• Exposure

  • ESSENTIALS

    • Use Manual Exposure Mode, raw files, Spot Metering

    • Determine the most important setting, shutter speed or aperture, lock it down

    • Determine the second most important setting, shutter or aperture, then lock it down

    • Then spot meter off a grayish surface and adjust only the ISO needed to center the light meter

       

  • WHY

    • Manual exposure gives us more control

    • We are smarter than the camera in any of the automatic modes (Av, Tv, P, Auto ISO)

    • In manual exposure we can lock down the settings that matter most for the situation

    • Shooting in RAW format gives us more flexibility in tough exposure situations

  • GOTCHAS

    • Watch your histogram for high contrast scenes that get clipped

    •  Watch for underexposed images, histogram bunched up on left side, don’t blindly raise ISO

    • Watch for overexposure, too many “blinky” warnings in LCD image review

    • Always remember that ISO does not let in more light or increase true exposure

    • Know your camera’s ISO versus dynamic range curve (ISO for diminishing returns ~ ISO 1600)

    • Low light scenes require tradeoffs on the exposure triangle (shutter speed and/or aperture)

       

“Cross Fox Stare”.  Canon 1DX + EF 600mm f/4L II + 1.4x III manual exposure 1/1250 sec, f/8, ISO 800 spot metered off face of the fox.

A noisy image with no blur is better than a blurred image with no noise!

Key Points

  1. Use Manual Exposure mode.  You are smarter than your camera.
  2. Use Spot Metering.  Meter off a neutral gray part of the scene.
  3. The correct exposure according the light meter is the light reflected from a grey target with 12.7% reflectance.
  4. There are only two adjustments to increase the exposure for a given scene:
    • Aperture.  Bigger = more light = higher true exposure
    • Shutter.   Slower = more light = higher true exposure
  5. Adjusting the ISO does not adjust the exposure
  6. The ISO varies the amplification of the photoelectron signal, and the range that current is digitized.
    • ISO is not sensitivity.
    • ISO does not give us more light, or exposure.
  7. Principle of reciprocity for camera exposure.
    • A click on the shutter speed dial = a click on the aperture dial = a click on the ISO dial.
    • Learn how to change these settings without taking your eye off the viewfinder.

Manual and Automatic Exposure Modes on Your Camera

There are two schools of thought on setting the exposure on your camera.  The first is letting the camera’s light meter and computer decide the best camera exposure and automatically set the shutter speed, aperture, or ISO to achieve this exposure.  In this “automatic” approach one is relinquishing some, or all, control to the camera.

  • Aperture priority (Av) is one of these automatic modes… all we control here is ISO and aperture
  • Shutter priority (Tv) is one of these automatic modes… all we control here is ISO and shutter speed
  • Letting your ISO float (Auto ISO) in “manual” mode, is actually an automatic mode…

Alternatively the photographer may take full control and set the shutter speed, aperture, and ISO that they feel is best.  This approach is referred to as fully manual exposure mode.  Most photographers begin with one of the automatic modes and eventually migrate to manual exposure photography.

The true exposure  is the total amount of light that reaches your camera sensor.  It is a function of how long the shutter is open and the size of the aperture (entrance pupil to be exact) of your lens.

  • ISO does not affect the true exposure.
  • ISO does not affect how much light reaches the sensor.
  • ISO does not affect the sensitivity of the sensor.

The camera exposure is what shows on the exposure meter and light meter of your camera.  It is affected by the shutter speed, aperture, and the ISO setting.  If you can remember that the ISO does not increase the true exposure, or the amount of light, then you will be able to make informed decisions in the field that will help you get higher quality images.

Manual Exposure Mode (M)

In this mode you control the shutter speed, aperture, and ISO independent of each other.  I recommend using manual exposure mode for 99% of your photography.  You are smarter than your camera is.

With a little bit of forethought you can set the shutter speed and aperture to achieve the optimal true exposure for the scene, and then adjust the ISO to properly set the camera exposure.

Manual exposure does not require you to memorize settings.  You still use the light meter in your camera to set the exposure.

