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OT: stops or EVs

 
 
Charles Schuler
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      01-27-2007

"Paul Mitchum" <(E-Mail Removed)0m> wrote in message
news:1hsehp4.1mwcr2d1uhiibkN%(E-Mail Removed)0m...
> Charles Schuler <(E-Mail Removed)> wrote:
>
>> I am teaching digital photography in my community and lean toward EVs as
>> opposed to stops. I know that the two are equivalent, but I also know
>> that they are not the same thing.
>>
>> My feeling is that f/stops or f-stops, technically speaking, should only
>> refer only to the aperture of the lens. I fully understand that an
>> equivalent exists by changing shutter speed or ISO or both. With all of
>> the folks now entering digital photography as a hobby, it would be
>> advantageous to clear this issue a bit.
>>
>> I also find that camera manuals lean towards EV.
>>
>> What are your thoughts?

>
> First of all, the concept is more important than the name.
>
> But secondly: The concept can be illustrated by explaining the
> difference between the two names.
>
> So you talk about stops and relative aperture size, and how opening and
> closing the aperture by stops changes the exposure. In the infancy of
> photography, changing the aperture was the primary way of altering
> exposure, which is why the term 'stop' came to refer to the amount of
> light exposed.
>
> And then you move on to the EV chart, showing how changing different
> settings can compensate for each other, yielding the same exposure (same
> exposure, but perhaps different picture). The wikipedia article on EV is
> pretty good. <http://en.wikipedia.org/wiki/Exposure_value>


It is, but the correction factor for ISOs other than 100 seems to be
inverted. Thanks.


 
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Charles Schuler
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      01-27-2007

"King Sardon" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> On Sat, 27 Jan 2007 16:18:08 -0500, "Charles Schuler"
> <(E-Mail Removed)> wrote:
>
>>
>>"Charles Schuler" <(E-Mail Removed)> wrote in message
>>news:(E-Mail Removed) m...
>>>I am teaching digital photography in my community and lean toward EVs as
>>>opposed to stops. I know that the two are equivalent, but I also know
>>>that
>>>they are not the same thing.
>>>
>>> My feeling is that f/stops or f-stops, technically speaking, should only
>>> refer only to the aperture of the lens. I fully understand that an
>>> equivalent exists by changing shutter speed or ISO or both. With all of
>>> the folks now entering digital photography as a hobby, it would be
>>> advantageous to clear this issue a bit.

>>
>>I know that stops are different things than exposure values. My point
>>was/is about discussing changes: delta stops = delta EVs (a change of one
>>stop is equal to a change of one EV). One can achieve an exposure change
>>by
>>varying aperture, shutter speed or ISO (or any combination). I like EV
>>because it is a more general concept and fits better with light meters,
>>etc.
>>
>>However, the common vernacular heavily favors stops. That doesn't mean it
>>is necessarily the best way to teach it.

>
> IIRC, a "stop" was originally (in the real old days) a small opaque
> plate with a hole in it of a particular size that was inserted
> transversely into a lens to control the amount of light. The stop
> wasn't the aperture, it was the opaque part. And the different hole
> sizes usually didn't vary by factors of 2.
>
> So today's meaning of stop = 2-fold change in aperture cross-sectional
> area isn't really accurate either.


Well, the concept of powers of two is mighty compelling these days. Thanks
for your contribution to the history of this subject! I learned yet another
thing about stops.


 
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Gisle Hannemyr
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      01-28-2007
Paul Repacholi <(E-Mail Removed)> writes:
> Gisle Hannemyr <(E-Mail Removed)> writes:


>> Knowing about absolute EV as a specific quantity of light
>> (see: http://hannemyr.com/photo/ir.html#ev ) is important.


> NO it is NOT! This is so far out, it is not even wrong! The
> related number for light values is LV. EV is only a speed/f#
> pairingand is not related at all to LV unless you specify
> a film speed as well.
>
> If you are going to teach it, get it 100% right, there is no excuse
> not to.


With all due respect, your terminology is outdated.

I am well aware of the fact that some people insists on calling this
"LV", but it never cought on, and IMHO serves no purpose. From a
teaching point ov view, it even confuses matters, because talking
about EV as a function of the speed/f-stop pair hides the fact that
exposure is controlled by the /triple/: shutter speed, aperture and
film (or sensor) speed..

If you had bothered reading the page I linked to, you'd found the
following note on it.

"Note: There is some confusion whether EV takes film speed into account
or not. Some insists that EV is only valid for ISO 100, and use a
different term, such as light value (LV), or actual EV (aEV) for a
number that is a function of the film speed, aperture and shutter
triplet. To cut a long story short, the most useful metric for
digital, where film speed is just as variable as the other two, is one
that takes film speed into account, so that is what I use and call
EV. Please note that the calculator featured in the excellent glossary
at DPreview is in agreement."

For calculator, see:
http://www.dpreview.com/learn/?/Glos...xposure_01.htm
--
- gisle hannemyr [ gisle{at}hannemyr.no - http://hannemyr.com/photo/ ]
------------------------------------------------------------------------
Sigma SD10, Kodak DCS460, Canon Powershot G5, Olympus 2020Z
------------------------------------------------------------------------
 
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