用相机测光（转帖） - I Love Montreal

LIGHT MEASUREMENT

Many orchid books mention 'foot candles' when discussing the light requirements required for orchids. Sophisticated instruments can be obtained to measure the light received in a glasshouse or other area, but this equipment is usually beyond the resources of most orchid growers. As an alternative you can obtain a reasonable estimate of foot candles by using a camera equipped with a light meter. There is some disagreement regarding the accuracy of such measurements, but I believe a useful indication of the light levels can be obtained, accurate enough for most practical purposes.

The procedure is as follows:-

INDICATIVE LIGHT INTENSITY IN FOOT CANDLES
Shutter speed	Aperture settings
	f 1.4	f 2	f 2.8	f 4	f 5.6	f 8	f 11	f 16	f 22
	INDICATIVE LIGHT INTENSITY IN FOOT CANDLES
1/4 sec	0.5	1	2	4	8	16	32	64	125
1/8	1	2	4	8	16	32	64	125	250
1/15	2	4	8	16	32	64	125	250	500
1/30	4	8	16	32	64	125	250	500	1000
1/60	8	16	32	64	125	250	500	1000	2000
1/125	16	32	64	125	250	500	1000	2000	4000
1/250	32	64	125	250	500	1000	2000	4000	8000
1/500	64	125	250	500	1000	2000	4000	8000	16000
1/1000	125	250	500	1000	2000	4000	8000	16000	32000
1/2000	250	500	1000	2000	4000	8000	16000	32000	64000

If you set an exposure time of 1/125 second and the aperture f.stop is f 11, from the table the light intensity is approximately 1000 foot candles.

This material was originally produced by the Swedish camera manufacturer Hasselblaad.

FOOT CANDLE / LUX CONVERSION
F/C 100 200 300 400 500 600 700 800 900 1000 1250 1500 2000 2500 3000 3500 4000 4500 5000	Lux 1076 2153 3229 4305 5382 6458 7535 8611 9687 10764 13455 16146 21523 26910 32292 37674 43056 48438 55820	Lux 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 12000 14000 16000 18000 20000 25000 30000 35000 40000 45000 50000	F/C 93 186 279 372 465 557 650 743 836 929 1115 1300 1486 1672 1885 2323 2787 3252 3716 4180 4645


Foot-candles divided by 0.0929 = lux		Lux x 0.0929 = Foot-candles

3.5 What is low light, moderate light and bright light fororchids ?

Let’s start with the maximum light : at noon, on a bright,cloudless summer day, the light level outdoors (in the northeast) will top10,000 fc.

We usually consider the light in a supermarket to bebright. If we measured that light levelwith a light meter we’ll find it’s intensity is about 200 to 300 fc, and forus, humans, these 200 or 300 fc of light intensity in the supermarket are quitecomfortable.

But 200 to 300 fcs of light is just a pittance for orchids.

To grow and bloom properly : Low end High end

• Low light orchids need 1,000 fc 1,500 fc

• Moderate light orchids need 1,500 fc 3,000 fc

• High light orchids need 3,000 fc 4,500 fc.

Please keep in mind these levels of light are for matureplants and the maximum light is for a limited time at the brightest moment ofthe day (noon in a bright, cloudless, summer day).

Please also keep in mind you can acclimate some of yourorchids to grow a little outside of these ranges, within reason of course.

The book “An Introduction to Orchids - A Guide to theGrowing and Breeding of Orchids” published by The South Florida OrchidSociety(Tel 305-226-4757) and the book “All About Orchids” published by OrthoBooks both have very clear charts about light requirements of orchids.

3.6 Where to grow you orchids at home

In practical terms what this means is that to provide theminimum light our orchids need to grow and bloom we’ll have to grow them :

-on the windowsill of an east window

-or within 1 foot of a west windowsill

-or within 2 feet of a south windowsill

-or within 1 foot of a south windowsill if it is shaded byother plants or by shade.

Medium light orchids

-on the windowsill of a west window

-within a foot of a south windowsill

-on the windowsill or within a foot of a south windowsill,

(as long as they donot get direct sun in the middle of the day).

Please keep in mind we are talking about windows that are not shaded by trees or other buildings.

文学城rokkorhunter写的：
拿出相机来，选择光圈优先（Aperture priority），ISO设100，光圈值定在16，爆光补偿exposure compensation设在0。这样你的像机就能当测光表用了。找个大晴天，让太阳光直射在你的手上，用像机对着手充满全画面，半按快门，这时像机测光表给出的快门数。应该在1/100秒左右，人的肤色有深浅，不同像机也可能有点差别，读数可以在1/50到1/200不等，不管你的像机读数是多少，假定是1/100，记住这个值。它就是太阳照到地球的光照强度，相当于EV16。冬天稍低点，夏天稍高些，雪天或沙滩反射强也会高一些。地球离太阳的距离没有大幅变动，这个光照值晴天相当稳定。阴雨天才会有显著降低。万物生长靠太阳，植物要是每天能有七八个小时这样的照度，就能种各种粮食水果。蝴蝶兰生长在热带雨林的树干上，一般没有阳光直射而是生长在树荫下。这样的照度，用像机测的话，得到的快门值大概在1/10到1/25秒，是1/100秒的10倍到4倍。也就是说，树荫下的光照度，只有全日照下的1/10到1/4。冬天阴天时我家离窗户一米之外，，测光给的快门值常常是1到2秒，也就是阳光下的百分之一不到！难怪人家要休眠。不过好在兰花并不像西瓜番茄需要全日照，前面说了1/10到1/4相对日光强度是蝴蝶兰正常生长条件，如果灯光能达到这个照度就好了。

这是我给兰花过冬提供的两只4尺日光灯，伸手离灯10厘米左右，测出快门1/25秒，20厘米，1/10秒。灯光跟太阳光的一大区别，灯光下光照强度，跟光源的距离平方成反比。离灯过远，照度就极度下降，所以大型植物如种大麻的，用日光灯是不管用的，但兰花小型草本就没问题。所以促花的第二个要点，把兰花叶片放在灯下10到20厘米的范围，刚好满足兰花的理想光照。这样冬天兰花保持快速生长，花期很容易提前两个月。