UV / Skylight filters and exposure at altitude

[BradS]

I've noticed that - or at least it seems that - my exposures at altitudes are whacked. It seems like everything is very over exposed when photographing above about 8500 feet....

Is it really possible that this is due to increased UV? and if so, would a UV or Skylight filter actually do something other than keep the dust off the front element in these situations?

Or is it yet another case of operator error?

 

[MattKing]

Well, light-headedness is certainly a possibility.
But I think that UV also contributes a lot.
Are you using TTL metering, or a hand meter? I ask, because most meters aren't sensitive to UV.

 

[jim10219]

How are you metering the light? Sunny 16 won't work up there. You don't have enough atmosphere filtering out the light, so it's actually brighter than it is at sea level. Some meters may pick up UV and others won't. So depending on your meter, it may or may not be thrown off by the extra UV. And UV response is also a characteristic of the film. Some films are more responsive to it than others. Plus, some lenses transmit more UV than others.

In any case, a UV filter would be a good idea. They were created specifically for film cameras. They don't do much good on most digital cameras, because most digital cameras have a UV filter placed directly over the sensor since they are especially sensitive to UV.

 

[BradS]

Well, light-headedness is certainly a possibility.
But I think that UV also contributes a lot.
Are you using TTL metering, or a hand meter? I ask, because most meters aren't sensitive to UV.

How are you metering the light? Sunny 16 won't work up there. You don't have enough atmosphere filtering out the light, so it's actually brighter than it is at sea level. Some meters may pick up UV and others won't. So depending on your meter, it may or may not be thrown off by the extra UV. And UV response is also a characteristic of the film. Some films are more responsive to it than others. Plus, some lenses transmit more UV than others.

In any case, a UV filter would be a good idea. They were created specifically for film cameras. They don't do much good on most digital cameras, because most digital cameras have a UV filter placed directly over the sensor since they are especially sensitive to UV.

Thanks guys.

FP4+ film in 35mm format.
Metering with the built in meter.
Hiking companion cameras are either a Nikkormat FTN with 50mm f/1.4 pre-AI or Minolta SRT-102 with 55mm f/1.7 PF.
The Nikon lens has a Nikon L39 filter - which, I think, is a "strong" UV filter suited to B&W. The Minolta lens is naked.
Never any exposure issues with the Nikon but seemingly always with the Minolta - but only at altitude... (Thus the question. I'm thinking the filter is the difference?)
No equipment related exposure issues with either at lower altitudes.

 

[Sirius Glass]

Yes, UV does affect the exposure, so

1. Meter without the sky in view, I do this all the time. OR
2. Use a spot meter using the Zone System.

 

[reddesert]

Take both cameras sometime and compare their meter readings of the same scene at low elevation and at altitude.

Filters are often named with reference to their cutoff wavelength - an L39 filter cuts off around 390 nm. (A Y48 yellow filter cuts off light shortward of 480 nm, etc.) The datasheet for FP4+ has a plot of its spectral sensitivity. https://www.ilfordphoto.com/amfile/...ilford_brochure&___from_store=ilford_brochure There is some sensitivity blueward of 390 nm, not that much, but they made the plot with tungsten ligh, which doesn't have much UV, and I don't know if they compensated for that.

Typically, it takes special glasses and coatings to make a lens that transmits UV well. I suspect that the average lens doesn't transmit well much blueward of 390 nm anyway, so the L39 filter shouldn't be making much difference. Cheap experiment: get a UV filter for the Minolta and try that. If I had to guess, it would be a difference between the light meters. Perhaps the Minolta's meter is more blue sensitive than the Nikkormat, or the Minolta's CLC meter circuit does something funny for very bright scenes, or the meter has gone non-linear at the very bright end (assuming that your problems are overexposure in full sun at altitude).

 

[GRHazelton]

An interesting thread! I remember some 60 years ago taking a 'Chrome of the Coast and Geodetic Survey marker on the top of Mt Evans in Colorado. Height 14.265 feet. Due to the weather, cold and spitting snow in July, there was no real view, hence the marker shot. Exposure set via a selenium meter, the results were good. At that time after reaching the summit by car there was a coffee shop up there, with coffee and donuts and oxygen dispensers for the needy. My Brother and I slogged up the final ascent for the picture opportunity and to prove that we'd been there. Our performance was affected, as was the car's, by the thin air.

 

[BradS]

Filters are often named with reference to their cutoff wavelength - an L39 filter cuts off around 390 nm. (A Y48 yellow filter cuts off light shortward of 480 nm, etc.) The datasheet for FP4+ has a plot of its spectral sensitivity. https://www.ilfordphoto.com/amfile/...ilford_brochure&___from_store=ilford_brochure There is some sensitivity blueward of 390 nm, not that much, but they made the plot with tungsten light, which doesn't have much UV, and I don't know if they compensated for that.

Interesting...good to know.

....Minolta's CLC meter circuit does something funny....

I'm starting to think this is the issue.

Meanwhile, I've alleviated myself of the problem by giving away substantially all of my Minolta gear.