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10-25-2010, 10:42 PM   #1
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Extension Tubes

Hi,


I have just purchased a set of Pentax 6x7 extension tubes on ebay. I have never used tubes before so I'm just trying to figure out a couple of things. Hope someone can help with the following questions...


  1. What scenarios lend themselves to using the tubes? Am I right in thinking these are best used for when you want to focus closer in on your subject almost like a macro lens. I am thinking these are not really designed to get longer reach which is what I believe Teleconverters are used for?
  2. Which Pentax 67 lenses are most suitable for using with the tubes? or is there no real limitations?
  3. What exposure compensation if any is required when using the tubes? Looking at the manual (http://www.pentaximaging.com/files/manual/6x7_EXTENSION_TUBES.pdf) it seems that for example using Tube #1 (14mm) on most lenses results in an exposure compensation of x1.6. Does this mean I need to add just over half a stop to my exposure (either aperture or shutter speed) to compensate for the tube?
  4. Is there any tangible loss of image quality when using these tubes? Is the effect cumulative (ie, more tubes = more quality loss)? As there is no optics in these tubes not sure about this...
  5. When using the tubes on a P67II body how is the metering on the AE Prism affected? And if I have the modification on my P67II to show the aperture in the viewfinder is that now not going to work when using a tube?


Thanks in advance for any help with these questions!


Rgds
Rick

10-26-2010, 07:29 AM   #2
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1. They will only make it possible to focus closer. Depending on the lens this may be for macro or just for portrait.

2. Sorry, no idea. Generally, wide angles won't give you enough working distance.

3. That sounds excessive, but it would depend on the lens. When your tubes are as long the focal length of the lens you add two stops. I don't remember the formula for this, I usually only use tubes on cameras that will measure through them, so I don't have to know.

4. It should be similar to cropping, but without the limitations of the film and different view (since you're closer). So it depends on the lens, but any good lens should be ok with a reasonable amount of tubes. (You're going to need to stop down more for reasonable DOF though, so in practice optical quality is worse than cropping.)

More tubes give worse quality of course, since it's like cropping more. And it's a bit worse than cropping unless the lens is optimized for close focusing, which it probably isn't unless it says "macro" on it. (And still isn't if it's a modern zoom that says macro, but that doesn't apply here.)

But of course it's better than cropping since the film isn't perfect.

5. The meter looks through the tube and sees the same effect the film will. So you don't need to know about compensation. I don't have a 67, so I don't know about what information remains. The 645 retains full information when using tubes.
10-26-2010, 09:30 AM   #3
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The Pentax 6x7 Extension Tube Manual

I also have the helicoid extension. I find it very useful with a 165mm lens when you want to get closer for a tight crop on a head shot without the camera being uncomfortably close . Normally, that lens is only good for a shoulders and head distance.

Last edited by tuco; 10-26-2010 at 09:44 AM.
10-26-2010, 04:27 PM   #4
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QuoteOriginally posted by rickbehl Quote
Hi,


I have just purchased a set of Pentax 6x7 extension tubes on ebay. I have never used tubes before so I'm just trying to figure out a couple of things. Hope someone can help with the following questions...


  1. What scenarios lend themselves to using the tubes? ...
  2. Which Pentax 67 lenses are most suitable for using with the tubes? or is there no real limitations?
  3. What exposure compensation if any is required when using the tubes? ... Does this mean I need to add just over half a stop to my exposure (either aperture or shutter speed) to compensate for the tube?
  4. Is there any tangible loss of image quality when using these tubes? ...
  5. When using the tubes on a P67II body how is the metering on the AE Prism affected? ...


Thanks in advance for any help with these questions!


Rgds
Rick
1) In general, extension tubes are just that: they extend the distance from lens to image, thereby permitting a larger reproduction ratio (i.e. magnification)

2) Again in general, the shorter the focal length of the lens, the greater the reproduction ratio, the closer the focus, and the greater the loss of light.

Bearing in mind that the whole idea of tubes is to enable close-up or Macro photography, then the shorter the focal length of the lens, the larger the image on the film for any given tube/lens combination.

Of course, it depends on how large you want your image. For example, if wishing to include the whole flower, rather than just the stamens, a lesser reproduction ratio would be better than a greater one.

Using tubes will limit your focusing distance range. Although the various combinations of tube/s and lenses will overlap, for any given combination, the distance range at which you can achieve focus will be different.

3) A guide to exposure compensation is the square of the extension ratio.

E.g., for a 100mm lens, adding a 100mm tube doubles the film to lens distance and quarters the illumination: so a loss of two stops (give +2 stops EC).

