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Rebel T 6 - Bulb mode


I just ordered the EOS Rebel T6 after upgrading from a PowerShot SX50. My friend uses the same camera and lenses to shoot really good astrophotgraphy and that's what I've been wanting to do forever, so I saved up enough to get one. However, the Bulb mode is confusing me. It seems to need something to focus on before it can take a shot... are there ways around this? Manual focus?

Also, any friendly/newbie tips would be much appreciated! I mostly do it as a hobby, and it's usually night/moon/astrophotography, with the occasional sunset and dog pictures. I have an extra lens coming (Canon Advanced VX Camera Ef 75mm-300mm Telephoto Zoom Lens), and I have a steady tripod.



@Kohaku wrote:

I just ordered the EOS Rebel T6 after upgrading from a PowerShot SX50. My friend uses the same camera and lenses to shoot really good astrophotgraphy and that's what I've been wanting to do forever, so I saved up enough to get one. However, the Bulb mode is confusing me. It seems to need something to focus on before it can take a shot... are there ways around this? Manual focus?

Also, any friendly/newbie tips would be much appreciated! I mostly do it as a hobby, and it's usually night/moon/astrophotography, with the occasional sunset and dog pictures. I have an extra lens coming (Canon Advanced VX Camera Ef 75mm-300mm Telephoto Zoom Lens), and I have a steady tripod.

Bulb mode can be confusing.  It is best used in conjunction with a remote shutter control.  Manual focusing is recommended.

If you want to get into astrophotography, there are two approaches to it.  The least expensive is use a very wide angle lens, and take landscape photos of the night sky.  The more expensive approach is to use a telescope for a lens.

I recommend going the wide angle lens, if you are just starting out.  I use a Rokinon 14mm T3.1 lens for night sky photos.  Many of the techniques and skills need to use a telescope effectively are best honed with a wide angle lens, and taking photos of the night sky.

"The right mouse button is your friend."

Most camera cannot focus well in low light. You should switch the lens to Manual focus. It would help to use live view, zoom in twice (10X) to hopefully see a star to focus on. If that's not possible, just focus at infinity (turn lens all the way to infinity then back off just a notch).

For astrophotography, it helps to have a fast lens so you don't have to crank the ISO up too much - This applies when you want to capture, say the Milky Way. You need to keep the exposure time to 30 seconds or less (20 is the norm).

This does not apply if you're going for star trails. The Bulb mode is typically use for that to expose for minutes at a time.
Diverhank's photos on Flickr


You need Tim Campbell.  He may see this thread and respond.

EOS 1DX and 1D Mk IV and several lenses!

Thanks for the tips! I'm not looking into getting anything pro, at least not right now. It took me half a year to save up for the camera and lens I wanted and the bone you suggested for astrophotography is $400+ Canadian. Maybe someday down the road if I come into some more money, I'll pick up that lens and give it a shot.

I also do want to try star trails, thank you for the info diverhank!


There are a few ways to do 'astrophotography' and varying degrees of equipment and difficulty involved.


The moon is possibly the easiest shot...  the key here is to use manual exposure mode.  The moon is so bright that the camera probably will be able to auto-focus on it... but the camera will be confused and probably not get the exposure correct.


Normally the metering system uses 'evaluative' metering.  Think of the whole image as being a grid (like a checker-board) and the camera has a metering sensor at each square on this grid.  It tries to meter the whole "scene" and find a nice middle exposure.  But since most of the scene is the blackness of the sky, it will assume it's just under-exposed and try to bring up the exposure in an effort to show more of the sky... which will result in a completely over-exposed moon.  So don't use auto-exposure when shooting the moon.  


The moon follows a very simple guideline called the "Looney 11 Rule" (I did not make that up ... that's the name).


This rule says that at f/11 (and only at f/11), the shutter duration for a correct exposure will be the inverse of the camera's ISO setting.  In other words if you use ISO 100, then the shuttter speed is 1/100th.  If you use ISO 200, then the shutter speed is 1/200th.  At ISO 400, the shutter speed would be 1/400th.  You get the idea.


