Pentax K- Spotmetering

On-camera Spotmetering with Your PENTAX DSLR

Course Overview

In this article, we will deal with the mid-size DSLR, Pentax K-5 and K-7 and the compact DSLRS, Pentax K-r and the K-x. All four of these models have meter modes of multi, center and spot. This course will help you to learn a systematic method to consistent photographic results.
Audience

This article intended to expand Pentax DSLR users understanding of the Spotmetering feature built into their camera and how they can gain better exposure control.

Before You Begin

Before starting this course, you should: Read your cameras manual especially the "Setting the Exposure" section.

·         Review the "Setting the Exposure" section on "Changing the Exposure Mode", "Selecting the Metering Method" and "Adjusting Exposure".

What You Will Learn

After completing this course, you will be able to:

·         Identify the dynamic range of a photograph both prior to taking it and on existing photographs.

·         Accurately adjust your exposure for consistent results.

·         Understand how to apply Zone knowledge to other metering methods as well

·         Know when HDR processing is required and what your options are.

 Important:

Expectation is that you have read your cameras manual in respect to how each of these metering modes operate. As to which mode is should be used is beyond the scope of this article but additional references will be provided in the Appendix.

Exposure Basics

Exposure is the combination of ISO, aperture and shutter speed. Confused, don't be we have all been there and will likely be overwhelmed by all the data but with practice it will get easier allowing you to master this information and apply it when you want it.

ISO

The ISO sensitivity (ISO rating, ISO speed) characterizes the sensor or film sensitivity to light. Formerly called ASA rating, digital cameras continue to use the same system introduced with film. It is common to use the term "stop" in photography and we will be using it a lot in this article. One stop in terms of ISO refers to a doubling of our sensitivity. ISO 200 is twice as sensitive as ISO 100 and is therefore "one stop higher." Unlike film based cameras where we would have to make a decision on the type of film needed before going out and "make do" if we made the wrong choice, digital photography is the capability to change the ISO settings as the conditions warrant down to each frame if the situation requires it.

Apertures

Aperture setting determine how large the lens opening will be that allows the light through to the film or sensor. Each "stop" in this case f-stop, doubles or halves the amount of light for instance f/4 is +2 stops larger then f/8 where f/11 would be -1 stop smaller opening.

 Note:

The smaller the aperture setting, higher f-stop number, the greater your depth of field(DOF) will be. Large aperture settings, low f-stop number, provide short DOF and render backgrounds out of focus.

Typical Apertures (f-stops) each stop moving left to right is half the amount of light as one preceding it. Depending on the lens you may have additional f-stops available to you.

1.00

1.40

2.00

2.80

4.00

5.60

8.00

11.00

16.00

22.00

32.00

Shutter Speed

Shutter speed determines how long the aperture remains open to let in the light to the film or sensor. The shutter is covering the film or sensor and only when the shutter-release is pressed, the shutter then opens for a pre-determined time to let light pass through our lens to the sensor. We also use "stops" to measure shutter speeds. Each stop doubles the time the shutter remains open and thus doubles the light sensitivity of our camera , 1/60 is twice as long as 1/125 alternately 1/250 is one stop faster letting in half the light of 1/125.

Typical Shutter Speeds (seconds) moving right to left each number indicates one full stop allowing half as much light are also shaded light blue.

1/8000

1/4000

1/2000

1/1000

1/500

1/250

1/125

1/60

1/30

1/15

1/8

1/4

1/2

1

2

4

8

15

30

     

Pentax  K-5, K-7 maximum is 1/8000 and 1/6000 for the Pentax K-r, K-x @ 1/3 EV steps adds additional steps of 1/3 less light, 2/3 less light then the typical 1/2 as much light.

