What's the issue with UV light?

I'm confused about the significance of UV light with respect to color film exposure. always thought that UV light needed to be compensated for when at high altitude for example but looking at the data sheet for Fuji Superia XTra 400 color film, it looks like the sensitivity of the film falls off very steeply below 400nm (see attachment, page 6). So is atmospheric UV even an issue with respect to in-camera film exposure and exposure calculations?

Based on the data Fuji provides, it doesn't seem to have much to say about UV but from what little it says there appears to be no problem in Fuji's eyes

It has been a while since I took Fuji Superia under what passes as strong U. V. in the U.K. at not more than a few hundred feet above sea-level but I don't recall seeing much in any in the way of blue shadows which is the usual cited problem with colour

Was this my brain compensating for what my eyes were really seeing or the fact that the UV was not intense enough at my latitude and height above sea-level? I just don't know

pentaxuser

pentaxuser said:

Based on the data Fuji provides, it doesn't seem to have much to say about UV but from what little it says there appears to be no problem in Fuji's eyes

It has been a while since I took Fuji Superia under what passes as strong U. V. in the U.K. at not more than a few hundred feet above sea-level but I don't recall seeing much in any in the way of blue shadows which is the usual cited problem with colour

Was this my brain compensating for what my eyes were really seeing or the fact that the UV was not intense enough at my latitude and height above sea-level? I just don't know

pentaxuser

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Where I live, high altitude refers to more than 7500 feet ASL.

BradS said:

I'm confused about the significance of UV light with respect to color film exposure?

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Take pictures of periwinkles and compare to the real thing. On film they come out more purple.

I know some aspects of the issue relate to a mismatch between spectral sensitivity of the light meter and film.

Since the outcome is a tendency to overexpose the issue is probably that film sees more deeply into blue than a meter

Bill Burk said:

Take pictures of periwinkles and compare to the real thing. On film they come out more purple.

I know some aspects of the issue relate to a mismatch between spectral sensitivity of the light meter and film.

Since the outcome is a tendency to overexpose the issue is probably that film sees more deeply into blue than a meter

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Thanks Bill.
I think you've hit upon it....the difference between the spectral sensitivity of the light meter and that of the film.

Yes...I think this must be it. thanks!

...which is probably why it seems so bewilderingly complex. Different lenses transmit different amounts of UV...probably CdS cells have different spectral sensitivity than SBC cells...etc.

The datasheet I found has a curve that stops at 400 nm--i.e., it doesn't say what the sensitivity is below that wavelength.

As far as I know, all film and most meter photocells are sensitive to UV light; at high altitudes, the scattered light has more UV than at lower altitudes, and can effect color and exposure, including UV flare. The easiest way to deal with it is to use a strong UV filter.

Practical testing tells you a lot. I've done a great deal of high-altitude photography and know the importance of testing under real world circumstances. Most color film will lose some of its acuity in distant scenes, as well as shift somewhat excessively bluish, due to high UV circumstances. That's what UV filters are designed for. But which exact type works best depends on not only the setting, but the specific film too. They all aren't exactly the same in that respect. They range all the way from clear special multicoatings to pale magenta and salmon amber-pinkish versions. I found the need to keep on hand several different kinds of UV and Skylight filters for the selection of films I have used over the years. No exposure compensation is needed unless the filter itself is so strong that it has a formal filter factor assigned to it; but in the case of skylight filters, that's only about a third of a stop.

Analogous UV-rich conditions can also occur at the beach, or at lower elevations where the air is exceptionally clear and clean. Most modern meters are geared to a peak sensitivity of around 550 nm (green), so are not designed to measure UV. TTL metering complicates any of this. In general, UV has little effect on the amount of exposure needed, and mainly on sharpness and hue rendition.

The "Exposure Guide" on that Superia sheet assumes that you do not have a meter, and that at high altitude under open sun, the air is cleaner and will simply be more bright overall than lower down where there's more atmospheric pollutants. It's a very rough kind of guideline.

But Drew, how does that UV get past the glass? I don't have quartz lenses.

Bill Burk said:

But Drew, how does that UV get past the glass? I don't have quartz lenses.

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Many glasstypes pass UV down to ~350nm which is where the absorption of gelatin kicks in and effectively cuts off the silver halide response. Lanthanum glass is an exception: It begins rolling off at 400nm (deep purple)

Another interesting tidbit: Modern film incorporates a UV blocking upper layer or dye which significantly reduces the effective UV response. This is why you see an effect under high-UV conditions, but will not see the 1-2 stop wide variations in effective speed over the course of a year like my dry plates (with their 19th Century style emulsion) have.

