Focusing 40D Live View on Stars

[larry h.]

<p>Last night, I tried my 40D to take wide view pictures of stars. I used to do a fair amount of this on film using an

old Minolta SRT102. This was my first time trying it with digital and autofocus.</p>

<p>On the Minolta lenses, merely setting the focus at infinity was good enough for my purposes. H

owever, on AF lenses, there is no infinity stop. So focusing at infinity is much more d

ifficult.</p>

<p>I tried using the Live View function (with 5x and 10x zoom) to focus manually on the

stars. However, I could see no stars on the LCD screen to focus on. Is there any way to amplify the image brightness

on the LCD screen so that I

can focus on the stars?</p>

<p>BTW, I was using a Sigma 30mm/1.4 lens. It did significantly better than

I expected. I took exposures (all 30 sec at ISO 1600) every 1/3 stop between f/1.4 and f/4.0. The stars were always re

latively sharp in the middle. The corners lost the classic astigmatic triangles at about f/2.0. Here is the f/2.0 shot for your

reference. Warning: the file is large. I wanted to preserve the

detail for those who<div></div>

[larry h.]

<p>... for those who are interested.</p>

<p>Note that the slight elongation from about 11 o'clock to 5 o'clock is due to the movement of the stars over a 30 second period, not a flaw of the lens.</p>

[larry h.]

<p>Here's the upper right corner detail at f/1.4. Note that I applied no noise reduction, sharpening or contrast at this point.</p><div></div>

[larry h.]

<p>Sorry, I did NOT mean to make that last link so big. Here's another try...</p>

[rainer_t]

<p>I believe you didn't nail focus 100% ... I append a shot (reduced to the same size as yours for easier comparison). That was taken with the 30/1.4 at f/1.4 8sec iso 800 with a 300D.</p><div></div>

[larry h.]

<p>Another try. Sorry.</p><div></div>

[larry h.]

<p>Wow, Rainer! That looks nice. Especially the Andromeda Galaxy.</p>

<p>I realize I didn't nail focus quite right. This was my first try. A couple of the shots were focused even worse.</p>

<p>How did you focus your shot? Obviously not Live View on a 300D. Even if I'd gotten the focus better, thought, I would have had more elongation because the exposure was 4x as long. Still, I welcome your tips on focusing with this or other AF lenses.</p>

[larry h.]

<p>While I'm at it, look at the corner detail on this previous shot with this lens at f/1.4. The view is of sky filtering through leaves. The bokeh (astigmatism) in this shot seems a lot worse than the corner detail of the stars. What gives? Anybody with any ideas?</p><div></div>

[rainer_t]

<p>I believe for such shots you need several things ...<br>

... a clear sky<br>

... no strong lightsource nearby (that is sortof "clear sky" as well)<br>

... tripod<br>

... accurate focus<br>

For the focus, I tried to <strong>manually</strong> focus on a lightsource in a village nearby (and set the camera to 3200iso for that)...after each shot, I controlled the result in the display. As said, focus was set to manual, so after I had sufficiently good focus, I changed to 800iso and took a number of shots from the sky. I believe AF is absolutely no help here.</p>

<p>I'm not completely sure, if your shot is slightly off focus, or if its due to the longer exposure time (in which the stars move), or if there was a thin layer of haze ... but the stars appear slightly blury.</p>

[rainer_t]

<p>After another look on your first shot, I believe the "problem" is the exposure time ... the stars have visibly moved in the 30seconds. Imagine how sharp the shot would appear if the trails would be only 1/4 as long. (And they even would <strong>not</strong> be less bright)</p><div></div>

[guy_howard]

<p>Larry - did you enable Live View exposure simulation? Page 112 of the 40D manual.</p>

[scherbi]

<p>When I took <a href="http://www.zooomr.com/photos/scherbi/4318822/">this</a> shot with my 40D connected to a starmax 127 maksutov on a driven equatorial mount, i took sample shots at ISO 1600 to nail the focus. My scope is f/12.1 so even with a full moon in eclipse, it's too dark to focus, too dark even for the live view exposure compensation to work right, iirc. I could take a reasonably well exposed shot at 1600 in under 1 second. This allowed my to tweak the focus with three or four shots. Then I backed down to ISO200 for the 6 second exposure.</p>

<p>I hope that helps.</p>

[j_smith6]

<blockquote>

<p>Note that the slight elongation from about 11 o'clock to 5 o'clock is due to the movement of the stars over a 30 second period, not a flaw of the lens.</p>

</blockquote>

<p>If Mikolaj Kopernik (Nicolaus Copernicus) heard that now, he'd have a heart attack :P</p>

[kasperhettinga]

<p>:D</p>

[larry h.]