1/8000

1/6400

1/5000

1/4000

1/3200

1/2500

1/2000

1/1600

1/1250

1/1000

1/800

1/640

1/500

1/400

1/320

1/250

1/200

1/160

1/125

1/100

1/80

1/60

1/50

1/40

1/30

1/25

1/20

1/15

1/13

1/10

1/8

1/6

1/5

1/4

0.3"

0.4"

0.5"

0.6"

0.8"

1"

1.3"

1.6"

2"

2.5"

3"

4"

5"

6"

8"

10"

13"

15"

20"

25"

30"

Pentax  K-5, K-7 maximum is 1/8000 and 1/6000 for the Pentax K-r, K-x @ 1/2 EV steps adds additional 1/2 stop of light then the typical one stop difference.

1/8000

1/6000

1/4000

1/3000

1/2000

1/15000

1/1000

1/750

1/500

1/350

1/250

1/180

1/125

1/90

1/60

1/45

1/30

1/20

1/15

1/10

1/8

1/6

1/4

0.3"

0.5"

0.7"

1"

1.5"

2"

3"

4"

6"

8"

10"

15"

20"

30"

 Warning:

Use a tripod if your shutter speed is less than the focal length of the lens used, with the in camera image stabilization up to four additional stops may be possible. Using a 300mm lens where previously any shot below 1/250 required the tripod you may be able to hand hold the shot at 1/30 or even 1/15. Strongly recommend you test for yourself where your limit will be.

Equivalent Exposures

Depending on the conditions or your expectations you may meter your shot and determine the correct exposure would be say f/8 @ 1/125 with a ISO of 400, but your shooting water and want that milky look. This means you need an equivalent exposure with a much slower shutter speed, no problem. For each stop you slow your exposure you increase the aperture which would mean that f/32 @ 1/8 of a second will give you the same exposure. But wait, my lens does not have a f/32, f/22 is high as I can go… no problem we simple drop our ISO from 400 to 200 which now shifts everything one stop down, in this case we drop the f-stop by one to f/22 which makes f/22@1/8  now the same. Here are the settings compared in table form.

ISO

f-stop

Shutter Speed

400

f/8

1/125

200

f/22

1/8

 More Information:

For more information about this topic, see:

THE ULTIMATE EXPOSURE COMPUTER article on the Fred Parker Photography site, http://www.fredparker.com/ultexp1.htm this site should answer any questions you may still have about exposure. The site has lot of information to digest, but well worth the time and effort.

What Does Spot Metering Mean?

Spotmetering is a way to measure exposure based on only a 1° field of view. Considering your basic 18mm-55mm kit lens field of view is approx. 77° @ 18mm and only 29° @ 55mm still a huge difference considering those fill the viewfinder.

Let us review what you see in the viewfinder really means…

Figure 1.  Figure 2.

Figure 1 gives us the full view of what see in your viewfinder. Figure 2 breaks down the basic coverage of the various metering modes with red outlining the 77 segments multi-mode offers on the Pentax K-5, green would get the center-weighted mode and the gray circle within the blue square will be our area of interest for the spot metering.

18% Gray


 Critical:

Meters see only one color 18% gray, no black, no white, no red, no green, no blue just the density of the gray tones.

The critical statement may seem a bit harsh but that is the reality of it, exposure meters are meant to find that perfect middle tone plain and simple based on the metering mode you select. Spotmetering takes this to a very definitive level by saying the item within the gray circle shown in the figures above I will make 18% gray and ignore the rest of the world outside this circle when determining a proper exposure.

So about now you may be wondering why on earth would I want to use a mode that is going to see a very tiny portion of my subject as only and then tell me it is always 18% gray? Consistency, unlike the other metering modes that take larger portions of the scene into consideration allows you the photographer control to create pictures where you choose what will be exposed and in what manner so that you capture the scene as you perceive it to be. This may not always be exactly as it is, but instead show how you interpret it to be.

Color to the exposure meter reduces color to gray densities; think back to B&W photographs, all you see is the gray densities or tones with the colors removed.

Zone System

The Zone System as formulated by Ansel Adams and Fred Archer provided film photographers as systematic method of precisely defining the relationship between the way they visualize the photographic subject and produce those results in the final picture. Even though they originated with black-and-white sheet film, the Zone System is also applicable to roll film, both black-and-white and color, negative and reversal, and to digital photography. While the original Zone system concentrated on the photographic subjects shadow details from a digital photography position we will like with slide (transparency) film concentrate on correctly exposing the photographic subject highlights, figure 6 helps to explain these relationships. The following chart  shows you how these different Zones equate the other concepts being promoted in this article.

