focus problems with ext. tubes

prez

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Any advice on focusing with a Panasonic GX85 using an Olympus 30mm macro lens with Fotiox ext. tubes.
Manual focus is almost impossible. Rotating the focus ring on the lens does almost nothing. It turns and turns but will not focus. The only way I can get a decent focus is by moving the camera in and out.
The manual focus with just the body and lens works quite well.
 

Bushboy

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You have to vary the camera to subject distance. Turning the focus ring will do practically nothing, as you found out. Just the way it is...
 

Bushboy

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Tubes are seldom necessary with that lens... what are you photoing???
 

piggsy

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Tubes are very likely pushing the lens to a point where it can no longer focus on anything at all. It already does 1.25x at ~4mm from the front of the lens! Possibly the focal point is inside the front element at the infinity position if you put a tube on it.
 

prez

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Thanks for your responses. I was hoping to get a little more distance from the subject (close up flower details). Wrong solution!
 

John King

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For increased distance to subject: Put a longer lens on your tubes. 50 - 60 mm. But you already thought about that.
Agree, Erik. e.g. FTs 50-200 with EX-25 (25mm extension tube), a very small flower from about 3-5m away. Correa Alba.

Subscribe to see EXIF info for this image (if available)
 

John King

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Thanks, Erik.

It was FTs 50-200 MkI wide open at 200mm. Losing 2 stops of light, so equivalent to a magnification of around an 800mm lens. Geez, the IBIS in my E-510 did a great job at 1/60th! However, also 12 years ago, and I was a LOT fitter, healthier, stronger and steadier than I am now (nearly 73 y.o. now).

Why don't you subscribe? Cheap as chips, and you get to see the EXIF data ...
 

Erich_H

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Thanks, Erik.

It was FTs 50-200 MkI wide open at 200mm. Losing 2 stops of light, so equivalent to a magnification of around an 800mm lens. Geez, the IBIS in my E-510 did a great job at 1/60th! However, also 12 years ago, and I was a LOT fitter, healthier, stronger and steadier than I am now (nearly 73 y.o. now).

Why don't you subscribe? Cheap as chips, and you get to see the EXIF data ...
You must have been using a tripod?

And, I'm thinking about it... 🤔

Just have to make a clean up (for money) of some superfluous equipment. I'm lazy, though.
Have to put up an auction listing and take pictures of boring, broke stuff...

Also, obviously, I'll have to get PayPal, if I didn't misread that page. And I hate new stuff I don't like. Giving out my numbers & sh*t to PayPal... Why no normal payment with MasterCard... Not your problem, though, my idiosyncrasies 🤨
 

archaeopteryx

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FTs 50-200 with EX-25 (25mm extension tube), a very small flower from about 3-5m away. [...] Losing 2 stops of light, so equivalent to a magnification of around an 800mm lens.
Pretty sure it wasn't. ;) Let's walk through some basic maths.
  • Correa alba flowers are about 12mm in diameter.
  • The 21.6mm image diagonal is five flowers across, so the magnification is about 21.6/(5*12) = 0.36x. This is a plausible estimate, as we'll see momentarily.
  • Without tubes, the Zuiko 50-200 f/2.8-3.5 has a maximum magnification of 0.21x and a minimum focus distance of 1.2m. It's therefore a 172mm lens at its minimum focus distance due to focus breathing by minor algebra involving the thin lens equation and definition of magnification.
  • Some further manipulation of the thin lens equation shows a 25mm tube increases magnification to 0.36x for a 172mm lens focused at 1.2m.
  • Essentially the same maths show the corresponding focus distance is 0.89m.
  • Wide open at 200mm is f/3.5. For magnification increases by extension the effective aperture is N(1 + M) = f/3.5 (1 + 0.36) = f/4.8. This is clearly a bit over a third of a stop, not two stops.
  • It follows from the thin lens equation, again with minor algebra, that the minimum focus distance of a lens is four times its focal length. Thus, it is not possible to focus an 800mm to 0.89m and no 800mm equivalent magnification exists (at least if you want a real image).
Use of the thin lens approximation introduces some error in the above. There are also some implicit assumptions about the pupil magnification ratio. If you still have the lens and tube, set them up, and are able to measure the error without doing anything particularly fancy I'll be impressed as I've never succeeded in measuring divergence from thin lens approximations below 1x.