<p>Rainer,<br>

Thanks. I think the 30 sec exposure definitely caused the trails, but I do still think that it was slightly out of focus, too. It was a very dark, clear night in a remote part of the Georgia (USA) mountains, with no major light sources. I thank you for reminding me that the stars would NOT be less bright in a shorter exposure, but I was hoping to pick up some of the luminosity in the Pleides. In retrospect, that was a mistake.</p>

<p>Guy, I did enable Live View Simulation. It did not help.</p>

<p>Bill, next time, I will start earlier (it was about 12:01 a.m. on New Year's Day) and do some test focus shots. I was just hoping that the Live View would help me focus.</p>

<p>Michael, you gave me a good laugh and an appropriate admonition. I actually start teaching an astronomy class next week at the high school I work at, so I should have known better. I should have been far more careful in my wording, but I used the common vernacular common when people talk about star trails.</p>

<p>Larry</p>

[myrjola]

<p>I don't know if this is of any use, but I also tried liveview focusing to stars yesterday for the first time with my 40D (the weather has been pretty bad around here, so I've been waiting for the opportunity to do this since I bought the camera a couple of weeks ago) and the stars were nicely visible on the LCD, even though I didn't have the liveview exposure simulation mode turned on. I tried this first with Aldebaran and then re-checked the focus with the brighter stars of Pleiades, so it certainly looked like the stars wouldn't even have to be very bright to be visible on the LCD. Perhaps your focus was initially off enough to make the stars too large and dim blobs to see on the LCD?</p>

<p>(The otherwise annoying longitudinal chromatic aberrations seem to be useful for finishing the liveview focusing to point sources like stars btw; once the stars are not surrounded by a green or purple halo on the 10x magnification, the focus is spot on.)</p>

[guy_howard]

<p>I went out last night to find out if I could duplicate the problem and confirm the effect of exposure simulation, and discovered that I had got it backwards in my first post, thanks to Mika. With exposure simulation mode off, the LCD system attempts to adjust the display to show an image with good contrast, and does not attempt to display what it believes will be the effect of the projected exposure. This gives a brighter LCD image in very low light.</p>

[larry h.]

<p>Mika & Guy,</p>

<p>Just to make sure, by turning off exposure simulation, I should be able to see and focus on the stars, right? That seems to be the solution. Hopefully it clears up here tonight so I can try it. This will be my last opportunity for dark skies for a while.</p>

<p>Thanks for the help, everyone! Larry</p>

[myrjola]

<p>While I didn't have the opportunity to try the exposure simulation with stars today, I tried it briefly indoors, pointing camera to a room lit only by some stray light from outside and from a lit room, starting from a fast shutter speed and increasing it gradually. After reaching some shutter speed (I think something like 1/4s or at least close to that) any further shutter speed increases didn't affect the live view display noticeably anymore (and switching the simulation mode off didn't seem to affect the brightness of things in live view noticeably either; perhaps some maximum gain had been reached?). So unfortunately this is not very conclusive information, but as I was able to focus successfully to stars with the simulation mode turned off, I think it's worth a try.</p>

[larry h.]

