Extension Tubes


rickbehl

Link Posted 26/10/2010 - 06:41
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

hefty1

Link Posted 26/10/2010 - 09:59
Hi Rick,

1. You are absolutely correct; they'll effectively turn any lens into a macro lens (the shorter the focal length of the lens, the higher the magnification you can achieve). They won't increase reach like a teleconverter.

2. No real limitations but you'll see more pronounced effects (higher magnification ratios) with shorter lenses.

3. Again your reading is correct.

4. No loss of quality will arise as there are no extra glass elements to contend with.

5. The prism metering is through-the-lens so should compensate automatically for the use of tubes. You need to make adjustments only if you rely on an external meter. Whether your viewfinder info still shows will depend on whether the tubes you have include pass-through contacts to relay the lens information to the camera - some do and some don't.

Other than macro work you'll also find tubes useful for general portraiture as the reduced minimum focus distance will enable tighter crops of your subject. And in fact any scenario where you've thought, "I wish I could get closer there..."
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Banjo

Link Posted 27/10/2010 - 02:43
rickbehl wrote:
Hi,


...Hope someone can help with the following questions...

...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 (magnification), 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 just how large you want your image to be. 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, so use a shorter tube.

On the other hand, the narrow DOF effect of longer focus length lenses will also transfer to the use of tubes so, of wanting OOF backgrounds, the longer focal lenght lenses will provide them, but for any given extension tube there will also be a reduced magnification. It's all roundabouts and swings.

(Bearing in mind that MF lenses, in general usage, have longer focal lengths than do 135mm lenses, in general usage, DOF using MF gear is likely to be narrower than in 135mm practice anyway. That is juat another practical difference between the two formats))

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. (You will, of course, lose infinity focus, while tubes are attached AND any lens extensiondue to the use of the lens' own helical forms part of the extension -i.e. adds to that of any tube/s used.)

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.

Banjo

Link Posted 27/10/2010 - 02:56
BTW there are two types of tube: "auto" and "manual".

The manual tubes require the use of stop-down metering and are best avoided due to inconvenience of use.
Last Edited by Banjo on 27/10/2010 - 02:57

rickbehl

Link Posted 27/10/2010 - 04:50
Thanks for the responses. 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 on the replies 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

Banjo

Link Posted 27/10/2010 - 05:32
rickbehl wrote:
Thanks for the responses. 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 on the replies 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

1) There's no way you would be able to take "distant" images with tubes attached, maybe a face/face and shoulders with a 200mm (35mm equivalent) lens and the shortest tube.

2) Yes. The use of tubes both reduces the lens' minimum focus distance and prevents the use of infinity focus.

3) You got it.

4) The situation is actually more complicated than that, as the use of tubes actually affects the size of the "effective aperture" (smaller than that indicated on the camera controls). But don't let it bother you, I have often used an indicated aperture of f/22 on 35mm film, and it doesn't show. Also, the number of tubes doesn't so much affect IQ as the available light to work with and all the attendant issues that brings.

If you have "manual" tubes, you can use a hand-held meter and apply the exposure factors you listed in point #3 (above): that should work.

If no hand-held meter, but the light is constant/reliable (e.g. sunny day or 100% cloudy) you can measure your exposure (grey card or substitute reading) with your TTL meter, then mount the tubes, apply the exposure factor, and shoot.

Another way (should work) is to set the camera to Aperture priority Auto Exposure and use the EC control to add exposure according to your exposure factors in #3, above. Focusing is still an issue...but you could pre-focus wide open, using a tripod.

As you can see, manual tubes are "messy" to use. Also, it is very difficult to focus at shooting aperture.

In general, tripods (outdoors) are messy and impractical.

However, using "auto" tubes is relatively simple and fun and easily done hand-held.

rickbehl

Link Posted 27/10/2010 - 09:30
I see. I've just taken another look at the extension tubes I have and I'm pretty sure they are the 'Manual' type and NOT the 'Auto' type...

I'm guessing that the main difference is that you need to remember to stop down the lens yourself (using the DOF lever) before taking the shot otherwise you will be probably end up with everything underexposed (metering wide open for what may actually be a stopped down shot)...

fatspider

Link Posted 27/10/2010 - 09:59
If the tubes are simply hollow rings with no mechanical parts then they are totally manual. "Auto" tubes will have the mechanical linkage for the aperture.

I dont know a lot about the Pentax MF system, did the lenses evolve electrical contacts for lens data transfer in the same way as the K mount? if so then dont confuse the term "Auto" with the prescence of contacs.

When it come to extension tubes the term "Auto" simply refers to the stop down linkage.
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davex

Link Posted 27/10/2010 - 14:03
Just to add a bit more confusion;
There are also Sigma AF tubes available in Pentax K mount. However, as they do not have a screw linkage they cannot AF standard lenses, not sure about SDM though.

Davex.
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joel67

Link Posted 29/10/2010 - 14:41
Hi Rick,
What an interesting topic this is but it seems to me a lot of people are confused about the use of extension tubes on P67 or P67ii.

The simple fact is it does not matter whatever the tubes are manual or automatic as there is NO LINK between the tubes and the METERING SYSTEM of either 67. You will always meter with the lens wide open until YOU CLOSE THE LENS MANUALLY with the DOF preview lever. The only link there is (auto tube) is a mechanical one that will close the diaphragm to the setting you have set your lens (not much of use in this situation)

Let me also say that will not lose any image quality with tubes (no matter if you use 1 tube or 2 or all 3 together or more...) The only things you are going to lose is DOF (big time) and light (lots of it). With the 67ii you can leave the setting on Automatic (as the camera will work out the correct speed for a given aperture)

For macro work with a 67 a good sturdy tripod with a good solid head will be essential as even in good light shotting speed will be very low.

To recap: leave your 67ii on Auto, compose, focus, close the lens manually, check DOF and metering in viewfinder, miror-up (essential) and shoot (using a cable release of course)

Hope this help
Regards
Joel

Banjo

Link Posted 01/11/2010 - 06:11
Would this link provide something useful to the discussion?

http://www.pentaximaging.com/files/manual/6x7_Auto_Extension_Tubes.pdf
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