[ismaelg]

Hello,

I was wondering: When shooting long exposures in Bulb mode like shooting for star tracks and such, Does the aperture setting makes any difference?

Thanks,

[Mike Cash]

Aperture always makes a difference.

What he said. ^

[netuser]

The fast the aperture the less time you need to expose.

[DAZ]

Aperture always makes a difference.

What he said. ^

The fast the aperture the less time you need to expose.

Aperture controls the amount of light through the lens. This plus the t/stop of the lens and any filters determines to total light the shutter lets though the lens. So for example you may want to stop down for fireworks so you get the colors instead of white streaks.

Aperture controls (along with lens length and focal point) the depth of field. This makes focus more critical. On most lenses focusing the lens all the way out is not focusing to infinity. Not counting that you may want something in the foreground to be in focus, trying to focus in the dark on stars at infinity can be darn near impossible at f/1.2. If you use f/4.5 it is not as critical.

Aperture controls lens resolution. Most lenses are softest wide open and improve by stopping down. A lens wide open can almost look like it is a little out of focus compared to the same lens stopped down 2-3 stops.

DAZ

[Mike Cash]

Not star trails, but here are a couple of shots which may help to illustrate Daz's second paragraph, taken from a tripod without moving the camera and identically framed:

Both were taken with the K20D and SMC Takumar 50/1.4. The first photo was focused on the leaves in the foreground and is 1/4 second @ f2. Notice the effect on the background and that it was fast enough to minimize the wind-induced motion blur on the leaves, but not fast enough to smooth the water. The second photo was focused on the rocks and is 15 seconds @ f16. Notice the much greater depth of field and that the shutter speed was slow enough to smooth the water, but the branch danced all over the place.

More directly related to the original question, though, if for example you need 10 seconds exposure at f4 you're going to need 20 seconds if you change to f5.6 (or 5 seconds if you go to f2.8). While in Bulb mode the shutter is open for a long time, in the end it is a finite amount of time. The total time the shutter is open is still a factor in exposure, just like with faster shutter speeds, and the same is true for aperture. Both are factors and the relationship between the two does not change.

[ismaelg]

Thanks all. My question was not clear. I do understand aperture and how it works.
My question is more specifically: In a 2+ hour bulb exposure, is there a rule of thumb to start with? something like set to minium, max, halfway? I'll be experimenting with that shortly.


Thanks,

[DAZ]

I am not aware of any “rule of thumb” for long exposure. All I can give is some of my thought when I take longer photos.

My first thoughts are am I taking the photo with a long exposure because it is dark and I would like it if I could take a shorter exposure. If I would be happier with a shorter then I look to how to do that. Faster aperture, faster ISO. But as I think you are looking for, if the longer exposure is what I am looking for, smooth water, streaking fire works, star trails then I look to optimize the other things. I you are looking for star trails in a 2+ hour long exposure you are going to need an extremely dark and clear sky. Just a little clouds over a near by town will give you sky glow. When I have gone for star trails in theory fast aperture and ISO will give you more stars. The problem is faster ISO will usually eat you up with noise so going slower is usually better. I know you are asking about aperture. Wide open should give you more stars (assuming sky glow doesn’t kill them) but for me I have better luck going to some thing like f/4. Part is the focusing on infinity in the dark and part is IQ. If I am trying to get some foreground then f/8 and calculate/set the lens to hyperfocal distance. When you take the shutter out of the equation for exposure and have to keep ISO down for noise then you have to use aperture for everything else. If there is some thing in the frame that is too bright you have to stop down. This is why some take many shorter exposures, say 15 seconds and stack the photos PP. Doing that you can open the aperture some and up the ISO a little, possibly getting more stars. With shorter exposures you can see what you are getting and make some adjustments with out wasting 2+ hours.

With shorter for like water, fireworks, cityscapes I usually start at best MTF for the lens then adjust for DOF or to not blow exposure/lights, what ever is most critical.

That’s about all my very little wisdom.

DAZ

[ajuett]

For a 2 hrs exposure I'd close the lens right up, F22 or slower. Lots of tips here; Astrophotography Techniques

[Mike Cash]

If you're considering a 2+ hour exposure with a digital, you have to take into account sensor heating/noise during such a long exposure, as well as the fact that your battery may or may not last that long. You may want to consider using an AC adapter.

If you're using film, you need to be aware of reciprocity failure.

[ismaelg]

If you're considering a 2+ hour exposure with a digital, you have to take into account sensor heating/noise during such a long exposure, as well as the fact that your battery may or may not last that long. You may want to consider using an AC adapter.

If you're using film, you need to be aware of reciprocity failure.

Thanks. What is reciprocity failure?

[Rustyoldbug]

I am no expert, far from it actually - but we were just out this last weekend doing star trails with my old film Pentax cameras and shooting directly through our telescope with the SO's "non-pentax" digitial.

Ran a bunch of test shots with the K1000s to figure out what worked the best via trial and error before we drug all the gear and cameras out to the desert. There is a lot of info out on the internet, but a lot of it conflicts. So I gave up and went out with several rolls of film and lenses and tried various aperture settings, film speeds, color and black and white, and exposure times starting at 30 seconds, then 1 minute, 5 minutes, 10 minutes, 30 minutes, 1 hr, 2 hr, & 4 hrs.

For star trails with film & long exposures I got the best results for really long trails:

*Using 100-200 film ( the 400 was a lot better for the short trails and picked up fainter stars in the distance better)

*B&W film is clean and nice, the color stuff would get weird shifts with the longer exposures which is cool in a way, but... that reciprocity comes into play here

*of course the darker the better (we are trying again next weekend with "new moon" making it really dark)

*Aperture opened all the way up or stopped down one from wide open

Get a really good tripod. The shots from the camera that was piggy backed on the telescope were perfect - but the tripod and mount alone are 60-70 lbs. The shots done with the 2nd camera on the smaller tripod which I thought was pretty sturdy would get shake from breezes and me picking up the end of the locking cable release.

Also going to setup again tonight (tomorrow morning really) for the Moon eclipsing Venus:

The Steamboat Pilot & Today: Jimmy Westlake: Moon to eclipse Venus

Other things we learned this last time out -
Bug repellent is good and bring snacks for the police if you are in a park that is suppose to close at sunset. Being very nice to the coppers - not only did they leave us be, but came back a few times to make sure we were ok and to look through the telescope.

[Sean Nelson]

Thanks. What is reciprocity failure?

Film gets less sensitive the longer you expose it. So a 2-hour exposure wno't produce an image "twice as bright" as a 1-hour exposure. This effect is negligible until you get into exposures measuring several seconds or more.

Digital sensors don't have this problem, but as mentioned above they do have noise issues, some of which stem from the sensor getting hot when powered up for a long period of time.

[ismaelg]

Thank you Sean!

[KungPOW]

For night shots of cityscapes etc, stop the camera down to get sharp pointy "stars" where the highlights and bright lights are.

Having an open aperture will give fuzzy highlights.

As for star trails...What the other guys said