Pentax 6x7 Exposure factors without tubes from Extension Tube instructions?

mweintraub · Oct 24, 2016

I was looking at the extension tube instructions and noticed that there are exposure factors for the lenses without the tubes in place... I'm confused by this. Why would there be?

Ie: 105mm 2.4 lens has a x1.3 factor.

Does this mean I should be compensating a little for these lenses? (I don't have a TTL prism and just use a hand held meter.)

Souce:
PDF: http://pentax67.net/wp-content/uploads/2015/02/P67-tubes.pdf
Transcribed to webpage: http://pentax67.net/?p=1158

shutterfinger · Oct 24, 2016

http://www.butkus.org/chinon/pentax.htm
several pentax 67 manuals, happy reading.

studiocarter · Oct 24, 2016

With a tube on the camera the lens would be further away from where it usually is, so, yes, more light is needed.

MattKing · Oct 24, 2016

mweintraub said:
I was looking at the extension tube instructions and noticed that there are exposure factors for the lenses without the tubes in place... I'm confused by this. Why would there be?

Ie: 105mm 2.4 lens has a x1.3 factor.

Does this mean I should be compensating a little for these lenses? (I don't have a TTL prism and just use a hand held meter.)

Souce:
PDF: http://pentax67.net/wp-content/uploads/2015/02/P67-tubes.pdf
Transcribed to webpage: http://pentax67.net/?p=1158

Only when you are focusing on close subjects.
At some of the close focus positions with the lens alone, you are able to achieve fairly high magnification, and therefore will experience the same sort of decrease in light intensity at the film plane that closely focused bellows cameras or macro lenses encounter.

mweintraub · Oct 24, 2016

MattKing said:
Only when you are focusing on close subjects.
At some of the close focus positions with the lens alone, you are able to achieve fairly high magnification, and therefore will experience the same sort of decrease in light intensity at the film plane that closely focused bellows cameras or macro lenses encounter.

At close focus. Ah, Missed that point. Thanks. Duh.

Leigh B · Oct 25, 2016

mweintraub said:
At close focus. Ah, Missed that point.

The whole question is how far the lens moves forward from its infinity-focus position.

The lens produces a cone of light.
It has a certain intensity at the film plane when focused at infinity.

As the lens moves forward, the area of that circle of illumination at the film plane increases, but the amount of light remains constant.
So you have less intensity per unit area than when the lens was focused at infinity.

This is true of every lens in every situation.

We usually ignore it except for macro work because the film's exposure tolerance hides it.
At a 1:1 (life size) reproduction ratio, you must increase exposure by two stops.

- Leigh

benjiboy · Oct 28, 2016

studiocarter said:
With a tube on the camera the lens would be further away from where it usually is, so, yes, more light is needed.

He's asking about lenses without tubes on.

Sirius Glass · Oct 28, 2016

It you meter through the lens, all these questions go away.

SKrasnov · Dec 28, 2016

I have to contribute to the thred a small drop -- my own online calculator for exposure factor with and without extension tubes for Pentax 6X7.
http://skrasnov.com/pentax-67/extension-tubes-exposure-compensation/
Let me know if you want see other lenses in the calc. Enjoy

Pentax 6x7 Exposure factors without tubes from Extension Tube instructions?

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