I think the main uncertainty is the actual diameter of the flower, though. Assuming the lens and extension tube are indicated correctly, it doesn't appear the flower can be much smaller than 12mm. But it might be somewhat larger. In which case the magnification will be correspondingly lower and the focus distance longer. Applying the thin lens equation with a 172mm lens focused at infinity finds the maximum focus distance is 1.55m with a magnification of 0.15x. It's rather unlikely the flower is more than twice the usual size but, even if it was, the focus distance would remain well below 3-5m.
 

piggsy

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Super confusing having telephoto lens lengths related to "magnification factor" which is a different thing entirely to "magnification ratio" we want for macro. For macro purposes there's nothing saying an 800mm lens magnifies anything better than a 50mm.
 

John King

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You must have been using a tripod?

And, I'm thinking about it... 🤔
Nope. Handheld :flowers_2: .
Just have to make a clean up (for money) of some superfluous equipment. I'm lazy, though.
Have to put up an auction listing and take pictures of boring, broke stuff...

Also, obviously, I'll have to get PayPal, if I didn't misread that page. And I hate new stuff I don't like. Giving out my numbers & sh*t to PayPal... Why no normal payment with MasterCard...
Erik, NEVER use a credit card on the Internet ... I cannot stress this enough!

Not your problem, though, my idiosyncrasies 🤨
I share those idiosyncrasies - retired CPA in my own practice; retired computer consultant in my own business; Neighbourhood Watch zone leader for about 25 years - Why would I be security minded?

Here's how we are organised, wife and I each have:

1) Internet account with attached debit Mastercard (or Visa). Transfer money into these as needed. Paypal and other Internet transactions go through these accounts. Minimal amounts balance.

2) Income account; no card; and

3) Savings account with no card access.

4) Term deposit account for our cash savings, no card access, branch access only.

5) Credit cards are not attached to any of the above accounts. Mine is with a different bank, where I have no money on deposit. We NEVER use our credit cards on the Internet - NOT EVER!

6) We set relatively low limits on daily transactions (around $2,000), and increase this when we need to for any single transaction, then immediately change back to basics. We can afford to lose a couple of grand. OTOH losing $50,000 would really hurt ...

NONE of the accounts with significant amounts in them can be accessed via any kind of card or Internet transaction, without logging in from our nominated devices.

We both have full access to all our accounts. The exception being our largish term deposit, which requires both of us to be physically present at the bank/branch, or present as executor.

Hope this helps you (and others) organise your finances in a safe and secure manner.
 

archaeopteryx

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Super confusing having telephoto lens lengths related to "magnification factor" which is a different thing entirely to "magnification ratio" we want for macro.
Why I usually use angles of view at low magnifications. Not sure why that isn't more common. Even in the somewhat unlikely case of not already having something which can do the arctangent there's loads of apps and online calculators.

For macro purposes there's nothing saying an 800mm lens magnifies anything better than a 50mm.
I'm trying to think of when an 800mm macro would be worth the cost, size, weight, and f/11+ effective aperture at 1x. Seems specialized to small and especially nasty things. For example, if you wanted to take macro pictures of molten steel I'd really hope the steel was controlled enough it'd be OK to have a lens closer than a couple meters away.
 

John King

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Pretty sure it wasn't. ;) Let's walk through some basic maths.
  • Correa alba flowers are about 12mm in diameter.
  • The 21.6mm image diagonal is five flowers across, so the magnification is about 21.6/(5*12) = 0.36x. This is a plausible estimate, as we'll see momentarily.
  • Without tubes, the Zuiko 50-200 f/2.8-3.5 has a maximum magnification of 0.21x and a minimum focus distance of 1.2m. It's therefore a 172mm lens at its minimum focus distance due to focus breathing by minor algebra involving the thin lens equation and definition of magnification.
  • Some further manipulation of the thin lens equation shows a 25mm tube increases magnification to 0.36x for a 172mm lens focused at 1.2m.
  • Essentially the same maths show the corresponding focus distance is 0.89m.
  • Wide open at 200mm is f/3.5. For magnification increases by extension the effective aperture is N(1 + M) = f/3.5 (1 + 0.36) = f/4.8. This is clearly a bit over a third of a stop, not two stops.
  • It follows from the thin lens equation, again with minor algebra, that the minimum focus distance of a lens is four times its focal length. Thus, it is not possible to focus an 800mm to 0.89m and no 800mm equivalent magnification exists (at least if you want a real image).
Use of the thin lens approximation introduces some error in the above. There are also some implicit assumptions about the pupil magnification ratio. If you still have the lens and tube, set them up, and are able to measure the error without doing anything particularly fancy I'll be impressed as I've never succeeded in measuring divergence from thin lens approximations below 1x.
Thanks for all your insight and calculations. Not sure that they are either correct, or particularly relevant.