That “modern” addition cuts speed variation from varying UV levels down to 1/2 stop or less. It also moderates the change in effective speed under indoor lighting to a stop or less (I see 3-stop change due to lack of UV under incandescent and LED)

Myself and others make prints using UV exposures thru the glass of the contact printing frames. UVA, and some UVB, I believe is transmitted by window glass -- no UVC, though...thankfully the atmosphere takes care of that, too!

Edit -- very little UVB gets thru window glass.

Bill - unless you are fairly high up where the air is really clean, atmospheric particulates as well as heat waves near ground level are going to disturb sharpness more than UV. But under ideal conditions, an appropriate skylight or UV filter will make a difference in that respect, especially is you are planning on a high degree of enlargement of the original shot. More often the symptom is a shift in color toward bluer. Just how much varies with specific film type. But most camera lenses still pass enough UV for this to be an evident problem in color photography. With most black and white work, much stronger contrast filter are in play, which eliminate the issue. The exception would be deep blue filters, which will give you an effect like old blue-sensitive plates. I'm not personally after any kind of old-time look, but sometimes do include blue filters in my kit to enhance atmospheric effects and the openness of shadows, versus the conventional "black sky" mentality. Since I do this mainly in conjunction with 8X10 film, a slight loss of film sharpness due to UV is so minuscule as to be visually nonexistent in a typical 2X to 3X enlargement.

There obviously were UV-transmitting lenses made for scientific purposes, including special microscope lenses. These are rather rare except in the medical field. Sometimes they were related to special astrophotography. But most ordinary camera lenses transmit a fair amount of UV anyway. That's why there's still an abundance of UV and skylight filters being made; even digital camera exposures are affected to some extent by UV.

Another example is so-called museum picture framing glass. Some of these are not only optically coated, but exist in sandwich style much like a Tiffen filter, with a thin pale amber film in between. But if you seriously test some of these materials under a strong UV source like direct window light, even all three cumulative layers of glass plus the internal UV film only slows down UV fading a certain amount. Transmission specifications don't tell the whole story, because every little bit of UV adds up, and the actual prolongation of the life of the print behind that glass might only be another 5 to 15%, depending.
In other words, you're eventually doomed if the display illumination source even carries UV. If you need something to block all of it, it will so alter the color of the displayed image itself as to be useless. Even mildly tinted amber or pinkish version spoil the saturation of blue and cyan hues.

UV destroys most pigments too, and will eat alive most of the colorants of an inkjet print over time, which is only partially pigment-colored anyway. If you want to see true permanent pigments, look at the surface of Mars - all red and ochre oxides, maybe some greenish chromium ones too. But we need a certain amount of UV to stay alive. Now there's evidence that even Neanderthals in the cold dark north probably had light skin color, blonde hair, and blue eyes to admit more UV.

I figured UV filters like camera cases were just profit making items for camera stores running on razor thin margins.
But UVC must be what my UV meter registers. Soon as I take it behind a sheet of glass it drops from whatever might be reading that day to zero.
Interesting that modern films have a UV blocking layer now, that may be one of the reasons we're not blowing out skies like before.

UV and related skylight filters still have an important function with color film. However, most people probably don't understand what that is in any specific sense, and buy them mainly to just to protect their lens itself, sometimes at rather high prices. What is amusing is that many of these are still the same thing, but have been relabeled as "digital filters" for sake on an ongoing market. They serve an actual purpose in those applications as well, besides just lens protection.

Bill Burk said:

I figured UV filters like camera cases were just profit making items for camera stores running on razor thin margins.
But UVC must be what my UV meter registers. Soon as I take it behind a sheet of glass it drops from whatever might be reading that day to zero.
Interesting that modern films have a UV blocking layer now, that may be one of the reasons we're not blowing out skies like before.

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No UVC gets through the atmosphere...probably UVB you are measuring. Actually, I am curious what fequencies your meter reads. One designed for medical or manufacturing use would measure UVC. Mine is limited to UVA

PS -- I also believe glass will block just about 100% UVB.

Bill Burk said:

Interesting that modern films have a UV blocking layer now, that may be one of the reasons we're not blowing out skies like before.

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that’s exactly what it is, Bill.