Apeture vs Sharpness In Astrophotography

[Speed]

I've been meaning to post these since Justin - JKeena - asked his question in the help filees. I've also noticed a few other posts regarding night/astrophotography.

I've been told time and again to stop down the apeture to get sharper photo's. True in terrestrial photography, but not really that applicable in astrophotography. So I ran this experiment, to prove that everything is in focus at f1.4 in astrophotography. All stopping down does is cost you light.

All of these photo's were taken with my N80 on a tripod, Nikkor 50mm f1.4 lens, 30 second exposures, and they were taken consecutively on the same roll of Superia Xtra 400. The only thing that was changed was the apeture. The first shot was taken at f1.4, the second at f2.0, the third at f2.8 and the last one at f4.0. As you can see, the stars didn't get sharper, but they sure got dimmer.

BTW, this is the constellation Cygnus, the Swan. It is also called the Northern Cross. And the Milky Way runs right through it, which is the glow you see at the wider apetures.

[Kokopeli]

All of these photo's were taken with my N80 on a tripod, Nikkor 50mm f1.4 lens, 30 second exposures, and they were taken consecutively on the same roll of Superia Xtra 400. The only thing that was changed was the apeture. The first shot was taken at f1.4, the second at f2.0, the third at f2.8 and the last one at f4.0. As you can see, the stars didn't get sharper, but they sure got dimmer.

Basically alll you did was keep the 30 second exposure constant and then reduced the light by stopping down the aperture? It only makes sense that the photos taken with the same exposure time and less light would be dimmer and the ones with more light getting to the film would be brighter.

Are you sure people weren't suggesting increasing your exposure time in conjunction with stopping down the aperture? That's the only thing that makes any sense to me, but since I don't do any real astrophotography, I wouldn't necessarily know what I'm talking about...

Interesting thread regardless. Thanks for sharing the results of your experiments!

Nikon Samurai #3
~Brian

[Speed]

Basically alll you did was keep the 30 second exposure constant and then reduced the light by stopping down the aperture? It only makes sense that the photos taken with the same exposure time and less light would be dimmer and the ones with more light getting to the film would be brighter.

Are you sure people weren't suggesting increasing your exposure time in conjunction with stopping down the aperture? That's the only thing that makes any sense to me, but since I don't do any real astrophotography, I wouldn't necessarily know what I'm talking about...

Interesting thread regardless. Thanks for sharing the results of your experiments!

Nikon Samurai #3
~Brian

All I did was stop down the apeture. Everything else remained the same. The obvious point was that there would be less light recorded, hence fewer stars and less of the Milky Way showing. The not so obvious point is that the stars are just as sharp at f1.4 as they are at f4. I've had numerous people tell me to stop down one or two stops for sharper images - on star shots. The nearest star is 25 Trillion Miles away. I am quite confident that everything, and I do mean everything, from that point on is in focus at f1.4. Which is why I conducted this experiment.

As for stopping down with long exposures, stopping down may (or may not) make a difference. When I get a piggyback camera mount for my Celestron, I'll be sure to experiment with that one too! :-)

I will readily admit that a cheap lens may (probably?) give blurred images when shot wide open. But a quality lens shouldn't be a problem. I get some vignetting in the corners when I take night shots with my 50mm wide open, but I don't consider it a major distraction on star shots.

As I mentioned, this is for anyone who has questions regarding night shots with stars. Metering For Night Sky Help
If you want pinpoints for stars, and lots of them, shooting wide open is the way to go. Faster film also helps, and 800 speed will pick up more than 400 speed (obviously), but I'd be wary of making prints larger than 8X10 with them. That's why I usually stick with 400 speed. If you want the most stars and glow that you can get, then 800 speed is the way to go.

Another interesting observation I made, if you shoot at f2.0 for 30 seconds, 400 speed film records about what your eye can see. Shoot wider than that, and you pick up more stars, nebula, Milky Way than your eyes can detect. Getting your photo back and seeing more stars on the print than you saw with your eyes is really cool. An example follows.

Look at the fuzzy "star" in the middle of the frame. That "star" is actually the Andromeda Galaxy, 2.3 million light years away. It is the farthest object that can be seen with the naked (unaided) eye. On a dark night you see a dim star. With a 30 second exposure at f1.4 you can actually see part of the arms. Now that is cool! In My Humble Opinion, of course. :-) Best of all, it didn't take fancy equipment. I used my 50mm lens for this shot as well.

