[Charles1]

I just bought a set of 3 extension tubes to use with my *ist ds. I am using a Tamron 75-300mm zoom. How do I determine correct exposure ? Can I use an external light meter and them use a formula to determine correct exposure ? I tried a few by trial and error and got some very good results, but would like a reliable way to determine correct settings......
Thanks... Charles1

[pacerr]

Just meter as you usually do. The camera calculates the correct exposure based on the light that passes through WHATEVER lens/combination you use.

If you're not satisfied with the results shown on the LCD/histogram, use the exposure correction function as explained in the user manual.


H2

[Tojar]

I have a problem with my extension tubes so I thought borrowing this thread instead of creating a new one.
Bought this for my K20D; DealExtreme: $9.77 Macro Extension Tube/Ring for Pentax SLR/DSLR Cameras
when I inserted it to my body, it went to such position in which the red dot exactly on the same position as the release button. When the button is pressed, the extension ring can be rotated counterclockwise to a point where the red dot is facing straight downwares but the ring cant be lifted off. How could the extension be removed from body?

[GerryL]

I have a problem with my extension tubes so I thought borrowing this thread instead of creating a new one.
Bought this for my K20D; DealExtreme: $9.77 Macro Extension Tube/Ring for Pentax SLR/DSLR Cameras
when I inserted it to my body, it went to such position in which the red dot exactly on the same position as the release button. When the button is pressed, the extension ring can be rotated counterclockwise to a point where the red dot is facing straight downwares but the ring cant be lifted off. How could the extension be removed from body?

Welcome to the forum Tojar!
Bummer..but I can't visualize what you have done.
Pictures might help as others might chime on this too once they see it.
My extension tube is soon to arrive so I have no idea yet on how to meter either with my chosen extension tubes.
I got the Vivitar AT-22 and I know it has the aperture/diaphragm levers for stop down metering (..i think??).
I need to follow this thread too when my tubes finally arrive and for me to test it out.

[Blue]

I have a problem with my extension tubes so I thought borrowing this thread instead of creating a new one.
Bought this for my K20D; DealExtreme: $9.77 Macro Extension Tube/Ring for Pentax SLR/DSLR Cameras
when I inserted it to my body, it went to such position in which the red dot exactly on the same position as the release button. When the button is pressed, the extension ring can be rotated counterclockwise to a point where the red dot is facing straight downwares but the ring cant be lifted off. How could the extension be removed from body?

There are 3 flanges on the tube and 3 slots on the camera mount that they fit into. they are of slightly different sizes. You need to rotate the flanges to the match up to the correct slot. Do this and depress the lens release and see if you can coax it out. I suspect there is a tolerance issue with one or more of the flanges on the tube. Do you follow my logic?

[Charles1]

Thanks for the info. These tubes are not compatible with auto focus or exposure setting in the normal fashion. If I meter with an external meter, is there a formula to tell me what would be the correct settings ?

Thanks........ Charles1

[Blue]

Thanks for the info. These tubes are not compatible with auto focus or exposure setting in the normal fashion. If I meter with an external meter, is there a formula to tell me what would be the correct settings ?

Thanks........ Charles1

There is a way to calculate it and then compensate for the appropriate number of f-stops based on the amount of extension you are using. However, if you are using a K0d, K200d or similar, I recommend taking test shots in the manual mode and using the histogram to get it where you need to be because in the end, you will need to do that anyway. This is one of those areas where there is no substitution for practice and patience.

[pdxbmw]

I have had great success with using the green button when I use my extension tubes.



If it wasn't for that damn blue flower right in the bottom middle, I would absolutely love this shot. Extension tubes are lots of fun.

[Tojar]

Here's some pictures of the situation:
The extension ring system whole with 50mm:

Only mount ring, in closed position:

And in opened position. Can't be lifted a little from bottom but none from upper side:

[Blue]

I just bought a set of 3 extension tubes to use with my *ist ds. I am using a Tamron 75-300mm zoom. How do I determine correct exposure ? Can I use an external light meter and them use a formula to determine correct exposure ? I tried a few by trial and error and got some very good results, but would like a reliable way to determine correct settings......
Thanks... Charles1

Here is a formula to calculate exposure increase factor for symmetrical lenses and nonmacro versions. This is the way I do it when using film. It is based on formulae in the Kodak book, Closeup Photography (1984).


EI = (m + 1)2 note: that is squared, the 2 is supposed to be superscript


Where:

EI = exposure increase factor
m = magnification


Magnification can be calculated by:
m = x/l

Where:
m = magnification
x = length of extension
l = lens length


Fore example when I use my 28.5 mm pentax tube,
On a 28mm lens I lose approx 2 stops (EI = 4 from above formula)
On the 50mm I lose just over 1 stop (EI = 2.46 from above formula)
On a 135mm lens you lose about 3/4 stop (EI = 1.47)
On a 200mm lens I lose about 2/3 stop (EI = 1.31)

Remember that exposure factor isn't the number of stops. You will determine that based on the fact that each stop lets in 2x the light the previous one did. Remember that you can compensate for this by the aperture, shutter speed or both.

Asymmetrical lenses require a more complicated formula because pupilary magnification of the lens has to be calculated.

This is the way I did it for bellows and tubes when I used film. I carried a cheat sheet.

Edit: I want to add that macro was the main motivation for me to go digital. I use the green button and histograms to get me where I need to be on digital. I used to shoot slides and would bracket. A good day for my scientific presentations were when I got 3 good slides out of a role of 24 or 4 out of 36 in the macro range.

[Blue]

Here's some pictures of the situation:
The extension ring system whole with 50mm:

Only mount ring, in closed position:

And in opened position. Can't be lifted a little from bottom but none from upper side:

I am out of suggestions other than looking in there with a dental mirror and looking for why the flanges won't fit back through the slots. I'm still guessing there are tolerance issues. Worst case scenario, you may have to send it to a shop to get it off.

[Blue]

Charles1,

Peter Zack shows how to calculate magnification when the magnification of a non-macro lens is known. It is Part 3 under extension tubes at

Shooting Macros Section 3 – The Math Enticing the Light

If you are using a Pentax k-mount lens for example, you could go to the Dimitrov site and look up the specs for a lens such as the M 28mm and get its magnification.

[Ratmagiclady]

I have had great success with using the green button when I use my extension tubes.



If it wasn't for that damn blue flower right in the bottom middle, I would absolutely love this shot. Extension tubes are lots of fun.

We really like that one, too. Nice going.