  Your radius is from the sensor to the artist, at the edge of the frame.  If you swing that imaginary string back to the center, the end of the string is behind the artwork as the artwork is closer to the camera the camera than the artist at the edge.

There are ways to ensure you don't get caught up in this focus and recompose trap.  First, re-position your main subject, in this case, the artist, in way that you would need a reduced depth of field.  Second, have sufficient depth of field to keep the closer artwork in the center in focus as well as the artist (and the artwork on the left edge which is the same distance as the artist) in focus, and three move your auto focus rectangle onto the artist while keeping the composition in its final form, i.e., focus without having to recompose.

The other pitfall is exposure.  Most cameras lock focus and lock exposure with a half-press of the shutter.  If you focus on our artist, above, press the shutter halfway to lock focus, you will most likely lock exposure also.  That may work when recomposing, but it may not.  The light may be different on the artist (there may be spotlights lighting the artwork but they don't light the artist) and your exposure may be off.  So, you have to be careful and cognizant of your technique to enjoy a higher level of success.

These are simple examples crudely put together just to remind you to keep in mind a couple of issues that you may find when photographing in this manner.  In the old film days, it was a real problem as there was no LCD to double-check your exposure, focus and depth of field.  Now, it is easy to check but you must magnify the image on your LCD to ensure you really have nailed the depth of field.

Thanks for looking.  Enjoy!

Dennis Mook

Many of my images can be found at www.dennismook.com.  Please pay it a visit.  Thank you.


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