Hello, my name is Beck and I have a question about shooting the night sky with my Rebel t31. I

[Beck]

Hello, I tried to take shots of the night sky and need some help. I turned the manuel focus off and was shooting between 10 and 30 seconds. The question is how do I get the f stop lower to increase the light??? I have done this before so I kind of know what I'm doing. Yes the camera was on a tripod, and I was using a wireless shutter release. I tried different sittings but could not get the F stop below 18?

I know somebody knows how to do the folllowing to make it work. I turned the ISO up to 3200 BUT I couln't get the combination of the following:

Wide open appeture, 20 to 30 second shutter, and manuel focus.

I love learning to take better pictures and look forward hearing what I could do to make me improve.

Thank you all- I think the camera was on the MV setting

[amfoto1]

Um... manual focus vs auto focus basically has nothing to do with image exposure. Two separate things.

Yes, it probably is best to set and use manual focus (turn AF off at the switch on the lens).

You should be using a tripod. I'd also recommend using either a remote release or the camera's self-timer so that you aren't touching the camera during exposure, to keep from accidentally shaking it during long exposures.  Usually with really long exposures and when using a tripod, it's also best to turn off Image Stabilization (IS), if your lens has it.

There also is no such thing as "MV" setting. It sounds as if you are trying to set exposure manually, so would want to set the camera to "M" on the mode dial. This lets you freely set the three factors of exposure: lens aperture size (f-stop), shutter speed, and sensor sensitivity (ISO).

I believe on the T3i (and most other Rebel models) you use the same dial to change the shutter speed (20-30 seconds in this case) and the aperture size (f18 is too small). To switch back and forth between aperture and shutter control, there's a button on the rear of the camera.

To change your shutter speed, you simply roll the dial right behind the shutter release button. To change your aperture, press the "Av" button on the back of the camera with your thumb and roll the same dial on top of the camera with your forefinger.

Another way to do it is to press the "Q" button and get the quick access screen on the LCD. This screen shows all the major settings of the camera at a glance. You also can navigate around that screen using the camera's dials and buttons. And, while any one setting on the Q screen is selected and highlighted, you can make changes to it. This can be most helpful when trying to set up the camera in the dark!

You'll need to experiment with exposure settings to find what gives you the results you want, shooting the night sky. I'd recommend a lens aperture of around f8 or f11. That's likely sharper than "wide open". And, at the other extreme, stopping down to very small apertures (like f18) will cause an effect called "diffraction" which robs fine detail from images.

Also experiment with shutter speeds. The stars and moon move surprisingly quickly, so shutter speeds need to be fairly short unless you want to shoot "star trails". It takes some practice and experimentation to learn how your images will look, at various shutter speeds.

ISO settings can be a little tricky, too. Too high and ISO causes digital "noise" in images. You'll have to be the judge of what's acceptable in your images.

Something you might want to use is Long Exposure Noise Reduction.... I believe the T3i has it. If it does, you enable it in the camera's menu, then the camera will use it with any exposure 1 second or longer. However, you need to know how it works, or you might make a common mistake. LENR works by the camera making a second exposure, right after the first one you took. This second exposure is done with the shutter closed, to make a "blank image" that's used to identify noise, which is then subtracted from the first image.

Here's the tricky part. If the first image is, say, 20 seconds long.... then when using LENR the second exposure will also be 20 seconds long. So it will take a total of 40 seconds actual time, to make that 20 second exposure. Now, if you forget that the camera is making that second exposure and interrupt it (such as by turning it off or removing the batteries), the camera will throw away both images! More than a few people making 1-second and longer exposures forget this and end up wondering why the camera was taking twice as long to take an image and where their images went when they turned the camera off!

Something else you might find handy is an Intervalometer.  This looks like and can serve as a simple remote switch, but it also has means of making both extra long exposures (more than the 30 second limit of the camera) and time-lapse multiple exposures, firing the shutter every so often at intervals and for a total number of images you choose. My cameras use a different one than T3i... But I'm certain something similar is available for use with your camera.

***********

Alan Myers
San Jose, Calif., USA
"Walk softly and carry a big lens."
GEAR: 5DII, 7D(x2), 50D(x3), some other cameras, various lenses & accessories
FLICKR & EXPOSUREMANAGER 

[TCampbell]

f/18... could it have been f/1.8?  e.g. the EF 50mm f/1.8 lens has a maximum aperture size of f/1.8 (it can't go larger than that.)

You normally always want to manually focus the stars.  The auto-focus system will not be able to perform auto-focus using a star (they are too small and usually too dim.)  Astro imagers take quite a bit of time up-front to carefully focus, shoot some test images, and evaluate the focus.  Ideally you'd bring a laptop with you and to "tethered" shooting which makes it much easier to evaluate focus.  But you could also evaluate focus using the LCD screen to review the images -- but zoom in to the 10x size to evaluate if your focus was accurate.

Since the Earth is spinning (stars move at about 15 arc-seconds (angular distance) in each 1 second of real time) the stars will appear to elongate and grow a "tail" if the image duration is too long.  Some people want that tail (in which case they usually capture many 30 second long images over a few hours and then merge them using a utility (there's a utility called "Star Stax" which is free.)   If you do not want the tail, then divide 375 by the focal length of your lens... the resulting value is the number of seconds you can image before the stars appear to grow a tail.  

E.g. if using a 50mm lens, then 375 ÷ 50 = 7.5 (seconds).  So here, a 20 or 30 second exposure would be too longer (your stars will not be pin-point, they'll be slightly elongated due to the movement of the Earth).

But if you had, say, the EF-S 10-22mm lens or the new EF-S 10-18mm and you use them at the "10mm" focal length end of the range, then it's 375 ÷ 10 = 37.5 seconds (a 30 second exposure is no problem.)

Images of the night sky look best when taken well away from city light (your human eye wont be able to see that the sky is really a muddy brown color caused by the light polution of urban lights... but your camera will and the image wont look very good (and will need extensive processing.)

Similarly... avoid taking such shots when the moon is in the sky.  It creates a tremendous amount of light pollution which makes the sky look muddy in long exposure images.

[Beck]

Thank you both! Great stuff. I shoot the night sky using a tripod, remote shutter release, and at the last dark sky area left in the northeast.
I will try everything mention and experiment.
Again, I thank you for taking the time to respond. Beck

[ScottyP]

Hey Tim.

Would this 375 calculation be something where a 1.6 crop factor is relevant, or not? Is the calf the same on FF and crop?

[TCampbell]

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@ScottyP wrote:
Hey Tim.

Would this 375 calculation be something where a 1.6 crop factor is relevant, or not? Is the calf the same on FF and crop?

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If using a full-frame sensor, the base is 600 (in astrophotography it's called the "Rule of 600").  The 375 value already adjusts for the 1.6x crop factor (600 ÷ 1.6 = 375).  Some imagers think that the "Rule of 600" is working a bit too closely to the edge of where the stars begin to elongate and prefer to use a "Rule of 500" to be conservative.

On the camera's rear LCD review screen the stars will look sharp.  On the computer screen at 100% zoom you may notice that the stars around "round" but are elongated if the exposure time was too long.

[Beck]

I was wrong. I had the camera set on TV instead of M. I will keep practicing and try what was suggested. Again, thank you all.
Beck