My Flash meter lies to me

Started Dec 20, 2004 | Discussions
Booga
New MemberPosts: 19
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My Flash meter lies to me
Dec 20, 2004

My flash meter suggests an exposure of approx 1 stop less than actual. I use a canon 20 D. The istogram showing correct exposure usually requires overexposure by 1 stop as opposed to the reading taken by my Gossen sixtomat flash meter. why is this so?

In any case I have learnt to rely on my camera histogram & a grey card for accurate exposure.

Any explanations or am I missing a fundamental truth regarding the flash meter?

Thanking you in advance.
--
M.S.Mehroke
Sydney, Australia

RDKirk
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Re: My Flash meter lies to me
In reply to Booga, Dec 20, 2004

Booga wrote:

My flash meter suggests an exposure of approx 1 stop less than
actual. I use a canon 20 D. The istogram showing correct exposure
usually requires overexposure by 1 stop as opposed to the reading
taken by my Gossen sixtomat flash meter. why is this so?

It may be a difference in the technique you're using (user error). Or it's possible your meter needs to be calibrated to match your camera.

It's quite common--the norm, actually--for two meters to disagree slightly(although being a full stop off is rather unusual). In older times, we simply marked a meter or camera as "+ 1 stop" or "-half stop" and adjusted the film exposure index accordingly.

It doesn't matter technically which you use as your standard, just that you determine which will be your standard according to the tests you've run, then set the other to match it.

These days, most handheld meters have an easy calibration facility--and cameras usually don't--so it's usually easier to calibrate the handheld meter to match the camera. Check your meter manual for the procedure in your case.

However, as I said, being a full stop different is rather unusual for a Gossen meter, so you might want to review your metering techique or your test technique.

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RDKirk
'TANSTAAFL: The only unbreakable rule in photography.'

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Louis4444
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No such thing as "true" exposure...
In reply to Booga, Dec 20, 2004

My flash meter suggests an exposure of approx 1 stop less than
actual.

What's the "actual" exposure?

I use a canon 20 D. The histogram showing correct exposure

How do you define "correct exposure" on the histogram?

usually requires overexposure by 1 stop as opposed to the reading
taken by my Gossen sixtomat flash meter. why is this so?

Many reasons are possible for this...

  • Light meter is maybe set to a different ISO than the camera

  • Make sure there no FEC is dailed in (neither in flash nor body nor

external meter)

  • Incident versus reflected light metering results can differ

  • Light meter of the camera might be calibrated differently (I have a

Sekonic,dont' know about Gossen). Either the camera or meter might
need recalibration

  • Metering characteristics of external meter might be very different from

characteristics of in-camera meter

In any case I have learnt to rely on my camera histogram & a grey
card for accurate exposure.

There are no "absolutes" in exposure metering; exposure metering is based on assumptions (luminance of scene measured averages out to some level of "middle grey", for example) and there is no "correct" solution to the exposure metering problem (for example, how should highly reflective surfaces in flash photography be treated?)

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jimcreative
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Re: My Flash meter lies to me
In reply to Booga, Dec 20, 2004

This would only be provable if you were photographing a gray card.

I am not familiar with the meter you are using. Is it a spot meter or an incident meter? If it is an incident reading then the meter has no way of knowing anything about the actual light values to are finding their way to your lens.

If I was photographing a table of black velvet scraps no incident meter in the world would get me close to an exposure that would work if I followed its readings. This is why I have used a spotmeter, exclusively, for the past 20 years for anything except digital.

Booga wrote:

My flash meter suggests an exposure of approx 1 stop less than
actual. I use a canon 20 D. The istogram showing correct exposure
usually requires overexposure by 1 stop as opposed to the reading
taken by my Gossen sixtomat flash meter. why is this so?

In any case I have learnt to rely on my camera histogram & a grey
card for accurate exposure.

Any explanations or am I missing a fundamental truth regarding the
flash meter?

Thanking you in advance.
--
M.S.Mehroke
Sydney, Australia

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Wild-Mike
Veteran MemberPosts: 5,910
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My flash meter is incapable of lying.
In reply to Booga, Dec 20, 2004

I have seen meters that are inaccurate or differently calibrated though. Never seen one that is capable of a lie though.

