cropped sensor and exposure time

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RobHD

Senior Member
Well after spending time on my bum doing little to nothing , ive finally got round to reading some of my bookmarks on astrophotography as well as my photography course
smile.png


most know about the 500 rule for working out exposure times to help eliminiate star trails

500 divided by the focal length of the lens.

Well reading up found some rather interesting issues, which may explain why i get trails, or the odd spike were its starting to trail.

ive been working with a 35mm lens and using the 500 rule

So 500/35 = 14.25 so 14 seconds

my nikon d5200 doesnt have 14 seconds so ive been using 15 or 10 and getting better results at 10.

Now ive just finished reading on my break an article from Grant Collier, great wide field and mosaic photography of the night sky.

the 500 rule works great for full frame cameras, but most i know of have cropped sensors, me included.

His article suggests that we multiply the focal length by the amount the sensor is cropped.

So for the Nikon on the fast 35mm lens it would work out as.

500 / (35x1.5)

giving 500 / 52.5 = 9.52

hence my better shots at 10secs.

So in theory with my kit lens at 18mm i should be able to get 18 second shots before star trails.

something else for me to try when i next get clear sky
 
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MartinCornwall

Senior Member
There are also the 400 rule for crop sensors where you divide 400 / 18 giving 22 secs. Or the 600 rule for full frame 600 / 18 giving 33 seconds. So your 500 rule with 18 seconds should have no trailing. If you see what looks like trailing in the corners only and also the stars looking a bit stretched out ( a like a bat) then this is not trailing but coma and is an issue with the lens.
 
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RobHD

Senior Member
i get loverly purple stars when there all in focus on the 35mm lens , finally figured out how to remove thatlast week in lightroom, yes i know its tken me months to figure it out and going straigh past it in processing :hopelessness:
 
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WayneF

Senior Member
MartinCornwall said:
There are also the 400 rule for crop sensors where you divide 400 / 18 giving 22 secs. Or the 600 rule for full frame 600 / 18 giving 33 seconds. So your 500 rule with 18 seconds should have no trailing.
Click to expand...

For FX, I vote for the 400 rule. Stationary 14 mm FX at longer than about 20 seconds will show stars as slightly elongated blobs. I think 25 seconds is objectionable

HOWEVER, this does also depend on the degree of viewing enlargement. I am speaking of 100% views, but tiny resamples to fit the monitor screen are very much more forgiving (the opposite of enlargement).

Just saying, longer focal length is an enlargement.
And DX requires 150% enlargement compared to FX.
And any viewing magnification to view it on a larger screen is certainly more enlargement.
Any enlargement lets us see blur better.
 
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RobHD

Senior Member
I've looked at the barn doors as well, then keep looking at the star adventure from skywatcher, but I've bought a eq1 telescope mount I'm going Frankenstein so I can mount the camera and RA motor as well as a finder , then I have my tracking scope mount for max £50
 
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TonyD315

Senior Member
I had the chance to take a workshop on Astrophotography last year and it's something that I've really enjoyed doing since. Here's a quick cheat sheet that was given to us. I shoot with a D7100 and the Nikon 12-24 f4 so I automatically go to 25sec exp at 12mm f4. The only thing that I adjust is the ISO since I don't want the stars to start to trail.

596027397ab1859a8cfbcb6dbdd4806a.jpg





Sent from my iPhone using Tapatalk
 
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salukfan111

Senior Member
RobHD said:
i get loverly purple stars when there all in focus on the 35mm lens , finally figured out how to remove thatlast week in lightroom, yes i know its tken me months to figure it out and going straigh past it in processing :hopelessness:
Click to expand...
I believe proper focus has your stars with a slight red tinge around the outside (can be removed in post).
 
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salukfan111

Senior Member
RobHD said:
Well after spending time on my bum doing little to nothing , ive finally got round to reading some of my bookmarks on astrophotography as well as my photography course
smile.png


most know about the 500 rule for working out exposure times to help eliminiate star trails

500 divided by the focal length of the lens.

Well reading up found some rather interesting issues, which may explain why i get trails, or the odd spike were its starting to trail.

ive been working with a 35mm lens and using the 500 rule

So 500/35 = 14.25 so 14 seconds

my nikon d5200 doesnt have 14 seconds so ive been using 15 or 10 and getting better results at 10.

Now ive just finished reading on my break an article from Grant Collier, great wide field and mosaic photography of the night sky.

the 500 rule works great for full frame cameras, but most i know of have cropped sensors, me included.

His article suggests that we multiply the focal length by the amount the sensor is cropped.

So for the Nikon on the fast 35mm lens it would work out as.

500 / (35x1.5)

giving 500 / 52.5 = 9.52

hence my better shots at 10secs.

So in theory with my kit lens at 18mm i should be able to get 18 second shots before star trails.

something else for me to try when i next get clear sky
Click to expand...
The crop factor shouldn't matter because it is a focal length thing. I too get better results with a 400 rule. Before you come to any conclusions back off some on the aperture (to reduce comma and other things) and do detailed testing of your own.
 
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J-see

Senior Member
It's not the crop but the pixel density of a sensor that makes a difference in the duration.

When using the same lens, I could shoot the D750 slightly longer than the D810 and that one again slightly longer than my D7200.
 
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