underexposed images with Helicoid extension tube

[wildbill]

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?

 

[wildbill]

help. anyone have experience using this tube?

 

[Chazzy]

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

 

[wildbill]

yup, no compensation!

 

[David A. Goldfarb]

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]

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]

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).

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