# 1/3 sensor Focal length.

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Hi, my (webcam) astro camera has a sensor of 1/3 and my telescope focal length is 1250mm what would the focal length be when then image sensor replaces the eye piece?

I know that canon aps-c is times by 1.5 or 1.6 what is the math for the 1/3 sensor.

Cheers

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13 minutes ago, Lordspace said:

Hi, my (webcam) astro camera has a sensor of 1/3 and my telescope focal length is 1250mm what would the focal length be when then image sensor replaces the eye piece?

I know that canon aps-c is times by 1.5 or 1.6 what is the math for the 1/3 sensor.

Cheers

Cameras don't have a focal length, just an image scale. Try using this field of view calculator to determine the scale of the image.

ps the focal length of a camera lens doesn't change when used with an APS-C sensor.

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18 minutes ago, Lordspace said:

Hi, my (webcam) astro camera has a sensor of 1/3 and my telescope focal length is 1250mm what would the focal length be when then image sensor replaces the eye piece?

I know that canon aps-c is times by 1.5 or 1.6 what is the math for the 1/3 sensor.

Cheers

The focal length will still be 1250mm.

John

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If I use a 18-55m lens on my 450d with a aps-c sensor on 55mm the the focal length or crop factor would actually be 82.5m so the same would happen with a telescope as it's just a lens??

If I attached my canon to the telescope the focal length or crop factor would be 1875mm

So what would it be for a 1/3 size sensor??

Surly it would increase the magnification??

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9 minutes ago, Lordspace said:

it would increase the magnification

Hi

No. The focal length -and so the 'magnification'-  is fixed, no matter what sensor you use. The main difference you'll see (or not see in this case) is that your webcam will cover less field of view. The objects themselves however will be exactly the same size whether you use your dslr or your webcam.

Similarly, objects taken with your 450d would appear exactly the same size as on a 6d, just that the latter will fit in more objects.

Cheers and HTH

Edited by alacant
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4 minutes ago, Lordspace said:

If I use a 18-55m lens on my 450d with a aps-c sensor on 55mm the the focal length or crop factor would actually be 82.5m so the same would happen with a telescope as it's just a lens??

If I attached my canon to the telescope the focal length or crop factor would be 1875mm

So what would it be for a 1/3 size sensor??

Surly it would increase the magnification??

No, the focal length of the lens doesn't change by using a different size sensor.  It is the field of view that changes, the focal length of the lens/telescope remains the same. The telescope still has a focal length of 1250mm. Use the field of view calculator in the link I posted to determine the field of view.

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It does'nt magnify as such, as above the focal length is the focal length but the FOV changes.
The crop factor is the equivalent field of view compared to full frame.

The crop factor for 1/3 is approx 7.3.

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I think the idea that focal length is connected with sensor size comes from the unfortunate term 'crop factor' which Mike defines above but which is best kept out of astrophotography entirely. Just stick to meaningful terms like focal length, sensor size, field of view and resolution in arcseconds per pixel. Avoid Crop Factor and Magnification.

Olly

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41 minutes ago, Lordspace said:

Surly it would increase the magnification??

As others have said it won't increase magnification.

Another way of looking at it is, if you have a small sensor and a large sensor that both have the same pixel size, the image in the large sensor will have a bigger field-of-view, so the object will appear to be smaller than the image taken with the small sensor.  However, if you where to crop the image taken with the large sensor, then you could effectively produce an image exactly the same as with the small sensor.  The magnification of both would be exactly the same as it's determined by the telescope optics, not the sensors.

John

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17 minutes ago, Starwiz said:

As others have said it won't increase magnification.

Another way of looking at it is, if you have a small sensor and a large sensor that both have the same pixel size, the image in the large sensor will have a bigger field-of-view, so the object will appear to be smaller than the image taken with the small sensor.  However, if you where to crop the image taken with the large sensor, then you could effectively produce an image exactly the same as with the small sensor.  The magnification of both would be exactly the same as it's determined by the telescope optics, not the sensors.

John

And, in fact, from a given focal length the size of a given object on your PC screen at full size depends on the pixel size of the sensor. Smaller pixels mean bigger image when each camera pixel is given one screen pixel. (That's what we mean by 'full size' or '100%.')

Olly

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You shoot Jupiter with a 640 x 480 pixel webcam and look at the results on a PC -

Wow, Jupiter looks big in that frame !

Same scope but using a 4000 x 3000 pixel DSLR and viewed on a PC -

Wow, Jupiter is tiny !

That webcam sure has a high magnification !

But if you crop the DSLR image to frame Jupiter the same as the webcam.....

Wow, Jupiter looks big in that frame !

Both original images have the same magnification, which is set by the scope, but have hugely different Image Scales.

MIchael

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What's a 1/3 sensor????

May mean something to a "photographer", but provides no information to us in astronomy.

Pixel size and frame size is what matters......

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