Author Topic: Long exposure problems  (Read 6941 times)

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Offline CraigularB

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Long exposure problems
« on: August 14, 2010, 03:52:33 AM »
Hey everyone,

I've been trying to do some long exposures, mostly to get some halfway decent pictures of the stars at night. I've been in places with little ambient light, and the stars aren't really visible. On top of that, with long exposures the top corners of the photos get REALLY bright and I'm not sure why.

So, my questions are these: how do I fix the bright corners, and what are some good settings for star field photos? The picture below (supposed to be stars and also has the bright corners) was taken w/ 25sec shutter, f/25 aperture (iPhoto lists Aperture and Max Aperture, and the max was f/4.0, so I'm not sure which one is which), 22mm focal length, no long-exposure NR, D-Rage optimizer was just set to D-R, and ISO was 1600. I was also using a UV filter.

One last question...Neither of my lenses have focus marks on them, so I'm not sure how to set focus for infinity for good star focus (and autofocus won't work because it's so dark haha).

Thanks!

-Craig

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Offline zekewhipper

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Re: Long exposure problems
« Reply #1 on: August 14, 2010, 05:37:00 AM »
I don't have any advice to give on exposure except that if your shutter time is 25 seconds, then you sure as heck don't need to be shooting at ISO 1600. 

Now for focusing to infinity, on many lenses you simply have to manually turn the focus ring all the way in one direction (whichever would give you infinity focus in the daytime) to set it.  The lens you are using may or may not work that way.
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Offline CraigularB

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Re: Long exposure problems
« Reply #2 on: August 14, 2010, 06:49:35 AM »
I don't have any advice to give on exposure except that if your shutter time is 25 seconds, then you sure as heck don't need to be shooting at ISO 1600. 

Now for focusing to infinity, on many lenses you simply have to manually turn the focus ring all the way in one direction (whichever would give you infinity focus in the daytime) to set it.  The lens you are using may or may not work that way.

Thanks for the focus tip! I'll be playing around with my lenses to figure out which way goes to infinity.

And I thought 1600 might be overkill but I was ready to try anything because I couldn't figure it out haha. I'm still tryin' :)

-Craig
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Offline vaughaag

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Re: Long exposure problems
« Reply #3 on: August 14, 2010, 08:07:21 AM »
Craig,

It looks like a hot sensor to me, also known as amp noise. I dont know if you can resolve it or PP it out.

Rgds,
Dave
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Offline harveyzone

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Re: Long exposure problems
« Reply #4 on: August 14, 2010, 09:51:27 AM »
I've been trying to do some long exposures, mostly to get some halfway decent pictures of the stars at night. I've been in places with little ambient light, and the stars aren't really visible. On top of that, with long exposures the top corners of the photos get REALLY bright and I'm not sure why.

So, my questions are these: how do I fix the bright corners
This is usual for many cameras with long exposures, picking up internal heat.

To remove it you need to be aware before it happens, as removing it later is dificult. Switch on long exposure noise reduction to do it in camera (but this will make each exposure twice as long as it is set for), or take a 'dark frame' exposure when you are shooting the stars to create a snap shot of the noise, and remove it manually later using dark frame subtraction techniques.

Tom Harvey

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Offline Rob aka [minolta mad]

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Re: Long exposure problems
« Reply #5 on: August 14, 2010, 08:46:13 PM »
Hi and welcome to the forum.
Already mentioned the what is happening is that the sensor is picking up heat and its being amplified (espescially as youre using iso 1600)
I wouldnt go above iso 400 (personally i use 200), but one thing i would do is reduce the appeture down to no more than f/5.6 and shoot at 30 secs. This should give you bright stars and very little niose and id be surprised if there was any brightness in the corners anymore.
I have shot some star shots using the above settings, and stacked 70 odd images to get the shot below.




Rob

Offline CraigularB

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Re: Long exposure problems
« Reply #6 on: August 14, 2010, 10:35:40 PM »
Quote from: Rob aka [minolta mad]
Hi and welcome to the forum.
Already mentioned the what is happening is that the sensor is picking up heat and its being amplified (espescially as youre using iso 1600)
I wouldnt go above iso 400 (personally i use 200), but one thing i would do is reduce the appeture down to no more than f/5.6 and shoot at 30 secs. This should give you bright stars and very little niose and id be surprised if there was any brightness in the corners anymore.
I have shot some star shots using the above settings, and stacked 70 odd images to get the shot below.

