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Old 02-11-2012, 11:19 PM
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Exposures for Diamond Ring

Apologies in advance for a long post, but I'm getting a little confused as to whom to believe when it comes to eclipse exposure calculations.

I had done my exposures based on one calculator, but after looking at some images posted recently on IIS that included exposure details, I decided to do some more research.

Rob posted a diamond ring shot at 1/2000, F9 at ISO400. I have a fixed aperture setup at F6.2, or 1 full stop faster, and would like to use ISO100 (2 stops slower) to maximise my sensor's dynamic range. Overall, the equivalent exposure for my setup would be 1/1000, F6.2 at ISO100 (1 stop slower).

The calculator I had been using gave suggested exposures of 1/80 second for the diamond ring - over 3 stops longer than what Rob used. It also recommends exposures that are one stop longer for partial phases through an ND5 filter, but 1.3 stops shorter for unfiltered exposures during totality, when compared to another calculator on the eclipse-chasers website (www.eclipse-chasers.com).

Today I did some test imaging with my setup on the setting sun. Through a hazy sky at low altitude, the images that I selected as having an "ideal exposure" based on their histogram, matched closely with the exposures predicted by the eclipse-chasers calculator, when I asked it to compensate for altitude and "humid" conditions. (This is what I would have selected for the Cairns eclipse, given I'll be on the coast)

Given that I was shooting through a very hazy sky (ie greater atmospheric extinction) and my exposures matched eclipse-chasers more closely vs the other calculator (which was suggesting longer exposures again for the partial phases) - I'm tending towards the exposures predicted by eclipse-chasers. At the NACAA workshop, the comment was also made by those who had photographed successive eclipses with digital cameras that their images were brighter in the more recent eclipses with newer and newer cameras/sensors. Maybe this suggests the recommended exposures should be getting shorter???

So, according to the eclipse-chasers calculator (allowing 2 stops for atmospheric extinction), I should be shooting the diamond ring at 1/250, F6.2 at ISO100. (2 stops of extinction was mentioned at the NACAA eclipse photography workshop and matchers the eclipse-chasers calculator) I realise the brightness will vary greatly during the moments before and after totality, so no one exposure will be correct for the diamond ring. Obviously a longer shot will expose more of the corona, but if it's not taken just prior to the sun being completely eclipsed by the moon, it will be drastically overexposed.

I'm not terribly concerned about the varying exposure predictions during partial phases or the rest of totality - I'll got the time to bracket exposures to allow for that. For me, the diamond ring is the "money shot" of the eclipse, hence my pondering of the issue.

At this stage, my script will be written to shoot a 4fps burst of exposures at 1/250 just prior to C2 and 1/40 just after C3.

So, my question is, what exposures are you planning to shoot for the diamond ring shot?

DT

Last edited by DavidTrap; 02-11-2012 at 11:21 PM. Reason: clarity
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Old 03-11-2012, 05:46 AM
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dannat (Daniel)
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Can you set exp bracketing?
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Old 03-11-2012, 07:15 AM
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Not during a burst of exposures. I also believe the light intensity will change so dramatically during those few seconds that bracketing won't "ensure" I get the right exposure at the right time.

My hope is that by taking a burst over several seconds I'll get one "correctly" exposed image at some stage.

Just wondering what exposure others are using as their starting point?

DT
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Old 04-11-2012, 12:08 AM
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OzEclipse (Joe Cali)
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Eclipse-Chasers exposure calculator

David,

Background on the eclipse-chasers calculator.

When the calculator was first written by Bill Kramer, there were some major problems with the extinction model being used which I pointed out to Bill. Calculating extinction isn't easy at the best of times because the number of air masses you "look through" is only ever an estimate. Pollution, humidity, aerosols all contribute.

I corresponded a lot with Bill about this and we iteratively tested the calculator against as many of our own real past exposure records to give as good an indication as is possible. We tested the extinction model against results from low altitude eclipses as well as the non-extincted exposure model against our results from high in the sky corona imaging.

The 15 degree extinction exposures correlate exactly ( surprise surprise) to my images from China in 2008 when I was at a sparsely populated non-air polluted site in NW China. The sunset brightness looked a bit like our sunsets here with the sun bright on the horizon not fading away 15 degrees above the horizon as happens in most cities in the east of China.

That doesn't answer your Q about diamond rings.

There is no one correct exposure for diamond rings. For starters, it depends how much photosphere is showing. The true diamond ring is the very last little ray shining through the last valley on the limb. But it's quick and hard to catch.

Diamond rings can be given a very wide variety of exposures - just depend on what you want the thing to show. You can (I'll call it over expose) to get a big spread like the image at the bottom of this page :

http://joe-cali.com/eclipses/PAST/TS...YA_Photo1.html
exposure details are on the photo

There are three images on this page ( about half way down the page )
http://joe-cali.com/eclipses/PAST/TSE2010/joe.html
Ignore the exposure details of the one through cloud showing shadow bands. The extinction by the cloud makes this exposure data irrelevant.

But the first is taken at prime focus of a WO M70 refractor f6.3 1/500s ISO100

The third image below is taken by Bengt Alfredsson. He has deliberately stopped down the lens to f8 to give the diffraction spikes. About 2/3 down the same page are 4 brackets in rapid succession from 1/500s - 1/60s ISO 100 70mm f6.3.




Some people go extreme like this one by Alex Birkener
http://www.astronation.net/index.php...3#!prettyPhoto[Sonnenfinsternis]/16/

I'm pretty sure Blende 14 means f14. 0.3s seems awfully slow for that image even at f14. No time now but one of these days I want to email Alex and check if his settings are correct.

There is no one correct setting so don't stress if you see different recommendations they are based on different but valid outcomes.

