Re: summer / winter film speeds (was: Presentation)

[Eckhard Stephan <eckhard.stephan@xxxxxxxx>]

> > Why is lower then summer the film ISO in winter?
> 
> In the summer the angle the earth is at with respect to the sun allows for a
> greater amount of infrared radiation to fall on the earth.  Therefore, you can
> generally shoot faster in the summer than in the winter.
> 

Of course you can shoot faster in summer than in winter, but I'm not so
sure if yours is the right explanation. In summer no doubt the sun rays
hit the earth in a steeper angle than in winter, which means that the
TOTAL amount of solar energy (including all wavelenghts from heat over
IR and visible light to UV), that hits the earth's surface is higher. In
terms of absolute quantity, we really do get more IR than in winter. But
we also get more visible light, UV, ... 
In fact, in winter the ratio of IR to visible light might even be a bit
higher than in summer. On a sunny day the ratio of IR to visible light
(not the absolute amount of IR!) is highest in the morning and an in the
evening hours, while the typical photographer unfriendly light at noon
contains more blue and UV than red/IR light. The higher the sun, the
more UV and blue, the less red and IR. This is due to the optical
properties of our atmosphere. If you consider that in winter the sun is
generally closer to the horizon than in summer, there should be even
more IR light (compared to visible light) in winter than in summer. Of
course, this is practically negligable...
I always thought the speed change of IR films was rather a matter of IR
REFLECTION: At least here in good old Europe, there is much less
IR-reflecting foliage in winter than there is in spring and summer. Less
IR-reflection of course means less exposure of IR films, which has to be
compensated for by a slower film speed. That's why many people shoot
their HIE faster in summer than they do in winter.
It may however be a little dangerous to simply change your film speed
with the season regardless of the subject you are photographing. In
situations where there is little IR reflection to be expected (e.g.
stoney landscapes, water, architecture...) I generally use a slower film
speed (ISO 200 in most cases). When there is presumably a lot of IR
reflection (lots of green foliage...), I choose a faster film speed (ISO
320-400), no matter what time of the year it is. A bracket over +/- 1
aperture stops around the metered value is of course always a good idea,
especially if you are not 100% sure about the correct exposure ;-)

All the best, 

Eckhard


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*          Eckhard Stephan             * 
*          Freiburg (Germany)          *
*     eckhard.stephan@xxxxxxxx         *	
*   http://www.eckhards-photobox.de    *   
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