DSLR plus lens - rule of thumb to avoid star trails?

[Dara]

Update : here is the tool I've written linked further down in the thread - Astrophotography Exposure Calculator

I've been trying to find a way of estimating how long an exposure should be for various lenses I have, to minimise star trails, on a basic tripod. Guidance seems generally woolly - "use a wide angle lens and set exposure time to 30 seconds", for example.

A bit of Googling (which can be tricky because all the results seem to be about maximising the length of the star trails!) came up with the following formula -

max exposure time = 1000 / focal length

which would give the following times (assuming a full frame 35mm camera body) -

10mm : 100 seconds

20mm : 50 seconds

50mm : 20 seconds

100mm : 10 seconds

300mm : ~3.3 seconds

Does this sound like it's anywhere near correct?

[Photosbykev]

slightly pessimistic imo but certainly close enough as a rule of thumb but I assume it is effective focal length rather than stated focal length.

I would expect to get reasonable images with 30 seconds on a 5D MkII and 24mm lens but the same lens on a cropped sensor like a 7D might be pushing it a bit.

[angusb1]

I would expect to get reasonable images with 30 seconds on a 5D MkII and 24mm lens but the same lens on a cropped sensor like a 7D might be pushing it a bit.

Wouldn't using a smaller sensor just reduce the FOV?

[Photosbykev]

Wouldn't using a smaller sensor just reduce the FOV?

reducing the FoV increases the magnification so the star trailing is more visible. It's the same effect as using a longer focal length.

[davew]

Just be aware that a wide angle lens covers more than one " Time zone " if you understand me

Dave.

Edit to remove attachment.

[Dara]

I was assuming it would be effective focal length, which is why I said it was for 35mm full frame cameras.

24mm lens on a 7D calculates from the formula as 26 seconds - so as you say 30 seconds would be too much. Looks like gut feeling and the formula are agreeing!

[Photosbykev]

I was assuming it would be effective focal length, which is why I said it was for 35mm full frame cameras.

24mm lens on a 7D calculates from the formula as 26 seconds - so as you say 30 seconds would be too much. Looks like gut feeling and the formula are agreeing!

sorry I missed the full frame detail

[angusb1]

I see. The trail the star makes as its image moves across the sensor covers the same distance from start to finish of the exposure on a smaller sensor as it does on a larger sensor, but it will take up a larger percentage of the total size of the sensor. If the images are shown at the same size the one from the smaller sensor will look like the trails are longer as the whole image will be larger but at a lower resolution if the sensors both have the same pixel size. So the length of the trail in microns on the sensor will be the same on both sensors but it will look longer on the shot from the smaller sensor.

Is this right?

[Dara]

If I'm understanding what you've written correctly, then yes (I think so!).

I've come across this formula which seems to fit the bill -

exposure time = (1000 * sensor size / 36) / (focal length * cos(declination))

I think I'll knock up a web app to do the calculation. Watch this space.

[kentronix]

excellent, this is great info to have, I am watching and taking notes

is there an easy multiplier to convert these 35mm figures to the APS-C size sensor ?

I am happy to take proof or example pics if the thread goes that way. (550d and a few lens options)

[Photosbykev]

excellent, this is great info to have, I am watching and taking notes

is there an easy multiplier to convert these 35mm figures to the APS-C size sensor ?

I am happy to take proof or example pics if the thread goes that way. (550d and a few lens options)

divide the 35mm exposure figure by the crop factor, typically 1.6

[TJSnapper]

I was experimenting with my 400 and 1DMkIIn (1.3 crop) last night and found that anything above 2.5 sec was inducing star trailing.

[jgs001]

From experimentation...

18mm you can get away with 30secs..

50mm I was able to use with 8 secs, I might have got away with slightly longer, but the trails were visible zoomed all the way in.

[Dara]

I've implemented a web tool to calculate maximum exposure times, as I mentioned above.

It can be found at : Astrophotography Exposure Calculator

Feedback, error corrections and ideas for improvement are more than welcome.

[AlexB]

I took this shot... Stargazers Lounge - AlexB's Album: Non-telescope shots - Picture with 135mm @ 8 seconds and it streaked. According to that rule of thumb, I should have been doing a 7 second exposure :/ I shall have to try that to confirm.

[Dara]

I think there is a further rule of thumb that to get pin-point stars, rather than just star trails being 'minimised', then the maximum exposure from the first rule of thumb should be divided by 3. Just something to keep in mind!

[Stephen]

I've implemented a web tool to calculate maximum exposure times, as I mentioned above.

It can be found at : Astrophotography Exposure Calculator

Feedback, error corrections and ideas for improvement are more than welcome.

Nice app

Can I ask for either the addition of Fuji S3 + Sigma 18-50mm f3.5-5.6 DC (not the OS model) - or explain to an idiot how to work it out?

Cheers!

[Dara]

I'll try to add the Fuji body + lenses this evening - might be a bit of a delay as I'm hoping to try to spot Jupiter in about half an hour, plus there is the wee fellow's bed-time to negotiate...

[Superewza]

Could always just use a barn door mount, then you're looking at what, 10 minutes with 200mm?

[TJSnapper]

Nice one Dara, i tried to enter parameters with the 1DMkIIn but i can't get a lens option, just a tick

[Dara]

Nice one Dara, i tried to enter parameters with the 1DMkIIn but i can't get a lens option, just a tick

1D Mk II / II n is fixed. Sorry about that (turns out there was an extra space in the middle so the search couldn't find it).

[Dara]

Can I ask for either the addition of Fuji S3 + Sigma 18-50mm f3.5-5.6 DC (not the OS model)

Done + done! (Although for the purposes of this app any lens of the correct length would have sufficed. Still, better to have it right )

[Stephen]

Done + done! (Although for the purposes of this app any lens of the correct length would have sufficed. Still, better to have it right )

Much appreciated

[Fred_76]

Dara

I tried you exposure tool to shoot the C/2011 L4 Panstarrs comet with the following settings :

DSLR : Canon EOS Rebel XS/1000D

Lens : EFS 55-200 set at 55 mm F/5.6

Declination of the comet : around 24°

Your tool returns an exposure time between 11 and 12 s depending on the area of the picture. But with those exposure time I have a big star trail on the comet nucleus and also on all the stars of the picture.

I found that the correct exposure time to get rid of trails was about 4 s. At 5 s trail is clearly visible. If you look at the photos of this event on the net you will see that most photos have been shot with 3-5 s exposure time to avoid trailing stars, not 10-15 s.

I believe the formula you use overestimates the exposure time.

I therefore tried to understand the "maths" behind trails, and wrote an article (in french) that you can read here (in french) or translated in english with Google here.

The rule of the thumb I come with is a little bit more complex but takes into account the main physics of lenses, sensors and light :

max exposure time (s) = [19.3 x pixel size (µm) + 13 x aperture + 0.1 x focal length (mm) ] / ( focal length (mm) x cos declination )

You can drop the 0.1 x f term if the sky is perfect (good seeing). With this formula, and my setup, I calculate an exposure time of 3.5 s which is quite in agreement with what I experienced.

Best regards

Fred

[Fred_76]

Mistake => decliantion of the comet should read 15°, not 25°...