You don't actually HAVE to use f/11... but that's the only f-stop where this inverse-relationship of ISO & shutter speed works correctly.




So f/11, ISO 100, and 1/100th would work great.  Easy shot to nail.  This shot was taken using the "Looney 11" rule (but I'm taking this through a telescope ... not a typical camera lens.)




The moon doesn't emit it's own light... it's being illuminated by the sun.  As in any photography... the light looks best when it's coming from an angle.  In the shot above near the 1st quarter moon, the Sun is illuminating from the right side (and low).  This means all the texture of the moon's surface will cast shadows and it results in the surface of the moon looking rather three dimensional.  If you took the shot near the full moon, the sun would be "behind you" (like using on-camera lighting) and that means the craters and mountains don't cast shadows.  This results in a flat two-dimensional look that isn't usually as interesting.


One last tip about the moon (but it could apply to anything).  There's a concept called "atmospheric extinction".   The idea is that the atmosphere above your head (if you are at sea level) is "1x air-mass"  But if you point your camera lower in the sky, the amount of air that light has to pass through is considerably thicker... 2x air-masses... 5x air-masses or more as it gets lower to horizon.  Air has particles in it that abosb light.  So when you shoot the moon very low in the sky, you might be getting only half as much light as when it's high in the sky.  This can require longer exposure times.  Actual adjustment will depend on air quality at the time.






Next up in the degrees of difficult & equipment are the nightime landscape shops with the starry sky (usually the Milky Way) overhead.


For this shot you simply need a solid tripod.  You probably also want a wide-angle lens (a lens with a very short focal length) and also usually a very low focal ratio.


The "problem" with this shot is that though the sky doesn't look like it's moving... it is moving (or rather the Earth is spinning).  It spins at a rate of 15 arc-seconds (angular rotation) in each 1 second of time.  In 4 seconds, the Earth spins 1 full arc-minute.    In 4 minutes the Earth will have rotated about one full degree.


This means that with the camera sitting on a stationary tripod, the stars will appear to move during a long exposure and this will cause the stars to appear to have "tails".


It turns out if the shot is short enough, you wont notice the movement.  


There are two different forumas to work out the exposure time ... I'll just describe the easier one.


The easy forumla was originally called the "600 Rule" but is more commonly called the "500 Rule" since camera sensors have improved resolution.  But this rule was meant for cameras that have "full frame" sensors.  "Full frame" means the digital sensor is the same size as a single negative of 35mm film.  That frame size is actually 36mm wide by 24mm tall (the term "35mm" refers to the full width of the film INCLUDING the area where the sprocket holes are located).  


Your camera is not a "full frame" camera.  It has an "APS-C" size sensor (that name was given because the sensor size is roughly the same size as APS-C film.  APS-C stands for "Advanced Photo System - Classic" size.  It was a bit smaller than 35mm film.  Your sensor is actually 1.6x smaller (meaning if you divide both 36mm and 24mm by 1.6 you get the size of your sensor... which is 22.5mm by 15mm.


This means that your camera will have a slightly narrower angle of view compared to a "full frame" camera when using the same lens.


I'll describe the rule... and then I'll describe the adjustment needed becuase you use an APS-C camera.


The rule says:  Divide 600 by the focal length of your lens and the result is the maximum number of seconds you can safely expose before you notice the stars are beginning to elongate.


The "500" base is more commonly used than the "600" base because photographers usually will see a tiny bit of elongation if they use the 600 base.  For extra sharp stars... they use 500.


Assume we have a 14mm lens... you'd divide 500 ÷ 14 = 35.7 (about 36 seconds).


BUT... that's for a FULL FRAME camera.


Since you have a camera with a sensor which is 1.6x smaller, you have to multiply the focal length of your lens by 1.6 to compensate.


