Manually changing the f-stop on this broken-on 14mm lens by 6 stops from f2.8 to f22 reduces
the light transmission much like adding a 6-stop ultra-density filter, but not exactly.
f-stops are a measure of the lens geometry, but t-stops are a measure of the actual lens-like
transmission.
Here's an example.
The first image was taken at f2.8 and 1-1-1-1 of a second.
The second image was taken at f5.6 and 1-25th of a second.
By the principle of reciprocity, both images should be equally bright.
I decreased the aperture area by a factor of 4 and increased the shutter time by a factor
of 4, but the exposures are not equal because I was using f-stops instead of t-stops.
When I change the f-stop during a time-lapse, GB time-lapse needs to know the actual t-stop
difference in order to adjust the exposure accordingly using reciprocity.
To measure the t-stop difference, I can calibrate the lens using GB time-lapse.
First I need a place with a constant low light level, like the inside of this closet.
The light should be constant so I can compare exposures at different times, and the light
must be low level so I can start with a 1-second exposure at f2.8.
In the camera's shutter tab, I choose bobbins at the time to 1 second.
In the camera settings control tab, I set the ISO to 100 and set the white balance to
tungsten.
In the setup tab, I'll set save to computer and set the image quality to small of fine.
When I take a photo, there's a warning, AV unknown.
GB time-lapse can't determine the AV value of a manual lens.
To fix this, I'll go to the camera profile menu, choose the advanced tab, and set the
default f-stop value to 2.8.
Now GB time-lapse will assume the lens aperture is f2.8.
I'll capture another photo.
This is pretty good.
The histogram is balanced with no clipping.
The luminance is near 128.
You can adjust the light by changing the bulb or by blocking the light somehow.
Because I do this frequently, I've added a dimmer to my closet light switch.
I'll use AutoRamp to do the calibration, and check the auto ramping on checkbox.
The AutoRamp wizard appears.
I'll check the custom radio button, check the box for bulb mode only, check the box to
lock the aperture at f2.8.
This means GB time-lapse will treat it as if it has a constant f2.8 value, and we're
adding ND filters.
I'm going to calibrate this lens at f5.6, f11, and f22.
Each two stops apart, so I'll enter a value of 2 for each of the three filters.
Changing from f2.8 to f5.6 is just like adding a two-stop filter.
Likewise, we're changing from f5.6 to f11, and we're changing from f11 to f22.
I'll click OK.
I'll set the target luminance to match the last image, 137.
It's not required, but I'll change the shift ISO times from 24 and 12 to 8 and 4.
This will make GB time-lapse also increase the ISO to keep the bulb time short.
I'll uncheck use sunrise sunset compensation, because it's indoors with artificial light.
I'll also go to the Project Settings menu, and under the AutoRamp tab, increase the sensitivity
from its normal 0.1 value to 0.75.
This will make AutoRamp respond more quickly when I change the f-stop.
I'll save this project with the name Calibrate Lens.
Next, I'll go to the Timelapse tab, set the interval to 6 seconds, and begin the time-lapse.
During the time-lapse, AutoRamp adjusts the exposure to maintain the target luminance.
Note the bulb time is near our 1.000 second starting value.
It should stay steady near the 1 second starting value, but it may change a little because
the light bulb doesn't produce exactly constant light.
I'll check the box to add the first ND filter, wait for the shutter to close, and then go
into the closet and change the f-stop from f2.8 to f5.6.
Then I'll click OK to let AutoRamp continue.
To compensate for the expected two-stop change in light, AutoRamp increased the bulb time,
but it overshoots the target luminance because the two-stop value isn't accurate.
As AutoRamp continues, it gradually adjusts the exposure to match the target.
The luminance is close to the target value, so I'll make a note of the ND EV value.
It's 1.18.
This is the exposure difference in stops from the starting 1 second ISO 100.
I'll check the box to add the second ND filter, wait for the shutter to close, and then go
into the closet and change the f-stop from f5.6 to f11.
And again, I'll click OK to let AutoRamp continue.
To compensate for the expected two-stop change in light, AutoRamp increases the bulb time
in ISO again.
Again it overshoots, but will gradually adjust down to the target level.
When luminance reaches the target value of 137, I again make note of the ND EV value
in this case 2.88.
Now I'll check the box to add the third ND filter, wait for the shutter to close, and
then go into the closet and change the f-stop from f11 to f22.
Again I'll click OK to let AutoRamp continue.
And again, it tries to compensate for the expected two-stop change in light.
This time I've undershot a little bit, indicating that the change was actually more than 2 stops.
There it's close to the 137 target value, so I'll make a note of this third and final
ND EV value, 5.06.
I'll click to pause the time lapse, wait for the last image to be captured, and click
End.
Here are the calibration results.
Changing from f2.8 to f5.6 is like adding a 1.18 stop ND filter.
Changing from f5.6 to f11 is like adding a 1.70 stop ND filter.
And changing from f11 to f22 is like adding a 2.18 stop ND filter.
Now if the lens is calibrated, I can use this data during a time lapse to change the aperture
without changing the image brightness.
This calibration is valid for a Canon 7D body.
A body with a different sized image sensor may have a different lens calibration.
You can use this same procedure to calibrate any manual lens with an aperture ring with
f-stop deconts.
