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PostPosted: Thu Apr 23, 2009 4:05 am 
Hi there,

Can anyone tell me if the image clarity (for solar photography) would suffer if I use a 2x teleextender with 55-250mm IS canon lens + Canon Rebel XTi? I am preparing for the coming Solar Eclipse.


This site gives Field of View and Size of the Sun's Image for Various Photographic Focal Lengths:
http://umbra.nascom.nasa.gov/eclipse/98 ... raphy.html


As per the above site, if I use a 2x Teleconverter then at the max zoom the focal length of the lens should be 500mm, which should be Ok.

Or should I have to purchase a portable telescope for this photography?


regards,
Phani Kumar.


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PostPosted: Thu Apr 23, 2009 9:34 pm 
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Hi Phani, I always use a 2x teleconverter when shooting the moon or the Sun through my telescope to boost the magnification. I used a Canon 400mm f5.6 plus 2x for one eclipse and my 504mm telescope plus 2x for another eclipse.

Of course a converter will also magnify any problems with the lens, so you have to take this into consideration. I've not used the 55-250mm so I can't say how well it will handle a 2x converter. I suggest trying some test shots on a distant subject like a building.

Also experiment with focusing at infinity and also triggering the shot - will you use a self timer, a cable release or handheld? Remember for the corona shots, your exposure may be too long to handhold without shake - even with IS. Also remember the Sun will move gradually across the frame, so watch out if you're using a tripod - you will need to adjust it during the eclipse.


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PostPosted: Fri Apr 24, 2009 3:01 am 
Gordan, thanks for your reply.

Yes, you are right. The lens seller also suggested that: "a 2x converter will decrease the quality of the image. Again it depends, sometimes it may work out and sometimes not".

Yes, I will use a Tripod and a IR remote (Photix, bought at ebay) or a self timer.

-> It will be helpful for me if you can suggest how exactly to check the quality of the 55-250mm and a 2x or 1.5x converter while buying? As you said, will a distant building alone will do (in which time of the day) or should I also try to photograph a celestial object?

regards,
Phani Kumar.


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PostPosted: Fri Apr 24, 2009 7:21 am 
Gordon Laing wrote:
Hi Phani, I always use a 2x teleconverter when shooting the moon or the Sun through my telescope to boost the magnification. I used a Canon 400mm f5.6 plus 2x for one eclipse and my 504mm telescope plus 2x for another eclipse......


would it be possible for you to post couple of the pics you have taken with the above combination?


thanks,


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PostPosted: Fri Apr 24, 2009 10:16 pm 
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Sure thing - here are some shots I took with a Canon EOS 5 (yes, the 35mm camera, not the digital!), during the 2001 Total Solar Eclipse in Zambia. I used the 400mm f5.6 lens with a 2x converter on a tripod with a fluid head to allow me to adjust the position without nudging the Sun out of the frame! There was no tracking here, so I had to wait for the camera to settle after adjusting, then take exposures no more than about 1 second to avoid motion.

The first two shots are uncropped, so show the area you'd capture at an effective focal length of 800mm f11. The thid shot is a cropped area that's been reduced slightly, so you're viewing at approx 66%.

The film as I recall was Fuji Provia 100F. The scans were made on a high-end drum scanner.

All were taken in quick successon but at different shutter speeds - don't ask me what speed - it was 8 years ago! But the series ran from about 1/100 to 1 second, halving the exposure every time.

Image

Image

Image


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PostPosted: Sat Apr 25, 2009 2:44 am 
Aaaah those are cool pics, Gordon.

So, if that is the Image size of the sun with 400mm+2x converter, then I should expect much smaller image with a 250mm+2x converter right?

And regarding the shutter, from your images i learnt that while at eclipse, one should take lots of photos with series from 1/100 or less to 1 sec depending upon the lens and ND filter which is placed on the lens..

Waiting for the ND (density 5, i suppose with this the image would be darker) filter which I ordered, once I get it I will take few snaps and will post it here..


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PostPosted: Sat Apr 25, 2009 9:20 pm 
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Hi phani, thanks!

Yes, I was working at an effective focal length of 800mm, whereas you'll be working at 500mm, so the Sun will be almost half the size on the frame - but it'll still look good, and will give you plenty of room to capture the corona and also to accommodate the Sun moving across the frame more so than mine. I had to adjust my tripod every few seconds, and every time I worried I would nudge it too far and lose the Sun! You will still need to make adjustments regularly though. Remember to switch IS off if you're using a tripod.

Note, do not use an ND filter for solar or eclipse photography.

If you want to take images before or after totality to show the partial eclipse, you WILL NEED a special solar filter. Anything else will damage your equipment and your eyes.

As soon as totality begins though, you can remove the filter. All of the the photos above were taken without a filter. But the instant totality ends, you need to fit a filter or put a lens cap on.