Zone Assignment

EV Compensation

Adams Density

18% Gray Density

Approximated RGB Value (completely subjective to conditions)

Zone 0

-5 stops

0

0.5625%

 

Zone I

-4 stops

.5

1.125%

 

Zone II

-3 stops

1

2.25%

 

Zone III

-2 stops

2

4.5%

28

Zone IV

-1 stop

4

9%

78

Zone V

0 metered default

8

18%

128

Zone VI

+1 stop

16

36%

178

Zone VII

+2 stops

32

72%

228

Zone VIII

+3 stops

64

144%

 

Zone IX

+4 stops

128

288%

 

Zone X

+5 stops

256

576%

 

This next charts below shows approximations of various films and the digital camera in terms of the number of stops coverage that are typical. You would need to test your camera to see actual range of coverage.

Type

Approximate Dynamic Range

Black and White negative

7

Color Negative

6

Digital Camera

5 - 7

Color Slide (Transparency)

5

Polaroid Pictures

3

Litho High contrast Film

2

Another rule-of-thumb view about dynamic ranges:

Type

DYNAMIC RANGE

STOPS

Typical outdoor, sunlit scene

100,000:1 or more

~17 EV

Human eye

10,000:1

~14 EV

Film camera

up to ~2000:1

~11 EV

Digital camera

typically ~400:1

~8.5 EV

Good computer monitor

500:1 to 1000:1

9 - 10 EV

Typical photo print

100:1 up to 250:1

7 - 8 EV

Our Digital Zone System

Our working scale consists of seven zones from Zone II to Zone VIII as shown in Figure 5. For our purposes, we will consider Zone II pure black and Zone VIII pure white from a digital perspective as no texture will be discernible merely tone:

Figure 5.

Exceeding Zone III or Zone VIII on your camera would be expected to yield no discernible details and would likely be considered pure black or blown out highlights. Every stop of exposure will be one stop +/- of the stop it currently occupies. The following  table helps describe the differences.

Zone Sample

Description

Zone III - This will be our black, requires -2 stops exposure than your actual spotmeter reading to be correct.

Average dark materials and low values showing adequate texture

Zone IV - This is our dark gray, requires -1 stop exposure than your actual spotmeter reading to be correct.

Average dark foliage, dark stone, or landscape shadows

Zone V - This is the base point of all-metering yielding an 18% gray no require no stop adjustments.

Middle gray: clear north sky; dark skin, average weathered wood

Zone VI - This is our light gray, requires +1 stop more exposure than your actual spotmeter reading to be correct.

Average Caucasian skin; light stone; shadows on snow in sunlit landscapes

Zone VII - This will be our white where we still see texture, requires +2 stops additional exposure than your actual spotmeter reading to be correct.

Very light skin; shadows in snow with acute side lighting, digitally probably our lightest tone with texture: textured snow

Using the Pentax K-5 for example, Figure 7, these zone shifts can easily be expressed using the cameras exposure adjustments as EV compensation, which can be seen in the following places:

 Figure 7.

In Figure 7 we see the monitor screen on back of the camera showing no EV compensation being used on the EV bar:

 if we had altered it we can see the full stops indicated by the numbers and in this case EV steps is 1/3 so we see the additional 2 exposure steps in 1/3 increments between the full step marks.

The viewfinder will tell us the actual compensation value, shows us increase of +1.5 stops which moves exposure from Zone V to between Zone VI and VII. This change also is shown in the LCD panel on the top of the camera as:

 

 

Pulling Together the Pieces

Up until now, we have covered several areas from what exposure is and how its parts work together to something called the Zone System we have interpreted for use with your digital camera. This leaves us with the underlying question why do you, or should you even care?

Why do you care?

