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07-02-2013, 08:13 PM   #16
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QuoteOriginally posted by rbefly Quote
I think Demp nailed it here. The darker photo is completely in focus, while the lighter one has the classic 'wide open' shallow DOF, only the front is in sharp focus. Also notice the glass jar. First photo, completely blurred out. 2nd photo, it's nearly in sharp focus.
Without knowing the data, I'd guess the top photo was shot at f/2.8 to (maybe) f/4.0. The second one? Much greater DOF, lens stopped down.
Ron
I see what you're saying and agree with your observations, but something is still not right... unless the camera is recording EXIF data wrong or something, both show f/8 and when taking the photos, I made sure that the settings matched...

07-02-2013, 08:41 PM - 1 Like   #17
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Do a little experiment with your 70-200. Keep ISO and shutter speed fixed, take a series of photos at various F-stops (2.8, 4, 5.6 and 8) and compare the results. You should be able to see the exposure and DOF changing. If all photos have the same overall exposure then the aperture does not close.
07-02-2013, 08:41 PM - 1 Like   #18
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To address f/stop differences you could re-shoot both wide open--it should address the possibility of sticky aperture blades.

But if the difference remains the factor I mentioned (p) is really a difference in "actual" f stop at higher values of maginification. If the difference is 2 stops exposure--it means 2 f/stops difference--and thus a fairly significant difference in depth of field.
07-02-2013, 08:46 PM   #19
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QuoteOriginally posted by demp10 Quote
Do a little experiment with your 70-200. Keep ISO and shutter speed fixed, take a series of photos at various F-stops (2.8, 4, 5.6 and 8) and compare the results. You should be able to see the exposure and DOF changing. If all photos have the same overall exposure then the aperture does not close.
QuoteOriginally posted by dms Quote
To address f/stop differences you could re-shoot both wide open--it should address the possibility of sticky aperture blades.

But if the difference remains the factor I mentioned (p) is really a difference in "actual" f stop at higher values of maginification. If the difference is 2 stops exposure--it means 2 f/stops difference--and thus a fairly significant difference in depth of field.
Demp, I was just doing that as you wrote this.

So here is report #2... results look pretty consistent to me...




Full-sized images of each are on Flickr: Flickr: ptaxlmtd's Photostream (they're the first 6 images after the comparison composite image)

Maybe the issue was dirty contacts and camera and lens not communicating together correctly?

07-02-2013, 08:52 PM - 1 Like   #20
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You also could set iso and shutter speed, and let the camera exposure system pick the f/stop--if the aperture blades are OK they should both be properly exposed. And see what the fstops chosen are
07-02-2013, 08:56 PM   #21
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QuoteOriginally posted by dms Quote
You also could set iso and shutter speed, and let the camera exposure system pick the f/stop--if the aperture blades are OK they should both be properly exposed. And see what the fstops chosen are
Ok, will do that in a second, thanks for the suggestion.

...and the results. Exposure looks okay to me on all three of them (except for the blown highlights, just left EV @ 0) even though the apertures the camera selected varied a bit.
Threw the Tamron 70-300 in there as a third comparison.

iso 1600, 1/40


Last edited by Julie; 07-02-2013 at 09:17 PM.
07-03-2013, 06:46 AM   #22
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Was it fluorescent lighting? Shutter speeds above 1/30 second with fluorescent lights can give very eradicate results.
07-03-2013, 10:42 AM   #23
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The latest tests show very consisted results as expected. If you cannot recreate the original issue, then another possibility is that when you mounted the 70-200, the aperture lever was somehow jammed and could not couple and move correctly with the body, so it stayed wide open all the time.

I have experienced the opposite where the level was not engaged properly, keeping the lens closed down to the smallest f-stop. That was easy to spot since the viewfinder was very dark. The easy fix was to un-mount and re-mount the lens.

07-03-2013, 03:12 PM   #24
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As Parallax said, with fluorescent lighting and an exposure time of 1/100 second, it would be very strange if you wouldn't get from time to time an underexposed shot.

1) To get the light the lightmeter thinks you will get, exposure time must at least cover a complete half cycle of voltage; in the US, this would be a minimum exposure time of 1/120 second.

2) If the curve of the emitted light was a sinus, you would get always good results with 1/120 second. But it isn't, as the fluorescent light sources do only emit light when the voltage is higher than some threshhold. So part of the cycle there is no light at all.

3) So, exposure will only be as predicted if the movement of the shutter curtains fits into the blackout time of the light source. If the first curtain starts moving near the maximum of light emittance, at 1/120 second also the movement of the second curtain will be near the (next) maximum. Then the overall amount of light the sensor will get will be significantly less than predicted.

It is just hit and miss.

EDIT:
Please, no one should argue that the shutter curtains movement is nothing special to this situation.
It is.
In most cases, you will have a light source constantly emitting light.
But fluorescent light is a kind of stroboscop. There is a difference between shots where the curtain movement happens during blackout (no influence on exposure) v during maximum light emittance.

Last edited by RKKS08; 07-03-2013 at 03:35 PM. Reason: Argument added
07-03-2013, 03:20 PM   #25
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QuoteOriginally posted by RKKS08 Quote
in the US, this would be a minimum exposure time of 1/120 second.
1/120th would only be 1/2 cycle. U.S. is 60Hz. Even at 1/60th of a second everything has to be in EXACT synch/time. If your particular camera's shutter is actually 1/61 instead of 1/60th you risk anomalies.
07-03-2013, 03:28 PM   #26
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QuoteOriginally posted by Parallax Quote
1/120th would only be 1/2 cycle. U.S. is 60Hz. Even at 1/60th of a second everything has to be in EXACT synch/time. If your particular camera's shutter is actually 1/61 instead of 1/60th you risk anomalies.
Of course. This is not contradicting what I wrote.

But the existing fluorescent light sources do not react on the switch of polarisation. In respect to light emittance, any half cycle is identical. So for our problem, 120Hz is the significant number for 60Hz power supply.

Last edited by RKKS08; 07-03-2013 at 03:33 PM. Reason: Information added and typing corrected
07-03-2013, 04:07 PM   #27
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With exposure times not being exactly a half cycle of the voltage (or multiples of this), I would expect to occasionally see some vertical graduation. From where the darker part is positioned, you could even guess whether the leading or the trailing curtain was the culprit.

The longer you set exposure time, the smaller the occasional effect of overall light reduction will be.
Influence is only coming from the first and the last captured cycle.
07-03-2013, 04:23 PM   #28
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Unless old style fluorescent lamps with magnetic ballast were used (which use line frequency of 60 Hz in the US), the new compact fluorescents with electronic ballast, operate in the 10,000 to 40,000 Hz range if not higher. At that frequency the exposure effect due to shutter speed is not an issue.
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