Robert, AKA Nikon Samurai #1

[Kokopeli]

Look at the fuzzy "star" in the middle of the frame. That "star" is actually the Andromeda Galaxy, 2.3 million light years away. It is the farthest object that can be seen with the naked (unaided) eye. On a dark night you see a dim star. With a 30 second exposure at f1.4 you can actually see part of the arms. Now that is cool! In My Humble Opinion, of course. :-) Best of all, it didn't take fancy equipment. I used my 50mm lens for this shot as well.

That is pretty cool Robert. Thanks for the tips and for sharing that image!
~B

[Sebastian]

Speed,

to me, the first shot is MUCH softer than the rest. Notice how much less blooming there is in the larger stars in the later images?

As for high quality lenses vs. cheap lenses, ALL lenses will be soft wide open, it's just a matter of how soft. You didn't pay big money for the 1.4 because it's sharp wide open, you paid for it because of the sole reason that it GOES to 1.4. I have never had a lens that is equally as sharp wide open as it is stopped down, nor have I ever read of one. Light being what it is will scatter at maximum aperture and minimum aperture, only in the middle does one get the maximum sharpness from a lens.

I would take a good look at the negs with a loupe, your finding go against everything else anyone has ever experienced.

I might be going out to Myrtle Beach sometime this year with my friend from NC, I'll keep you posted if I make my way down there.

Keep up the work, I love seeing how different yet similar astronomy and photogramy are, and it's all thanks to your posts here. Thanks!

[Speed]

Speed,

to me, the first shot is MUCH softer than the rest. Notice how much less blooming there is in the larger stars in the later images?

As for high quality lenses vs. cheap lenses, ALL lenses will be soft wide open, it's just a matter of how soft. You didn't pay big money for the 1.4 because it's sharp wide open, you paid for it because of the sole reason that it GOES to 1.4. I have never had a lens that is equally as sharp wide open as it is stopped down, nor have I ever read of one. Light being what it is will scatter at maximum aperture and minimum aperture, only in the middle does one get the maximum sharpness from a lens.

I would take a good look at the negs with a loupe, your finding go against everything else anyone has ever experienced.

I might be going out to Myrtle Beach sometime this year with my friend from NC, I'll keep you posted if I make my way down there.

Keep up the work, I love seeing how different yet similar astronomy and photogramy are, and it's all thanks to your posts here. Thanks!

I may be crazy! :-)
BTW, Billy Joel came up with that one (Glass Houses), not Asylum Steve. Though I highly respect Steve for using it in his signature line!!! I'm really dating myself here...

Yes, I see what you mean. I'll have to check the negative with my loupe. The question to me is, are the stars "soft" from the lens being wide open (probably a factor), or are they "soft" from more photons hitting the negative (probably a factor as well). A point source of light, such as a star is going to show up as a larger object with more exposure. And the brighter the star, the more pronounced the effect. I've seen enough star shots to know that. And I've already mentioned the vignetting I get. It happens with the best of lenses. The question is, is it objectionable or is it tolerable in the photo?

I know that all lenses are soft when shot wide open. I just recently read in a book by Art Wolfe that most of them are soft when stopped all the way down as well. Blew me away. I always wondered why guys like Galen Rowell shot at f8 instead of f16 or f22. Now I know.

Yes, all lenses are sharpest when shot somewhere in the middle, generally f8 or f11. When I get my piggyback mount, I'll try some shots with different apetures again. The Orion Nebula should make a wonderful subject to try that one on! It will probably take me days or weeks to complete that project, but boy will it be fun. :-)

Let me know when you got to Myrtle Beach. It's only 2 1/2 hours from my house. Easy day trip! Actually, Dawn and I are going to Myrtle Beach the week of April 18th through the 25th, for our annual get away. :-)

Yes, photograrphy and astronomy are different, yet they overlap. In fact, they overlap so much, that a friend and I are planning on starting an Astronomy/Photography Club. Not every photographer is interested in astronomy, but nearly every astronomer I know has some interest in photography. It's a convenient arrangement, as it allows me to combine two of my favorite hobbies!

Thanks for the comments my friend!