·         Consistently predictable results

·         No more blown out highlights and loss of detail

·         Increased creativity

Consistently able to produce pictures where you know you will have details as expected in the final result. By Spotmetering on the highlight in your shot and adjusting your EV compensation so that brightest subject falls in Zone VII will do this.

Spotmetering snow or a beach with texture and setting your EV compensation in Zone VII will make your snow white and beaches bright. Likewise if no texture concerns but want that bright snow pure white, kick it up +3 stops to Zone VIII.

If your photography a very high contrast scene that exceeds dynamic range of you camera, a.ka. you spotmetered the max highlight and your shadows and cannot cover  the EV range in a single shot

Why should you care?

You actually may not nor should you, if on vacation and unless it's one of those once in a lifetime chances where you will never have the chance again… turn everything to automatic and go for volume of pictures.

Camera setup for vacations or "general" events for me usually means center-weighted metering, and shooting modes in order of preference Auto/Program/Av/TV or Manual on the Pentax K-5.

Remember photography should be fun and a work of love, now go take more pictures J

Appendix

Appendix 1: EV Exposure Rules

Light Conditions

Here is a basic set of lighting conditions with the corresponding EV recommendations as taken for the http://en.wikipedia.org/wiki/Exposure_value  site with the exposure chart adapted with that sites information and other opinions on the internet.

ISO Based EV Adjustments

So what if I am not using ISO 100 as the chart is based on? Not a problem, you simply adjust your EV number up or down by the values in the following chart:

So mathematically speaking a different ISO speed S, increase the exposure values (decrease the exposures) by the number of exposure steps by which that speed is greater than ISO 100, formally

\mathrm{EV}_{S} = \mathrm{EV}_{100} + \log_2 \frac {S} {100} \,.

For example, ISO 400 speed is two steps greater than ISO 100:

\mathrm{EV}_{400} = \mathrm{EV}_{100} + \log_2 \frac {400} {100}
= \mathrm{EV}_{100} + 2 \,.

To photograph outdoor night sports with an ISO 400–speed imaging medium, search our "Lighting conditions" table for “Night sports” (which has an EV of 9), and add 2 to get EV400 = 11.

For lower ISO speed, decrease the exposure values (increase the exposures) by the number of exposure steps by which the speed is less than ISO 100. For example, ISO 50 speed is one step less than ISO 100:

\mathrm{EV}_{50} = \mathrm{EV}_{100} + \log_2 \frac {50} {100}
= \mathrm{EV}_{100} - 1 \,.

EV Based Camera Settings

Now find your EV in the following table to see your suggested exposure look up the EV number in the left hand column of the chart then read across to you desired f-stop to discover the suggested shutter speed.

Appendix 2: Simple Named Exposure Rules

Sunny 16

Sunny 16 rule means it’s a sunny day, your intended subject is sunlit, and its between roughly 9:00 am - 3:00 pm, they you set your aperture to f/16 and your shutter speed to match your ISO setting on your camera close as you can. So let's say my camera ISO is 100 then I can set my exposure to f/16 @ 1/125 and shoot away. However if  you're out in the snow or at the beach I would suggest you increase by +1EV and either use f/16 @ 1/60 or preferably f/22 @ 1/125.

Cloudy 8

Variation of the sunny 16 rule, aperture fixed at f/8 with shutter speed set to ISO close as possible

Hazy 11

Variation of the sunny 16 rule, aperture fixed at f/11 with shutter speed set to ISO close as possible

Overcast 5.6

Variation of the sunny 16 rule, aperture fixed at f/5.6 with shutter speed set to ISO close as possible

Sporting 5.6

This is based off using f/5.6 and setting your shutter speed to 10x your ISO setting. Assuming ISO 100 then 10x100=1000 or 1/1000 for your shutter speed.

Moonstruck

Found this on the web somewhere. Exposure starts out at a one hour exposure with the f-stop based on the ISO being used. The time can be halved but for each halving of time you must open your aperture +1EV.

