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03-19-2010, 12:42 PM   #1
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Quick macro tube question

I just ordered myself a Pentax SMC M 1.7 50mm and a Viv 28-70mm RL Edition Macro, I would like to use some macro tubes to get things nice and close. I just wanted to know if I'll get a 1:1 (or better) by doing this.

I have no idea if the viv can already get 1:1 (I got it on the cheap from eBay, looked like a good score for 20 bucks)

Thanks

03-19-2010, 01:21 PM   #2
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No, your Viv won't go 1:1. Yes, you can get 1:1 by adding extension equal to the focal length of any lens. Put 50mm of tubes on a 50mm lens, 100mm on a 100mm lens, etc. My old manual Viv 90/2.8 macro achieves 1:1 by extending out another 90mm. It's a beast! Sure is crisp though.

NOTE: Putting tubes on a zoom isn't the best idea, because you'll get much better IQ with a prime.
03-19-2010, 04:56 PM   #3
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By coincidence, just last night I tested out my A50/1.4 with a $10 tubes(no contacts of course) and was favourably impressed. It was no harder to work with than my dedicated 1:1 100/3.5 lens since I use it entirely manual when shooting bugs anyway.

Hmmm...100mm of extension tube on a 50mm lens sounds dark. Wonder if there is a handy web page calculator to guestimate the lhow many stops of light are lost when adding 100% or more of the FL in tubes?
03-19-2010, 06:09 PM   #4
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QuoteOriginally posted by imtheguy Quote
Wonder if there is a handy web page calculator to guestimate the lhow many stops of light are lost when adding 100% or more of the FL in tubes?
No calculator, but there are tables in these manuals showing the exposure factors:

Auto Extension Tube Set K

Auto Bellows Slide Copier K

03-19-2010, 07:42 PM   #5
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QuoteOriginally posted by RioRico Quote
Yes, you can get 1:1 by adding extension equal to the focal length of any lens. Put 50mm of tubes on a 50mm lens, 100mm on a 100mm lens, etc.
QuoteOriginally posted by imtheguy Quote
Hmmm...100mm of extension tube on a 50mm lens sounds dark.
It would be dark. That's how you get to 2:1 tho. The magnification formula: MAG = EXT / FL

where MAG is magnification (M),
EXT is the tube extension in mm,
and FL is the lens focal length in mm.

QuoteQuote:
Wonder if there is a handy web page calculator to guestimate the lhow many stops of light are lost when adding 100% or more of the FL in tubes?
I don't know the calculator, but here's the calculation (in effective f-stops): EA = I * (M+1)

where EA is the effective aperture,
I is the aperture marked on the lens,
and M is the magnification.

So when you add 50mm tubes to a 50mm lens, M = 1.
And if the lens f-stop is 11, the effective aperture is f/22.

And when you add 100mm tubes to a 50mm lens, M = 2.
So if the lens f-stop is 11, the effective aperture is f/33.

And when you add 150mm tubes to a 50mm lens, M = 3.
So if the lens f-stop is 11, the effective aperture is f/44.
Et cetera.

And it works the other way too.
So when you add 25mm tubes to a 50mm lens, M = 0.5
And if the lens f-stop is 11, the effective aperture is f/16.

Of course the easy way would be to just meter the subject without tubes, then add tubes and meter again.
03-20-2010, 12:37 AM   #6
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SOldBear, you are the coolest. Not easy to read for neophyte like be but if I understand correctly it says that a A50/1.4 lens with 50mm tube requires about 5x the exposure as without the tube. Although its faster for me to just adjust as I go, if I had to set it up right I would simply meter without the tube then (for a static subject) cut the shutter down from, say 1/150s to 1/3s. Hopefully its not referring to 5x stop adjustment so I will read it cover to cover. Thanks for the reference.


QuoteOriginally posted by SOldBear Quote
No calculator, but there are tables in these manuals showing the exposure factors:

Auto Extension Tube Set K

Auto Bellows Slide Copier K
03-20-2010, 12:40 AM   #7
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QuoteOriginally posted by RioRico Quote
It would be dark. That's how you get to 2:1 tho. The magnification formula: MAG = EXT / FL

where MAG is magnification (M),
EXT is the tube extension in mm,
and FL is the lens focal length in mm.



I don't know the calculator, but here's the calculation (in effective f-stops): EA = I * (M+1)

where EA is the effective aperture,
I is the aperture marked on the lens,
and M is the magnification.

So when you add 50mm tubes to a 50mm lens, M = 1.
And if the lens f-stop is 11, the effective aperture is f/22.
By "effective" aperture does that mean only exposure or DOF also?
Yes, I could try and work this out in practice but its way more fun to get schooled by experience. Thanks.
03-21-2010, 05:44 AM   #8
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QuoteOriginally posted by RioRico Quote
I don't know the calculator, but here's the calculation (in effective f-stops): EA = I * (M+1)

where EA is the effective aperture,
I is the aperture marked on the lens,
and M is the magnification.
Thanks for posting this formula. That's a question I wanted to ask myself...

