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How-to's

Exposure variables – Making the most out of your camera

Hal Becker (HBB)


Keywords: photography, aperture, shutter, speed, iso, illumination, luminance, dof, blur, digital, sensor, film, camera, mode, metering, sekonic, candela, lumens, lux, ev, hbb

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Introduction

Four variables are involved in capturing an image photographically: Shutter Speed, Lens Aperture, ISO, and Illumination.

Some instructors believe that students should not be exposed to all four of these variables in the initial phases of their photographic journey.  Some will present shutter speed and aperture alone, leaving ISO and Illumination for later.  Others will present shutter speed, aperture and ISO, as the three variables controlled by the camera, leaving illumination for later.  Some feel that, since illumination is not controlled by the camera, it should not be included in exposure discussions.  From a somewhat broader perspective, illumination can be considered the sine qua non of photography.  Without illumination, there can be no photography.

There is no single teaching method that is right or wrong, or better than others.  The approach that works for an individual instructor, leading to a firm grasp of exposure by the student, is the one that should be employed.

The following article provides a description of the four variables and their relationship to each other.

Section I provides a definition and brief discussion of the variables.

Section II discusses the four variables and presents a non-numeric chart that describes their interrelationships. 

Section III explores the arithmetic used to calculate the variables for reflected illumination metering systems:  Given any three of the variables and their numeric values, the fourth can be determined.  This section also includes a brief discussion of the equation required for use with incident exposure meters.

Section I:  Basic Definitions

Shutter Speed is the time that the shutter is open, exposing the film or digital image sensor to light passing through the lens.  Shutter speed may range from very short intervals (1/8000 of a second: 0.000125 second) controlled by the camera, to several minutes or more using manually controlled time exposures.  Slower shutter speeds are frequently used in static situations, where the subject is stationary and illumination is adequate: portraiture, landscapes etc.  Faster shutter speeds are used for freezing subjects in time: sporting events, racing, etc.

Lens Aperture is a variable opening in the camera that lens determines the intensity of the illumination that passes through the lens on its way to the film or digital sensor while the shutter is open.  Large apertures (small f/number, f/1.4 for example) will permit a greater intensity of illumination to pass through the lens (think brighter image) and a shallow depth of field, while smaller apertures (f/11.0 for example) will allow proportionally lower intensity of illumination to pass through the lens (think dimmer image) and a greater depth of field.

ISO (International Standards Organization) indicates the relative speed at which the film or digital sensor can absorb enough of the photons of light passing through the lens while the shutter is open to render an acceptable exposure on the film or digital sensor.  Lower ISO numbers (100 for example) are considered slower, as they require more photons to capture the image, resulting in higher image quality.  Higher ISO numbers (6,400 for example) can capture an image with considerably less photons, resulting in lower image quality.

Illumination consists of photons of light in the visible portion of the electromagnetic spectrum that illuminates the subject.  It may be ambient (sunlight, indoor lighting, etc.), or it may be supplemental (studio lights, on-camera speedlights, etc.), or a combination of ambient and supplemental (using fill flash to illuminate the shadow side of a subject standing in the shade outdoors, for example).  Illumination reflected off the subject, passing through the lens and shutter and striking the film or digital sensor, creates the image. 

(19 Votes )
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Originally written on January 25, 2016

Last updated on April 25, 2016

Hal Becker Hal Becker (HBB)

Hal is an expert in several areas, including CLS Awarded for his excellent article contributions to the Resources. Donor Ribbon awarded for his very generous support to the Fundraising Campaign 2015

Phoenix, USA
Basic, 8889 posts

17 comments

Jeanne Walker (sailingne1) on April 28, 2016

Nicely done!

Hal Becker (HBB) on April 27, 2016

Hal is an expert in several areas, including CLS Awarded for his excellent article contributions to the Resources. Donor Ribbon awarded for his very generous support to the Fundraising Campaign 2015

Bo: Thanks for helping me correct the error in the chart. Try as I might, it seems to be impossible to catch every error in my lengthy articles. I will try and do better next time, which will be the article that I recently started on shutter and speedlight relationships, including the mysterious second curtain sync function used with moving subjects. Regards, Hal

Hal Becker (HBB) on April 27, 2016

Hal is an expert in several areas, including CLS Awarded for his excellent article contributions to the Resources. Donor Ribbon awarded for his very generous support to the Fundraising Campaign 2015

Bo: Thanks for helping me correct the error in the chart. Try as I might, it seems to be impossible to catch every error in my lengthy articles. I will try and do better next time, which will be the article that I recently started on shutter and speedlight relationships, including the mysterious second curtain sync function used with moving subjects. Regards, Hal

Bo Stahlbrandt (bgs) on April 25, 2016

One of the two c-founders, expert in several areas Awarded for his valuable Nikon product reviews at the Resources

Editorial note re wrong chart data "The Equations" has been removed. The chart is ok.

