Steaphany
Well-Known Member
This was on my mind for a while. I did some exposure tests regarding how the SD14 performed when shooting IR photos to get a handle on just how sensitive the Foveon imager is in the IR region of the spectrum and how far off the SD14's meter was for such conditions.
Well, when I entered the measurements into a spread sheet, I ended up with some strange results which I had to wait for my replacement IR blocking filter before I could run some detailed exposure tests. I just completed my analysis of 45 exposure test imagess and I found some interesting results.
Since I can't post the actual OpenOffice spread sheet, here is a screen capture:
View attachment 1744
First some background on how I gathered the data presented here.
I set a moderate light level in my office which would remain constant through out the duration of my shoot. I set up my X-Rite ColorChecker as the primary object in the field of view and used my 28mm DG EX Aspherical Macro lens. The SD14 was on a tripod to ensure that every frame had the same scene and the SD14 was set to Sun Light White Balance and ISO 100. The default exposure turned out to be 1/13th at an f/1.8. I then switched the exposure to manual setting 1/13th @ f/1.8 as my starting point.
I then shot a exposure for each shutter speed value from 1/13th through to 1/4000th. I then returned to 1/13th and proceeded to shoot from 1/13th through to 2.5 Seconds. Lastly, I shot another exposure at 1/13th.
This gave me a collection of 45 images, three being at 1/13th which served as a consistency check to ensure that the room lighting actually remained constant through out the shooting period.
I bulk extracted the JPG images from the X3F files so that they would be easy to work with and I could be sure that every image had exactly the same post processing treatment.
Using Photoshop, I opened each JPG and measured the luminance of the Neutral 5 gray square which ideally would yield 127 when photographed in ideal conditions. The measurements were made by selecting a region of the Neutral 5 Gray square, choosing Luminance, and reading the mean value measured.
I also verified these measurements in Paint Shop Pro Version 7.02 since this version has no color management and would only report luminance values from the actual file data.
Now, to the spread sheet.
Column A is the decimal shutter speed.
Column B is the Exposure Value departure from the SD14 metered 1/13th Second. Since the ISO and Aperture remained constant, the EV is solely based on the shutter speed. Note that I achieved a significant exposure range of -8.3 through to +5.0.
Column C is the Photoshop Mean luminance measurements of the Neutral 5 Gray Square.
For a point of reference, I adapted the Ansel Adams zone exposure system to the 0 to 255 scale, the center values of each of the 11 zones are listed in Column D.
The chart has the Exposure Value along the X axis and the Neutral 5 Gray square image value along the Y. The divisions of the Y axis correspond to the Image Values corresponding to the boundaries of the Ansel Adams zones.
The Violet line which spans EV values of -5 to +5 defines the Ansel Adams zones. Note that the Violet line crosses a vertical exposure value line at the center of each Image Value Zone.
In when using the Ansel Adams zone system in film photography, an ideal exposure would span -3EV to +3EV for the bulk of the image with high lights and deep shadows extending into the -4EV and +4EV ranges. Exposures extending into the -5EV and +5EV are areas of the image which have no recognizable detail.
Note that I have 11 shades of Gray to the right of the chart whose values correspond to the zone center image value.
The SD14 exposure performance is shown by the Blue line.
The first thing to note is where the SD14's exposure line crosses the 0EV point. The SD14's metered exposure is slightly shifted to the right, instead of 127, the Neutral 5 gray square yield a image value of 138.
Second, the slope is actually more drastic than the Ansel Adams zone line meaning that the SD14 would yield a more contrasty image.
Third, the SD14's exposure is not symmetrical about the 0EV point. Moving towards a brighter image value, the SD14 is nicely linear but it will saturate at only +3EV. If you ever use a separate spot exposure meter, any object brighter than +3EV from the SD14's settings will be washed out. Moving from the 0EV point towards the darker image values, the SD14's exposure is only linear to about -2EV. The Red straight line is a linear regression line of the measured image values over -2EV through to +3EV showing that this region of the SD14's exposure curve is truly linear. Beyond -2EV, the SD14's exposure plateaus. Where the linear regression predicts the darkest image value zone being achieved at -3EV, in reality that point is not reached until going as far as -8EV.
This dark image value plateau also explains why the SD14 is prone to noise in low light or in dark shadows.
How to work with this information.
First off, I'm going to set my exposure compensation to -0.3EV to bring objects with Neutral 5 gray values closer to the desired 127.
When shooting to the right, It's best to be conservative on how far you go. With only 3EV of head room, it would not take much of a bright scene to end up with loss of detail in the high lights.