Joe
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Old 04-11-2012, 06:37 AM
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Not about what you asked but I wanted to mention it. How does your camera respond to a large number of images taken close together?

I say this as my camera froze last weekend when I tried to do a 20 shot mosaic portrait. I had changed my memory card settings to SD card primary and CF card as secondary. I think I will swap it around to CF as primary as they read and write faster.

You don't want a freeze in the middle of a sequence.

Also consider the time to save all these images and then the resulting slower frame rate afterwards.

As far as exposures go with D800 you would be smarter to slightly underexposed than overexposed. Highlights are lost but as you know with D800 you have 2-3 stops of usable shadows you can recover. So a smart strategy would be to slightly underexpose and preserve the highlights and recover the shadows in post processing.

Greg.
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Old 04-11-2012, 09:28 AM
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Thanks Joe & Greg for your responses,

Joe, your comments confirm a lot that I had suspected about exposures and the eclipse calculator. My hunch is the previous calculator I was using was based on the long published tables.

Looking at your examples,

The Libyan eclipse was high in the sky so extinction can be ignored. 1/60 at F7.5 (and assuming ISO 100), is almost equivalent to the 1/80 at F6.3 predicted by the first calculator I was using. I agree that looks overexposed, but a burst of exposures might capture the moment when the proverbial "last little ray shining through the last valley on the limb", and yield a nice coronal display as well

You said the diamond ring lingered during the 2010 eclipse and one of your images was shot 20seconds before C2. I can only assume that is because the duration of totality was significantly longer during that eclipse. The Bailey's Beads simulator on Solar Eclipse Maestro suggests 7-10 seconds before and after C2/3 will be the timeframe for this shorter eclipse - any comments?

The 1/500 at F6.3 (again assuming ISO100) shot, if taken a few seconds later might be "exactly what I'm after"! It was also interesting to look at the effect of cloud on exposure. Hopefully that doesn't affect us in Port Douglas, but if it does, my plan is to try the exposure compensation in Solar Eclipse Maestro - looks like light cloud might need ~2 stops of compensation.

After reading your pages, I think I might proceed with my planned burst runs, fast on the way in and slow on the way out. I might keep the run shorter prior to C2, so as Greg suggest it doesn't tie up the camera while it clears the buffer. It doesn't really matter how long the camera is tied up for after totality ends, so I might shoot that sequence from well before the predicted C3, in case timings are a little out! I have ordered a new CF card that should be faster than my existing SD cards. I'll have to do some testing to see how fast the buffer clears when writing my scripts. With the automatic scripting for camera control, you must wait for the buffer to clear before it can accept a new command from the computer. Xavier tells me this is a consequence of the way the internal camera software is written, no way around it. I do intend to switch the camera to DX mode for these burst runs, so the file size is substantially smaller than full-frame, meaning the buffer will clear sooner. I won't need the field of view provided by full frame to capture the diamond ring - assuming I have the sun centred in my frame!

Greg - I agree with your comment about "underexposing slightly" with the D800. You can definitely pull detail out of the shadows, but with every camera, highlights are gone.

Overall, preparation for the eclipse has been a fascinating process. In a lot of ways I'll be glad when it's over - I can get back to a lot of other things I've been neglecting.

See you in Cairns!

DT
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Old 04-11-2012, 01:08 PM
geoffsims (Geoff)
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Hi David,

Here are my 3rd contact diamond ring shots (and exposure details) from 2010 - taken about 100m away from Joe!

They may assist you in deciding what exposures to use when.

Geoff
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Old 04-11-2012, 10:12 PM
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Very handy. What does C3 stand for?

Greg.
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Old 04-11-2012, 10:17 PM
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3rd Contact!
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Old 05-11-2012, 10:27 AM
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Geoff - thanks very much for that sequence of images. I'll certainly use those as a reference when deciding on my exposures.

Yesterday, I did some testing in clear skies. Rain overnight had cleared the atmosphere of haze, and my exposures were substantially different to Friday. I now have a better feel for how my scope, camera and filter relate to the published calculators. The unknown of course is how it will work without the filter in place.

I've also gained some experience on how much a hazy sky changes the exposure, as well as how the histogram moves with bracketing. I've setup my exposure sequence during totality and just need to see how the camera performs with a new memory card to adjust timings for the diamond ring burst sequences. I am consistently using shorter exposures vs those quoted by the calculators for the partial phases, so will adjust my diamond ring exposures in a similar fashion.

I've probably shot a thousand or more exposures testing the script sequence...

Thanks again for the advice - fingers crossed for fine weather,

DT
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Old 05-11-2012, 11:55 AM
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Very interesting, David... I look forward to hearing your thoughts on exposure times afterwards.

When do you remove/replace the solar filter? Is it part of the script, i.e. is there a message saying "remove solar filter NOW"?
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Old 05-11-2012, 12:35 PM
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Thanks Dave,

There are voice prompts in the script to say things like: Check Focus, 60 seconds, remove filters, replace filters. You set them up to occur at what ever time point in the script you want. For example, I have Check focus set every ten mins, as I figure the telescope will heat up in the morning sun.

Hopefully it all just works and I can enjoy the show!

DT
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Old 05-11-2012, 05:23 PM
Bob Jones
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Hi guys

Same question as naskies. When do you remove/replace the solar filter?
Is it safe to make exposures 30 seconds before/after C2/C3 ?

Thanks Bob
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Old 05-11-2012, 09:39 PM
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Quote:
Originally Posted by Bob Jones View Post
Hi guys

Same question as naskies. When do you remove/replace the solar filter?
Is it safe to make exposures 30 seconds before/after C2/C3 ?

Thanks Bob
yes 15-30s before after. exposures are short so it is safe. don't look through the viewfinder.

cheers

joe
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