<p>Mika & Guy (and anyone else),<br>

I was back in dark skies last night. This time I tried my new Canon 17-55mm/2.8.<br>

I tried Live View Exposure Simulation on and off. The only thing I could focus on was Venus. The Pleides and Hyades (and Alderbaran) were too dim for me to focus on in either mode. Do you have any further thoughts on focusing on stars in Live View?<br>

The new lens seems great, BTW. Here is a shot that includes the Andromeda Galaxy at the lower left and the Double Cluster (I think) at upper right. 35mm, f/2.8, ISO 3200, 8s. (Thank you Rainer for reminding me that a shorter exposure would not make a difference in stars.)<br>

Last question: I notice lots of noise and banding. I set ISO to 3200, and enabled Long Exposure Noise Reduction and High ISO Noise Reduction. Should I have used a lower ISO? Should I turn off those two noise reduction modes and do it in DPP or Photoshop instead? I appreciate any help you may give me.<br>

Thanks again, Larry</p><div></div>

[rainer_t]

<p>Focussing ... if Venus was bright enough, that should be it. (Since Venus is as infinite as all other objects in the sky ... at least for the camera).<br>

The 17-55 has performed very well ... and from your picture, focus was spot on. But since it's f/2.8, you were forced to use 3200iso to gather the same amount of light that you got with the 30/1.4 at 800iso.</p>

<p>I believe "long exposure noise reduction" is good for you, since (at least in my understanding), it takes a second shot (with shutter closed) and substracts the "dark-frame" from the image. I don't know what the camera does with "high ISO noise reduction".</p>

<p>Ideally, you would have taken shots at 800iso and 30sec and had mounted the camera on a telescope to track it (even manually). That would give you a plus in exposure while you still get the superb quality of the lens.</p>

<p>Focussing again...If you know the focal length you'll be using, you could also focus to infinity daytimes and stop playing with the focusring. If its very cold, there might be a bit of a shift unfortunately. (But then again, it should be cold daytimes as well.)</p>

[myrjola]

<p>The focus looks quite good here, and especially considering the lens has been used wide open, I second Rainer T's comment about it performing very well.</p>

<p>One alternative to using a tracking telescope mount might be making a so-called <a href="http://en.wikipedia.org/wiki/Barn_door_tracker">barn door tracker</a> . You could also take several short exposures and 'stack' them with suitable software such as Iris or Deep Sky Stacker; they can calculate the apparent movement of stars between images and re-align the individual images to produce a higher-quality final result. These applications support subtracting a combination of multiple dark frame from the images to achieve better noise reduction than the built-in camera feature amongst many other nice tricks. This can also be combined with the use of telescope mount or a barn door tracker to push the limits even further.</p>

<p>Regarding the focusing problem with things dimmer than Venus, I must admit I'm confused as for me even the stars in Pleiades were easily visible on the Liveview. I'll have to test this again myself when the weather is good and I have some spare time, perhaps some non-obvious setting is causing the difference. If there are any street lamps or something similar that are at least a few hundred meters off I think using them for focusing should provide good results too, at least with moderate focal lengths/apertures.</p><div></div>

[larry h.]

<p>Hi Rainer & Mika,<br>

Thanks again for your help. Thanks also on your compliments to my focus and new lens. I focused on Venus then shifted aim, but it won't always be around.</p>

<p>Mika said, "Regarding the focusing problem with things dimmer than Venus, I must admit I'm confused as for me even the stars in Pleiades were easily visible on the Liveview. I'll have to test this again myself when the weather is good and I have some spare time, perhaps some non-obvious setting is causing the difference."</p>

<p>I think you're right--there must be some sort of non-obvious setting causing the difference. I could see nothing except Venus and the bright rectangle on the screen. Maybe the rectangle was the problem. Do either of you know how I can get rid of that on the screen? That might help.</p>

<p>Also, does anyone know of another website where I can ask about the problem? I used to know where there were decent astrophotophotography forums, but I lost track after I went digital. I'm looking for somewhere that concentrates on digital astrophotography, preferably wide-field and preferably with Canon equipment.</p>

<p>Thanks again, Larry</p>

<p>PS--Here's another I took of the Pleides and Hyades, 55mm, f/2.8. 8s, ISO 3200.</p><div></div>

[myrjola]

<p>Larry,</p>

<p>while I'm not personally active on <a href="http://www.cloudynights.com/">Cloudy Nights</a> , I'm under the impression that it also features an active astrophotograhy forum including plenty of DSLR astrophotography talk, so it might be worth checking out.</p>

[larry h.]

<p>Thanks Mika. I am in the process of registering on the site now. -- Larry</p>