underexposed images with Helicoid extension tube

wildbill

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• Jun 18, 2009

• #1

I got a helicoid variable extension tube for my Pentax 67II to replace the ext tube set I hadn't been using much. With the old tube set, I'd use the apeture preview lever to stop the lens down then let shoot in aperture priority mode. No problems with accurate exposures.

I've downloaded the Helicoid manual but I'm not seeing my error.
I've only shot one roll with the helicoid but they're underexposed 1/2 to 1 stop. I did some quick tests with and without the tube attached and it seems that the camera may not be recognizing the ext tube's ext factor. What am I doing wrong?

 

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wildbill

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• Jun 19, 2009

• #2

help. anyone have experience using this tube?

 

Chazzy

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• Jun 19, 2009

• #3

I don't understand why your meter isn't compensating for the extension. I guess that's your question.

 

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wildbill

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• Jun 19, 2009

• #4

yup, no compensation!

 

David A. Goldfarb

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• Jun 19, 2009

• #5

I find that with various cameras TTL metering can become non-linear if the light is too low, and if you're shooting macro using a small aperture, you're likely to be in the non-linear zone. Try metering at full aperture and stopping down for the exposure, or just use a handheld meter and calculate the exposure factor.

 

Pupfish

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• Jun 24, 2009

• #6

The most obvious question to ask is whether you're metering at infinity or less than the extension you're using for the actually image-taking. There is always light loss due to macro extension, ordinarily a one to two stop loss between half life-size and life size (for a symmetrical lens design).

 

John Koehrer

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• Jun 24, 2009

• #7

Pupfish said:

The most obvious question to ask is whether you're metering at infinity or less than the extension you're using for the actually image-taking. There is always light loss due to macro extension, ordinarily a one to two stop loss between half life-size and life size (for a symmetrical lens design).

Click to expand...

Shouldn't matter if it's metering through the lens.