Multiply 14 x 1.6 = 22.4


NOW divide 500 ÷ 22.4 = 22.3 seconds (about 23 seconds)


Notice how you can't expose for quite as long because your angle of view is a bit narrower?


So this is where "bulb" mode comes in handy because your camera wont actually have a built-in shutter speed for 23 seconds (but you could do that with a bulb timer.)


This also implies that now you're on a quest to capture as much light as you possibly can in just 23 seconds (assuming that you were using a 14mm lens).  This means it would be helpful if that lens had a low focal ratio (e.g. an f/2.8 or faster lens)


Canon makes a 10-22mm which means you could shoot at 10mm (that would give you 31 seconds) but it's only an f/3.5 lens.

The 14mm is f/2.8 which gives you 23 seconds BUT in that 23 seconds it collects more light per second.  When you work out the math, it can capture roughly as much light as an f/3.5 10mm lens could capture in 38 seconds (but it captures it in just 23 seconds).  So the 14mm f/2.8 would collect more light.


Canon's 14mm lens is very good... and also VERY expensive.


Canon also makes a 24mm f/1.4 lens (also VERY expensive).  At 24mm (24 x 1.6 = 38.4) you can only expose for 13 seconds... BUT it's an f/1.4 lens which means that you'd collect FOUR TIMES more light than an f/2.8 lens (which is already collecting 60% more light than an f/3.5 lens).  13 x 4 = 52.  So it's "as if" you could take a 52 second exposure with an f/2.8 lens... but this lens does it at f/1.4 in just 13 seconds.


Basically the game here is to collect as much light as you can without the stars and other deep-sky structures starting to smear due to the rotation of the Earth.





When you do astrophotography, it's pretty much always manual focus.  There's just not enough light (not even the brightest stars) to allow the camera's auto-focus system to work.  Also, the exposure is always manual.  You manually set the aperture and the shutter speed is typically in Bulb mode.  (On the T6 you rotate the shutter speed until you get to 30 seconds... and the next click beyond 30 is "bulb".)  This means you'll need a remote shutter release (more on that in a moment.)





Since it's always manual everything when shooting astrophotography, there's little point in having auto-focus lenses.  


Rokinon makes some lenses which are completely manual... they don't even communicate with the camera.  You manually focus and there's a ring on the lens to set the aperture.  They're fairly cheap lenses so they wont cost nearly as much as the Canon lenses.


The good news is that the optical elements in these lenses are actually pretty good.  If you get a good copy, you get pretty sharp images.  The bad news is that getting a good copy is a bit hit & miss.  Their qualtiy control isn't the best.  The #1 complaint with this lenses is "de-centered" optics.  In other words if you pointed the camera lens at something with fine detail on a completely flat surface (say... a rough texture brick wall ... or a sheet of newsprint taped to a wall (completely flat) and you center the lens so that it's pointed perfectly straight at that wall (not tilted upward/downward/left or right) then you should get reasonably even focus across the frame.


The truth is that the focus "plane" (for any lens) isn't really flat.  It's slightly curved (even though lens makers try to correct for this -- some amazing lenses actually do exceptionally well at this but those will be expensive lenses).  So a subject near the center of the frame might be fractionally better focused than something near the  corners.  BUT... if you compare the quality of focus in each of the four corners, it SHOULD be about equal (even if it's not quite as perfect as the center).  If you notice the focus appears noticeabley better in one or two corners than on the opposite corners AND you're sure your camera was pointed STRAIGHT at that flat surface (not tilted on an angle so there's no reason one corner should have been closer than any other) THEN you probably have a lens with the "de-centered" optics probelm (you can't fix this yourself... you have to send it back and try another).  


This can be a bit frustrating but if you manage to get a good copy, they're pretty good lenses.


If "anything" in space is correctly focused then "everything" in space is correctly focused.  


For this reason, when you are trying to manually focus the camera lens on the night sky, there are a few tricks.


