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Physics

dudler

Time for an update: I still use film, though. Not vast quantities, but I have a darkroom, and I'm not afraid to use it.

I enjoy every image I take: I hope you'll enjoy looking at them.
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Physics

15 Nov 2020 8:43AM   Views : 490 Unique : 314

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They say you canít fight city hall: but even city hall canít fight physics. Iíll tell you why this matters on a Sunday, particularly, laterÖ

Yesterdayís blog was about lighting, and those with a bit of knowledge of the area will have noticed that I was using lights rather close to the subject, which is how people often do it. Itís just that itís not the best way, because of physics.

Physics underpins so much of photography, and it shouldnít come as a surprise that physics isnít always on your side. It certainly isnít when you put a light a couple of feet away from your subject, because of the Inverse Square Law.

The picture at the top illustrates what the Law means for lighting. The wall thatís close to the flash unit is burnt out: off to the right, itís getting underexposed. If you are using light like this for a portrait of two people standing shoulder to shoulder, one will be much more exposed than the other. If the light is two feet from one face and four feet from the other, thereís going to be a difference of two stopsí exposure between them.

Pull the light back to fifteen feet away, and youíve got a very small difference. I like big studios and powerful flash units because of this! The sketch below may make this clearer. I hopeÖ

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You donít have to understand the physics, but you do need to accept it, and account for it. Itís why checking exposures with a meter, or perhaps studying the histogram view of a test shot matters so much. If you have the space available, use it, and move the lights back, unless you want to use the exposure difference creatively.

Whyís this relevant to a Sunday? Remember school homework? I have a strong memory of an essay that had to end ĎAnd with a sigh, he tossed the black box over the edge of the cliff.í I think Iíd spent most of Sunday afternoon not writing it. It may be my autistic tendencies, but the problem simply didnít compute. I still have the same problem with some sorts of test, exercise or competition, but I have worked out strategies that would let me write the story now.

But for a lot of people, physics was the big block between Sunday and going back to school for a new week. I look forward to hearing the best excuse for not handing in homework in first thing on a Monday morningÖ

And should you have a handy flashgun (or desk lamp), you may want to do your own homework by setting up a subject with heavy light falloff.

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Comments

JuBarney Avatar
JuBarney Plus
12 36 7 United Kingdom
15 Nov 2020 9:50AM
Beautiful portrait .. and lighting!
mistere Avatar
mistere Plus
10 37 8 England
15 Nov 2020 4:49PM
Physics is easy, people are difficult. It's all relative, or relatives. Probably has something to do with Quantum as well. Smile
A well timed blog, John. I've been mucking around with the inverse square thingy. Keeping
the settings on the camera the same and moving the lights. I know it's not the easiest way to
sort out the exposure but it is interesting. Tape measures are a lot cheaper than light meters as well.
chase Avatar
chase Plus
18 2.5k 682 England
15 Nov 2020 5:47PM
I always handed my homework in on time......Wink

Shifting the lights around makes such a difference to the intensity, I was faffing around with the darkroom light recently, too close yuk, too far away, hardly noticeable, it took me a while to work it out.

The drawing is fabulous, bet you didn't do Art at school Wink
dark_lord Avatar
dark_lord Plus
19 3.0k 836 England
15 Nov 2020 6:37PM
I liked Physics.
The low point was discovering the school's Zenit E Wink

As it's Sunday, lets put things into some sort of perspective.
The STEM community were tasked with carrying out some cutting edge research.

The engineers came up with a wonderful device with levers and cogs, hydraulic rams, flailing arms and wot-nots. With many cuts and bruises they wer ready.
They destroyed a building.

The chemists toiled long and hard mixing this, that and the other. Many singed eyebrows and burns in uncomfortable places later they set off their experiment.
They destroyed a town.

The biologists set themselves up behind perspex barriers and wearing face shields and masks. Large fermentation vats to replicate viruses were commissioned.
They destroyed a planet. Well, life thereon.


The physicists sat and smiled, then pressed a button.


They destroyed a galaxy.

So, physics matters. Or anti-matters as the case may be Smile
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