...

Rob

Rob and everyone,

Thanks for the replies! I will definitely turn down the ISO and tweak the aperture next time I try these shots (It's been really cloudy here, unfortunately). Any tips on how to focus for a shot like this in the pitch-black? I read that turning the A300 off and back on resets the focus for infinity, so I may try that first.

2 more questions: Should I remove the UV filter before attempting these pictures? I read that the UV filter that most people have on their lenses for protection can reduce the amount of visible stars. Would it be worth trying?

Also, would I be right in guessing there are some tutorials here/elsewhere on the net on how to stack images like Rob said? Are these fairly easy to find or do you have any personal favorites that work well for you? I have Photoshop CS5.

Thanks again! Also, thanks Rob for moving the post. I wasn't sure if it was camera-specific or just taking photos.

-Craig
« Last Edit: August 15, 2010, 04:26:27 AM by CraigularB »
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Offline winjeel

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Re: Long exposure problems
« Reply #7 on: August 15, 2010, 01:27:23 AM »
You're not going to like this, but my answer is to use film. With film you can expose for any length of time, even over an hour or more if you really wanted. If you use iso100 in digital can reduce noise, or if you use film you'll avoid grain.
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Offline harveyzone

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Re: Long exposure problems
« Reply #8 on: August 15, 2010, 08:58:56 AM »
You're not going to like this, but my answer is to use film. With film you can expose for any length of time, even over an hour or more if you really wanted. If you use iso100 in digital can reduce noise, or if you use film you'll avoid grain.

But you may encounter other problems like reciprocity failure.

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Offline paul_b

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Re: Long exposure problems
« Reply #9 on: August 15, 2010, 12:41:00 PM »
For Focus could you get the camera to focus on the moon, which has plenty of light, then turn off AF. Alterantely experiment with noting where infinity setting is during the day on distant objects and then set to the same setting using MF at night.



Exif data ISO 200 F8 1/400 or 1/800 so plenty of light.
« Last Edit: August 15, 2010, 12:55:23 PM by paul_b »
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Offline Clive

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Re: Long exposure problems
« Reply #10 on: August 15, 2010, 05:00:06 PM »
Hi Craig

Welcome. No major comments. Just posting two star shots with EXIF info. May be of use.

Did not have probs with ISO 1600 on the bottom 18-sec shot. Used low ISO on the 302-second image. These were taken with the A700.

BTW .. my old KM 7D would produce similar hot spots on ONE corner on long night shots. It was theorized that the hot area was where the sensor was closest to the battery that emitted a wee bit of heat. Maybe. No solution is offered.

Good luck

Clive



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Offline Rob aka [minolta mad]

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Re: Long exposure problems
« Reply #11 on: August 15, 2010, 09:32:21 PM »
The UV filter wont affect the images at all, or i have never found it too.
CS5 is great for stacking images.


Rob

Offline CraigularB

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Re: Long exposure problems
« Reply #12 on: August 15, 2010, 11:46:53 PM »
The UV filter wont affect the images at all, or i have never found it too.
CS5 is great for stacking images.


Rob

Do I just stack the images in layers and set the layer blend option to Overlay? Or is there a special procedure for stacking star photos?

-Craig
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Offline Rob aka [minolta mad]

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Re: Long exposure problems
« Reply #13 on: August 16, 2010, 04:26:46 PM »
In CS4 its file < scripts < load files into stack.
I havent got CS5 on my laptop so will have to check if its the same on CS5 later ???


Rob

Offline CraigularB

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Re: Long exposure problems
« Reply #14 on: August 16, 2010, 04:31:36 PM »
In CS4 its file < scripts < load files into stack.
I havent got CS5 on my laptop so will have to check if its the same on CS5 later ???


Rob

Rob, just did a quick check in CS5 and that option is still there. Next time I get a chance I will try this out! Thanks for the help :)

-Craig
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