extension tubes

Hi all,

I have recently acquired a set of extension tubes as I would like to do more macro photography as I don't have the time to do wildlife and landscape photography any more (Try taking a good photograph whilst trying to stop your 2 year old with a magnetic attraction to all things water related from jumping / falling into a lake? But at least the baby can't escape from the buggy... yet )

Any how, back on topic.
What is the effect of extension tubes?
This is what I have noticed so far, for the same distance from lens to subject with and without extension tubes:
Depth of field apears to remain unchanged
The subject is larger in the viewfinder
Minimum focal distance appears to be the same
Loss of 1-2 stops of exposure

What sort of focal length would you normally recommend, about 100mm? I will probably experiment, but I guess a wide angle would give vingetting and a long focal length would still give me the same problem of lack of depth of field and having to be 2 meters from the subject.

Thanks, Gazen

Extension tubes have the advantage that they do not interfere with the optical construction of the lens, so do not reduce quality in themselves.

You are increasing the distance of the lens elements from the film plane, which is what you do when you focus any conventional lens closer (forget internal focusing for now, to avoid complication) so using the tubes will enable you to focus much closer.

The subject is larger because you are closer and have increased magnification. This means there will be very little depth of field and you will need to stop down as much as possible. For best quality in large prints try to avoid apertures smaller than f16 because quality drops of at the smallest stops.

As you have observed, you need more exposure the longer the tubes you use, because of light fall off as the lens moves further away. This happens in any lens as you rack it outwards to focus closer, but only becomes significant at magnifications greater than around 1:10. This is why a, say, 100mm lens will usually focus to around 1m. Only Macro lenses routinely focus closer.

Good luck with the macro photography!

Just to add to John's excellent reply...

If you use a short focal length lens, you have to get physically closer (you will not get vignetting). If the focal length is too wide and/or the extension tubes too long you can find the focus point is actually inside the lens - and you won't be able to focus at all!

If you use a long focal length lens, you are physically further away but you will get less magnification. This is useful for "normal" subjects to make a telephoto focus closer. (So you won't be limited by a 2m distance! In fact, with all tubes loaded up you still need to go pretty close!)

So it depends what you want... if you want super small objects, use a 50mm lens (or less) with all the tubes and get really close; if you are isolating a subject you can use a longer focal length.

I have also tried adding a 1.7x AF adapter too (Body:1.7x:extensions:100mm lens) and have got some interesting results!

Go ahead and experiment!!

Matt

Once you get hooked, you'll probably want to get a dedicated macro lens.
The good news is that just about all prime macro lenses (even Sigma!) are very good. The Pentax ones are all superb, and the Tamron 2.5 90 is very highly regarded.

Ignore macro zooms. Sharpness is paramount.

Longer macro lenses are generally better for bugs, which is why second-hand ones sell for more than the otherwise excellent 50mm lenses.

Auto-focus and aperture-automation tend to be pretty unimportant, so something like the Pentax-M 100mm f4.0 is a pretty good, and pretty economical, choice.

And, of course, you can still use the extension rings if you want to get REALLY close.

George

Thank you for your comments.

I will keep an eye out for those lenses at my favourite second hand shops (and on ebay)

I like Matt's attitute to experimenting. I will definately do that when I save up enough for a Pentax DSLR until then, more frugal use of Fuji slide film is going to have to do.

Gazen

http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=3872332150&ssPageNa...

LCE Group have a Pentax 100mm f2.8 macro autofocus secondhand at the moment:

http://www.lcegroup.co.uk/item_details.php?second_hand_equipment_id=3595
(At their Exeter store)

Matt