Follow these steps before you start to take photos:

  1. Think about the scene you are photographing…
    • Will you be photographing something moving fast (birds in flight, etc.) ?
    • How close will you be to the subject (up close requires more depth of field, smaller aperture)?
    • Are you photographing an expansive landscape (small aperture to have sufficient depth of field)?
    • Do you have a tripod with you?
    • Are things moving in the scene (wind blowing the grass, clouds moving, …) ?
  2. Set the most important true exposure setting (shutter speed or aperture) first
    • Birds in flight  –> set shutter speed to 1/1600 sec or faster.  Lock down the shutter speed.
    • Landscape —> set the aperture to give enough depth of field.  Lock down the aperture.
  3. Then set the other true exposure setting, if you first set the shutter speed, then set the aperture based on the scene you are photographing (or vice versa if you first set the aperture).
  4. Once you have locked down the aperture and shutter speed use spot metering on a grayish target (fence post, gravel, rock, etc.) and adjust only the ISO while centering the light meter reading on that grayish target.  Now the camera exposure is set for your scenario are ready to take photos.
    • If I am photographing birds in flight I set the shutter speed to 1/1600 sec and then the aperture to f/6.3.  I do this when I first arrive at the location.  I then find a gray subject and set the ISO to center the light meter using spot metering.  Now I am ready to photograph birds in flight (if those birds are big and up close I set my aperture to f/7.1 or f/8).
    • For landscapes with my tripod I first set the aperture to f/8 or f/11 (depending on what is in the scene), then I set my shutter speed low ( < 1/100 sec) since I have a tripod.  If the wind is blowing or moving clouds are in the scene I increase the shutter speed (> 1/100 sec).  I then spot meter off a gray target and center the light meter with the ISO.
    • If the light is very low, at night, or in a dense forest, then I may give a little on shutter speed and/or aperture, however I still think about which true exposure is most important (motion or depth of field).

When I first started photography I shied away from manual exposure.  After awhile I became frustrated with missed images due to reliance on the camera to figure out the exposure for me.  Blurry wildlife images resulting from the camera slowing the shutter since it did not realize there was an animal zipping across the field of view can be very disheartening.   “That blurry object is a mountain lion running across the trail in front of me…”  No thanks!

They say that “you get better photos when using a tripod, since it slows you down and makes you think about what you are doing…” .  I think the same is true when photographing with manual exposure.  It makes me think and plan accordingly.

Fully manual exposure mode.  Here the shutter speed is 1/1250 sec, aperture is f/5.6, and ISO = 500.  Spot metering, RAW file storage, daylight white balance, and high-speed burst shutter mode are also set.  Low quality iPhone (glare) image of my Canon 7D2.

“Manual” exposure with a floating ISO is really not manual exposure.  In this scenario you letting the camera automatically set the ISO to give a camera exposure for a gray subject.  I never use this mode since I want to manage shutter speed, aperture, and ISO myself based on the scene I am photographing. Low quality iPhone (glare) image of my Canon 7D2.

Aperture Priority Exposure Mode (Av)

In this mode you lock down the aperture setting and the ISO setting and let the camera decide what shutter speed is needed to properly expose a neutral gray subject. If you also set the ISO to “AUTO”, then you are also letting the ISO “float” and the camera will also automatically set the ISO in addition to the shutter speed.

Aperture priority is the most popular mode for those that do not use manual exposure.  Even so, I find manual exposure to be better for both wildlife and landscape photography.  I want to control the shutter speed.  This is especially true in wildlife photography, or in landscape photography where wind or cloud motion is occurring.

On very rare occasions I use aperture priority mode. One example was when I was lucky enough to be within 20 meters of a family of bobcats eating a cottontail rabbit.  This occurred at sunset and although the light was beautiful and warm it was changing very fast.  My excitement to be laying down on the ground with a 600mm lens next to these beautiful bobcats got the better of me.  I was in manual exposure mode, and properly setup for photographing wildlife at a further distance than this (1/1250 sec, f/6.3, ISO 640).

