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View Full Version : Reducing Aperature vs. Reducing shutter speed; which improves Moon sharpness more?



spaceboy0
2008-Feb-24, 09:57 PM
Some of my Moon photos have turned out a bit fuzzy. Will reducing the aperature (dropping the f to f 5.5 or f 8) achieve the same effect as reducing the shutter speed?

Will reducing aperature improve sharpness more than just reducing shutter speed? In other words, if I operate at f 5.5 or f 8 at 1/30 speed, will that achieve the same sharpness as f 1.4 (wide open) at 1/60 speed?

RickJ
2008-Feb-24, 11:13 PM
Without seeing the pictures I can't say what was causing the fuzzy moon. I take it you are using a camera with some lens rather than a telescope to take the moon. What lens focal length, what are you mounted on, is it tracking the moon or stationary? Most camera lenses perform better stopped down one or two f stops. As you increase focal length of the lens the exposure time usually lengthens as longer lenses usually are slower. Yet to stop the earth's rotation if on a fixed tripod, you may need shorter exposures with longer lenses just when you need longer ones to get enough light. Still at 1/30th a second a 400mm lens should still work.

Vibration can be a big enemy of astro photos. Just touching the camera to trip the shutter can blur an image with a long lens. So to will the flop of the mirror if using a DSLR. Lock the mirror up then trip the shutter either with a cable release or use the delay timer so all vibration dies out by the time the shutter takes the image.

Each time you stop a lens down one stop you must also double the exposure time. So if you are using 1/60th of a second at f 1.4, you'd need 1/2 second at f/8 not 1/30th! That's a 5.5 f stop change. 1/30th of a second would be appropriate for f/2 not f/8!

Rick

spaceboy0
2008-Feb-24, 11:28 PM
yes it's a 35 mm film SLR on a tripod. I'm using a 200 mm lens with a 2 x doublet.

Strangley at 200 mm the image is okay, but as soon as I introduce the 2 x doublet, the image is a bit fuzzy. Not really fuzzy, just a bit.

So I figure it must be because the 2 x doublet is a bit crappy, so mabey I should stop it down to f/5.5 to make the image sharper?

Then again, by stopping down the lens, are you not just improving the quality around the edges of the image and not so much in dead center? I don't see how stopping it down is going to help if I keep the Moon in the dead center of the field.

RickJ
2008-Feb-25, 01:25 AM
You didn't mention that extender before!

Yes, these "barlows" fuzz up an image. I've never seen one that didn't. Stopping down helps but not a lot. I've never seen one suitable for astronomy use.

A camera lens is normally designed to just provide enough resolution for a sharp image on fine grain film at a couple stops down from its wide open f stop. Double the focal length doubles the resolution needed for a sharp image and suddenly you have a fuzzy image.

So yes, stopping down a couple stops does usually improve a lens' image quality. Same as you see more clearly in bright light with your pupil stopped down than you do in dimmer light with it more open. Eye exams are done in the dark for this reason.

Since your lens is already working to the best of its ability at 200mm doubling this just brings out the defects you weren't supposed to see. While your tele-extender may be part of the problem, I doubt even the best will work very well for you. Lenses aren't normally designed for it, the cost would be too high with no gain for most users.

So why are lenses not designed to be sharpest wide open? Cost for one thing. But there's more to it. Lenses are rarely used wide open. They are normally used stopped down some. Once you stop a lens down from its sharpest point it gets fuzzier. Most photographers don't want a lens to be less sharp at the settings the use most. They want them sharpest where they use them the most. So they are designed with that in mind. Those of us that use lenses wide open most of the time quickly find we need very high quality, usually expensive, fixed focus (not zoom) lenses that don't fall off very much when wide open. Though today's lenses are far superior to the those I used 50 years ago its still a major issue. Still, I've not found one that can handle a focal length extender and still be as sharp as it was without it.

Enlarging a fine grain film image usually provides a much more satisfying image than using an extender. At least that was my experience in my film days.

Rick

spaceboy0
2008-Feb-25, 03:33 AM
For the Venus transit in 2004 I used a 2 x and a 2 x piggybacked together (giving 800 mm?) and I was also photographing through Mylar film.

Would these two reasons result in the poor result I got? Mylar film does fuzz it up a bit yes?

RickJ
2008-Feb-25, 04:41 AM
Mylar isn't necessarily bad. That used for commercial solar filters is fine. They make very good filters often superior to glass ones. So if you were using a commercial mylar solar filter that is fine. But one extender pushes a lens too far as you found with the moon. Two is disaster! Also it pushes most tripods too far as well. Unless you locked the mirror and used a cable release or delay vibration also likely made a bad situation worse.

Rick

spaceboy0
2008-Feb-26, 02:43 AM
Would you say I'm better off with one single 400 mm lens rather than a 200 mm lens coupled with a 2 x doubler?

Am I better off with a single 600 mm lens than a 200 mm coupled with a 3 x?

spaceboy0
2008-Feb-26, 02:47 AM
I just noticed something. In my photos with the 200 mm coupled with the 2 x doubler, the star on the edge which is Saturn looks like a comet.

This must indicate coma. Instead of a pinpoint light, Saturn looks like a comet.

This must be the coma effect caused by the 2 x doubler. In the 200 mm photos the image is sharp.

Mind you you wouldn't think the coma would be a problem in the center of the image.

RickJ
2008-Feb-26, 03:23 AM
Would you say I'm better off with one single 400 mm lens rather than a 200 mm lens coupled with a 2 x doubler?

Am I better off with a single 600 mm lens than a 200 mm coupled with a 3 x?

After all that dialog about how they can't help but only ruin the image you still insist on trying to add yet another one. And it's not just the extender's fault. Lenses are designed to just work at their designed focal length to keep costs down. Extending them just shows the defects.

Now three times you've had bad results with them. Learn from that.

LOSE THE EXTENDERS

For 400mm and longer consider a small ED refractor or cassegrain mak scope or lens.

Rick

Dave Mitsky
2008-Feb-26, 05:15 PM
LOSE THE EXTENDERS

For 400mm and longer consider a small ED refractor or cassegrain mak scope or lens.

Rick

I agree. Also, FYI, there is no a in the word aperture.

Dave Mitsky

spaceboy0
2008-Feb-26, 07:07 PM
At football game you see photographers on the sidelines with big telescopic lenses. Just curious what FLs they use for those great shots you see of plays in the newspapers.

fotobits
2008-Feb-29, 11:13 PM
Usually they use 400/2.8 or 600/4 lenses for those sideline shots. You'd also be surprised how much cropping happens before publication.

Regarding the discussion above, I've been a professional photographer for 20+ years, have tried just about all the equipment out there, and have never seen a 2x converter I considered acceptable. Even 1.4x converters will soften an image, and all but the most expensive introduce color artifacts.