Adding 50 mm of tube to a 100 mm lens is increasing the distance by 1.5, so (1.5x1.5)=2.25, so +one and a bit stops EC required.

The use of TTL metering can largely (but not always) compensate. You need to consider just what the meter is registering on and how you want that to come out in the negative (which zone?). (The notions underlying The Zone System are helpful here).

4) Yes and no: you lose an immense amount of DOF. This makes focusing enormously tricky (out of focus issues) and -generally- imposes the use of very small (the smallest available) apertures (pushing you into diffraction issues?) and the slower shutter speeds (camerashake/subject movement issues).

Again, in general, you focus by rocking back and forth with your camera, rather than by turning the focusing ring. For cameras with AF, I understand the AF mechanisms find it difficult to cope and tend to operate very slowly.

Anyway, its all fun!

Have a go.


Last edited by Banjo; 10-26-2010 at 04:32 PM.
10-26-2010, 08:52 PM   #5
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Thanks

Thanks for the respons. I'll try and get out with the camera and tubes in the next day or two to try them out. A couple of thoughts and conclusions so far...

1) Seems best to use these for close-up/macro type work rather than trying to use them for the 'extra reach' (eg wildlife type images)

2) Looks like general consensus is that the tubes are optimally used with focal lengths in the range of 75-165mm (75mm, 90mm, 105mm, 135mm, 165mm) although of course they can be used practically with all lenses with maybe less than optimal results.

3) I think I get it now with the EC. When the manual says x2.0 it translates to 1 extra stop (ie, double '1' ONE time) and when it says x4.0 it means 2 extra stops (ie, double '1' TWO times). If I summarise the data from the tables in the manual I believe that as a rule of thumb to remember the EC:

Tube #1 (14mm) : Add 0.5 Stop
Tube #2 (28mm) : Add 1 Stop
Tube #3 (56mm) : Add 1.3-1.5 Stops

Again using the TTL meters obviously helps removing the need to remember these numbers.

4) Looks like there should not be too much loss of image quality as long as tubes are used individually and stacking is avoided. Generally it sounds like I should be using very small apertures to get maximum sharpness throughout? As f/22 may introduce softness it sounds like f/13-f/16 is optimum? Also, as noted and probably dictated by these shutter speeds a tripod sounds mandatory.Conversely using a tube with longer lenses should help to produce nice bokeh (probably good for portraits with the 165mm?)

5) A bit confused here. After a couple of tests it looks like the tubes do not allow the aperture modification to be used (pentax service centre mod to display aperture instead of the frame number in the viewfinder display). This is not a big issue really. However just so I am clear on how to measure exposure. It sounds like I cannot use Aperture Priority mode? And I'm not too familar with the term 'stop-down metering' (excuse my lack of knowledge here). Am I right in thinking the procedure for metering with these tubes are:

i) Switch metering mode to manual (ie NOT 'A' mode)
ii) Compose and focus the scene
iii) Set desired aperture for required DOF.
iv) Use the DOF Preview lever to stop down the lens to desired aperture and then set shutter speed to correct exposure (using viewfinder meter to get to Center Mark '0').
v) Press 'Meter Lock' to lock exposure or keep DOF lever locked at stopped down aperture. (However the P67II manual says that the 'ML' button cannot be used when the camera is in Metered Manual Mode?)
vi) Release shutter

Does that sound correct?

Thanks again for all your help!

Rgds
Rick
10-26-2010, 10:00 PM   #6
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Using Tubes

Rick - you are grasping these concepts fairly quickly. Exposure compensation and/or metering on close-up photography will continue to bedevil long after you have solved your stability and composition issues. Try using a film with a wide exposure latitude(eg. B&W) to get a usable image. OTOH, If you are feeling bold you might try using tranparency film and bracketing.

On lens choice, most lenses are optimized for shooting at infinity. As you focus closer and closer you are departing from that ideal. Choosing a lens such as the 135 (which is probably optimized for 1/10 life sized) would provide for better optical quality. It has the secondary virtue of providing a greater working distance from the front of the lens to the subject, compared to lenses of a shorter focal length. A common alternative is the 105 with its faster maximum aperture and good optical properties.

As you experiment with the tubes, don't be afraid to use them in combination. That is part of the purpose of the kit. However, lack of light will become a limiting factor and you may resort to flash. This will introduce a whole new range of questions.