I still have the lens, camera and EX-25.

With the EX-25, the exposure on an easily centred object at 200mm is 1/50th at f/3.5 (reported by the camera).

Without the EX-25, the exposure is 1/125th at f/3.5 (reported by the camera).

Therefore a bit over a stop difference caused by the EX-25 (loss of effective light). This accords with what Olympus states (and other sources), IIRC now.

What I was saying is that the IBIS did a fantastic job, given the slow shutter speed and the 1/(FL * 2) reciprocal rule. With no EX-25, this means that the minimum SS for a normally steady shot would be 1/(200 * 2) = 1/400th second. 1/60th is a lot slower than the reciprocal rule would dictate. I was just under 61 y.o. when that photo was taken, so hardly my youthful steadiness ...

BTW, I was remembering the minimum focus distance wrongly, as the Correa photo was taken about 12 years ago. The minimum/maximum focus distance of the 50-200 at 200 mm is roughly 0.5 to 1.2m from the front element (add another ~292mm to the FP).

I think the main uncertainty is the actual diameter of the flower, though. Assuming the lens and extension tube are indicated correctly, it doesn't appear the flower can be much smaller than 12mm. But it might be somewhat larger. In which case the magnification will be correspondingly lower and the focus distance longer. Applying the thin lens equation with a 172mm lens focused at infinity finds the maximum focus distance is 1.55m with a magnification of 0.15x. It's rather unlikely the flower is more than twice the usual size but, even if it was, the focus distance would remain well below 3-5m.
I don't think that we are at DPR, unless I'm dreaming ... ;)
 

archaeopteryx

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Therefore a bit over a stop difference caused by the EX-25 (loss of effective light). This accords with what Olympus states (and other sources), IIRC now.
That's surprising, particularly as I can find no such statement from Olympus. Citations and a specific description of the measurement procedure would be of interest.

I do see mentions adding the EX-25 to take the Zuiko 50mm 1:2 "macro" to 1:1 is a stop of loss. That's within rounding error for N(1 + M = 0.5) = 1.5N versus N(1 + M = 1) = 2N in a simplified statement. However, changing the focal length of the lens alters the difference in magnification created by the tube.

The minimum/maximum focus distance of the 50-200 at 200 mm is roughly 0.5 to 1.2m from the front element (add another ~292mm to the FP).
You'd need to measure the working distance with at least centimeter accuracy (and include the 25mm from the EX-25 if it's not part of the 292mm).
 

John King

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That's surprising, particularly as I can find no such statement from Olympus. Citations and a specific description of the measurement procedure would be of interest.

I do see mentions adding the EX-25 to take the Zuiko 50mm 1:2 "macro" to 1:1 is a stop of loss. That's within rounding error for N(1 + M = 0.5) = 1.5N versus N(1 + M = 1) = 2N in a simplified statement. However, changing the focal length of the lens alters the difference in magnification created by the tube.
The camera light meter is telling us everything that we need to know, theory notwithstanding ... The EX-25 loses around a stop at 200mm and wide open. No amount of fiddling around the edges will change that figure dramatically.

This is also in line with my experience of other extension tubes used over more than 50 years. The longer the tubes, the greater the magnification, and the greater the light loss.

You'd need to measure the working distance with at least centimeter accuracy (and include the 25mm from the EX-25 if it's not part of the 292mm).
Sorry, but no.

I have osteoarthritis in (but by no means limited to) my lumbar spine. I live on painkillers as I cannot take anti-inflammatories due to lifetime Warfarin use. It was difficult enough just doing what I've already done, and I will now need to rest for the rest of the day. Getting old ain't for sissies ...

I am far more interested in practical photography and the heuristics involved than I am in arguing the toss about technical aspects that have little or no relevance to me.
 
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