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NPF rule to avoid star trails

dslr imaging tripod
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#1 Fred76

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Posted 09 April 2016 - 04:59 AM

Hello!

 

When shooting with wide angle lens and a camera on a tripod, i.e. without motorized equatorial mount, we have two choices, whether long lasting exposures to see the stars trailing in the sky, or short exposures to avoid the star trails. The NPF rule is a new way to calculate the exposure time to avoid star trails.

 

There are some "rules of the thumb" to calculate the exposure time, such as the "rule of 500" (or 450 or 600), taking into account the crop factor or not, the declination or not... but though these methods work sometime, they fail in several occasions.

 

I developped a new rule which is more consistent to modern sensors and gives results that are reliable with any sensors. I called it "the NPF rule", N standing for aperture, P for pixel size (in ┬Ám) and F for focal length (in mm) ... and delta the declination angle.

 

There are two formulas, the easy one that gives a quick result and correct time most of the times and a second one, a little bit more complex, but really good.

 

Here is the "easy" one :

npf-simple60.png

 

And the complex one :

npf-complete.png

 

I developped an online tool to calculate the exposure time for many DSLRs. You just choose your DSLR among the list (406 listed from 19 brands : this list is almost exhaustive), then you enter the focal length and aperture of your lens, and some details on the field you intend to photograph (direction, elevation above horizon, your latitude).

 

10293-1445013182.jpg

 

Then you just press "Calculer" and you get the result for 9 areas of your picture so that you can choose the exposure you want, depending on the scenery you shoot :

 

10293-1445013183.jpg

 

 

This is in french but you can let Google translate it for you.

 

The link (in french) : http://www.sahavre.f...-file-d-etoiles

 

The link (in googlish) : https://translate.go...t-text=&act=url

 

The maths behind the NPF rule is explained after the online tool.

 

Clear sky to all !

 

Fred


Edited by Fred76, 09 April 2016 - 05:09 AM.


#2 Chilihead

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Posted 09 April 2016 - 05:35 PM

That's awesome Fred, thanks.

 

I've just started getting into DSLR astrophotography. This will certainly help me out.



#3 17.5Dob

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Posted 09 April 2016 - 05:55 PM

Nicely done, thank you !!



#4 Fred76

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Posted 19 April 2016 - 05:30 AM

Now almost 410 interchangeable lens cameras in the database, from 19 different brands.

#5 Poochpa

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Posted 23 April 2016 - 01:36 PM

Now almost 410 interchangeable lens cameras in the database, from 19 different brands.

Fred, I don't see any place to select a lens from the database.



#6 Fred76

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Posted 23 April 2016 - 03:50 PM

You shall activate javascript in your browser...

 

Then you will see a list with all trade marks, and a sub list with the cameras of this brand in the first blue row of the page.

 

The link (in french) : http://www.sahavre.f...-file-d-etoiles

The link (in googlish) : https://translate.go...t-text=&act=url


Edited by Fred76, 23 April 2016 - 03:51 PM.


#7 Poochpa

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Posted 23 April 2016 - 06:53 PM

I have JavaScript enabled in Safari on my IPad, but still don't see any lens database.



#8 Fred76

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Posted 24 April 2016 - 04:57 AM

It is not a data base of lenses, but of "interchangeable lens cameras" (ILC) which is a generic name for cameras where the lens can be changed.

 

You first select the trade mark, then you select the model. After that you need to enter the lens characteristics (focal length, aperture) and some details of the area of the sky you want to photograph.

 

I tested this webpage with Safari and Chrome on my IPhone 3GS and it works well. I tested it also from my computer with Chrome, Firefox, IE and it also works.

 

It is impossible to build a lens database, there are too many, it was already quite challenging to build this ILC database...

Attached Thumbnails

  • npf.jpg

Edited by Fred76, 24 April 2016 - 05:00 AM.


#9 Poochpa

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Posted 24 April 2016 - 01:01 PM

I misunderstood. It works fine and is very useful. Thanks!



#10 mikeyL

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Posted 27 April 2016 - 10:14 PM

Very nice tool - and very generous to make it available! I like your rule, as I have used the 500 rule with my FF DSLR before and still had what I considered to be unacceptable trailing. Your tool looks to be more conservative and clearly would yield better results.

 

ML



#11 Fred76

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Posted 13 July 2016 - 04:26 PM

Could some photographers give me feedbacks on how they found the NPF rule ?

 

That would be great to improve the tool.

 

Fred



#12 Poochpa

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Posted 16 July 2016 - 11:19 AM

I tried using it, but found it too conservative. Using a Canon T4i ("Canon" translates to "gun" in googlish, btw) and the kit 18-55mm lens at 18mm, to image the Milky Way from the horizon to about 30 deg., your calculator gave me 7s. The Rule of 500 gave me 28 sec. I shot using 15 and 20s exposures without trailing.

Mike




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