Focal Planes & Focal Length

John G Evans c 2019

Through my honest attempt to comprehend the photographic focal planes within the camera lens I consulted a brilliant photography colleague of mine whose name I shall not reveal at this time having no consent to mention her name. However, she brought to light the mystery hidden within the complex world of photographic focal planes. Essentially speaking the higher the f/ number the more focal planes we come to know as being in focus. So, I encountered a PDF online entitled Basic Principles of Photography by David Capel.

While Capel explains illustratively that the Latin ‘Camera Obscura’ = “Dark Room” he explains also that “light passing through a small hole produces an inverted image on the opposite wall. Leonardo DaVinci and other artists during the 15th century made use of this creative technique to sharpen their artistic imaginings.

Creative refinements were designed were utilized by 18th century artists as Canaletto. The first original photograph was created by Joseph Niepce in 1825 upon light sensitive silver chloride plates. Thus, I suspect the originating feature whereas silver film became a useful tool for creating the image upon. I learned this factor working for a micrographics firm creating silver film and diazo film images we now call microfiche. I did this for 6 years working in a micrographics production facility. Looking back, I can now visualize what I was creating, how, and why.

Capel goes on to further demonstrate that the first production camera (Daguerreotype), designed by Louis Daguerre (1840’s) became the prototype we all benefit from in our world today. So, if we fast-forward 150 – years we discover that early photographers used silver halides on a mirrored – polished hard surface we call film. But as David goes on to explain present day fundamentals of photographic reception, we learn digital cameras employ electronic sensors consisting of large number of square cells or pixels.

A smaller aperture or opening means we receive little light or subject matter visibility. A larger aperture or opening means we receive more light rays or subject matter visibility. With the lens we add to the camera we receive rays of light on a singular point creating a sharper image.

Capel also demonstrates as I have aspired to explain as correctly as I am able as a somewhat novice/want-a-be-portrait-photographer a formula (that which I have discovered useful), that shows us 1/D’ + 1/D = 1/f. This formula translates to points at distance D are focused on D’ plane results in the subsequent turning of the focus ring to move the focal plane rendering us a more focused image (whether on film or a censor or even in a pinhole camera a wall). So, the field of view depends upon the focal length f that is a distance from the aperture to the censor (or film).

By doubling the focal length, we receive half the view. Essentially, a short focal length we see a wide field of view, but, a long focal length we see a narrow field of view. For a known or given film size s and focal length f plus the angular field of view the formula would look as such:

                                                a = 2 arctan s/2f

With a Perspective Convergence your short focal length (wide FoV), images tend to exhibit pronounced perspective effects. A wide FoV near to subject parallel lines converge, and with a narrow FoV far from subject parallel lines remain parallel. By increasing focal length and moving further from subject we can drastically change the composition.

The problems I encountered over a specific composition I was working with first at f/1.2 then f/5.6 and still being out of focus as I was told by a colleague was motion blur as opposed to focal planes, but in the process of discovering why my focus was not sharp, I thought more light was necessary on the subject to be able to focus clearly on her closest eye. Once in focus everything within that particular focal plane should be in focus. f/1.2 gave me a wider aperture opening to receive more light rays and subsequent image transferred TTL (through the lens), and when I modified the focal plane to f/5.6 I remained out of focus with the exception of the model’s face. So, my friend instructed me to photograph at f/4.0, 1/160 s, ISO 100, with a lamp power decrease to 1/64s. This was a super starting point.

With a determination to manipulate the camera and lens to work to my advantage I begin at this point. Adjustments are made until I am completely satisfied with the end result.

At this point I wish to thank David Capel for his photographic evaluation of focal planes and focal lengths through his valuable article in a PDF format. Without his research I never would have understood more clearly. All credit is and should be rendered to David Capel for assisting me to write out the measures he has taken to research, write, and illustrate this 44 – page PDF article I used to try and understand more about focal planes and focal length. Thank you, David Capel.

Published by John Gregory Evans

Poetry is truth as we share our words that express our stories. I have been writing poetry over twenty years and have been published within many venues, including two books of mine. As poets we speak our truths and become known by our words. If you are a poet...keep writing. There remains joy and love in the solace of the written word.

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