Exposure compensation

[slat]

I was taking some astrophotography pics the other night. I am using a Canon SL1. I used three different lenses. The 18-55/3.5-4.5 kit lens, the 24/2.8 pancake and the 10-18/4.5-5.6. My question is that no matter what lens I was using I had to increase my exposure compensation in order to keep the f/stop wide. If I kept the compensation at zero I would have to go up to on my f/stop. From what I've read the compensation is supposed to be zero, but I'm not sure what to do to keep f/stop wide and compensation at zero. The photos seem to look ok, but if I can make them better I'd like to do that. Any help or ideas would be greatly appreciated.

 

[photo1x1.com]

Hi,
to get the most control over your camera, use manual exposure. That way you set aperture, shutter speed and ISO yourself. It sounds more intimidating than it actually is.
Using the 10mm lens, 30sec | f4.5 | ISO 1000 should be a good point to start. If that´s too bright or dark, adjust the ISO. Be sure to shoot in RAW to get the most out of your image in your editing software.
I´m sure you read everything else about astrophotography, like the 500 rule, etc.? If not, do some reading. You´ll love it!

 

[benhasajeep]

I agree about needing to go into manual. Your fighting the camera's software. It's trying to expose to what it thinks is right.

 

[TCampbell]

For astrophotography you would use ONLY manual exposure and manual focus (manual everything). Automatic metering and focusing capabilities don't work for shots of the night sky -- there's just not enough light compared to what the camera was designed to expect.

You will normally use:

a) the lowest focal ratio (f-stop or "Av") that your lens will allow.
b) usually also the widest focal length your lens will allow
c) you calculate the maximum shutter length if you are _not_ using an astrophotography "tracking" head such as a Sky Watcher "Star Adventurer" head or an iOptron "SkyTracker Pro" head. That is calculated (for your camera) by dividing 375 by the focal length you are using (for a 10mm lens that would be 375 ÷ 10 = 37.5) to arrive at the max number of seconds you can expose. At that value you may notice some elongation of stars if you look closely. So you could be more conservative and drop the duration by 20% and that will probably eliminate the star trails.
d) Since the camera won't let you use any shutter speed longer than 30 seconds nor will it let you use odd shutter speeds (not neat 1/3rd stop increments) you would normally put the camera into "Bulb" mode and use a remote shutter release to control the shot time.
e) You will use manual focus -- and this is a somewhat lengthy process that involves dialing in the focus as best as you can, maybe use live-view and magnify to 10x on a particularly bright star to see if you can refine focus... then take some test shots, adjust, more test shots, adjust, etc. until you are happy that you've nailed the focus (it's probably a 10 minute process). Once you are happy... that focus can be used all night. But your patience to get it right before you start shooting is rewarded.

You control the exposure primarily through ISO adjustments (since the aperture is just set to max and the shutter is just set to the longest you can manage without getting star trails... the only thing left you can change is the ISO.)

 

[petrochemist]

I was taking some astrophotography pics the other night. I am using a Canon SL1. I used three different lenses. The 18-55/3.5-4.5 kit lens, the 24/2.8 pancake and the 10-18/4.5-5.6. My question is that no matter what lens I was using I had to increase my exposure compensation in order to keep the f/stop wide. If I kept the compensation at zero I would have to go up to on my f/stop. From what I've read the compensation is supposed to be zero, but I'm not sure what to do to keep f/stop wide and compensation at zero. The photos seem to look ok, but if I can make them better I'd like to do that. Any help or ideas would be greatly appreciated.

Initially I missed the 'astrophotography' and so was thinking Av... It's only when I got to Tims reply that I saw my error.
For astro manual is definitely the way to go. Your camera can't accurately meter that low, and 18% grey is probably not the average you want either.

I don't know where you read EC should be kept at zero, but I would consider that TOTAL garbage. EC should be at whatever value that gives the result you want. In everyday shooting close to zero is likely to be right, but even then high key or low key subjects will cause deviations to be needed. Many photographers prefer to keep EC around -½ to reduce blown highlights & allow processing of their raw files to recover the difference. I wouldn't call that bad practice.

 

[petrochemist]

c) you calculate the maximum shutter length if you are _not_ using an astrophotography "tracking" head such as a Sky Watcher "Star Adventurer" head or an iOptron "SkyTracker Pro" head. That is calculated (for your camera) by dividing 375 by the focal length you are using (for a 10mm lens that would be 375 ÷ 10 = 37.5) to arrive at the max number of seconds you can expose. At that value you may notice some elongation of stars if you look closely. So you could be more conservative and drop the duration by 20% and that will probably eliminate the star trails.

I believe the 375 Tim is using here is for Canon APSC cameras & is an update of the old '600 rule' (375=600/1.6 Canons crop factor). In other forums I've seen astro photographers comment along the lines of 'with digital images (& the greater tendency to view large/crop) slower speeds than 500/equivalent focal length are now recommended'.
Although they are far from ideal, I tend to use my MFT for astro, as they are light enough to have with me when I go somewhere with proper dark skies. For these I definitely see signs of trails working to 250/focal length.

As with the old handholding rule adjustment to your own preferences/hardware is helpful.

 

[slat]

So by using manual everything that takes exposure compensation out of the options since it won't be relevant at that point? Is that correct?