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11-18-2013, 06:33 AM   #1
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Metering with extension tubes - limited exposure?

I have been taking macro shots of watches that I have restored. Generally I am quite pleased with the result, using a light tent and extension tubes with an old 50mm prim lens on my K-X.
I have sanded down the contacts on the tubes to enable metering.
My problem is that I cannot get exposure times beyond 0.8 second.
I switch to manual focus and manual exposure. When I turn on the camera I am asked what focal length lens I am using and I indicate 50mm.
I set the f stop on the lens.
I half press down the shutter to activate the info in the viewfinder. I can then press the green button to stop down the lens and get an exposure value. When I press the shutter I get a good exposure.
This all works fine until I reduce the lens aperture so that the exposure would be longer than 0.8 second. When I stop down with the green button the exposure never gets longer than 0.8 second and the images get progressively under-exposed as I decrease the lens aperture.
I can live with this but there are times when I would want to increase the depth of field using a smaller lens aperture.
Any thoughts on why I cannot get a longer exposure?
Thanks
Chris

11-18-2013, 06:42 AM   #2
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Light is leaking in through your viewfinder. put some tape over it or cover it with a cap and you should get better results.
11-18-2013, 06:55 AM   #3
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QuoteOriginally posted by mattt Quote
Light is leaking in through your viewfinder. put some tape over it or cover it with a cap and you should get better results.
Good idea. I will try it at the week-end (when there is some light again!)

Thanks

Chris
11-18-2013, 08:18 AM   #4
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QuoteOriginally posted by mattt Quote
Light is leaking in through your viewfinder. put some tape over it or cover it with a cap and you should get better results.
QuoteOriginally posted by chris141058 Quote
Good idea. I will try it at the week-end (when there is some light again!)

Thanks

Chris
Well, it may help - but I am afraid that the problem sticks deeper than that: My experiences with several different camera bodies are that the built-in light meters aren't really geared for such low-light metering. You would probably be better off by measuring the light wide open and then compute the increase in exposure time required when you stop down your lens.

11-18-2013, 09:45 AM   #5
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QuoteOriginally posted by Stone G. Quote
Well, it may help - but I am afraid that the problem sticks deeper than that: My experiences with several different camera bodies are that the built-in light meters aren't really geared for such low-light metering. You would probably be better off by measuring the light wide open and then compute the increase in exposure time required when you stop down your lens.
My experience too
11-18-2013, 01:04 PM   #6
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QuoteOriginally posted by Stone G. Quote
the built-in light meters aren't really geared for such low-light metering.
What he said. The lower range of sensitivity for your meter is 1 EV(100). This corresponds to the amount of light on the subject that would require an exposure time of 2s at f/2 for ISO 100. Below that light level, the sensor response is non-linear and the meter may indicate a much shorter exposure than is really required. With stop-down metering using non-A-contact lenses the threshold may be reached even sooner, particularly with extension tubes or bellows or even a dedicated macro lens. (The extension results in less light to the sensor, even wide open.)

As noted above, metering wide open and calculating for the working exposure is an option. M-mode is your friend here. A hand-held meter capable of incident light readings* is also an option. Do your light measurement with the meter at the location of the subject, sensor pointed towards the camera. Replace the meter with your subject and transfer the meter settings to your camera, focus, and shoot! Remember to compensate for the extension tube. Again, M-mode is the way to go. Good luck!


Steve


* reflected light reading off a gray card is a suitable alternative.
11-18-2013, 05:39 PM   #7
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(As already said)

the light level is inadequate (w/ the lens closed down). Typically the camera will give the same default value--I believe it is 0.4 seconds on my K20d.

Either work from a more open fstop and adjust exposure in manual, or take a shot and looking at image and adjust the exposure. The subject isn't going anywhere, and that's the beauty of digital--and you will learn about exposures if you think about what you are seeing.
11-23-2013, 06:58 PM   #8
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Use 5-shot burst mode adjusted so that the "brightest" exposure is the one the camera suggests, with an exposure interval of 1 or even 2 EV, mounted on a tripod. LV. Trigger exposure with a wireless remote. Archive the best exposure and the one that is the next darker one. Delete the other three.

11-23-2013, 07:35 PM   #9
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I pondered about what I just said and realised that it was maybe not detailed well. I tend to do this sorta stuff on the fly, muttering to myself as I make adjustments and decision. Often in that order. (" let's try +2. No that's wrong its -2. And so on)

OK here goes. 5 shot burst. Interval will be 1EV. So you need to use +/- button to set adjustment to minus two, double the interval. (-4 if interval is 2 EV) Fire the burst. Voila!

I have never shot product shots of watches or anything, but I would imagine that the workflow would be quicker for a short batch if you just replaced watches one by one without chimping the results, and checked all the shots after the complete process. Especially because the detailing and face colour of different watches might require different exposures. Also refocussing for watches of different thicknesses would be a pain in macro IMO, is it worth using some sort of adjustable base or shims to hold the watch face at the same distance? I ask this because I am considering doing some macro batch work soon.
11-28-2013, 08:40 AM   #10
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After reading this topic I decided to do a quick check with my K-5 with some extension tubes and dim light to see how it metered for long exposures. Not caring about the subject or results I set up a 4-tube (auto) combination, with a Pentax 50mm tube at the camera so the chrome mount would activate camera contacts so the lens would stop-down. (The Vivitar auto set has anodized back mounts. Find an Asahi set if you can.) This made quite an extension, and I used a 50 f2.8 SMC-A macro stopped to f22 to get the dimmest image for metering, and used low ambient to get long exposures. Here's the setup as I went mouse-hunting for a subject:
Name:  K5-Ext.jpg
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Using the viewfinder instead of live-view the K-5 would meter to expose 10-13 seconds using the green button. However, I've noticed that the green button method typically underexposes manual lenses the more they are stopped-down. Manually stopping using the lever under the shutter release gives longer exposures. With this setup, both methods underexposed at least 1-1/2 stops. Metering with the eyepiece blind in place increased exposure only about 30%, so it's not entirely due to light leakage through the finder. Better exposure was 30 seconds in this case:
Name:  Mouse 30.jpg
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As an old film user, I always have used the eyepiece (often with a 90-degree finder) for macro work. I also tried live-view this time, and it does give a nice view for setting up in these dim conditions. However, the camera would lock-up when I tried to meter in live view, requiring battery removal.
Of course, this flimsy tripod doesn't give good results. My Leitz table-top and large ball-head is ideal - but it was packed away.
I agree there are metering issues using manual lenses for macro, especially when stopped down significantly. Knowing this, you can compensate manual exposures and get good results.
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