Photography

If you represent f-stop, shutter speed, ISO, luminance (reflected light), and illuminance (incident light) as a Base 2 log value, you can simplify the exposure calculation process...  The  "Additive System of Photographic Exposure (APEX)" is explored in a previous post (Ev addendum... November 2013).

From the detail explanation we can see that Ev=Av + Tv... the Exposure Value (Ev) can be represented as the sum of the Aperture Value (Av) and the Time Value (Tv) using the following formulas:

Ref : 01

Below is a test shot of a local apartment complex taken with a Fujifilm X100T and a partial list of the exif data.

ISO : 100
Shutter Speed Value : 1/150
Aperture Value : 4.9
Brightness Value : 7.57
Light Value : 11.9

Ref 02 : Test shot of a local apartment complex




Ref 03 : APEX formulas applied to the test shot exif data

In the preceding we can see the application of APEX rules to the test shot metadata.  The shutter speed value and aperture value are calculated to be Tv=7.23 and Av=4.59, the sum of which is an Ev of 11.82 at ISO 100.
__________

As a reminder Ev0 is defined as f/1.0 at 1 second and all other pairs that give an equivalent exposure...  each stop of aperture or each doubling/halving of the shutter speed will increase (or decrease) the Ev value by one. Ev is ISO specific... Ev(ISOxxx)=Ev(ISO100) + log2(xxx/100).  Ev15(200) are the same pairs of f-stop and shutter speed as Ev14(100).  By definition the chart shown in Ref 04 defines the exposure pairs for Ev at ISO 100.

Ref 04 : Ev(100) Reference Chart

__________

Let's go back to "Ref 03 : APEX formulas applied to the test shot exif data" for a moment.  Given that the Brightness Value (Bv) is provided by the test shot metadata and since we now have a calculated Ev=11.82, we can determine the actual ISO since we know that Sv=Ev-Bv... we end up with an actual ISO value of 59.5 (not ISO 100 as reported by the camera).  As a cross check we can see that the reported Light Value of 11.9 is more or less equal to the calculated Ev=11.82 (at ISO 100).  In the case of the X100T, the Light Value=the Exposure Value (Ev) at ISO 100.

Let's look at example of a cloud photo where where I took a base line photo and then another photo with -1 Compensation

Ref 05 : Test shot of some clouds above the city




Ref 06 : Base line and -1 Compensation detail with APEX calculations

The cloud test photo was reported to be shot at ISO 200.  Since I have 'determined' that the Light Value is actually the Ev at ISO 100, I have adjusted the Light Value by one step to show the EV at ISO 200.  In the base line photograph (with no compensation) we can see that the calculated ISO is 119.9 rather than the reported ISO 200.

The cloud test metadata for the photograph with -1 Compensation shows how the exif information is adjusted.  The photographs were taken a few seconds apart, so you can see that under the same lighting conditions the Light Value is adjusted based on the Exposure Compensation setting.  Please note that a negative compensation will increase the Ev value where as a positive compensation will have the reverse effect on the Ev value (see Ref 04 : Ev Reference Chart).

The other point of interest is that the "actual" ISO has chanced between the base line photo and the photo with -1 Compensation.  By comparing the two calculated ISO values we can see that a camera setting of -1 Compensation is in fact equal to about +2/3 Ev or -2/3 actual Compensation.

For the test shots shown and other testing I have done, I use the  X100T metering option: Multi.  I have to do further testing with "spot", "average", and "Face Detection" metering modes to see the resulting effect on the metadata.  As well I was using in-camera jpg conversion and need to test if shooting RAW gives different results.

Some thoughts on what this means...

01) One of my favorite TV shows is "The Big Bang Theory" so it's nice that the numbers crunch in a logical and explainable way (at least until proven otherwise).

02) The exif data reported by the mfg. is not necessarily correct.  It's well documented that the mfg. tend to over rate ISO values.  While the reported f-stop and shutter speed seem to be accurate (at least they are non-standard) values, the ISO value always seems to be reported as a "standard value" (not fractions thereof).

03) The exif data from one camera may not be equal to the data from a second camera (given the same light and composition).  The use of some exif tags tend to be mfg. specific.

04) Using the compensation feature (i.e. -3 to +3) may not give the same results as manually adjusting the f-stop, shutter speed, or ISO.

05) [Digital] ISO settings are relative...  The X100T is a so called "ISO-Less" camera, which means you can process images taken at different ISOs to have the same look and feel.  Whereas the results from a camera like the Canon 5D III are reported to be "ISO dependent" and cannot always be adjusted to look the same in post processing.

Thanks for reading,
Casey

No message body

Here is a look into the exif data from a low light photograph.  It's a January 2017 photo of Aomori City walking back to the hotel after dinner.

Ref 01 : a lone taxi in Aomori City after dark (c) CLH - January 2017




Ref 02 : Exif and APEX reference data

We see that the Brightness Value is a negative number, and I used -1 Compensation, but the numbers still crunch properly.  A Bv of -0.38 works out to be 2.628 candelas/m2.  According to the reference charts that amount of light is the same as: floodlit buildings, monuments, and fountains, which seems about the correct level.

As a reference a Bv of -3.09 (.04 candelas/m2) is the amount of light you get from phosphorescent markings on a watch dial after 1 hour in the dark.

Here is the same Aomori street location from Google Street View as of September 2015... the same taxi and definitely the same white vehicle parked on the right... unbelievable  ; ) 

Ref 03 : (c) Google Street View - September 2015

Casey

when your camera is on automatic ,  low light setting or any other  setting aside from manual , those are the calculations your on board computer makes based on the amount of light  it measurers, distance , focus etc.?

The two choices the photographer has is where to stand and when to press the shutter.  There are however technical details to consider... such as your metering and focus point choices.

Metering options...  The Fuji X100T has the following options, which will be slightly different by make and model.

01) Multi: automatic scene recognition is used to adjust exposure for a wide range of shooting conditions.

01) Spot: measures the lighting conditions in the selected focus area or center of the frame

03) Average: exposure is set to the average of the entire frame

04) Face Detection: exposure is set to the face area (or other)

05) White Balance: not strictly a metering option.  How automatic WB settings are arrived at can be a complicated process.  I am not sure which method the X100T uses, but you can find some general information here:  Auto White Balance and a Digital Camera - July 30, 2015.

AutoFocus... I really like using face detection to focus when people or animals are in the scene, but on many cameras you can select the auto focus point(s).  I tend to use only the center point, set the focus, and recompose.  Zone focus is also an option that works well for street photography.  For the film equipment that I have, I  use manual focus.  Some cameras also have a focus tracking option.

The on-board computer will determine the exposure etc. based on the above.  Why a digital camera in auto mode chooses a specific f-stop, aperture, WB, and ISO is the magic algorithm of each mfg ; ).  The photographer can take manual or partial control by fixing one or more of the choices (e.g. aperture priority).

I hope that answers your question...
Casey

so much sophistication in a tiny aluminium box , optics in conjunction with electronics. It's like an art in itself someone must of done all the possible calculations or at least came up with the mathematical formulas. Thank you very much for this reflection Casey Edward

and each maker has their own approach and their own idea as to what makes the "best" photo