Extension tubes??

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Passage

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I have a Hasselblad 501C/M and would like to do some close-up photos of ....well, whatever looks good close-up. I (think) that I understand the concept of extension tubes, however I don't understand the overall technical use of the different sizes; 8mm, 16mm, etc., and what to expect when used with different focal length lenses, such as additional exposure. I have a 50mm, 80mm, and 180mm lenses. Can anyone give some advice?
 

Dave Parker

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The longer the extension tube the larger the image on the film plane and the longer the exposure, the longer the focal length the larger the image, so combine a longer extention tube and a longer focal length the larger the image on the film plane, you will have to take into account exposure factors when metering as the 501 does not have built in TTL metering, unless you have a metered finder, also take into account the closer the macro work the shorter the depth of field, which means you will have to stop down more to attain better depth of field, which again results in longer exposure times, which in turn can induce resiprocity failure in your film.

Dave
 

rbarker

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My suggestion would be to think in terms of 1) focal length vs. amount of extension, 2) potential problems due to proximity of the lens to the subject, and 3) finally any exposure compensation that might be needed.

At infinity, assuming a conventional lens design, the focusing mechanism of the camera will place the typical lens at approximately its focal length away from the camera. For 1:1 reproduction, that distance doubles. So, for example, with the 80mm lens, you'd need a total extension of 160mm (the combined distande of the extension tube and the focusing mechanism within the lens). The lens to subject distance is similarly 2x the focal length for 1:1 magnification.

Thus, the shorter extension tubes (e.g. 8mm, 16mm, etc.) are really intended to allow somewhat closer focusing of standard lenses for non-macro shots. For these purposes, the Proxar close-up lenses are another alternative. I use a 24mm extension tube, for example, with the 180mm lens in order to get tight headshots that would otherwise fall inside the lens's closest focusing range.

For "true" macro work, you may find the Hassy bellows will provide far more flexibility. But, extension tubes are much less expensive.

Within your set of lenses, the 80mm is probably the most suited, even though it is not optimized for close work. With the 50mm lens, you'll be right on top of the subject, making it more difficult to light the subject well. The 180mm would provide far greater distance, but require too much extension to be practical for work approaching 1:1.

For an in-depth discussion of the compensation needed because of the additional extension, take a look at http://www.largeformatphotography.info/bellows-factor.html

There's also a PDF on the Hasselbald site at Dead Link Removed that includes helpful information on the various extension tubes, along with exposure compensation figures.
 

BradS

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I need a chalk board....

Basic idea: extension tubes increase the image distance (obvious). The implication is that, for a given focal length, one is able to decrease the object distance (i.e. get closer to the object). This relationship is dictated by the equation:

(1/focal lenth) = (1/image distance) + (1/object distance)

so, for any given lens (i.e. focal length), increasing the distance between the lens and the film (the image distance) requires the the camera to be closer to the subject (object distance decreases) and of course, the closer you are, the bigger the image on the film. Simple...right?

The tricky part is adjusting the exposure for the increased image distance. The relationship that I like to use for this calculation is:

exposure factor = (image distance / focal length) ^2

This will give you a number larger than one that you can apply just like a filter factor (e.g. multiply the shutter speed by this number).

The trouble with SLR lenses is that, it is difficult to accurately measure or estimate the image distance. Technically, the image distance is defined as the distance between the nodal plane of the lens and the film plane. The difficulty arises in locating the nodal plane...luckily, we don't really need to.

We can use the fact that the image distance is equal to the focal length when the lens is focussed at infinity.

Mount the lens on the camera with an extension tube. Set the lens at infinity (notice, I didn't say focus at infinity - cause you probably can't do that with the extension tube in there...I mean set the focussing ring to infinity and leave it there. focus by moving the camera). Now, the image distance is one focal length plus the length of the extension tube.

Let's look at an example, take the 80mm lens and the 16mm extension tube...set focus at infinity. The image distance is now 96mm = 80mm + 16mm so your extension factor is:

factor = (96/80) ^2 = (96 * 96) / (80*80) = 1.44

if your meter reading suggests f/16 at 1/125 second you need to use something like 1/90 second instead because...

1.44 * (1/125) = (1/86)

Alternately, you can open up one half stop. This too can be calculated accurately as follows....

stops = log(factor) / log(2)

Continuing with the example,

stops = log(1.44) / log(2) = 0.53 ==> 1/2 stop


Does it make sense? Any questions?
 
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KenS

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Passage said:
Thank you all for the education

One way to relieve some of the stress of carrying a chalk board around with you then finding you have left the chalk in your other jacket, might be to invest in a Palm Pilot (or similar), install the freeware "Photographer's Vade Mecum".

I have made good use of this function for close-up and photomacrography with 35mm, Medium and & Large Format for quite a number of years.

Ken
 

BarrieB

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Passage said:
I have a Hasselblad 501C/M and would like to do some close-up photos of ....well, whatever looks good close-up. I (think) that I understand the concept of extension tubes, however I don't understand the overall technical use of the different sizes; 8mm, 16mm, etc., and what to expect when used with different focal length lenses, such as additional exposure. I have a 50mm, 80mm, and 180mm lenses. Can anyone give some advice?
Hasselblad publish a fold-up chart that explains it all: ie the image size or the object distances near and far for each combo. really worth having one.
Barrie B.
 
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