About exposure equivalence

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FSK - 26 Jul 2003 21:12 GMT

Hi,

I was shooting a birthday cake (white color) from the top under fluorescent
light
using the program setting with my EOS 620, 50 mm f1.8 prime lens and ISO 100
film plus a flash. The metering set the following exposure values: 1/60 at f16.

In order to avoid camera shake I wanted a faster speed. Because I know there is
such thing as exposure equivalence, I switched to shutter priority and set it to
1/125. This was one stop faster than 1/60. I expected the camera cpu to set the
aperture at f11, one stop faster again, but instead it selected f1.8 and the
green light in the
viewfinder was still blinking to warn about underexposure !

The EOS 620 and the canon flash can go upto 1/250.

Can please anybody explain why the exposure equivalence did not work here ?

Regards.

FSK

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Jim Nason - 04 Aug 2003 02:53 GMT

>Hi,
>
[quoted text clipped - 9 lines]
>green light in the
>viewfinder was still blinking to warn about underexposure !

No clue about the specifics, but if things were set as you say... it
should have worked.. On the other hand, if the automated exposure was
1/60 at f16... camera shake is not a problem. At this exposure
setting, the flash (and hence exposure) duration is shorter than an
concievable shutter speed (probably measured in 1/10,000s of a
second). In simple language.. 1/125 is many times longer than the
flash duration.. and hence the exposure.

Jim

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Jim Davis - 08 Aug 2003 13:07 GMT

>In order to avoid camera shake I wanted a faster speed. Because I know there is
>such thing as exposure equivalence, I switched to shutter priority and set it to
>1/125. This was one stop faster than 1/60. I expected the camera cpu to set the
>aperture at f11, one stop faster again, but instead it selected f1.8 and the
>green light in the
>viewfinder was still blinking to warn about underexposure !

Are you sure it wasn't warning about overexposure?

Hey, white cake, flash up close, you'd have to stop way down not to
overexpose. Likely the shutter speed had little to do with the
calculated exposure, the flash being the main light.

Also you wouldn't have to worry too much about camera shake either
with flash being pretty fast and using a light lens.

I'd be more concerned about direct flash making it look like crap no
matter what the exposure. You might want to think about a tripod and
bounce flash.

Jim Davis
Nature Photography
http://www.kjsl.com/~jbdavis/

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B. Skelton - 09 Aug 2003 08:18 GMT

> >In order to avoid camera shake I wanted a faster speed. Because I know there is
> >such thing as exposure equivalence, I switched to shutter priority and set it to
[quoted text clipped - 4 lines]
>
> Are you sure it wasn't warning about overexposure?

In an EOS camera, if the largest aperture number for the lens is blinking,
it means that it can't open up enough to obtain a "correct" exposure.
Therefore, underexposure would result.

> Hey, white cake, flash up close, you'd have to stop way down not to
> overexpose. Likely the shutter speed had little to do with the
> calculated exposure, the flash being the main light.

With A-TTL and shutter priority, the flash is *not* the main light, only a
fill light. The shutter speed/aperture combination the camera was trying
to achieve was for the ambient light. Do some research on this and you'll
understand...

Bruce

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Jim Davis - 09 Aug 2003 13:52 GMT

>In an EOS camera, if the largest aperture number for the lens is blinking,
>it means that it can't open up enough to obtain a "correct" exposure.
[quoted text clipped - 8 lines]
>to achieve was for the ambient light. Do some research on this and you'll
>understand...

Well, I don't understand why it did that. I mean a white cake up
close, surely there wouldn't be a problem getting enough light.

Why would it pick f16 is a big question too. I thought in Av mode, the
user picked the fstop.

Jim Davis
Nature Photography
http://www.kjsl.com/~jbdavis/

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