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Extension tubes theory

simonpg

simonpg

Active Member
I've never much used extension tubes to enable macro or closer focusing. But, I wonder if some one can guide me to a general understanding of the rules of thumb with regards closer focus distance.

I think I understand the degree of magnification general guide eg to make a 1:1 image with an 80mm lens add a couple of tubes to achieve 80mm extension as a general guide.

What I am wondering about is the relationship between getting a shorter minimum focus distance. For ex&le if I add a 32mm extension tube to a 180mm lens, what will the closest focus distance reduce to.

I hope this makes sense, and my apologies if it seems a stupid question. Thanks for the help.
 
Simon, Somewhere in one of the Hasselblad websites,(their own}}) is more information than you can think of regarding their lenses and the use of 1,2 or 3 extension tubes. The chart also covers the use of the bellows with various ext.tube configurations. If I find it before you do, I'll let you know.
Colin
ps. There are NO stupid questions
 
Many thanks Colin. I'll have a search. Should have thought of that myself.

But, specific experiences / dos / donts from personal experience are welcome.

I'm planning to get a 120mm Makro one day so experience in the meantime will help - no so much for full on macro work just closer shooting for detail. I can imagine that with 6x6 negs the results from Zeiss optics would be sensational. Also sometimes I'd like to get my 50mm and 80mm lenses in closer.
 
Hi Simon, there's a useful pdf file called "Close-up photograhpy, V system" on the Hasselblad global site (www.hasselblad.se) which you can download by going to "Docs & document" & scrolling down the page. The file covers lens & tube combinations & DOF, amongst other things. Cheers, David
 
Simon,

The H'blad PDF doesn't have minimum focus distances, but from memory the macro 'calculator' does. The 'calculator' is the one in the blue plastic lining...I was given one by Kennedy's years ago. It is sort of useful if you can pick one up.

However, min. focus probably isn't the most important thing with an SLR (as opposed to a rangefinder) as you can focus through it directly. It is often better thinking in magnification (or 'field width' as H'blad does). The PDF has this data.

'Field width' is easily converted to magnification given the width of the neg is 56mm. Viewing/photographing a ruler in the frame will help visualise it too!

The other big issue is exposure compensation.
If you use a TTL meter, there is no problem beyond balancing background and subject etc. If not, then compensation for the extension is needed.

There are formulae for working out exposure compensation, too, if you want to pursue the first principles line. The PDF has the data though.

Remember that racking the lens out adds extension in the same way as a tube, so it must be added to the tube (hence the min. and max. data on the PDF). A lens at infinity has no extension.

The 180 will have v. narrow dof with extension (might be good!), but I hope you have a GMS mirror! I am a bit of the belief that unless one REALLY wants to get into 'macro' then tubes plus an 80 or 150 is pretty good.

The benefits must be marginal (especially if the edge of the frame is out of focus as it often is) and the usual provisos apply, ie the larger the enlargement the more the issue. Of course, many like their 120 for normal photography too, so it is dual purpose.

On experience, the results are great and the square looks even more 'different' than for normal photography, but it is clumsy with the bigger camera over a 35mm.

Lastly, the 50 mm is not recommended for macro as a retrofocus design, although there is nothing stopping you using it and you will get more mag. for the same extension. Short working distance so be careful of the filter/front element!
 
Many thanks Gilbert and David. I've got the 'Blad doc and will read it tonight and see if I can make good sense of it. Nick thanks for the tips.
 
Any basic book of the theory of photography describes how to calculate DOF, hyperfocal distance, magnification, light loss, pupillary magnification, etc. by your self. It is pretty easy and adds your understanding of photography. Try!

Kerkko K.
 
>Since someone has said that there is no such thing as a stupid question,  here is my question... Is it better to use an extension tube to shoot a subject which is closer than infinity, but focused at infinity on a lens that is optimized for infinity ? Kommini
 
> There is no difference whether you reach the required extension with an extension tube or with the focussing helicoid of the lens. The only exception are lenses with floating lens elements. These should be used at their close distance setting for optimized close-focus performance. If I remember it right, there are only 50 and 40 mm-lenses with this feature available.
 
Thanks Ulrik. Yes the 40 and 50 mm lenses are the only 2 with FLE. I use a 50mm version.

What I was wondering about was the mathematical effect of adding various extension tubes to lenses of specific focal lengths. For ex&le does adding a 16mm tune to a 180mm lens reduce the closest focus distance by a specific factor?

The Close-Up PDF on Hasselblad's web site does not refer to that but just image sizes and exposure compensation (unless I don't understand how to read the data they give!).
 
Simon,

The old:

1 / focal length = 1 / object distance + 1 / image distance

and:

(image distance - focal length) / focal length = magnification

or:

added extension / focal length = magnification

should get you a long way.

The "special case" of unit magnification, where image distance and object distance are equal, and both 2x the focal length provides a point of departure for calculations.

Obviously the first equation only works in this simple form for thin lenses.

And you will have to remember that the distance printed on the lens' focussing scale, the distance also specified everywhere as "focussing distance", is the sum of object distance and image distance, plus possibly (not specified) the distance between principal planes.

As was mentioned, calculating fields of view and/or magnification is more practical.
 
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