ISO

Approximate Exposure

50

60'@f/4

100

60'@f/5.6

200

60'@f/8

400

60'@f/11

800

60'@f/16

1000

60'@f/22

Sunrise/Sunset

To fully capture sunrise or sunset I highly recommend that you take your metered exposure value and set is -1EV for more dramatic results. The -1EV tends to improve foreground silhouettes and provide better color saturation. Also, shoot "lots" of shots as these shots can change dramatically within seconds.

 Tip:

Use a tripod

 Tip:

Try some long exposures shortly after the sun appears to have set, the results may surprise you.

Appendix 3: HDR

Shooting HDR images provides you to expand your dynamic range well in excess or what's possible within a single shot.

Some additional conditions:

1.       Use a tripod

2.       Set your mode to Av

3.       Manually choose your white balance (WB), see "Adjusting White Balance Manually" in your owner's manual

4.       Manually set your ISO to lowest setting, do not leave set to AUTO, see "Setting the Sensitivity" in your owner's manual.

In-camera HDR

Pentax K-5, K-7, K-r and K-x users have an additional option besides the automatic exposure bracketing, in camera HDR. In camera HDR capture will take three images -3EV à 0EV à +3EV

Externally processed HDR

All your cameras do perform "Exposure Bracketing" which simplifies the process. Your first stop  if interested in exploring this  I suggest open you camera manual and look up "Exposure Bracketing" , on the Pentax K-r and K-x you can automatically bracket three consecutive images at three exposure levels:

Step Interval

Pentax K-r/K-x Bracket Values

1/3 EV

±0.3, ±0.7, ±1.0, ±1.3, ±1.7, ±2.0, ±2.3, ±2.7, ±3.0

½ EV

±0.5, ±1.0, ±1.5, ±2.0, ±2.5, ±3.0

·         3 images: 0EV à -1EV à +1EV (where 1 = bracket value chosen)

The Pentax K-5/K-7 has fewer bracketing values but can select between two, three or five bracketed exposures:

Step Interval

Pentax K-5/K-7 Bracket Values

1/3 EV

±0.3, ±0.7, ±1.0, ±1.3, ±1.7, ±2.0

½ EV

±0.5, ±1.0, ±1.5, ±2.0

·         2 images: 0EV à -1EV or 0EV à +1EV

·         3 images: 0EV à -1EV à +1EV

·         5 images: 0EV à -1EV à +1EV à -2EV à +2EV

Where 1 and 2 based on bracket value chosen.

 Note:

Examples based on ½ EV step intervals so whole steps used.

Expanding your exposure brackets

If the scene requires it you can expand these bracketed shot ranges. After selecting Exposure Bracketing take your initial group of shots now adjust your EV Compensation so the bracketed shots move either to the right to gain +EV or left to provide additional -EV will allow you to capture the full range of the image you visualized.

Example:

•    5 images: 0EV à -1EV  à +1EV  à -2EV  à +2EV

Now if I press the  button and watch my status screen either in the viewfinder, LCD or live view if available I can adjust the EV till I see +3.0 or +5.0 depending on your camera, basically you want your left hand bracket marker to be on one stop above your previous max. Now when I now take my bracketed picture will result in:

·         5 images: +5EV à +4EV  à +6EV  à +3EV  à +7EV

Now we repeat this only we set our far right hand bracket marker to one step lower, -3.0 or -5.0 this results in:

•    5 images: -5EV à -6EV  à -4EV  à -7EV  à -3EV

This gives us a 15 stop range total which is pretty amazing and likely far in excess of what would be needed. This of course could be modified to expansion of your dynamic range within a less expansive way with any duplicated exposure values deleted.

Then combine these photographs using the HDR software of you choosing, I prefer using Photomatix, http://www.hdrsoft.com note when used in trial mode, Photomatix versions are fully functional and never expires, but adds a watermark to the saved images.

 Note:

Photomatix if purchased does provide mechanism to remove the watermarks from images already created so all your trial work is recoverable less the watermarks.

A couple of excellent tutorial I highly recommend the one located at www.stuckincustoms.com  and the republished article on http://www.naturescapes.net/docs/index.php/articles/189 the second one also downloadable as at 10MB PDF file.

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