Since the cats were so close I knew I had to stop down my aperture to get more depth of field.  Meanwhile the light was getting dimmer.  As the sun dropped below horizon I ended up just putting the camera into Av mode and letting the camera decide on shutter speed.  The results were blurry images.  When I realized the shutter speeds in Av mode were too slow for avoid motion blur I went back to manual mode, knowing I could fire a burst at 1/125 sec shutter speed handheld and hopefully get one image that was sharp.

To be clear, even though I switched to Av mode in this scenario, it did not work well, the shutter speeds the camera was automatically setting were way too low.

Bobcat kitten eating a cottontail rabbit.  Canon 1DX, EF 600mm f/4L II + 1.4x III at 1/125 sec, f/5.6, ISO 2500. Note the low shutter speed for this long of a lens (840mm). 

Manual exposure mode shines (no pun intended) when the light is low.  If there is plenty of light, in all areas of the scene, then settings the camera automatically uses are not too far off.  When the light level is low however the automatic exposure modes, like Av, can result in too slow of a shutter speed — especially when wildlife is the subject.

Aperture priority exposure mode.  I have locked down the aperture to f/5.6 and locked down the ISO 500 settings.  I have no idea what the shutter speed will be.  The camera will set a shutter speed depending on the available light.  The resulting shutter speed will be what the camera thinks gives a correct exposure for a neutral gray subject with the aperture at f/5.6 and the ISO 500. Low quality iPhone (glare) image of my Canon 7D2.

Shutter Priority Exposure Mode (Tv)

In this mode you lock down the shutter speed and the camera will automatically set the aperture, and the ISO if you let that also float.

Usage of shutter priority mode is pretty rare.  This is due to most photographers, and photography scenarios, requiring a specific depth of field.  Without knowing what the aperture will be leaves us clueless about the depth of the field.

Shutter priority mode, rarely used, lets me lock down the shutter speed and let the camera set the aperture (and possibly the ISO if that is set to “AUTO”).

Understanding Camera Exposure in Digital Cameras

The amount of light reaching your camera sensor, from a given scene, depends on two things:

  1. The length  of time the shutter is open,
  2. The size of the aperture used inside your lens .

These two adjustable settings on your camera-lens are the only settings that we can use to vary the amount of light incident on your camera’s sensor (known as the true exposure ).

The sensor in a digital camera contains a grid of very small pixels.  Each pixel absorbs photons from the light incident on it.  These photons are converted into electrons in the sensor via the photoelectric effect in the semiconductor material of the sensor.   The electrons are then collected from each pixel  to form an electrical signal, which is then amplified.  The amplified electrical signal is then digitized.  The end result is a numerical (integer) signal value for each pixel that relates to the amount of light incident on each pixel.

Conceptual drawing of a pixel and how light (photons) are converted to electrons.  The electrons are collected into an electrical signal which is amplified and then converted from an analog electrical signal to a digital value.  This digital value relates to the exposure level at the pixel.  The digital image is recorded onto your memory card as the digital exposure value for each pixel.  Image from https://www.ptgrey.com/white-paper/id/10912 .

Camera Exposure

Camera exposure, is a relative measure of exposure.  The pixels in our digital cameras can fill up with photo-electrons.  Smaller pixels fill up faster than larger pixels for a given incident light density.  Full pixels are interpreted as high exposure values.

The camera exposure is a relative level, strongly affected by the gain applied (ISO) in amplifying the photoelectron signal and the level of photoelectrons chosen to be the maximum (full pixel) level (also ISO). The actual light recorded depends on the aperture of the lens and how long the shutter stays open.

A “technically correct” exposure is one that captures details in the brightest parts of the image, and also captures details in the darkest parts of the image.  The correct exposure will also minimize motion blur and field blur (too narrow a depth of field).  In a “perfect” exposure no information from the image is lost.  This is not always possible.

Each sensor, or film, has a certain range of exposure it can record detailed information.  If the range of exposure for a scene exceeds what the sensor can record (the sensor’s dynamic range) then information will be lost.  Details in the whites will be lost (“blown out”) if the image is overexposed.   Conversely, an underexposed image will lose detail in the blacks or shadows.

