Using a camera light meter to determine approximate %VLT

[OmarJ]

I have been unable to locate the %VLT on the following lenses:

PRIZM TR22 Black Iridium

PRIZM Snow Black Iridium

PRIZM Snow Jade Iridium

PRIZM Snow Rose

So it occurred to me that I might be able to get an approximate value by using the one tool I have available to measure light, a camera light meter. Before I get into what I did and what the results of the experiment showed, I think I should provide an explanation of how a camera light meter works for those that have never used a manual or semi-automatic camera.

Photographers and uninterested parties can skip the next section…

There are two values that go into determining exposure. Shutter speed and aperture. There is a third setting that tells the light meter the sensitivity of the film or sensor you are using (ISO) but that is not relevant to this discussion. Shutter speed and aperture in combination determine how much light enters the camera. For our purposes we really are not interested in shutter speed except that we have to be able to force it to a given value, hence the need for a semi-auto or fully manual camera. It is aperture that we are interested in. Aperture is measured in f-stop(s) which are a series of numbers that denote how wide the lens opens to allow light in. The smaller the f-stop the more light is let in and vice-versa. If one knows the first two f-stops in the series the rest can be calculated.

1.0, 1.4, 2.0, 2.8, 4, 5.6, 8, 11 (actually 11.2 but we drop the decimal point), 16, 22, 32

At each point in the sequence going to the next value (larger number) lets in half the light while going to a smaller number lets in twice the light. If you select a given shutter speed the camera will tell you what f-stop should be used for a correct exposure. This might be a little confusing when I start listing the values returned by my experiment, so I’ll give an example. Let’s say you have a correct exposure with a f-stop of f-32 and then you put a grey sunglass lens in front of the camera lens that cuts out 50% of the light (has a %VLT of 50) the camera which was just asking you to set an aperture of f-32 for a correct exposure will now recalculate the exposure based on less light arriving at the lens and will now tell you that you need to set the aperture to f-22. That is the next f-stop down in the sequence and will allow twice the light to enter the camera lens (which it now needs because you have cut out 50% of the light by putting a sunglass lens with 50% VLT in front of it). By changing the f-stop to 22 you are now back where you started. You have cut out 50% (half) of the light with the grey lens and you have added that back by doubling the light allowed in due to the larger aperture. Sorry if this is confusing, I don’t know how else to explain it. Anyway, the important thing to remember is that if you start at an f-stop of f-32, f-22 will give you half the light, f-16 will give you 1/4th, f-11 will give you 1/8th, etc.

So now to the experiment…

I set the camera on a tripod and set the shutter speed to give me an expected aperture of f-32. I then took several lenses that I knew the VLT for and placed them in front of the lens to test my theory.

Grey gave me an f-stop of f-13 (not a ‘normal’ f-stop since the camera is capable of 1/3rd settings). This is about 2 and 2/3 f-stops down from my original f-32. Going to f-22 would half the light (50%), going to f-16 would half it again (25% getting through). f-13 indicates that I am getting less than 25% of the light through the lens but more than 12.5% (f-11), pretty much in line with the known VLT of an Oakley grey lens (18%).

TR45 gave me f-20. f-22 would have indicated half the light getting through, and the slightly lower f-stop of f-20 (1/3rd down from f-22) is right in line with my expectations since the TR45 allows only 45% light transmission.

TR22 gave me f-14. f-14 (1/3rd stop below f-16) is also right in line with my expectations. A TR22 lens allows 22% of the light through the lens. 25% light transmission would have resulted in a f-stop of f-16 so a slightly lower value matches my expectations for this lens. As an additional confirmation the value for this lens is 1/3rd f-stop greater (more light) than the f-stop for the grey (18%) lens.

Now I tried several lenses that I do not have official %VLT values for…

PRIZM Snow Rose gave a f-stop of f-14. Identical to the TR22 lens, which tells me that the Rose lens has a VLT of about 22%.

PRIZM Snow Jade Iridium gave a f-stop of f-11. This indicates that the lens falls somewhere between a 12% and 13% VLT. f-11 indicates that 1/8th (12.5%) of the light is getting through.

PRIZM Snow Black Iridium gave a f-stop of f-10. f-10 is 1/3rd f-stop below f-11 and would indicate a VLT of less than 12.5%.

These are obviously all approximations since the environment was not completely controlled and the camera light meter has definite limitations when trying to determine exact VLT values. But the results were consistent with known values, and so would appear to offer some value in determining approximately where a lens’s VLT falls.

 

[flyer]

Well done, Mr. Wizard!

I had tried this years ago but was unable to relate readings to percentages...

 

[OmarJ]

Well done, Mr. Wizard!

I had tried this years ago but was unable to relate readings to percentages...

Thanks!

