Night photography help

[iamsomnath]

Arshad bhai, I am not sure if I agree to what you said. I have searched numerous threads on web that confirm your claim though.
According to my understanding, angular movement of star is same no matter what is the sensor size (pixel density being same)
A crop sensor camera is showing you a smaller portion of the image of what would be captured by full frame sensor, so technically angular movement of stars should be same for both formats.
hence no need to use the "crop ratio" in "rule of 500"

Just thinking loudly .... may be the change in effective "angle of view" comes into consideration?

 

[Ankur003]

True Angular movement remains the same
But the crop sensor magnifies the movement by 1.5 times for APS-C.
If I try 25 seconds on APS C it clearly shows movement
While 18 seconds shows none

Crop sensor doesn't magnify the image (or movement for that matter)
You are confusing full frame equivalence with image magnification.

However, this is right for both formats.

You see there is star movement even for a 5 second exposure
Its just that we cant notice it.

Even on a FF camera if you zoom in to the image, you will see star trails.

A lens made by manufacturer at 50mm rating is 50mm. The sensor behind the lens doesn't change the focus length rating of the lens. It is just that crop sensor will recieve only a smaller portion of image project by the lens.

Let me show what i wanted to say through this image...

 

[Roadie..]

You mean for a given lens at a given focal length the angle of view remains same for both sensors?

 

[Roadie..]

You see I have never tried FF

 

[Ankur003]

Apparent "Angle of view" is certainly narrower in APS-C due to smaller sensor. But this angle of view is not result of lens characteristics. This is due to cropped image gathering sensor at the back of lens.
Another analogy that i can draw is- consider I click a image on FF camera and later crop it by 1.5 times. Will this image be same as we would have got from cropped sensor camera of same pixel density?(using same lens)

PS:
I am no expert. Just sharing my understanding. Either i will get corrected or spread right info.
I have a 50mm lens and both APSC and FF cameras. Let me find a clear sky. I will test both and publish results.

 

[Roadie..]

Yes Cropping the FF image by 1.5 times will be similar to that taken by aps-c

 

[iamsomnath]

Apparent "Angle of view" is certainly narrower in APS-C due to smaller sensor. But this angle of view is not result of lens characteristics. This is due to cropped image gathering sensor at the back of lens.
Another analogy that i can draw is- consider I click a image on FF camera and later crop it by 1.5 times. Will this image be same as we would have got from cropped sensor camera of same pixel density?(using same lens)

Your understanding is perfect. Yes the Field of view does change because the crop sensor does use only the central part and the corners are cut. So theoretically within that central "FOV" the rendering will be identical and would really not need any adjustment.

My thought is about the corners. Specially in wide angle lenses we see higher degree of distortion at the sides. If you mount a 20 m lens on a ape-c DSLR , it will use the central circle which is relatively distortion free. Mount the same 20 mm on a full frame and it will use the whole image including the sides ( including the magnified distortions thereof) . So perhaps it will also be an issue to factor in.

Try as you have said. Let's see the result.

cheers
Somnath

 

[Ankur003]

Yes Cropping the FF image by 1.5 times will be similar to that taken by aps-c

That implies exposure duration can be kept same for both formats. However both images should be viewed on same sizes. if you magnify stars start trailing.

 

[Ankur003]

Your understanding is perfect. Yes the Field of view does change because the crop sensor does use only the central part and the corners are cut. So theoretically within that central "FOV" the rendering will be identical and would really not need any adjustment.

My thought is about the corners. Specially in wide angle lenses we see higher degree of distortion at the sides. If you mount a 20 m lens on a ape-c DSLR , it will use the central circle which is relatively distortion free. Mount the same 20 mm on a full frame and it will use the whole image including the sides ( including the magnified distortions thereof) . So perhaps it will also be an issue to factor in.

Try as you have said. Let's see the result.

cheers
Somnath

Very valid facts Somnath Da. corners see horrible distortion as compared to center...
I have D3100 and D750. A fully conclusive result will require sensor of same pixel density. But i can use my D750 in DXmode... Great!
Let's compare and dissect all 3 set of images soon....

 

[Roadie..]

That implies exposure duration can be kept same for both formats. However both images should be viewed on same sizes. if you magnify stars start trailing.

Yes that ways the Image remains same.
Only some corners are cut

The magnification I mean is that viewing a cropped version on the same screen appears bigger.

But since I haven't tried that on ff, I cant confirm the exposure for the same.

But for my crop sensor 20 -22 secs max is what I found practical to capture still stars.

You may please confirm the practical shutter speed on ff which gives you satisfying results.
Just forget the 500 rule and try