Watch the darks and shadows. Since the SD14 exposure is not symmetric between the positive and negative values from 0EV, it really would not take much to become "too dark".
Well, when I entered the measurements into a spread sheet, I ended up with some strange results which I had to wait for my replacement IR blocking filter before I could run some detailed exposure tests. I just completed my analysis of 45 exposure test imagess and I found some interesting results.
Since I can't post the actual OpenOffice spread sheet, here is a screen capture:
View attachment 1744
First some background on how I gathered the data presented here.
I set a moderate light level in my office which would remain constant through out the duration of my shoot. I set up my X-Rite ColorChecker as the primary object in the field of view and used my 28mm DG EX Aspherical Macro lens. The SD14 was on a tripod to ensure that every frame had the same scene and the SD14 was set to Sun Light White Balance and ISO 100. The default exposure turned out to be 1/13th at an f/1.8. I then switched the exposure to manual setting 1/13th @ f/1.8 as my starting point.
I then shot a exposure for each shutter speed value from 1/13th through to 1/4000th. I then returned to 1/13th and proceeded to shoot from 1/13th through to 2.5 Seconds. Lastly, I shot another exposure at 1/13th.
This gave me a collection of 45 images, three being at 1/13th which served as a consistency check to ensure that the room lighting actually remained constant through out the shooting period.
I bulk extracted the JPG images from the X3F files so that they would be easy to work with and I could be sure that every image had exactly the same post processing treatment.
Using Photoshop, I opened each JPG and measured the luminance of the Neutral 5 gray square which ideally would yield 127 when photographed in ideal conditions. The measurements were made by selecting a region of the Neutral 5 Gray square, choosing Luminance, and reading the mean value measured.
I also verified these measurements in Paint Shop Pro Version 7.02 since this version has no color management and would only report luminance values from the actual file data.
Now, to the spread sheet.
Column A is the decimal shutter speed.
Column B is the Exposure Value departure from the SD14 metered 1/13th Second. Since the ISO and Aperture remained constant, the EV is solely based on the shutter speed. Note that I achieved a significant exposure range of -8.3 through to +5.0.
Column C is the Photoshop Mean luminance measurements of the Neutral 5 Gray Square.
For a point of reference, I adapted the Ansel Adams zone exposure system to the 0 to 255 scale, the center values of each of the 11 zones are listed in Column D.
The chart has the Exposure Value along the X axis and the Neutral 5 Gray square image value along the Y. The divisions of the Y axis correspond to the Image Values corresponding to the boundaries of the Ansel Adams zones.
The Violet line which spans EV values of -5 to +5 defines the Ansel Adams zones. Note that the Violet line crosses a vertical exposure value line at the center of each Image Value Zone.
In when using the Ansel Adams zone system in film photography, an ideal exposure would span -3EV to +3EV for the bulk of the image with high lights and deep shadows extending into the -4EV and +4EV ranges. Exposures extending into the -5EV and +5EV are areas of the image which have no recognizable detail.
Note that I have 11 shades of Gray to the right of the chart whose values correspond to the zone center image value.
The SD14 exposure performance is shown by the Blue line.
The first thing to note is where the SD14's exposure line crosses the 0EV point. The SD14's metered exposure is slightly shifted to the right, instead of 127, the Neutral 5 gray square yield a image value of 138.
Second, the slope is actually more drastic than the Ansel Adams zone line meaning that the SD14 would yield a more contrasty image.
Third, the SD14's exposure is not symmetrical about the 0EV point. Moving towards a brighter image value, the SD14 is nicely linear but it will saturate at only +3EV. If you ever use a separate spot exposure meter, any object brighter than +3EV from the SD14's settings will be washed out. Moving from the 0EV point towards the darker image values, the SD14's exposure is only linear to about -2EV. The Red straight line is a linear regression line of the measured image values over -2EV through to +3EV showing that this region of the SD14's exposure curve is truly linear. Beyond -2EV, the SD14's exposure plateaus. Where the linear regression predicts the darkest image value zone being achieved at -3EV, in reality that point is not reached until going as far as -8EV.
This dark image value plateau also explains why the SD14 is prone to noise in low light or in dark shadows.
How to work with this information.
First off, I'm going to set my exposure compensation to -0.3EV to bring objects with Neutral 5 gray values closer to the desired 127.
When shooting to the right, It's best to be conservative on how far you go. With only 3EV of head room, it would not take much of a bright scene to end up with loss of detail in the high lights.
Watch the darks and shadows. Since the SD14 exposure is not symmetric between the positive and negative values from 0EV, it really would not take much to become "too dark".
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