Optimum Exposure: An optimum exposure should place a fully lit middle-grey tone at a luminance value of about 114 Levels. This assumes a 2.2 gamma colour space such as Adobe RGB or sRGB set in your camera RAW processing software. If you use a 1.8 gamma colour space such as ProPhoto RGB or Colourmatch RGB then the figure should be close to 94 Levels. These figures are with all processing controls set to zero in your RAW processing application at time of processing. To read for these values, place your cursor over the middle grey swatch in the Shirley test and take note of the resulting densitometer RGB values displayed.
To understand how to work with the findings from a Shirley test, let’s
suppose that the test revealed that the image capture entitled120 ISO was
the optimum exposure rather than the 100 ISO capture. The procedure is,
leave the camera set to the ISO that you tested for (in this case 100 ISO)
and set the handheld meter to the ISO revealed in the test (in this example
120 ISO). Let me explain further – when you set the ISO on a digital SLR you
are setting the sensitivity level for the imaging chip and you are setting
the sensitivity of the in-camera meter at the same time, they cannot be
separately controlled. But this is okay when you use a hand-held meter
because you ignore the readings of the in-camera meter anyway – this is
assuming your camera is set to manual. If you have to use in-camera meter
readings instead of hand-held meter readings (ie for travel shots), then you
should still set your camera to 100 ISO and then compensate for the
discrepancy (1/3 of a stop in our example) with the plus/minus,
over-/under-exposure compensation on your camera (see Image 6). This is also
how you should work if you are using Auto, Program, Shutter Priority and
Aperture Priority modes since they rely on the in-camera meter.
Now I’m Really Confused
ISO Test Brightness – backwards from reality: In our Sensitivity Test, the
darker exposures represent higher ISOs and the lighter exposures lower ISOs.
Since this is somewhat confusing let’s take a close look at how it really
works. If both meter and camera are set to 100 ISO and we manually set the
camera’s exposure (camera set to M for manual exposure) to the hand-held
meter reading captured at the subject plane, let’s say f11 at 1/60, we
should theoretically be able to capture a correctly exposed image of our
Shirley test set-up. If, however, the camera is out of whack, then our
exposure of the test will appear wrong. If, for instance, the image appeared
a little too bright in this f11 at 1/60 exposure, we would be able to tell
by how much by looking at the other exposures and finding the one that
places the middle grey swatch at 114 or 94 levels. Let’s say that happens to
be the exposure taken at f12 (f11 1/3). F12 allows 1/3 of a stop less light
into our cameras than does f11. This handy bit of information tells us that
this camera/hand-held meter combination needs 1/3 of a stop less light for a
correct exposure. To allow a 1/3 of a stop less light into your camera using
ISO means you need to work with a higher ISO setting on your meter. In our
example stated above, set the meter to 120 ISO instead of 100 ISO. Believe
me this is much easier than having both meter and camera set to 100 ISO and
then having to transpose each reading in your head before setting on your
camera. Setting the meter to 120 rather than 100 ISO is telling the meter
that the camera has a higher sensitivity to light and so doesn’t need as
much light for a correct exposure. So this is why the higher ISOs (120 to
200 ISO) are represented in our series of test images as darker exposures
and why the lower ISOs (80 to 50 ISO) by lighter exposures.
If I needed to shoot at a higher sensitivity for low light conditions, say
400 ISO, then I would set my camera to 400 ISO and my meter to 420 ISO.
Or…for in-camera metering, camera set to 400 ISO and the plus/ minus
compensation to minus 1/3.
Just to make sure you really got it, suppose that the fifth exposure labeled
80 ISO was the optimum exposure in the test, you should then set your camera
to 100 ISO and the hand-held meter to 80 ISO. For in-camera metering, set
your camera to 100 ISO and the camera’s plus/ minus exposure compensation
feature to plus 1/3.
And that is all there is to creating a sensitivity test – in less than 30
minutes you can perform this test and by doing so will have eliminated one
large variable in a field of infinite possibilities plus you will have an
optimal RAW file to create the perfect pre-set for automating RAW
processing. In retrospect I guess I could have created a more interesting
Shirley if I used a bikini-clad Shirley and I could have explained to
offended parties that we need the flesh tone, a lot of it, as much as we can
get… and getting back to Hawaii, it seems I will have to go back to shoot
some more in April due to last trip’s cloudy weather. Oh well, we all have
our crosses to bear.
Dave Montizambert
Dave Montizambert lectures internationally on lighting, digital photography
and Adobe Photoshop. He is also a published author having written two books
on lighting and digital photography (publisher Amherst Media) plus numerous
magazine articles on these topics in North America and in Europe. Dave also
creates Photoshop tutorial CDs & DVDs for www.software-cinema.com. Dave is
available for lectures and workshops in your area and can be reached at
montizambert@telus.net or www.montizambert
If you would like to learn more about creating simulated sunlight for those
overcast days, check out Dave's book on lighting called, Creative Lighting
Techniques, available through Amherst Media and the SWPP bookstore.
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