Questions about extension tubes

[olroy2044]

I have some questions about extension tubes.
1)  Is this the correct way to determine the "size" (don't know the correct term--Length?  ") of a tube?



2) How does one determine how the ratio is changed when a tube is added?

3) How is the ratio affected by crop factor?  For instance, if a lens is marked "1:2.8" for 35mm film, how does the crop factor of my 20d (1.6) affect that ratio?

The more I mess with this stuff, the more I DON'T know

Thanks
Roy

[mickeyobe]

Roy,

I have used extension tubes quite a bit without going into the why and wherefore. Math always overwhelms me.

I currently have a set of three that is marked 1, 2 an3.
1 is 10mm thick.
2 is 23mm thick.
3 is 30mm thick.
They can be stacked in any order to give me a maximum of 63mm extension.
When you measure the thickness of each section do not include the male thread.

They have been quite adequate for any macro photography I do.
With through the lens metering I no longer have to get a migraine computing my light loss.

I went to "Extension Tubes" On Google to try to find some answers for you.
There is so much there. Too much. Much too much.
This seemed most promising but it might not be what you need.
photography-on-the.net/forum/showthread.php?p=104627
There are, however many more entries.

You might try it.
Happy hunting.

Mickey

[mickeyobe]

May 11, 2013 16:30:46 GMT -5 mickeyobe said:

Roy,

I have used extension tubes quite a bit without going into the why and wherefore. Math always overwhelms me.

I currently have a set of three that is marked 1, 2 an3.
1 is 10mm thick.
2 is 23mm thick.
3 is 30mm thick.
They can be stacked in any order to give me a maximum of 63mm extension.
When you measure the thickness of each section do not include the male thread.

They have been quite adequate for any macro photography I do.
With through the lens metering I no longer have to get a migraine computing my light loss.

I went to "Extension Tubes" On Google to try to find some answers for you.
There is so much there. Too much. Much too much.
This seemed most promising but it might not be what you need.
photography-on-the.net/forum/showthread.php?p=104627
There are, however many more entries.

You might try it.
Happy hunting.

Mickey

P.S
You might try reverse mounting your standard lens. It works beautifully.

[olroy2044]

Thanks for the link Mickey.  My head is spinning around in circles with all the math. 

The set of tubes I have pretty much corresponds with yours for length.  Think I'll try the old tried and true formula--Try it see what happens

I've never tried the reversed lens.  Tubes first!

Roy

[Stephen]

As the enquiry is mainly about digital, the exposure factor is not needed with TTL exposure control.... but be careful with digital and tubes and bellows, the exposure system tries to compensate by opening the lens aperture and lowering the shutter speed and adjusting the ISO setting, or all three at once, so it is best to put the camera on Auto with aperture priority, set the required aperture with depth of field in mind, and also set the ISO to manual at about mid range, or lower ISO end, and let the electronic shutter speed control the exposure.

In particular with the Olympus Pen cameras,(and others), always use the spot metering, which can be aimed easily by the back control buttons, at the point of interest for exact exposure. The averaging auto exposure systems in Digital cameras are not so good as spot. Also break the old habits of expensive film, and bracket the shots, Olympus can take a sequence +/- correct, making life easy.

There are tables of exposure factors for each tube etc., but these relate to film use, and are reliable only to about 1:1, beyond that reciprocity failure sets in, with the need to add extra exposure on an approx log scale, till the point is past when reliable extension factors fail. In other words the exposure is always much longer than expected!!!

Stephen.