03-21-2010, 09:12 AM   #9
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QuoteOriginally posted by imtheguy Quote
By "effective" aperture does that mean only exposure or DOF also?.
Use the effective aperture to calculate exposure. The DOF is already so thin that it hardly matters. I mean, 50mm extension on a 50mm lens with focus at 50mm and lens aperture of f/11 gives a DOF around 49.6-50.4 -- that is, DOF is under 1mm thick. If going to 75mm or 100mm extension on that 50mm lens gives DOF around 49.8-50.2, that's under 0.5mm thick, are you really going to notice the difference?

This is MACRO photography. Factors on a MICRO-photographic scale (<1mm) are effectively irrelevant.

As for exposure: With a 50mm lens stopped to f/11, using 25mm extension for 1:1.5 magnification gives effective aperture of f/16, doubling the exposure time. Using 50mm extension for 1:1 MAG gives EA= f/22, so calculated exposure time is 4x. (Please note that 4x exposure is NOT 4 f-stops; it's 2 stops.) Whether that theoretical time is adequate, is for you to judge. As always, bracket bracket bracket.

NOTE: I got the DOF numbers from some online calculator, which may or may not be accurate (but at this scale it hardly matters). The magnification and exposure formulae come from a text of biblical scope published by WH Freeeman (Scientific American), FIELD PHOTOGRAPHY by AA Blaker. All Blaker says about DOF in macro photography is, there effectively ain't none -- he gives no formulae.
03-21-2010, 09:28 AM   #10
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QuoteOriginally posted by RioRico Quote
Use the effective aperture to calculate exposure. The DOF is already so thin that it hardly matters. I mean, 50mm extension on a 50mm lens with focus at 50mm and lens aperture of f/11 gives a DOF around 49.6-50.4 -- that is, DOF is under 1mm thick. If going to 75mm or 100mm extension on that 50mm lens gives DOF around 49.8-50.2, that's under 0.5mm thick, are you really going to notice the difference?
Yes, I will notice when trying to get all 8 eyes of a spider in focus its nice to know the relationships if not the calculation.
03-21-2010, 11:19 AM   #11
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QuoteOriginally posted by imtheguy Quote
Yes, I will notice when trying to get all 8 eyes of a spider in focus its nice to know the relationships if not the calculation.
[shrug] Various online DOF calculators exist. Whether one should trust their accuracy isn't my problem. The ones I've tried are woefully buggy (spiders not included).

I'd recommend running your own tests with your own gear, and compile tables based on the scale you're working at. If I was photographing the eyes of small spiders, I'd probably want a nice heavy optical bench to work at, with tethered dedicated camera, calibrated bellows, big monitor, etc. Well, not this year...
03-21-2010, 11:35 AM   #12
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Keep in mind that when adding extension tubes to macro lenses, the above formulas aren't going to work the same for them.
03-21-2010, 12:37 PM   #13
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QuoteOriginally posted by Blue Quote
Keep in mind that when adding extension tubes to macro lenses, the above formulas aren't going to work the same for them.
That's absolutely correct. You need to consider the TOTAL extension. So (for instance) if you have a 100mm macro lens, extended to the 1:1 position, that extension is itself 100mm. If you add another 50mm in tubes, that gives you a total extension of 150mm. Now your magnification is 1.5:1. Plugging that into the formula, EA = I * (M+1) -- if you have the lens stopped to f/22, the effective aperture is 22*(1.5+1) = f/55. Hey, we're almost in Ansel Adams territory!

NOTE: If the lens tells you the magnification but not the amount of extension, you can calculate that by: EXT = FL * MAG -- so that 100mm lens at 1.25:1 has a total extension of 125mm.

XTRA: How to get to Ansel's f/64 from a 100mm lens that's stopped to f/22? As EA = I * (M+1), then M = (EA/I) - 1 -- so (64/22) - 1 = 1.91 -- so you need a total extension of 190mm. Might as well round it off to 200mm. That gives f/66 -- hey Ansel, eat my dust!
03-21-2010, 01:03 PM   #14
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QuoteOriginally posted by imtheguy Quote
Hmmm...100mm of extension tube on a 50mm lens sounds dark. Wonder if there is a handy web page calculator to guestimate the lhow many stops of light are lost when adding 100% or more of the FL in tubes?
For those that are Android smart phone users there is an app called “Photo Tools” that has 17 photo calculators one of them is a bellows calculator (same as extension tube only not fixed). Many of the calculators in this tool I have not been able to find on line like a ND Filter calculator or flash calculator. Some like AoV , FoV or DoF you can find but having them on the phone when out in the field makes it worth will to have. The tool is not for everyone, as it is not going to hold your hand so you have to know what to put in to get out something that makes sense. It also costs a little money (I can’t remember exactly how much but I think less the $5) but you only pay one time (per phone). Like all tools of this kind it is only going to give you a starting place that is close and you will still need to make changes to your tastes but it will help you know how far you have to go to get close to where you want to be.

DAZ
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