Light Variation During Stacking

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malachai
Posts: 1
Joined: 17.04.2018 16:19

Light Variation During Stacking

Post by malachai » 19.04.2018 17:13

I am taking Macro photo stacks of specimens. During the photo shoot, the light varies significantly. I cannot figure out why.
I am using a Nikon camera with 2 extension tubes on it on a suspended table with a rail. There are two florescent light sources being diffused via a white cloth that surrounds the whole set up. The door is closed to the room (which is small), and the vents are all closed to eliminate most air circulation. The only other source of light is from my laptop screen and small TV screen.
As I am taking macro photos for a stack, the light starts out at the exposure I set it at, but then it cycles through a series of exposures getting extremely dark, and then back to a moderate exposure, then extremely dark again.
Has anybody been experiencing this issue? If so, how have you combated it?

Thanks,
Malachai

BobStone
Posts: 27
Joined: 03.06.2017 10:13

Re: Light Variation During Stacking

Post by BobStone » 19.04.2018 21:04

That can happen if an exposure bracket is set. On a Nikon, the BKT remains even after turning the camera off. See if the BKT symbol is shown on the LCD. Hope it's that simple..

sbuerger
Posts: 23
Joined: 05.06.2010 15:31

Re: Light Variation During Stacking

Post by sbuerger » 22.04.2018 14:02

Doesn't really sound like exposure bracketing because if so, the cycle should at least contain three phases (normal, darker, lighter) instead of two. However, if bracketing was used, you can easily judge this from the exposure values in the EXIF data of your shots even if the setting in the camera has been changed meanwhile.
A less convenient possible explanation: Did you make sure that the ratio between exposure time and your luminescent's (no, it's not "fluorescent") ignition frequency is at least 10:1? Many, epecially cheap, lamp ballasts ingnite at 200 or even 100 Hz - meaning that, (for example) in the case of 100 Hz, with an exposure time of 1/10 s you may "catch" 9, 10, or 11 light ignitions randomly (error rate 10%) - but with 1/50 s it will be 1, 2, or 3 ignitions (error rate 50%) which obviously won't lead to satisfying results.
Generally, I would not recommend to use luminescent light sources for sequential photography. Of course there are ballasts that work with a suitable frequency (flatbed scanners, for example, use luminescent tubes that are ignited at 30,000 to 40,000 Hz, but their power output is far too low for photographic needs), but today it is definitely a lot cheaper and better to use studio flashes or LED panels that are either constant voltage or high frequency PWM driven.

MikeCT
Posts: 104
Joined: 08.04.2014 19:56

Re: Light Variation During Stacking

Post by MikeCT » 27.04.2018 00:59

I described exactly this problem last week, seeing in Remote. Also using a Nikon, but with LED lighting. Exposure bracketing was NOT on.

The reply was “Helicon Remote does not manipulate live view image, except for the case of "Live view brightness increase" option enabled in the settings. So, whatever it is, the camera is doing it, not the program. I can't say what camera setting might be causing it.” Apart from the fact that the cycling was seen in the stored image, not the live view, this puts the blame on the camera.

Have other Nikon users seen a similar cycling?

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Stas Yatsenko
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Joined: 06.05.2009 14:05
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Re: Light Variation During Stacking

Post by Stas Yatsenko » 27.04.2018 14:36

I wouldn't say the blame is on the camera in the sense that it's faulty, but on the camera settings and/or environment. Very good comment from sbuerger there (thanks!), the stroboscopic effect is very likely to manifest itself with luminescent lights. Or with LED lights that use low frequency PWM.

Clipping path
Posts: 21
Joined: 26.09.2017 14:33

Re: Light Variation During Stacking

Post by Clipping path » 10.06.2018 17:11

I think Photoshop is the best for photography image editing.I personally use Photoshop because I love this feature.

el48tel
Posts: 18
Joined: 22.06.2016 16:05

Re: Light Variation During Stacking

Post by el48tel » 10.06.2018 22:35

That now explains why HF said I had a 17% variance in the exposure ---- the LED source!

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