#1 ... look at the focus ring on the lens and manually rotate to "infinity".  This will NOT be the correct focus (not by a long stretch) but it should be close enough to see the stars (strongly out of focus stars would just disappear and that makes focus really difficult).


#2 ... switch the camera to "live view" mode.


Canon has an interesting feature called "exposure simulation" in live-view mode and this feature is enabled by default.  SO... you'll crank the ISO up to max (I think that's ISO 6400 for the T6 but if it lets you go to ISO 12,800 then do it).  Set the shutter speed to 30 seconds.  


This is NOT the exposure you will use when shooting the night sky... we just want to focus.  Using these settings will amplify the image brightness on your live-view screen well enough to help you focus.


#3 ... point the camera to the brightest star you can find in the night sky.    This time of year that might be Vega or Arcturus or Altair or Deneb.  (Sirius is technically the brightest night-sy star but that will be setting).  This is JUST TO ESTABLISH FOCUS ... so even if this isn't the part of the sky you plan to shoot... just point it at one of these stars to help you focus.


VERY CAREFULLY rotate the focus ring to try to get the star as pin-point small as possible.  Also... try to perfectly center the star.  This is  still merely "closer" to focus... but not perfect.


#4 ... Use the magnifier in live-view to magnify the image by 10x.  


Now try to improve on the focus ... getting the star as pin-point as possoble.  


There are focusing aids such as the "Sharp Star" focus mask by Lonely Speck (not required but can make life easier).


#5 ... now that you're happy with focus... remember that if ANYTHING is in accurate focus in space then EVERYTHING is in accurate focus.  You can re-point your camera to the area of the sky you want to shoot.


If you used a focus mask, don't forget to remove it from the lens.


Don't forget to return your exposure settings to the setting you plan to use when shooting.  (e.g. ISO 800 or 1600 instead of ISO 6400) and return the shutter speed to Bulb mode.




It's nice to have either a computer using remote tethered shooting (e.g. USB cable) -OR- a "wired" remote shutter release cable.  Canon only makes one cable for your camera and that cable doesn't have a timer.  That means you would have to press & lock the shutter button down and manually keep track of the time until you end the shot.


There are third parties that make wired remotes for your camera that have built-in timers.  You can dial in any number of seconds you want.  (Check any of the major Internet camera stores such as B&H Photo or Adorama, etc. and you'll find a selection).  Be aware these SAME timers come with jacks for many different camera models.  You want the timer with the tip that works with Canon EOS "sub-mini" connection (it looks like the 1/8" stereo headphone jack).  All EOS Rebel and EOS mid-range cameras use that same connector.  Pro bodies use a different 3-pin connector.




One more tip... for this type of shot to work (Milky Way, etc.) you need to get as far away from light pollution sources as possible.  If you live in the city, this likely means you'll be doing a drive out in the countryside to get away from urban light pollution.  ALSO... the moon is a huge source of light pollution.  This means you'll want to take these types of shots when the moon is not visible in the night sky (near the "new moon" phase... when the moon is mostly up during daytime hours.)


I actually don't do this type of photography (even though it's one of the more popular forms) *because* I have to travel quite a bit to get away from the light pollution and I leave far enough north that I don't get much Milky Way (except for the months of many July & August) and often the atmospheric conditions don't cooperate.  


Tim Campbell
5D III, 5D IV, 60Da


To take even LONGER exposure and even NARROWER areas of sky, you need a mount that compensates for the rotation of the Earth.


There are "tracking" heads that can attach to a photo tripod (hopefully it's a sturdy tripod).


The two major vendors for these heads are Sky Watcher and iOptron.  Both companies make two different heads (a total of 4 choices).  Their low end heads are designed to hold about 5.5 lbs.  Their high-end heads claim to hold 11 lbs. (exactly double the weight).   The lower weight versions cost about $300 and the higher end versions are closer to $400.