Mike

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Booga
New MemberPosts: 19
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Re: My Flash meter lies to me
In reply to jimcreative, Dec 21, 2004

Yes I am photographing a grey card. I assume that when the histogram lies in the centre of the LCD screen, we have a perfectexposure & my pictures are proof. I am using the lastolite ezy balance grey card. This grey card gives a narrow spike.

If I relied solely on the light meter readings then my pictures are decidely darker.

I have checked & there is no correction dialed into the light meter.

It is an incident light meter. Gossen sixtomat. My camera is a canon 20 D set on manual exposure. The studio strobes are fired via a RF transmitter.

I used the tutotial on the following link to get my exposure & it works beautifully.

http://www.luminous-landscape.com/tutorials/dig-exp.shtml

I.e my product pictures are sharp & show proper colour. I also use a properly calibrated monitor(moniter spyder), hence I know the colours are correct when I view them on my monitor.

I am still at a loss to understand - why does the incident light meter vlaue suggest an inaccurate exposure value?

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M.S.Mehroke
Sydney, Australia

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Julio Balaguero
Contributing MemberPosts: 601Gear list
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Flash meter is just a tool...
In reply to Booga, Dec 21, 2004

Learn to use your tool. Just like a hammer, do you say that your hammer lies to you everytime you miss the head of the nail?

Your camera, flash, flash meter, etc. are just tools you need to master to become a master craftsman. Just my 2cent's worth of wisdom.
Best Regard,
Julio

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RDKirk
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Meter may be more honest than histogram
In reply to Booga, Dec 21, 2004

Booga wrote:

Yes I am photographing a grey card. I assume that when the
histogram lies in the centre of the LCD screen, we have a
perfectexposure & my pictures are proof. I am using the lastolite
ezy balance grey card. This grey card gives a narrow spike.

If I relied solely on the light meter readings then my pictures are
decidely darker.

You are presuming that a centered spike on the histogram is a correct exposure. I've read that several places, however I've also read that a correct exposure on the histogram should be slightly to the left of center.

This dispute is connected to the dispute over whether meters are really calibrated to reproduce 18 percent or 12 percent gray tones. Peeople claiming to be experts say that an 18 percent gray reading should be in the center of the histogram; a 12 percent gray reading should be a half stop to the left.

From my tests, it appears the 12-percenters are more correct--that exposure (putting the spike to the left of center) produces an image of a gray card that looks on my calibrated monitor the same gray as the card--Zone V. A centered spike produces a gray that is clearly lighter--Zone VI instead of Zone V.

So your handheld meter (like my Sekonic) is probably telling you the truth.

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RDKirk
'TANSTAAFL: The only unbreakable rule in photography.'

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Wild-Mike
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About histograms
In reply to RDKirk, Dec 21, 2004

RDKirk, check this link out for understanding histograms.

http://www.luminous-landscape.com/tutorials/understanding-series/understanding-histograms.shtml

It's pretty informative.

Regards,
Mike

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RDKirk
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More or less Gray?
In reply to Wild-Mike, Dec 21, 2004

Wild-Mike wrote:

RDKirk, check this link out for understanding histograms.

http://www.luminous-landscape.com/tutorials/understanding-series/understanding-histograms.shtml

It's pretty informative.

Regards,
Mike

Reichmann is of the 18% Gray group, which means he believes that the spike of a properly exposed gray card would be centered on the histogram. He's opposed by the 12% Gray group, which believes the spike of a properly exposed gray card would be left of center.

Sooner or later you have to stop reading and start taking pictures.

Even though I've been a card-carrying member of the 18% Gray group for decades (having been raised on the Zone System as a wee bairn), my tests with my 20D indicate that the 12% crowd may be right, as far as digital photography is concerned. That half-stop less exposure seems to be more correct for this particular medium.

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RDKirk
'TANSTAAFL: The only unbreakable rule in photography.'

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Booga
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Re: My Flash meter lies to me
In reply to Booga, Dec 21, 2004

Thank You all for your replies.I am still learning. I find that exposing to the right of the histogram works better for me. Hence I decided to use the flash meter as a starting guide & then let the grey card take over.

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M.S.Mehroke
Sydney, Australia

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