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Narrowband

Anyone understand the "Rule of 600"?????


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I have some questions for anyone who understands this concept. I have worked it out to some degree but need some help figuring out another angle of it. Thank to anyone who dare take this on. My brain refuses to work anymore!

Carl

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Divide 600 by your focal length for a 35mm/full frame (FF) sensor = exposure time in seconds without capturing star trails.

If your sensor is smaller, say APS-C, multiply your focal length before dividing 600 by it.

E.g., FF camera fitted with a 50mm lens gives:

600 / 50 = 12 seconds maximum exposure length

Or APS-C camera fitted with a 50mm lens gives:

600 / (50 * 1.6) = 7.5 seconds maximum exposure length

Cheers

Ian

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It may be me but that makes little sense.

A view through a 50mm lens will move the same amount if the sensor is 35mm, APS-C or anything else. The sensor just detects more or less of the image produced by the lens.

The sensor size has no input to the image produced through the lens or the movement that is produced.

In both instances it is a 50mm lens.

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It may be me but that makes little sense.

A view through a 50mm lens will move the same amount if the sensor is 35mm, APS-C or anything else. The sensor just detects more or less of the image produced by the lens.

The sensor size has no input to the image produced through the lens or the movement that is produced.

In both instances it is a 50mm lens.

You've forgotten about the crop factor sir ... a 50mm lens on an APS-C sensor is the equivalent of about 80mm on a 35mm full frame.

http://en.wikipedia.org/wiki/Crop_factor

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My DSLR is an APS-C so I if take my 35mm lens, multiply by the crop-factor now giving me a  52.5mm focal length.  the the "600Rule" gives, 600 ÷ 52.5 = 11.43  secs max exposure.

If I chose a 50mm lens I would need an 8 second maximum exposure.


For a FULL FRAME (TRUE FOCAL LENGTH) DSLR its just 600 ÷ (focal length lens)= shutter speed ( no crop factors to worry about?)


You can also use 500 as your rule? This gives even better image results. Note Canon has a crop factor of 1.6 against the Nikon's 1.5 

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So is it like saying that when using a crop sensor it is a smaller area being captured so it in effect magnifies field rotation. It is not a true magnification like with a zoom lens. The net result is the same affect on field rotation visibility whether zoomed or cropped.

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The crop factor is, IMNSHO, a complete red herring.  It does not change the focal length at all.  If you have a 50mm lens you can put whatever you want on the back of it and it will remain a 50mm lens.

What does change is the field of view of the sensor.  If you have a 50mm lens on a 1.6 crop factor camera sensor then you will have the same field of view as you'd get with an 80mm lens on a full frame camera.

The things that really matter when deciding how long an exposure you can take before star trailing becomes an issue are the focal length and the sensor pixel size.

James

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So is it like saying that when using a crop sensor it is a smaller area being captured so it in effect magnifies field rotation. It is not a true magnification like with a zoom lens. The net result is the same affect on field rotation visibility whether zoomed or cropped.

No.  Just forget about the whole crop factor thing.  It isn't relevant.

James

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I agree with James personally. A crop factor sensor doesn't magnify the view over a full frame sensor just changes the FOV. Then add the various other camera types, four thirds, compacts etc.

Which is why the rule is garbage as far as I'm concerned. It's not a quality rule like say the Sunny16 rule (even that has shortcomings but at least it's a useful starting point with most set ups).

I also don't understand why many people hang off these rules. Majority of photographers these days are digital, so why not just try it? Following a rule will just restrict you.

Cheers

Ian

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The crop factor is, IMNSHO, a complete red herring.  It does not change the focal length at all.  If you have a 50mm lens you can put whatever you want on the back of it and it will remain a 50mm lens.

What does change is the field of view of the sensor.  If you have a 50mm lens on a 1.6 crop factor camera sensor then you will have the same field of view as you'd get with an 80mm lens on a full frame camera.

The things that really matter when deciding how long an exposure you can take before star trailing becomes an issue are the focal length and the sensor pixel size.

James

Calculating how fast a star travels across a sensor due to the earths rotation.

For stars on the celestial equator.

d = t * f / 13750

d= pixel to pixel distance of sensor

t= time

f= focal length

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Learning something new all the time?..........Ive used "The Rule" before, its in print everywhere, with tables and charts to help you, but obviously I've not  fully understood it. Now after noting threads #3 &  #8 and a bit more study on the net, I'm starting to re-assess, and now agree. A lens is a lens, no matter what camera is behind it! Still  more to study.


Its a bit like the the 2xBarlow issue!  A Barlow doesn't change the Ep from an 8mm to a 4mm. It changes the focal length of the telescope, as seen by the EP. Its still an 8mm EP!

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Learning something new all the time?..........Ive used "The Rule" before, its in print everywhere, with tables and charts to help you, but obviously I've not fully understood it. Now after noting threads #3 & #8 and a bit more study on the net, I'm starting to re-assess, and now agree. A lens is a lens, no matter what camera is behind it! Still more to study.

Its a bit like the the 2xBarlow issue! A Barlow doesn't change the Ep from an 8mm to a 4mm. It changes the focal length of the telescope, as seen by the EP. Its still an 8mm EP!

If you had a 2x Barlow would a star drift out of view faster than without it (with the same eyepiece)?

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Crop sensor cameras tend to have a smaller pixel size than full-frame, thus depending on how many pixels you elect to have to activate to cause a star-trail a crop-sensor camera will have a trail in a shorter time. So I'd say that a shorter exposure is necessary for a crop-camera. What really matters is the pixel density/size.

Sent from my iPad using Tapatalk

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If you had a 2x Barlow would a star drift out of view faster than without it (with the same eyepiece)?

..... I would say yes. The higher the magnification (power) of the scope, the faster objects will seem to drift out of the field of view.  Telescope F-1200 a with 8mm EP = 150x . Same EP with2x Barlow, telescope is now F-2400 giving 300x. same EP, but the object would appear to drift quicker.

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