These heads have a motor (with adjustable speeds).  One speed is designed to rotate at EXACTLY the same speed that the Earth spins (known as "sidereal" rate) (looks like "side real" but actually pronounced "sid eery al".  "sidero" is a greek word meaning "of the stars".  So "sidereal" rate means "the rate of the stars".  It's actually about 4 minutes less than a 24 hour day.  This because the Earth actually completes a 360 spin on it's axis in just 23 hours, 56 minutes, 11 seconds (and a fraction of a second).  NOT 24 hours.  But after that much time, the Earth will have moved forward by about 1° in it's orbit around the Sun (360° in a circle... 365 days in a year ... so it's about 1° per day).    Once the Earth has progressed forward by nearly 1°, it has to keep spinning an extra 4 minutes to return the Sun to the midpoint in the sky (from noon one day to noon the next day).  This is why we think of a day as being 24 hours even though that's NOT the rate of the Earth's spin.


The idea behind the tracking head is that you align the head so that the axis of rotation is precisely parallel to the Earth's axis of rotation.  In other words you point the head to a spot in the sky which is very nearly pointed at Polaris (the North Star).  Polaris is really about 2/3rds° off true north but the head comes with an alignment aid that helps you find the right spot).  This way as Earth spins from West to East, the tracking head will spin from East to West at EXACTLY the same rate.  Since the two axis (earth and the tracking head) are parallel... the head will cancel out the rotation of the Earth and allow you to take VERY long exposures of the night sky.


This will let you get shots that look like this (Canon 60Da, Canon EF 135mm f/2L USM lens and a tracking head.  This is an HDR using 2 minute exposures, 1 minute exposures, 15 second exposures, and 3 second exposures).  But I also took an 8-minute test shot to make sure the head was aligned (stars will appear to "drift" if the head alignment isn't good).  


Orion HDR.jpg


Continuing to move up in the gear requirements and degree of difficulty.  You can attach your camera to a telescope.


To do this, you typically buy a camera nose-poiece which allows you to attach the camera to the telescope in such a way that the telescope becomes the camera lens.  Telescopes normally expect to have an eyepiece attached.  The two industry standard barrel sizes for these eyepieces are 1.25" barrels or 2" barrels.  They make a camera nosepiece in either size... and the back of it has the same bayonet mount that any lens would have.  This lets you lock it to your camera and slip it into the eypeice holder for your telescope.


Most telescope types will work EXCEPT Newtonian reflectors.  There's a category of newtonian reflectors that are designed for this (they are called "Newtonian astrographs).  


But the MOST important thing in this type of photography is that the telescope MUST be equatorially or "polar" mounted.  It must NOT use an alt/az type mount (otherwise you get a problem called "field rotation")


This would let you use significantly longer focal lengths that telescopes use.  But a problem is that it does need to be a very high quality mount (if you spend less than $1000 on the telescope mount (nevermind the cost of the telescope... I'm just talking about the price of the mount) then the mount is probably not going to be up to the task.  My general guideline is that mounts *start* to get good once you exceed the $1500 price range.  The more expensive it is... the better (no kidding).


Serious astrophotographers know that the mount is the most critical part of the whole rig... it is generally the most expensive piece and costs more than the telescope or camera.


As the field of view can get very narrow, it's helpful to do "auto-guiding" ... which involves a special guide-camera and often a second (small) telescope attached to the same mount (there are actually lots of ways to set up an auto-guider camera and software).  The idea is that the guide-camera images a single star every couple of seconds and compares the pixel position of that star to a master frame to see if the star appears to be drifting.  If it is drifting, the guider will send a correction to the mount to keep the telescope tracking on target.  This is more difficult than you might imagine and it's the reason astro-imagers spend a lot on these mounts and use computer software to manage the tracking accuracy.


Doing that you can take much longer exposures.  This was taken using more than an hour's worth of 8 minute exposures.


Andromeda & Companions


That is NOT what the image would look like straight out of the camera.  The raw image data would have appeared to be mostly black & white (it has hints of color).  Loads of processing work went into combining all the data to get that result.



Tim Campbell
5D III, 5D IV, 60Da

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