Playing with a Meike MK-C-UP multifunctional extension tube

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LordV
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Joined: Thu Nov 22, 2007 10:28 am
Location: UK

Playing with a Meike MK-C-UP multifunctional extension tube

Post by LordV »

First few goes with this adapter which can either be used as an odd electronic extension tube or as I was using a lens reversal adapter that maintains full electronic control of the lens. Tried it with the kit lens. One oddity I noticed during shooting with ETTL flash was the shots were around 1 stop overexposed at my normal flash settings. I think this is because the reversed lens has an aperture around one stop wider than the set aperture. This means needing to shoot with the aperture on the lens about 1 stop smaller than you would normally at any mag and also setting the FEC on the flash about 1 stop -ve to your normal setting.
Mag range with the kit lens was just under 1:1 at 55mm and about 5:1 at 18mm. I hadn't expected to get focus at that focal length but the focus distance was 25mm in front of the adapter ring.

Brian V.

The setup
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Magnification - min =0.86:1
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Magnification max =5:1.
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Single shots at min and max mag
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Image
www.flickr.com/photos/lordv
canon20D,350D,40D,5Dmk2, sigma 105mm EX, Tamron 90mm, canon MPE-65

rjlittlefield
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Post by rjlittlefield »

Brian, thanks for bring this adapter to our attention. This is very interesting because it's less than 1/3 the price of the Novoflex EOS Reversing Lens Mount that was previously the only off-the-shelf unit that would do this function.

As for the optical performance, what you're getting is consistent with what I described at "Shooting with a reversed 18-55 mm Canon kit lens".
I hadn't expected to get focus at that focal length but the focus distance was 25mm in front of the adapter ring.
To make sense of this, remember that the lens is designed so that it will focus at infinity when mounted normally on the camera, with 44 mm between the lens mount and the sensor. Turn it around, and that becomes at least 44 mm between the len mount and the subject. Subtract 19 mm for thickness of the adapter ring, and there's your 25 mm.

The 1-stop difference in exposure is due to that "pupil ratio" thing. It may be different at different magnifications -- would have to measure to be sure.

--Rik

LordV
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Post by LordV »

Thanks Rik for the comments and info- yes not really new stuff but it was an awful lot cheaper than the novoflex and worth a play. It also does seem pretty well made and comes with 58, 67,72 and 77mm adapter rings

Brian V.
www.flickr.com/photos/lordv
canon20D,350D,40D,5Dmk2, sigma 105mm EX, Tamron 90mm, canon MPE-65

TheLostVertex
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Post by TheLostVertex »

LordV wrote:One oddity I noticed during shooting with ETTL flash was the shots were around 1 stop overexposed at my normal flash settings. I think this is because the reversed lens has an aperture around one stop wider than the set aperture. This means needing to shoot with the aperture on the lens about 1 stop smaller than you would normally at any mag and also setting the FEC on the flash about 1 stop -ve to your normal setting.
rjlittlefield wrote: The 1-stop difference in exposure is due to that "pupil ratio" thing. It may be different at different magnifications -- would have to measure to be sure.
I recall when I shot with a reversed 18-55mm lens and a flash on ETTL I would also experience this problem. The lens was not mounted with electronic connections.

The camera meters a pre flash before the exposure and uses this to determine the exposure. So why does this effect happens?

I do not know the details of how canon's ETTL works, but I assume the flash pulse is several levels lower than the exposing flash. So the camera must scale its pre flash reading by a set amount to determine the final flash value. If this is the case, then maybe a large change in pupil ratio from what the camera expects causes the scaling to be different than what it should be.

Does any one know if this is what is actually happening, or is it something else that is going on?

rjlittlefield
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Post by rjlittlefield »

TheLostVertex wrote:I recall when I shot with a reversed 18-55mm lens and a flash on ETTL I would also experience this problem. The lens was not mounted with electronic connections.
I don't think we can't blame pupil ratio in that case. Whatever the lens does to the real flash it should do to the metering flash also, because nothing in the optics changes. With no other information, I'm inclined to suspect that it's the absence of electronic connections that causes the camera to treat the preflash differently. If you were using a chipped adapter, then there's also the possibility that the camera thought it could physically stop down the lens when actually it couldn't. I recall that with one of my adapters I have to be careful to set the camera's f-number to match the adapter's f-number, or ETTL exposures get weird.

LordV's case is a little different because the optics actually do change between metering and shooting. At least with my 18-55, when the lens is wide open the iris is not the limiting aperture. Stopping down the lens makes the iris become the limiting aperture, and as a side effect the pupil location moves longitudinally within the lens. My best guess is that it's this effect that alters the metering, rather than a static pupil ratio that happens to be different from 1.

When I answered the first time, I had overlooked that LordV's exposure difference was with ETTL. Thanks for making me think harder about this problem.

--Rik

Pau
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Post by Pau »

I've just tested the ETTL flash camera behavior with my 7D and I think I could have found the possible reason: The measuring preflash is done with the diaphragm wide open* and the diaphragm just closes for the actual exposure, so the camera needs to compute the lens aperture. If the actual effective aperture is different than expected because the lens reversed position the calculation is not accurate. Do you need to make corrections without flash?

If I'm right, when shooting wide open the measurement must be right, could you test it?

*you can see the preflash through the optical viewfinder
Pau

rjlittlefield
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Post by rjlittlefield »

rjlittlefield wrote:This is very interesting because it's less than 1/3 the price of the Novoflex EOS Reversing Lens Mount that was previously the only off-the-shelf unit that would do this function.
Oddly (but pleasantly!), the new device is also being sold HERE by Amazon.com under the "Neewer" brand, for around $50 from multiple vendors. The pictures clearly show Meike MK-C-UP. It would look like an advertising error, except that multiple reviewers say they actually received Meike, not anything labeled Neewer.

Guess I'll find out next week, when the package arrives.

--Rik

LordV
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Post by LordV »

Pau wrote:I've just tested the ETTL flash camera behavior with my 7D and I think I could have found the possible reason: The measuring preflash is done with the diaphragm wide open* and the diaphragm just closes for the actual exposure, so the camera needs to compute the lens aperture. If the actual effective aperture is different than expected because the lens reversed position the calculation is not accurate. Do you need to make corrections without flash?

If I'm right, when shooting wide open the measurement must be right, could you test it?

*you can see the preflash through the optical viewfinder
Pau - I may be way off base here, but I started asking about this in another thread here http://photomacrography.net/forum/viewtopic.php?t=26144 because someone else reported that the DOF of the reversed kit lens seemed shallower than the MPE-65 at the same magnification and set aperture. So I assume it may well affect non flash shots exposure- I will have to try it.

[EDIT]
A quick test with natural light did not really show any exposure difference between a normal 90mm 1:1 shot and the reversed kit lens 1:1 shot. Shots taken in Av mode of mid scale grey printed card.

Brian V.
www.flickr.com/photos/lordv
canon20D,350D,40D,5Dmk2, sigma 105mm EX, Tamron 90mm, canon MPE-65

rjlittlefield
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Post by rjlittlefield »

My copy of the adapter came today. In quick test it works fine; story at http://www.photomacrography.net/forum/v ... hp?t=26404.

Thanks again for bringing this adapter to our attention!

--Rik

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