Note that the dynamic range of your sensor is a function of your ISO setting.  Increasing the ISO reduces the dynamic range.

Often we must make subjective tradeoffs between what we want to emphasize in the photograph and what we are willing to let go of.  Perhaps we do not care as much about details in the deep shadows of the forest as much as we care about capturing the details in the bright white clouds in the sky.  In this case we adjust our shutter speed and aperture to slightly underexpose the image — making sure we do not blow out details in the bright clouds.

The sequence of images below were taken in burst mode and exposure bracketing.  The shutter speed varies between frames {1/160 sec, 1/500 sec, 1/1600 sec}.  The shooting conditions in these photos was very challenging (high dynamic range in the scene, moving water, moving bear, black bear with detail in its fur hard to capture).

Canon 1DX + EF 100-400mm f/4-5.6L USM II handheld, 1/160 sec, f/6.3, ISO 3200.  This exposure is over-exposed.  The red areas indicate clipping where detail is lost in the whites and highlights.  The slow shutter speed also shows motion blur in the water.  However, now we can see detail in the shadows and on the bear’s fur.

Canon 1DX + EF 100-400mm f/4-5.6L USM II handheld, 1/500 sec, f/6.3, ISO 3200.  This exposure is very good based on the histogram, we have full coverage across the dynamic range from blacks to whites.  You can see a very tiny area of over-exposed highlighted in red in the lower RHS.  The shutter speed is fast enough to retain somewhat sharp detail in the water drops.  More importantly I can see details in the most of the shadows and in the highlights in the water.

Canon 1DX + EF 100-400mm f/4-5.6L USM II handheld, 1/1600 sec, f/6.3, ISO 3200.  This exposure is under-exposed, the shadows are pushed up against the blacks.  Details are lost in the shadows and the bears fur.

Increasing the exposure in post-processing of this image will most likely result in noise.  Why?

Exposure Triangle

The Exposure Triangle is a handy mental model to help with manual exposure strategy.  I first decide which corner of the triangle to “lock down” first.  If there is fast movement (e.g. bird in flight) then I set the shutter speed to 1/1600 sec or faster.   If there is a large object, or the object is very close to me, then the depth of field is most important, so I lock down the aperture corner first.  If I set shutter speed first, next I think about depth of field relevant to the composition, set the relevant aperture, and lock that down.

Once I lock down the shutter speed and aperture I spot meter on a grayish part of the scene (rock, gravel, fence post) and adjust the ISO to center the exposure.  I then leave things as they are set and start photographing.

If the light is plentiful then I have more room to increase the shutter speed in a wildlife action scenario, if needed; or in a landscape scenario I may want to stop down the aperture to improve depth of field.  It is much easier when there is a ample light.

The challenges occur when the light is low.  This is when manual exposure really helps.  If I am underexposed on the light meter for ISO values less than < 1600 then I have to be careful I do not amplify noise instead of signal. I might decide to relax my ideal settings for shutter speed and aperture to increase the true exposure, before I start to crank up the ISO.    Perhaps for the birds in flight I am willing to fully open up the aperture and not worry about depth of field as much, or instead of shooting at 1/1600 sec I am willing to go down to 1/800 second shutter speed.  Manual exposure allows me to make these tradeoffs in a controlled, knowable, manner.

Most important though I ask myself what is more important to me for the image I am working on.  Perhaps I am not willing to lower the shutter speed or the open up the aperture.  I will then raise the ISO and rely up on using de-noising methods in post-processing.  Uniform areas (sky, water, snow) are pretty easy to clean up noise on.  Areas of the composition with detail that you want to retain (fur, feathers, grass, foliage) sharpness can be degraded in post-processing de-noising.

It is better to have a non-blurry noisy image, than a blurry image with less noise.

Exposure Triangle as a mental model for planning your photographs. Image from PetaPixel (note my correction on exposure triangle diagram… ISO does not affect the amount of light. It only amplifies the post-sensor signal and changes the digitization range of photoelectrons).

