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33 replies to this topic

#21
russellhq

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Looking at this link gives the impression they really shouldn't have the same FOV:

http://stargazerslou...iew-calculator/

Maybe there's an error in the assumptions?

Russell

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#22
JamesF

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I'm half-convinced there is, but I can't see where it is. The fields of view do look fairly similar in those two shots. Mine isn't cropped and I'd have expected it to be the significantly wider field. The sensors in the cameras are the same to within 1mm. I freely admit to being mystified :)

James

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#23
russellhq

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I did a quick overlay with the two images (yours is the upper image) and the FOV's look quite similar:

MW_Overlap.PNG

Russell

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#24
JamesF

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That demonstrates things very neatly. I get the feeling we're missing some vital bit of information that makes the reason for this obvious. I wish I understood it. I've been really tempted to buy the 50mm lens, but not if it radically reduces the field of view on my 450d. I had the impression from these images that it wouldn't, but I'm concerned that might have been a naive view. It even had me wondering if I had my lens nearer the 50mm end than the 18mm end when I took my images, but absent-minded as I am sometimes, I'd struggle to believe I'd be that bad. Besides, I've just checked the exif data in one of the original image and it confirms an 18mm focal length.

Perhaps someone who understands these things better can explain it.

James

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#25
htj

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Hi, the censor size on the 450d and 60d is exactly the same. The cannot account for the difference.

I believe I read somewhere that the 18-55 mm kit lens was more close to 18-47 mm. I don't have a source, sorry (and I am to lazy to google right now).

The FOV should really only depend on the censor size and the focal length (assuming image cirle is big enough). However focal lengths are not always 100% accurate (nor focal ratios).

If you want something wider than 50 mm, the new 40 mm f/2.8 pancake lens looks interesting and fairly affordable, though I'd read some reviews first.

#26
davew

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This thread is confusing me :)

Are you saying an image taken at 18mm is the same field of view of one taken with a 50mm lens ?

Canon's own figures for the 18 / 55 lens give the two extremes as 74* at 18mm and 27* at 55mm.

Someone or something is reporting incorrectly.

Having imaged at both 18mm and 50mm I can ease your mind and say the two fields of view are nothing like.

Dave.
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#27
russellhq

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Thanks dave, but that's pretty much what we are saying. James took the image in the upper left of the overlay above at 18mm with his 18-55mm lens, whereas another member took the lower right image with the canon 50mm prime lens: http://stargazerslou...tempt-with-dss/

They both appear to have similar FOV's and that's why we are confused :confused:

Russell

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#28
davew

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Thanks for the link.

I now feel it's " Someone " that's wrong.

That image is what I would expect to see from an 18mm lens. A 50mm lens will do Orion and a bit to spare.

Dave.
https://sites.google...ldastroimaging/

EM 200. Moravian G2 8300. FS78. Various camera lenses.
Website very much in progress.

#29
JamesF

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I've dropped the poster of the second image a PM to see if (s)he can confirm the settings used. Meanwhile I'm going to try to work out the field of view from another image I've taken that doesn't suffer from LP.

James

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#30
JamesF

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Settings and lens, obviously. Doh.

James

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#31
davew

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I think your 18mm settings are ok James.

Dave.
https://sites.google...ldastroimaging/

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Website very much in progress.

#32
JamesF

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Measuring off another of my 18mm images, which is a little tricky, 74 degrees doesn't look unreasonable as a figure for the field of view across the diagonal.

James

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#33
JamesF

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I've dropped the poster of the second image a PM to see if (s)he can confirm the settings used. Meanwhile I'm going to try to work out the field of view from another image I've taken that doesn't suffer from LP.


The poster has responded saying that theirs is in fact an 18mm focal length image and not 50mm at all. That's that cleared up then :)

I'm not sure about committing to the 50mm lens now. I might have a look around to see what's about around the 20mm mark. I don't even care if it's an old lens that has to be used on manual, for fairly obvious reasons. Be nice to see some 50mm wide field images posted for comparison though.

James

Skywatcher ST80, ST102, ST120, ED80, Skymax 127, MN190, 200P, Celestron C9.25, Coronado PST, several pairs of binoculars, 10" dob
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Planets: Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune
Messier Objects: 110
Lunar 100: 41


#34
russellhq

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Thanks James, mystery solved!

Russell

Skywatcher 80ED + 0.85 FR/FF, HEQ5 Pro Synscan (4:1 belt mod), EQMOD,
Atik One 6.0 with Atik OAG and Atik GP
Hutech IDAS LPS-P2; Baader Ha 7nm; OIII and SII; Baader LRGB CCD
AstroTortilla, PHD, PHD Lab, APT, DSS, Photoshop





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