But now that I have had more time to think about it I realize that I made this far more complicated than it needed to be. Having taken tens of thousands of photographs, working with aperture values (f-stops) is intuitive for me. It isn’t for most people. It would have made far more sense for me to fix the f-stop and allow the shutter speed to vary and use that to measure the approximate VLT. Doing it that way would have been both easier to explain and easier for a reader to understand. When I get a chance, I will update the original post to show that approach as well. I will also try to make the experiment a little more accurate by using artificial lighting that will not vary during the measurements rather than trust to natural ambient light.

 

[cacatman]

Fascinating read @OmarJ!!! Great job!!!

Did you use controlled lighting (sun may be variable over 1-2 hours of testing)? Also, what camera did you use (obviously full frame like the Canon 1 series or Nikon D5 would possibly be more accurate with metering than their base entry models).

Your numbers came out pretty close to the the official numbers...

Standard Prizm Lenses
Prizm TR22 22% 2 Brighter Light Conditions Contrast Tunes light to maximize contrast resulting in sports tuned colors that enhance contours and details. TR22 is best suited for brighter light conditions, eliminating 78% of ambient light and blocks 100% of UVA, UVB, and UVC light.

Prizm Daily Polarized 14% Base tint : Grey. Running, everyday driving. Dull colors become warm and rich for sharper contrast and a more pleasing, relaxing visual experience. Daily lenses will maximize contrast and enhance visibility so you can have an experience that is relaxing and pleasing to the eye throughout all your day’s activities. See what you’ve been missing every day. Deeper reds, Enhanced blue sky, Blooming hues of green, HDPolarized lenses that block 99% of reflected glare

Prizm Shooting TR45 45% 2 Shooting. Prizm™ TR45 tunes light to maximize contrast resulting in sports tuned colors that enhance contours and details. TR45 is best suited for brighter light conditions, eliminating 55% of ambient light and blocks 100% of UVA, UVB, and UVC light.

Prizm Deep Water Polarized 12% Watersports. Prizm Deep Water Polarized lenses filter out the shades of blue that overwhelm your vision on open water. The deep water lenses filter out specific shades of blue that crowd your vision on the open water so you can see below the surface. Strengthens the reds and greens colors that will give you a better view of the activity below the surface

Prizm Field 15% Baseball. Prizm™ Field accentuates the blue of the sky to enhance the contrast of a fly ball

Prizm Golf 30% Golf. Prizm™ Golf improves contrast for better separation of color and gives you more depth cues to gauge distance and grass conditions. Base tint : Ruby

Prizm Road 20% Cycling, Running, Driving. Prizm™ Road lenses enhance vision in both bright light and shadows, helping riders spot changes in the texture of road surfaces. Improved vision in both bright light and shadows. Traffic lights become more vibrant, and white and yellow lines are more clearly differentiated. Blues and greens are enhanced for a more pleasant ride

Prizm Shallow Water Polarized 15% Watersports. Prizm Shallow Water Polarized lenses help keep whites bright so you can spot the flash of fish and the flies on the surface. Boosts green and copper hues that define hiding spots. Maintains bright whites to easier spot the fish and flies on the surfaces of the water. Helps define shadows of the fish

Prizm Trail 36% Base Tint : Grapefruit. Are perfect for trail running, hiking, and mountain biking because it enhances the colors on the trails along with the light conditions under and out of shadows. You will be able to see and react faster to the creases and patches of sand, roots, rocks, and other obstacles that are crucial to avoid. Enhances reds and browns. More vividly represents plants and foliage colors. Improved vision in both bright light and shadows

Prizm Baseball Infield 20% Base Tint : Mocha. Gives an infielder the ability to see the greens and reds which allow the ball to stand out more against the grass and dirt

Prizm Baseball Outfield 15% Base Tint : Magenta. Enhances the sky colors to yield better contrast for fly balls

Snow Goggles
Prizm Torch Iridium 11-20% Partly cloudy to bright light Contrast Partly cloudy, medium light, sunny, bright light

Prizm Black Iridium 5-10% Partly cloudy to bright light Partly cloudy, medium light, sunny, bright light

Prizm Jade Iridium 11-20% Partly cloudy to bright light Partly cloudy, medium light, sunny, bright light

Prizm Sapphire Iridium 11-20% Partly cloudy to bright light Partly cloudy, medium light, sunny, bright light

Prizm Rose 21%+ Cloudy to medium light Cloudy, low light, partly cloudy, medium light

 

[OmarJ]

Did you use controlled lighting (sun may be variable over 1-2 hours of testing)? Also, what camera did you use (obviously full frame like the Canon 1 series or Nikon D5 would possibly be more accurate with metering than their base entry models).

No, I didn't use controlled lighting and that was one of the problems with my first experiment. Even though the entire process took less than 1 hour there probably was some variability. I plan to do the experiment again, basing the results off of shutter speed changes rather than aperture (which I believe would be more intuitive for reader) and also use a controlled light source for the second effort. I will add those results to the original post when I get time to perform the test. I am also waiting on a TR22 Black Iridium lens (another one for which I can't find VLT figures) and will add that one to the test.