During totality I would recommend taking a series of shots at slower and slower shutter speeds, starting at approx 1/100 at 100 ISO and reducing half or 1/3 of a stop at a time. So you could try:

1/100, 1/80, 1/60, 1/50, 1/40, 1/30, 1/25, 1/20, 1/15, 1/13, 1/10, 1/8, 1/6, 1/5, 1/4, 0.3, 0.5 and so on.

I would recommend trying to do this now with any subject, pausing for a couple of seconds between each shot to see how long it takes and to make sure NONE have camera shake. Obviously you need to make sure you can do it all in time during totality.

And please do save a few seconds to watch it with your eyes, because it is an amazing sight! If anything goes wrong with your equipment, just forget it and watch it instead. If you try and fix your gear, you will lose the moment.


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PostPosted: Sun Apr 26, 2009 3:30 am 
Yes, I have ordered for a Solar Filter and as soon as it arrives I have planned to practice the same what I would do at the time of Solar Eclipse. And you suggested the same, which means I am tracking in the right path :)

I have taken print out of few imp points that you have said! Its worth it for me or anyone who is planning for TSE.

I have one more question for you. Can you comment on the quality and the usefulness of the Scope for this purpose - Celestron C65
http://www.celestron.com/c3/product.php ... 5&CatID=30

Aperture: 65 mm (2.56 in)
Focal Length: 835 mm (32.87 in)
Focal Ratio: 12.85

I tried to check the usability of this one for the solar eclipse and I couldn't find much information on this..

If this is good then, I will not order the 55-250mm lens! Otherwise I will go for the lens.


thanks,


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PostPosted: Sun Apr 26, 2009 9:23 pm 
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Hi Phani, you may be ok with the zoom - remember my camera was 35mm, so (like a full-frame model) had no field reduction factor. So a 400mm with a 2x converter was working at an equiv of 800mm.

Your XTi has an APS-C sized sensor, which means all focal lengths are effectively multiplied by 1.6x. So a 55-250mm with a 2x will give 500mm, but on your DSLR, it will effectively be 500x1.6=800mm!

So this lens will give the same filed of view as my photos above on your camera.

That spotting scope looks fun, but I'm not sure what the quality would be like with a camera, or even if you could mount a DSLR to it - and remember, the focal length would be a bit too long as it would be 835x1.6=1336mm. That's great for normal Sun and Moon shots, but it would crop the corona of a total solar eclipse, and that's what you really want to capture...


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PostPosted: Mon Apr 27, 2009 2:37 am 
OOOOOh... Ok..

I dint knew these calculations before. I have heard of APS-C and full frame sensors dint consider it for the scope!

"Your XTi has an APS-C sized sensor, which means all focal lengths are effectively multiplied by 1.6x. So a 55-250mm with a 2x will give 500mm, but on your DSLR, it will effectively be 500x1.6=800mm!

So this lens will give the same filed of view as my photos above on your camera." Then I think I should go for 55-250mm. Later I can use it for other purposes also.

Did you post any article on these calculations any where before or is there a link where I can read more about it? I want to know few things like why it should be multiplied by 1.6x and how should I know what image size a particular lense would produce etc.



thanks,


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PostPosted: Mon Apr 27, 2009 5:56 am 
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Yep! Check out our lens guide here!

http://www.cameralabs.com/lens_guide/Le ... uide.shtml


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PostPosted: Mon Apr 27, 2009 10:06 am 
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Those are great shots indeed Gordon, I love the third one!

- Bjorn -

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PostPosted: Mon Apr 27, 2009 4:57 pm 
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Good stuff, however, I too have a question:
How can you make sure you focus at infinity? With my lens, twisting the focus ring completely to the right makes stars blurry. Twisting it a bit back makes them sharper. I also noticed that autofocus does still has some "play" left in it, so it doesn't completely focus at infinity when I focus on stars?

18-105 VR kitlens, no distance markings.

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PostPosted: Mon Apr 27, 2009 5:02 pm 
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Zoomed in live view with manual focus seems best. Without that, you'll just have to take test shots and make small adjustments to help get it perfect. Of course you have to make sure to do this before the event to maximise time available, and not to knock it OOF before then. But I wonder how much earlier you can do it, in case of zoom creep and maybe even thermal expansion?

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PostPosted: Mon Apr 27, 2009 9:47 pm 
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Focusing is one of the hardest parts in this equation. It's much easier now with magnified assitance in Live View, but those shots above were done with a film camera through the viewfinder by eye. Another issue was I was taking partial shots beforehand with a filter and once I removed this during totality, I had to slightly readjust the focus - still not sure it's 100% spot on, but what can you do?!

Phani has a non-LV camera, so will have to do it by eye too. I suggest practising on the moon first. Doing so will also show you how quickly the moon will move across the frame and how often you'll need to adjust it - and how long you'll need to wait for it to stop wobbling before taking the shot!

It's really quite stressful especially when you know you only have a minute or so to get all your shots. Next time I'm tempted to just watch the whole thing. But then I've never captured an eclipse with a DSLR...!


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