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Author Topic: Crop Factor  (Read 5716 times)

Marc Lauper

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Crop Factor
« on: August 31, 2010, 05:36:38 am »

Hi there

I think about buying a Lumix LX5 for familly snapshots and streetfotography.
I dont look for perfect quality (still use film for that).
Though I have some questions about the sensor crop factor.

The LX5 has a crop factor of 4x.
That means, the f2.0 lens is a f8.0 on this cam?
This goes for the dof it delivers, as well as for the light strenght?

Thanks

cheers
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Dick Roadnight

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Re: Crop Factor
« Reply #1 on: August 31, 2010, 07:47:19 am »


The LX5 has a crop factor of 4x.
That means...
...that a 20mm lens will give you the same angle of view as an 80mm lens would on a 24 * 36 mm full frame camera.

DOF will be increase due to the smaller format.
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Marc Lauper

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Re: Crop Factor
« Reply #2 on: August 31, 2010, 08:18:58 am »

Thanks!

So only the focal lenght is directly related to the crop factor.

In the same light condition the f2.0 of the lumix gives me the same shutterspeed
as with a f2.0 on full frame?

And due to the smaller sensor I just got more DOF and noise.
But no relation to crop factor/f-stop how much more DOF..
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Dick Roadnight

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Re: Crop Factor
« Reply #3 on: August 31, 2010, 11:29:30 am »

Thanks!

So only the focal lenght is directly related to the crop factor.
Yes
Quote
In the same light condition the f2.0 of the lumix gives me the same shutterspeed
as with a f2.0 on full frame?
Yes
Quote
And due to the smaller sensor I just got more DOF and noise.
But no relation to crop factor/f-stop how much more DOF..

See DOF calculator
« Last Edit: September 01, 2010, 10:01:42 am by Dick Roadnight »
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Marc Lauper

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Re: Crop Factor
« Reply #4 on: August 31, 2010, 12:17:30 pm »

Thanks for your answers!

cheers
Marc
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RFPhotography

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Re: Crop Factor
« Reply #5 on: August 31, 2010, 01:20:52 pm »

No.  DOF is not affected by the size of the sensor. 

DOF is affected by aperture and magnification.  If you stand in the same place, use the same focal length and the same aperture with a full frame camera and a cropped frame camera, your DOF at the sensor plane will be the same.  Print the two images to the same size and DOF in the print from the cropped frame sensor will be less.

DOF is affected on cropped frame cameras because in order to get the same scene framed in both cameras you have to either (a) move further away from the subject, (b) use a shorter focal length or (c) a combination of the two.  Whatever method you use, magnification is reduced.  Lower magnification means greater DOF at the sensor plane.  Print the two images to the same size and DOF from the cropped frame sensor will be about the same.

The DOF will be about the same in a print of the same size because the size of the out of focus elements that is considered 'acceptably sharp' gets smaller as the film/sensor format gets smaller on a relative basis. 

Example:

Assume:  50mm lens, f8
Distance to subject:  10 feet (full frame), 20 feet (2X crop)
Circle of Confusion size:  .025mm (full frame), .013 (2X crop)
DoF (at sensor plane):  5.09ft (full frame), 10.75ft (2X crop)

You have to double the print size of the 2X crop image to make it the same size as the print from the full frame camera.  Doubling the print size will double the size of the CoC, making it the same as the full frame camera and halving the DoF in the print of the 2X crop image.  You're essentially making magnification of the two images the same again by printing them the same size.
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Dick Roadnight

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Re: Crop Factor
« Reply #6 on: August 31, 2010, 04:03:03 pm »

And due to the smaller sensor I just got more DOF
Correct - for the same angle of view, (as the magnification will be less).
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Marc Lauper

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Re: Crop Factor
« Reply #7 on: September 01, 2010, 04:24:44 am »

Very interesting.

This would mean if I want 15/10cm prints:

On full frame I need 1772/1181px @ 300dpi
For 2X I need 3543/2362px @ 600 dpi
For 4X I would need 7086/4724 @ 1200 dpi (which is not possible with the LX5)

Again: Thank you both for the infos!
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