500 rule for M4/3 help needed

Swissy

Swissy

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When i've done astrophotography previously i used a full frame Canon and i worked out the 500 rule myself and it worked.

I'm now about to try some astro with my Olympus EM1 and a Canon EF fit Rokinon 12mm f2.8 manual lens, fitted via a Focal reducer, and i'm struggling to work out what will be the maximum exposure time so as to avoid star trails.

Anyone help me please?

Thanks.
 
Just figure out the effective FF equivalent and use the 500 rule with that.

So 12 x 2 = 24mm and then factor in the effect of the focal reducer and jobs a goodun.

Equivalence still works here.
 
Thanks for that seems easy enough.

But how do i factor in the focal reducer?

These are the specs of the focal reducer

'Camdiox focal reducer speed booster turbo lens adapter is suitable for Olympus / Panasonic Micro Four Thirds (Micro 4/3) cameras.

Fits to Canon EOS (EF mount) lens and Micro Four Thirds (M4/3 mount) camera.

The focal length is magnified to 1.5x equivalent to 35mm film format, close to 4/3" sensor, crop factor reduced from 2x to 1.44x.

The optical element of adapter reduces focal length of camera lens by 0.72x magnification, close to near 1:1 magnification in APS-C sensor, e.g. 35mm x 1.5 x 0.72 = 37.8mm.

Since more light is transmitted, it helps to increase the exposure to 1 stop, and you can use greater speed to shoot same image by 1 stop.

Focus infinity.'

The bold bits are what i'm struggling to factor in.
 
So the final crop factor is 1.44x (as it states) so its 12 x 1.44 = 17.28 (so call it 18mm).

Or, to do it the long hand way it would be 12 (original focal length) x 2 (crop factor) x 0.72 (reducer magnification) = 17.28
 
Many thanks, so the 18mm is what i use with the 500 rule, and ignore the x 0.72?
 
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Swissy said:
Many thanks, so the 18mm is what i use with the 500 rule, and ignore the x 0.72?
Click to expand...
Yes.

Alternatively you could just go out and test it. Shoot with exposures of 5, 10, 15, 20,... seconds and inspect the results. You could have done that in the time this discussion has been running.
 
StewartR said:
Yes.

Alternatively you could just go out and test it. Shoot with exposures of 5, 10, 15, 20,... seconds and inspect the results. You could have done that in the time this discussion has been running.
Click to expand...

Oooh you're so clever, yes i could have gone out to test it but you can't see stars through clouds, or do you have a clever answer for that too?, and i wanted to know exactly what setting would be ideal rather than p***ing around in the dark on location.

We're not all 'experts' like you, it's people like you on this site that stops members from asking questions for fear of looking daft, i would appreciate it if you maybe but out of my threads in future.

k
 
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Swissy said:
Oooh you're so clever, yes i could have gone out to test it but you can't see stars through clouds, or do you have a clever answer for that too?, and i wanted to know exactly what setting would be ideal rather than p***ing around in the dark on location.

We're not all 'experts' like you, it's people like you on this site that stops members from asking questions for fear of looking daft, i would appreciate it if you maybe but out of my threads in future.

k
Click to expand...
I'm just trying to be helpful. You'll note, I hope, that I did actually answer your question. And I offered an alternative. It was a nice clear evening where I live. I don't know where you live.

In truth though, if you want to know "exactly" what setting would be ideal, there's no simple answer because the 500 rule is only an approximation and it doesn't hold true in all circumstances. It's safe, in the sense that if you use 500 you're probably not going to see any star trailing. But it's not always optimal. If you're pointing the camera at stars which are around the celestial pole rather than the equator, you can get away with longer exposure times. (Reason: stars near the pole rotate in smaller circles around the pole, so they move slower in the sky, so they don't trail as much.) Also, the 500 rule depends on assumptions about how big you want to view the image. If you want to make a big print, or view the image at 100% magnification on screen, then the 500 rule will work. But if your output is smaller - for example if you want to print no larger than A4, or if you want to view it on screen - then you can tolerate a longer exposure than the 500 rule gives without seeing any trails.

Hope this helps.
 
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