Extension Tube

Thinking about getting one of these for use with my Tamron Di90 Macro Anyone got one, are they any good. What sort of magnification can I expect? I'm thinking of using the combination to take pictures of small wild flowers about 8 - 10mm across. How many stops do you lose as well. Thanks in advance.

http://srb-photographic.co.uk/pentax-k-auto-extension-tube-5913-p.asp

Extension tubes are not lenses. They have no glass in them.

Therefore they do not change the magnification you get from the lens you use. Consequently they don't change the exposure either.

The only thing they do is allow whatever lens you're using to focus closer than it can without the tubes.

Closer subject = bigger image.

That's all.

The thing about that one, apparently, is that AF lenses will still AF with it, as well as letting the auto-exposure information pass through as well.

Ha! Ha! .... I thought this was the comedy channel for a minute

Don't you even know the basics of photography? .......

Extension tubes and bellows increase the focal length and
lose light due to the aperture no longer being what is was
for the shorter focal length.

The old rule of thumb with a 50mm lens used to be a stop
an inch. So 2 inch of tubes to give 1:1 would lose 2 stops.

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With any lens 1:1 macro will always be 2X the focal length
sensor to lens and also 2X the focal length subject to lens.
It will always lose 2 stops. (Factor of 4).

1:2 would need 1.5X FL and lose just over a stop
(Factor of 2.25).

The PDF manuals for Pentax tubes should give the
info. for each separate tube.

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I obviously phrased this wrongly as I'm aware extension tubes are err, tubes ... I've had a set of bellows for many years but they don't offer auto focus etc like the tube linked. Appreciate if anyone that has one of these would comment as they seem to be pretty good. Think it said somewhere its made of brass so should be pretty solid.

If this is in the wrong place by the way, please move as required...

Yes, sorry. Of course the exposure changes - if you magnify the same subject area over more pixels on the sensor, the light coming off the subject is spread over more pixels, so the exposure per pixel is less.

Sorry for the confusion, posted too quickly there.

However, the tubes don't affect the focal length of the lens. How can they? You might just as well say that the lens has a different focal length in your hand than on the camera.

Magnification, of course, is another matter. Of course tubes increase the magnification.

But the additional magnification comes from the subject being closer, not because the lens is suddenly a different focal length.

I hope I haven't started an argument between you two ...

Thanks for replies so far!

No, I was wrong to say that the exposure doesn't change. Algi was right to point that out. Albeit in his own inimitable style

The other thing about that one is that it's too short to be useful on anything other than a macro lens. Also, AF is all but useless for macro photography. I would go instead for a full set of tubes with the aperture contacts, but no AF. There's a couple of Vivitar sets on ebay at the moment.

Regards
David

Yes, very much agree.

As I mentioned in the OP I only intend to use it on my Tamron 90mm Macro.
I thought it would still be relatively handy out in the field so to speak.
If I need any more extension I'd use the bellows unit I have.

Your Tamron will already focus to 1X (1:1) which means it will
cover a subject 23.7mm x 15.7mm. On the K-5 you can also
safely crop. A 25mm tube (if my rough calcs are right)
would give you 1.25X or a subject field of 19mm x 12.5mm
not a big gain. 12.5mm diam. flower instead of 15.7mm
flower

This article may be of some interest about how the effective
FL and aperture change with macro photography. IOW the aperture
you set on the lens isn't the aperture in use. Nikon actually
display the correct aperture. The article is a bit misleading
in parts such as discussing the format. 1:1 is 1:1 on APS-C
and FF in other words the format doesn't matter. You even
see people saying the 1:1 macro lenses are only 1:1 on FF


This is a crop of a K-5 + Tokina 90mm macro at f/2.5. I've only
cropped it because it won't reduce to 800 pixels wide
properly. The flowers are tiny.




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Thanks very much for that Algi. I think you're right, probably not much to be gained on the Tamron. I'll have a look at that article, thanks.

Vic.

Nice shot by the way!

The magnification gained by extension tubes is dependant on the focal length being used. Those SRB tubes could give some significant magnifications with the kit lens, though the optical quality of any zoom will struggle when magnified like this.