Calculating exposures

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Calculating exposures

Postby Charles Twist » Sat Nov 05, 2011 9:40 am UTC+00:00

I have a problem with my light meter in that I am failing to trust it. It occasionally gives a spurious metering so I am going to get it fixed and buy a back-up meter. In the meanwhile, I am going to rely on my digital camera to provide a reference point. So I decided to look at the SLR reading versus a Sekonic borrowed from Dave Parkin (thanks!).

The SLR was in spot-metering mode with a 105mm lens which simulated the 1° spot quite accurately. It's not ideally set up: the SLR reads at 400 iso and the meter at 50iso, but that's a straightforward conversion. More tricky is that the SLR is in aperture priority and the Sekonic in exposure priority. It's not that big a problem, I thought - I'll just convert to EV to check that the two agree. The internettells me that EV=log(N^2/t) where the log is base 2, N is the f-number and t the exposure (see the link for detail). Plugging that lot in to Excel, I find that the two meters agree very well, +/- 0.2 stop which might be due to metering errors.

What gets me though is that converting on paper doesn't work so well:
First metering is pretty close:
SLR: 400iso, t=1/2.5sec, f=16 (equivalent to 3.2sec at 50iso)
Sekonic: 50iso, t=2sec, f=11.6
But the second is way out:
SLR: 400iso, t=2.5sec, f=16 (equivalent to 20 sec at 50iso)
Sekonic: 50iso, t=8sec, f=11.6
Anbody any idea what causes this discrepancy?

Thanks,
Charles
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Re: Calculating exposures

Postby Joanna Carter » Sat Nov 05, 2011 11:09 am UTC+00:00

One factor that comes to mind is the order in which Excel is evaluating the formula (operator precedence). I'm sure you know this Charles but, for the sake of others who may not know:

1 + 2 x 3 = ?

A lot of people will evaluate that to an answer of 9 but, according to the rules of operator precedence, the answer is 7.

If you wanted to go against the order of precedence, you would need to write the question as (1 + 2) x 3.

Why? Because multiplication and division are evaluated before addition and subtraction.

Excel honours the rules of precedence but I have come across some cheap calculators which think like a human being :roll:

On the other hand, it might have nothing to do with operator precedence but, at least, I have provided a useful maths lesson 8)
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Re: Calculating exposures

Postby John Brewer » Sat Nov 05, 2011 11:35 am UTC+00:00

If you want to borrow my light-meter, something like this, http://www.robertwhite.co.uk/product.as ... &PT_ID=179 while the foundry cast you a new part, I can pop it in the post. You know us wetplate photographers don't need light-meters we just look up to the sky, pull the finger from our mouth and raise it in the air, hmmmm, a 3 second day ;)
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Re: Calculating exposures

Postby Emmanuel Bigler » Sat Nov 05, 2011 12:57 pm UTC+00:00

Charles
What you need to compute various EV and other combinations of speeds and f-numbers is not an energy-hungry, CO2-emissive computer fitted with some horrible software increasing your "digital slavery".
You simply need a good ol' slide rule in your pocket.
Like this one, made specially for you (French IL = British EV )
Including the old speed scale 1/10 1/25 1/50 1/100 1/200 ...
http://www.cijoint.fr/cjlink.php?file=cj201106/cijri0E8PM.pdf

Offered to you for free not like most commercial spreadsheet programs :mrgreen:
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Re: Calculating exposures

Postby Neil Barnes » Sat Nov 05, 2011 10:27 pm UTC+00:00

Am I the only one old enough to remember: 125ASA - bright sunshine, 125th@F16? Or the little pictures printed inside the film box?

It was *years* before I got a light meter, or had a TTL meter, but I'm still using that old Weston Euromaster 4.

Of course, B&W is a lot more forgiving, as a rule...

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Re: Calculating exposures

Postby Joanna Carter » Sat Nov 05, 2011 10:29 pm UTC+00:00

Neil Barnes wrote:Of course, B&W is a lot more forgiving, as a rule...

Unless you happen to be using Ilford's direct reversal paper :oops:
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Re: Calculating exposures

Postby John Brewer » Sat Nov 05, 2011 11:14 pm UTC+00:00

Yep, I remember the 'sunny 16 rule'! A great get out of jail free method. Not for velvia tho unless bracketing :)
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Re: Calculating exposures

Postby ajmiller » Sun Nov 06, 2011 4:29 pm UTC+00:00

Emmanuel that looks like a useful pdf especially as I have the old speed scale on a lens - unfortunately the link doesn't work for me.

Appreciate another try if possible.

Merci!