Rule of Thumb Ranges for Shutter and Aperture

  • Birds in flight
    • My ideal shutter speed is 1/1600 sec or faster.
    • If the light is very low, then I will open up the aperture all the way, not worrying as much about depth of field.
    • If the light is very low, then I will lower the shutter speed, with a lower limit of 1/640 sec, or moving from 1/1600 sec to slower shutters speeds in a stepwise manner:
      • 1/1600 sec –> 1/ 1250 sec –> 1/1000 sec –> 1/800 sec –> 1/640 sec
    • I would rather avoid motion blur than noise!
  • Bird sitting on a perch
    • I try to keep my shutter speed > 1/100 sec.
    • I usually try to blur the background, open up the aperture (f/4 or more).
  • Mammals (bear, bobcat, coyote, wolf, bison, wild horse, …)
    • How close it to me?  If close, then stop down to f/8 or smaller aperture.
    • What focal length lens?  If close and long focal length (> 300 mm) then stop down even more.
    • If mammal is moving I try to keep shutter speed > 1/500 sec.

If I am forced to use a lower shutter speed on moving wildlife, then I fire off many images in burst mode.  From the burst of images I pick out the ones where the motion blur is minimal.

Handheld at 1/10sec, f/11, ISO 100.  I took a burst of images quickly on this bear, out of this burst there were a few where the bear was still enough to get detail on his fur.  (Note, my goal on this image was to intentionally have the water blur and the bear be sharp, hence the low ISO and small aperture).

Practice this Technique

A given setting of shutter speed, aperture, and ISO on your camera results in a certain camera exposure (shown by camera’s light meter).

The important principle of reciprocity, that the exposure is equal to the time the shutter is open times the area of aperture, means that if we increase the shutter speed (reduce exposure) we can regain the same exposure by increasing the aperture.  Cameras are designed so that one “click” change in shutter speed is balanced in true exposure by one “click” in aperture.  If you double the shutter speed, while also halving the area of the aperture, then the exposure is the same.

Ok, enough theory, how do you benefit by this?  Look through your viewfinder at your subject while keeping an eye on the light meter scale in the viewfinder.  Lets say the exposure is perfect (i.e. the light meter is centered on 0), but you want to increase the shutter speed.  Increase the shutter speed by one click (without taking your eye off the viewfinder!), then open up the aperture by one click (don’t move your eye…).  Now your exposure will be the same (i.e. centered).  The camera is designed (check this) so that if you change the ISO by one click, you can balance the exposure by one click on either the shutter speed or aperture.

The more you can practice this the easier time you will have in photographing wildlife.  It helps to remember your starting point on the exposure triangle, then keep track of where you are relative to the initial point by counting clicks, and keeping track of the direction of the clicks based on your camera dials.  There have been many intense moments while I have been photographing a rare wildlife situation with no time to take my eye off the viewfinder, make an adjustment, then reacquire focus on the subject.  In this tense moments this technique has saved the day, three clicks faster shutter speed, three clicks larger aperture — go.  No looking any menus or dials!

The default amount of exposure change with each click of the shutter speed, or aperture, is normally 1/3 of an EV (stop).  The increment can be usually be changed to 1/2 of an EV (stop) if you so desire.  Make sure to check your ISO increment so it is the same size step as the shutter speed (see figure below).

The default amount of exposure change with each click of the ISO is normally 1/3 of an EV (stop).   I recommend using the same increment of 1/3 EV on shutter speed, aperture, and ISO.  In this way each click along the exposure triangle is equal.

Stops Making Sense?

  • Exposure is measured in units of “stops”, or “exposure values (EV)”
  • A full stop = EV
  • An increase in one stop doubles the exposure
  • A decrease in one stop halves the exposure
  • Your camera is probably setup with an exposure interval of 1/3 stop, in which the exposure changes by 26% between each 1/3 stop change.

The full stop values for shutter speed, aperture, and ISO.  Note the factor of two involved in each step. For each full stop change, the light on the sensor changes by a factor of two.  (Normally your camera will be set up in increments of 1/3 a stop between settings)

The 1/3 stop values for shutter speed, aperture, and ISO. For each change of 1/3 stop the exposure on the sensor changes by 26%.

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