Final thoughts - Flat objects such as the veins in a leaf may be easy to capture. 3-D subjects such as the stamens in a flower will be much more difficult due to the dramatically reduced depth of field. Have Fun - Frank
11-22-2016, 01:44 AM - 1 Like   #7
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QuoteOriginally posted by rickbehl Quote
What exposure compensation if any is required when using the tubes? Looking at the manual (http://www.pentaximaging.com/files/manual/6x7_EXTENSION_TUBES.pdf) it seems that for example using Tube #1 (14mm) on most lenses results in an exposure compensation of x1.6. Does this mean I need to add just over half a stop to my exposure (either aperture or shutter speed) to compensate for the tube?
I've made a simple calculator that helps to equate exposure factor with extension tubes and Pentax 67 lenses (four right now): Extension tubes & exposure compensation ? SKrasnov.com

Just select lens and extension tubes combination. That's all.
11-22-2016, 08:44 AM   #8
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QuoteOriginally posted by Sasha Krasnov Quote
I've made a simple calculator that helps to equate exposure factor with extension tubes and Pentax 67 lenses (four right now): Extension tubes & exposure compensation ? SKrasnov.com

Just select lens and extension tubes combination. That's all.
Welcome to the forum!

Good stuff, very useful

Phil.

---------- Post added 11-22-16 at 07:50 AM ----------

A couple things to add to the original post for using extension tubes on a Pentax 6x7 body:

1) This is the biggest "gotcha" I learned when first using the "inner" bayonet Pentax 6x7 extension tubes. YOU MUST SET THE LENS DOF PREVIEW LEVER TO “MAN” TO GET A METER READING. THE 6X7/67 TTL OR 67II AE METERS WILL NOT GIVE A READING IF THE DOF LEVER IS IN “AUTO”. IF YOU ARE USING APERTURE PRIORITY ON THE 67II/AE YOU MUST LEAVE THE LEVER IN THE “MAN” POSITION WHEN YOU PRESS THE SHUTTER. THE AE METER WILL DEFAULT TO CENTER-WEIGHTED METERING WHEN USING AN EXTENSION TUBE.

2) There is also a set of "outer" bayonet extension tubes that are required for the older Takumar 6x7 400mm, 600mm, 800mm & 1000mm lenses.

Phil.

11-22-2016, 09:17 AM   #9
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QuoteOriginally posted by rickbehl Quote
As f/22 may introduce softness it sounds like f/13-f/16 is optimum?
Using a 67 or 67II for macro work, you will find that DOF is quite limited. Trying to stay in the f/8 to f/16 range because of diffraction fears will be difficult. You will need to use smaller stops on many occasions just to be able to get everything in focus in the frame. I use f/22 to f/45 in most of my macro work but your shooting style may be different and you may find that you like a more wide open look. My most commonly used lenses for macro work-- 165 LS, 200 Pentax, 90-180 zoom. Have used the 45, 75, 105, 55-100, 150 and 300 with tubes but they all have limitations, so I stopped using them for macro work.

If you are using a lens that is not designed for macro work and think it will not perform as well as a macro lens, here is something to consider. When shooting at small stops, the aberrations that a non-macro lens would have at close distances are negated by the diaphragm. Even though spherical aberration is greater at close distance with a non-macro lens, that aberration is highly reduced with stopping down. The Pentax non-macro lenses are proven with tubes.

Last edited by desertscape; 11-22-2016 at 09:30 AM.
11-23-2016, 01:02 AM   #10
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QuoteOriginally posted by gofour3 Quote
Welcome to the forum!

Good stuff, very useful

Phil.

---------- Post added 11-22-16 at 07:50 AM ----------

A couple things to add to the original post for using extension tubes on a Pentax 6x7 body:

1) This is the biggest "gotcha" I learned when first using the "inner" bayonet Pentax 6x7 extension tubes. YOU MUST SET THE LENS DOF PREVIEW LEVER TO “MAN” TO GET A METER READING. THE 6X7/67 TTL OR 67II AE METERS WILL NOT GIVE A READING IF THE DOF LEVER IS IN “AUTO”. IF YOU ARE USING APERTURE PRIORITY ON THE 67II/AE YOU MUST LEAVE THE LEVER IN THE “MAN” POSITION WHEN YOU PRESS THE SHUTTER. THE AE METER WILL DEFAULT TO CENTER-WEIGHTED METERING WHEN USING AN EXTENSION TUBE.

2) There is also a set of "outer" bayonet extension tubes that are required for the older Takumar 6x7 400mm, 600mm, 800mm & 1000mm lenses.

Phil.
Thank you Phil!

I've Extension Tube #1 for outer bayonet that I've adopted for old Unbranded Petzval lens! But it will another post
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