Tony


Emmanuel Bigler wrote:Charles
What you need to compute various EV and other combinations of speeds and f-numbers is not an energy-hungry, CO2-emissive computer fitted with some horrible software increasing your "digital slavery".
You simply need a good ol' slide rule in your pocket.
Like this one, made specially for you (French IL = British EV )
Including the old speed scale 1/10 1/25 1/50 1/100 1/200 ...
http://www.cijoint.fr/cjlink.php?file=cj201106/cijri0E8PM.pdf

Offered to you for free not like most commercial spreadsheet programs :mrgreen:
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Re: Calculating exposures

Postby Joanna Carter » Sun Nov 06, 2011 5:11 pm UTC+00:00

ajmiller wrote:unfortunately the link doesn't work for me.

Appreciate another try if possible.

Tony

The page should come up alright, but then you need to click on the link on that page for the file. At least, that's how it works in Firefox on a Mac.

If that doesn't work, try right-clicking on the link in Emmanuel's post and then choose to save the target of the link.
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Re: Calculating exposures

Postby ajmiller » Sun Nov 06, 2011 6:28 pm UTC+00:00

Thanks Joanna - it works now for me - must be this new Chrome browser! :?

Thanks Emmanuel!

Tony

Joanna Carter wrote:
ajmiller wrote:unfortunately the link doesn't work for me.

Appreciate another try if possible.

Tony

The page should come up alright, but then you need to click on the link on that page for the file. At least, that's how it works in Firefox on a Mac.

If that doesn't work, try right-clicking on the link in Emmanuel's post and then choose to save the target of the link.
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Re: Calculating exposures

Postby Charles Twist » Sun Nov 06, 2011 9:18 pm UTC+00:00

Sunny 16 works really well. I have used it before with Provia. But that was in summer. Now with the low sun, it ain't so easy. And with shooting at all hours, it's not practical. Especially with Direct Positive Paper which I am using on the current project, as Joanna has intimated.

What you need to compute various EV and other combinations of speeds and f-numbers is not an energy-hungry, CO2-emissive computer fitted with some horrible software increasing your "digital slavery".

Emmanuel, that's great. I'll chop a tree down to print that out and then use my fingers.

Thanks John for your kind offer. PM sent.

Regards,
Charles
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Re: Calculating exposures

Postby IanG » Mon Nov 07, 2011 2:16 pm UTC+00:00

The little Russian Leningrad lightmeters are a good cheap backup solution, I had one sat idle for years,in fact I'd never usedit, but then tested it a couple of years ago before loaning it to a board member, mine is surprisingly accurate and they can be found for less than £5, being selenium cell there's no risk of a flat battery in the field.

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Re: Calculating exposures

Postby Emmanuel Bigler » Mon Nov 07, 2011 2:44 pm UTC+00:00

Hello all

I hope that you could download my pdf "sunny-16" slide rule from cijoint.fr, otherwise I'll of course place another copy on another repository.
In French the word "Repositoire" does not exist, and worse, COULD NOT exist since it would be too funny ;)

Regarding the sunny-16 rule, I have a pleasant story about it.
I recently joint a local photo club in my hometown. The club was founded in the 1890's but today all members have forgotten about what film could be. Hence I'll probably present some day what a large format camera is to club members, but I'm afraid that it is like "Mission : Impossible". So I started by speaking about medium format, to begin with.
Well, as a new member I was asked to show some images. I decided to extract some 6x6 colour slides [OFF TOPIC since not large format] from my collection and I insisted to project them optically and not digitally. In the same session, people had devised for a long time about what could be the best & most appropriate exposure, and I was surprised to discover that, as of 2011, auto-everything digital cameras can still burn the highlights like I routinely do with my faithful colour slides on film (I simply avoid to mention it too often on my favourite photo forums ;))
I had, of course, carefully chosen to project 3 images with a correct exposure and no burnt highlights, and I was asked how I chose the proper exposure time. The 3 images ware taken in the 1990s during a vacation trip to the US West, the usual places in Utah and Arizona.

I could not resist to cheat and be a bit provocative: "Well, it's just exposed like bright-sunshine-in-your-back, you know, like our mothers & fathers always did in the past for the family albim, I simply used the sunny-16 rule; you know, in Utah the sun is generous"
One club member of course asked asked "But what is the sunny-16 rule?" (La Règle de Seize par Soleil Brillant dans le Dos)

[END OFF-TOPIC, moderators please forgive me]
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Re: Calculating exposures, "sunny-16" circular slide rule

Postby Emmanuel Bigler » Tue Dec 13, 2011 10:18 am UTC+00:00

The repository http://www.cijoint.fr/ seems to be out of order as of Dec. 2011, and the previous document http://www.cijoint.fr/cjlink.php?file=c ... i0E8PM.pdf is no longer accessible.

Hence I'm placing another copy of my "Sunny-16" and EV circular slide rule on another repository.
I have added one page of instructions in English.

Click here to download: EV-circ-slide+instructions.pdf
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Re: Calculating exposures

Postby Tom van Kan » Mon Mar 24, 2014 3:41 pm UTC+00:00

I just use a Weston master 4, works a treat! Mind you, one tends to 'Bracket' when using Transparency, but certainly with HP5 and Portra it is excellent.
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