Hal Becker (HBB) on April 8, 2016

Hal is an expert in several areas, including CLS Awarded for his excellent article contributions to the Resources. Donor Ribbon awarded for his very generous support to the Fundraising Campaign 2015

Please Note: The Excel and conventional format equations for the Aperture in the above chart are both correct now. Please disregard the editors note at the bottom of the chart. Sorry for the confusion. Hal (HBB)

J. Ramon Palacios (jrp) on March 25, 2016

JRP is one of the co-founders, has in-depth knowledge in various areas. Awarded for his contributions for the Resources

(Edited by jrp Friday, 25 March 2016 ) Paschal, just make your questions at the D7000 Series forum. We will get you answers.

Paschal Okoye (padosky4eva) on February 15, 2016

wow!!! I just improved to d7100 but am still finding it difficult to use can someone help me out

Hal Becker (HBB) on February 9, 2016

Hal is an expert in several areas, including CLS Awarded for his excellent article contributions to the Resources. Donor Ribbon awarded for his very generous support to the Fundraising Campaign 2015

Mike: Nice catch! I need to remember to engage brain before turning fingers on. Excel equation for Lens Aperture should be: ((L*S*T)/(K))^(2) Thanks for noticing. With a long post it is difficult to catch minor typos like this, no matter how many times I edit it. JRP or Rick: Any chance this can be corrected above? Regards, Hal

Somnath Adhikary (Somnath adhikary) on February 9, 2016

I have acquired lot of knowledge aboutfour components specially illuminstion

Mike Muyal (Mik) on February 9, 2016

In your last table, shouldn't the Excel formula be to the power of two ie (^2) instead of (^0.5) ?

User on January 28, 2016

Hal, Well done; you've obviously done some extensive research for this article. Something useful I found that dovetails perfectly with your article is this online animated version of the principles you've detailed. http://camerasim.com/apps/original-camerasim/web/ I look forward to your next article. ~ Greg

Tony Wright (karton) on January 27, 2016

Thanks Hal, for this masterfully written piece. My youngest granddaughter is just learning photography and I will certainly pass it along to her. Keep up the good work! Tony W.

Hong Chow (hongkchow) on January 27, 2016

Hi Hal, Thank you so much for your great article particular with the illumination. Hong

Kerry Standifer (Kerry S) on January 27, 2016

Very nice article. There is a lot of information in just 3 pages. I like the mathematic relationship which is often lost in the discussion of exposure but is certainly the basis. Also often lost as stated by the author is the variability of light. The camera is simply the instrument for the capture of light. Understanding the ways to control light (when practical), and the ways to find the best conditions are key. It is all about the light, and the earlier that is put in the mix the better.

Peter Stokes (PAStime) on January 27, 2016

Thanks Hal - a great refresher. Peter

Dianne Clark (Dianne340) on January 27, 2016

Thank you. The first time "illumination" has been introduced to the standard triangle equation and that provided the missing link in my understanding

KENT M. WHITNEY (KMWHITNEY) on January 26, 2016

Donor Ribbon awarded for his very generous support to the Fundraising Campaign 2014 Donor Ribbon awarded for his generous support to the Fundraising Campaign 2015

Hal, Thank you for the refresher. It was a joy to go back a bit and rethink all that you provided. I remember when I was a Photog student and starting out with a pure Photography Corriculem in college and the blessing it was to learn from really great photography instructors, All Masters of the Art of Photography. For 3 years and 6 semesters later, they all had one thing in common, "Everything on our camera was in the manual mode, no exceptions...! While shooting film, if a student tried to cheat, our all knowing instructors always knew, they were amazing!! Then when we got into digital that ever present rascal of the embedded "Metadata" would tell the whole story, LOL After being required to shoot only in "Manual Mode" for 5 semesters and that becomming very natural by that time, we were allowed to start using the other Auto Options. I at the time found it more difficult to use the Auto modes except for ISO. Shooting manual became second nature and I didn't have to think much about it, just did it and the type of capture dictated the settings, be it low or high light levels, action or static, lens used and DOF needed or wanted. I enjoyed your lesson/refresher, had to use my noodle again, and that felt good, knocked off a bit of dust and rust I'd say :-) I must admit that there is one time that I usually set up in full Auto and that is when I am at a family function and do not want to be the photographer because when behind the camera we tend to miss the party, LOL! Thank you again Hal and many Blessings to you and your family... V/R Kent

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