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PostPosted: Fri Aug 20, 2010 3:32 pm 
Hi all,

I want to ask about focal length of lens in DX/APS-C system. If the topic has been covered before, please kindly refer me to the link.

What I know is, DX/APS-C has a smaller sensor, when compared to the FX or traditional 35mm film.
so if I mount a 50mm FX lens on a DX camera, I will get an apparently 75mm lens, which means that the angle of the view, the composition of the picture and magnification of the object will be the same as if I am using a 75mm FX lens on a FX camera.

While if I mount a 50mm DX lens on a DX camera, the final focal length should still be 50mm. (please correct me if I am wrong)

But I have questions about other feathers, like DOF, closest focusing distance etc. If I mount a 50mm FX lens on a DX camera, should I expect the lens to behave like a 75mm or 50mm lens? (in term of DOF and focusing distance etc) and why?

Thanks a lot for your help!!!


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PostPosted: Fri Aug 20, 2010 4:29 pm 
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Question: why are you asking? If you have previously shot film and want to know equivalents, fair enough. If you're new to all of this and moving straight to digital without having gone film, you don't need to think about it.

Firstly, the focal length is a lens characteristic and never changes with the sensor size. Only the effective field of view does. A 50mm DX lens is exactly the same focal length as a 50mm FX lens. As you correctly say, a 50mm lens on DX will give the field of view of a 75mm lens on FX.

The minimum focus distance is a lens characteristic, and wont change with different body.

Depth of field can get rather complicated, but as a rough approximation, people often apply the crop factor to the aperture value too e.g. a 50mm f/1.4 lens on DX would look similar to a 75mm f/2-ish lens on FX assuming the camera position is not changed relative to subject. Not exact, but it gives you the idea. Note that is only for effective depth of field, and does not apply to the exposure calculation, where the actual value still applies.

The way to think about this is, the lens doesn't know or care what is collecting the light at the other end. So if it is a lens characteristic, it wont change with sensor. This applies to things like minimum focus distance.

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Lenses: EF 35/2, 50/1.8, 85/1.8, 135/2+SF, 28-80 V, 70-300L, 100-400L, TS-E 24/3.5L, MP-E 65, EF-S 15-85 IS
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PostPosted: Fri Aug 20, 2010 4:51 pm 
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The behaviour of a lens depends on it's focal lenth.
Both 50mm FX and 50mm DX have a focal length of 50mm
The angle of view and DOF are a function of the size of the sensor AND focal length.
So, no matter what type of lens you associate to a sensor, it's focal length will remain unchanged, the angle of view (magnification if you wish) and DOF will be according to the size of the sensor AND focal length.
The only difference is that a DX lens is optimized for the cropped sensor and the image it projects is exactly the size of the sensor(because the distance between tha lens mounting and the sensor is different). If you can mount a DX lens on a full frame, because the sensor is larger, you'll see in the picture your immage bordered part of the lens' mounting side which is also captured in your picture. Appart from that, the angle of view and DOF will be like an FX lens.
Hope that this makes it more clear.

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Radu
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Canon 50D , SIGMA 10-20 f3.5 ,Canon EF 24-105 L IS USM, Canon EF 100/2.8 macro Canon EF 50/1.4 ,Canon EF 85 f1.8,Canon EF-S17-85 4-5.6 Old Tamron 28-300 inherited from my Canon Rebel G film camera
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PostPosted: Fri Aug 20, 2010 4:53 pm 
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:)
popo we answered at the same time.

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Radu
Canon PowerShot S100
Canon 50D , SIGMA 10-20 f3.5 ,Canon EF 24-105 L IS USM, Canon EF 100/2.8 macro Canon EF 50/1.4 ,Canon EF 85 f1.8,Canon EF-S17-85 4-5.6 Old Tamron 28-300 inherited from my Canon Rebel G film camera
Canon580EXII
http://www.errre.net


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