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05-17-2018, 07:03 PM   #1
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Further Vivitar 120-600mm test : the Moon

Here's a quick and dirty test shot of the Moon with the Vivitar 120-600. Shot with the K-3, on a cheap tripod (not good enough, I think). ISO 1000, 1/25 seconds, 10 seconds timer. Focus at fully at infinity, but it almost seemed like the lens would have benefited from some more leeway in this regard.

Shot in RAW, processed in Lightroom (fixed CA, added clarity, cropped).

Click on the image for a full size version. Enjoy!



05-17-2018, 07:33 PM   #2
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Well, nice one but in future I would set f/8, about 1/200s and iso100.
05-18-2018, 05:13 AM   #3
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QuoteOriginally posted by nobody Quote
Well, nice one but in future I would set f/8, about 1/200s and iso100.
It was at F11, I wanted to see if it was significantly sharper than F8.

The settings you propose would have required heavy adjustments in post-processing, bringing up the equivalent ISO/gain to more than what I used. There was not enough light to use 1/200. I agree that it would have yielded a sharper picture still.

I'll keep looking at the Moon and try to take advantage of its brightening in the following two weeks.
05-18-2018, 06:22 AM   #4
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Nope. You are wrong..

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05-18-2018, 08:10 AM   #5
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I'd tend to agree with Nobody. Unless you have a tracking mount with a Lunar setting, 1/25 is pushing it for the moon. It moves surprisingly fast. 1/60 is usually as slow as I'll go.
05-18-2018, 09:04 AM - 1 Like   #6
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QuoteOriginally posted by SteveinSLC Quote
I'd tend to agree with Nobody. Unless you have a tracking mount with a Lunar setting, 1/25 is pushing it for the moon. It moves surprisingly fast. 1/60 is usually as slow as I'll go.
Actually, 1/25 sec should be fine for lenses up to 600mm or so. The moon moves less than an arcsecond, which is less than a pixel.

What exposure length you need is easy to calculate - at least in order to avoid blurring due to motion of the moon during an exposure. Of course, other factors like focus setting and tripod stability are important too. Be sure to check your lens for focus at infinity! Virtually every lens I've ever checked does not focus at infinity when the focus ring is set to the "infinity" stop limit. They go "beyond" infinity!! Use live view on the moon to set your focus.

Although the moon is moving with respect to the background stars at a rate of about 12 degrees per day, you can pretty much just use the sidereal motion rate (the amount by which the sky "moves" due to the rotation of the earth) of 15 arcseconds per second. (The lunar orbital motion means an error in this rate of 12/360 - or about 3%.) (Even more technical: the moon is always within 30 degrees of the celestial equator which is at declination zero degrees. As you move away from the equator, the actual angular rate decreases by the cosine of the declination. Since cosine(30degrees) = 0.87, the lunar rate will not really be much less.)

If you know the arcseconds per pixel scale for your camera (which will be about 1.4 arcseconds per pixel for your K-3 / 600mm set up. See Easy Determinations of Camera Scale Factors (arcseconds per pixel) - PentaxForums.com for how to do this. Or just check K-1 Scale Factors - Arcseconds per pixel for several lenses - PentaxForums.com where I mention my results for for a 300mm lens on a K-3.), you can estimate how much pixel blurring your exposure might incur.

At 600mm, the moon will move one pixel in about 1/10 second. Your 1/25 should be OK in this regard.

Last edited by AstroDave; 05-18-2018 at 09:07 AM. Reason: Fixed lunar rate
05-22-2018, 05:15 AM   #7
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QuoteOriginally posted by nobody Quote
Nope. You are wrong..
About what? About a thin sliver of the Moon, lit from the side by the Sun, being darker than an almost full Moon lit almost straight on? I don't think so.

QuoteOriginally posted by SteveinSLC Quote
I'd tend to agree with Nobody. Unless you have a tracking mount with a Lunar setting, 1/25 is pushing it for the moon. It moves surprisingly fast. 1/60 is usually as slow as I'll go.
Again, not much to do about that exposure thing

See below, a better lit Moon gives much better exposure settings.

QuoteOriginally posted by AstroDave Quote
tripod stability
Yep. As I wrote, I will improve on that front. Below are more shots from this week-end, where I hung about 2 pounds under my current tripod. I think it helped, but not as much as you'd think.

QuoteOriginally posted by AstroDave Quote
Be sure to check your lens for focus at infinity! Virtually every lens I've ever checked does not focus at infinity when the focus ring is set to the "infinity" stop limit. They go "beyond" infinity!! Use live view on the moon to set your focus.
With the Vivitar, at 600mm, I feel that infinity is not quite infinity. It seems I'd be better off with just a tiny bit more movement.

BUT...

I ran comparative tests, a few seconds apart, at 600mm and about 550mm. The difference is stunning. I used 10 seconds timer, electronic shutter, manual everything. Here goes:

600mm


550mm


I think using the Vivitar just below 600mm will be the way to go. That is, unless I find the way to adjust its infinity focus at 600mm. Which, given the rarity of that lens, I don't think I'll find information about.
05-22-2018, 11:14 AM   #8
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You sure it's not focusing past the moon at 600?

05-23-2018, 05:07 AM   #9
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QuoteOriginally posted by ZombieArmy Quote
You sure it's not focusing past the moon at 600?
If anything, I'm focusing in front of the Moon. When adjusting the focus, it seems it would be better if it could go just a bit further, but there's a hard stop. Maybe I'll find my courage and play with that a bit. But I could mess up the whole lens, so I'm not sure.
05-23-2018, 08:26 PM   #10
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Good luck with that B. It looks really nice @550mm though!
05-23-2018, 08:32 PM   #11
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QuoteOriginally posted by bdery Quote
If anything, I'm focusing in front of the Moon. When adjusting the focus, it seems it would be better if it could go just a bit further, but there's a hard stop. Maybe I'll find my courage and play with that a bit. But I could mess up the whole lens, so I'm not sure.
Might be worth getting someone to look at it. Fixing the focus might make things better at infinity across the range, not just 600mm.
05-24-2018, 05:09 AM   #12
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QuoteOriginally posted by ZombieArmy Quote
Might be worth getting someone to look at it. Fixing the focus might make things better at infinity across the range, not just 600mm.
I'll consider that. The difference between 600mm and 550mm makes me think the problem might be limited to 600mm, but I can't be sure. I looked (and haven't found) any information on how to adjust the focus on that lens, I might find some courage and try to do it myself, or maybe pay a local repair shop in town.
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