The camera used was an old semi-pro Canon model (EOS 20D) with the meter set to spot metering. It should be quite accurate within the limits of exposure values. If I can locate my hand held meter I will try it with that. I have not used that toy in quite some time.

Thank you very much for the Snow values. I could not find those. I assume that those numbers hold true for the M Frame Alpha snow lenses?

 

[cacatman]

I assume that those numbers hold true for the M Frame Alpha snow lenses?

I guess so. If they are different, I think they'd probably change the name of the lens.

 

[OmarJ]

Let me start off by saying that I could not figure out how to get the columns below to display correctly. I tried Rich Text, HTML, and then tried aligning everything by hand, but no matter what I did the columns all got mashed together when I posted. In the end I put a pipe (bar) symbol between the columns in an effort to make it readable.

As promised, here is the update to my earlier experiment. There were two changes that I wanted to make to my original methodology. The first was to use shutter speed rather than aperture to calculate the VLT as I thought that this would be more easily understood by most readers that are not intimately familiar with photography. The second modification I wanted to try was to use a controlled (artificial) light source.

As explained earlier, a combination of shutter speed and aperture are used to give one the correct exposure for a photograph. If the shutter speed is fixed and the light meter is allowed to adjust the aperture, one can use the aperture selected by the camera to estimate the sunglass lens VLT. On the other hand, if one were to set the aperture and allow the shutter speed to vary the VLT can be estimated by using the light meter selected shutter speed. The reason that this may be simpler is that shutter speeds are not seemingly random numbers but are instead a relatively easy progression. Keep in mind that when a shutter speed is displayed by the camera it is actually the invers of that value. For example, if a camera says that the selected shutter speed is 4000 it means that the shutter stays open for 1/4000 of a second. If one starts with a shutter speed of 4000 (1/4000 sec) and then halves the incident light the camera will adjust the shutter speed to 2000 (1/2000 sec). If you halve the light again the camera will now display a shutter speed of 1000, halve it again and the light meter will display 500. So, if the camera displays 4000 with no sunglass lens in front of the camera lens you get the following:

Shutter speed | VLT%
4000 | 100
2000 | 50
1000 | 25
500 | 12.5

As with aperture, there are also intermediate values at 1/3 intervals. Here is the full sequence from 4000 down to 500 (the righthand column shows approximate VLT’s)

Shutter speed | Approximate VLT%
4000 | 100
3200 | 83
2500 | 67
2000 | 50
1600 | 42
1250 | 33
1000 | 25
800 | 21
640 | 17
500 | 12.5

The second part of the experiment was to try to use a controlled (artificial) light source. The results of this effort surprised me (although maybe it shouldn’t have). When I first reran the numbers using a neon lamp as the light source the numbers I got were VERY different from those I had seen in my original experiment when I used natural light and aperture. I tried again using natural light and the numbers were back to what I had seen initially. I believe that the discrepancy is because an artificial light source has a truncated wavelength range (so area under the curve, so to speak, is not equal to natural light). Or said another way, you are not getting light across all wavelengths, since an artificial light source is going to have an output spike over a very narrow range. This throws off the calculation. In fact, across the board, using artificial light I got higher VLT (more light apparently getting through the lens) values than I did with natural light. Since we generally use our sunglasses outside it made sense to me to return to using natural light. The experiment took me about 20 minutes to run for all lenses and in-between each test I rechecked the unfiltered light reading to make sure it read 4000. Below are the numbers. Just as a point of interest the first set are the values and approximate VLT’s I got with artificial light. The second set of reading are taken with natural light. The lenses in both tables are presented in the same order and are ordered by VLT from lowest to highest in natural light (the first table is not in perfect descending order). Where I know the official value from Oakley material I have included that alongside the lens.

Sunglass lens tint | Shutter speed | Approximate VLT%
No lens | 4000 | 100
PRIZM TR45 (45%) | 2500 | 67
PRIZM Snow Rose | 1600 | 42
PRIZM TR22 (22%) | 1250 | 33
Grey (18%) | 1000 | 25
PRIZM TR22 Black Iridium | 800 | 21
PRIZM Snow Jade Iridium | 1000 | 25
PRIZM Snow Black Iridium | 800 | 21

The same lenses with natural light give the following (note how they universally have lower tested VLT’s than the same lens in artificial light):

Sunglass lens tint | Shutter speed | Approximate VLT%
No lens | 4000 | 100
PRIZM TR45 (45%) | 2000 | 50
PRIZM Snow Rose | 1000 | 25
PRIZM TR22 (22%) | 1000 | 25
Grey (18%) | 800 | 21
PRIZM TR22 Black Iridium | 640 | 16
PRIZM Snow Jade Iridium | 640 | 16
PRIZM Snow Black Iridium | 500 | 12.5

My wife owns a sun lamp (one of those things that is supposed to make it easier to get up in the cold, dark north). Since we no longer live in that part of the country, the lamp is in a box somewhere. But if I do come across it I plan to try the measurements using it as a light source.