K-3 - Setting the focal length of a full frame lens

handlep · 08-20-2021, 04:05 AM

OK, so I'm a) not sure how to do this and b) not sure if I need to anyway :-)

When fitting a lens on the K-3 which does not pass data to the camera, and with SR switched on, I have to input the focal length of the lens. No problem with that. But, I have a Sigma 70-300 APO DG, and as that is a full frame lens, on an APS-C camera, technically it acts like a 105-450 lens (I think I have that right?). However, the lens passes it's actual focal length to the camera (i.e. 70-300) and I am not asked to set the length, nor can I if I go in to the menu (that option is greyed out).

The question is, do I need to? If the lens is acting as a 105-450mm, is the SR actually set incorrectly, in that it is stabilising as if it is a 70-300? Or does it not matter?

Any advice on this appreciated, many thanks.

Fogel70 · 08-20-2021, 04:18 AM - **1 Like**

It does not matter what format the lens is designed for, focal length of the lens is always the same.
It is only FOV that change between FF and APS-C, focal length on the lens does not change.

So you do not have to care of what focal length the lens communicate to the camera, as it should always be the same no matter what sensor size the camera has.

Just as when you manually input focal length it should always be the actual focal length of the lens.

handlep · 08-20-2021, 04:24 AM

Originally posted by Fogel70

It does not matter what format the lens is designed for, focal length of the lens is always the same.
It is only FOV that change between FF and APS-C, focal length on the lens does not change.

So you do not have to care of what focal length the lens communicate to the camera, as it should always be the same no matter what sensor size the camera has.

Just as when you manually input focal length it should always be the actual focal length of the lens.

Great, many thanks for your help :-)

Oldbayrunner · 08-21-2021, 12:54 PM

Your best bet is to forget that effective focal length garbage for Aps-c. It only effects the field of view which of course makes the subject appear closer even though the lens focal distance remains the same. Where it comes to play in thinking is if your trying to capture a wider field of view then you need to step farther away or change to a different wider lens then if you were using it on a full frame camera.

handlep · 08-22-2021, 02:07 AM

Originally posted by Oldbayrunner

Your best bet is to forget that effective focal length garbage for Aps-c. It only effects the field of view which of course makes the subject appear closer even though the lens focal distance remains the same. Where it comes to play in thinking is if your trying to capture a wider field of view then you need to step farther away or change to a different wider lens then if you were using it on a full frame camera.

Thank you.

08-20-2021, 04:05 AM	#1
handlep New Member Join Date: Jan 2011 Posts: 11	K-3 - Setting the focal length of a full frame lens OK, so I'm a) not sure how to do this and b) not sure if I need to anyway :-) When fitting a lens on the K-3 which does not pass data to the camera, and with SR switched on, I have to input the focal length of the lens. No problem with that. But, I have a Sigma 70-300 APO DG, and as that is a full frame lens, on an APS-C camera, technically it acts like a 105-450 lens (I think I have that right?). However, the lens passes it's actual focal length to the camera (i.e. 70-300) and I am not asked to set the length, nor can I if I go in to the menu (that option is greyed out). The question is, do I need to? If the lens is acting as a 105-450mm, is the SR actually set incorrectly, in that it is stabilising as if it is a 70-300? Or does it not matter? Any advice on this appreciated, many thanks.

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08-20-2021, 04:18 AM - 1 Like	#2
Fogel70 Pentaxian Join Date: Sep 2009 Location: Stockholm, Sweden Photos: Albums Posts: 3,062	It does not matter what format the lens is designed for, focal length of the lens is always the same. It is only FOV that change between FF and APS-C, focal length on the lens does not change. So you do not have to care of what focal length the lens communicate to the camera, as it should always be the same no matter what sensor size the camera has. Just as when you manually input focal length it should always be the actual focal length of the lens.

08-20-2021, 04:24 AM	#3
handlep New Member Join Date: Jan 2011 Posts: 11 Original Poster	Originally posted by Fogel70 It does not matter what format the lens is designed for, focal length of the lens is always the same. It is only FOV that change between FF and APS-C, focal length on the lens does not change. So you do not have to care of what focal length the lens communicate to the camera, as it should always be the same no matter what sensor size the camera has. Just as when you manually input focal length it should always be the actual focal length of the lens. Great, many thanks for your help :-)

08-21-2021, 12:54 PM	#4
Oldbayrunner Pentaxian Join Date: Mar 2013 Posts: 1,665	Your best bet is to forget that effective focal length garbage for Aps-c. It only effects the field of view which of course makes the subject appear closer even though the lens focal distance remains the same. Where it comes to play in thinking is if your trying to capture a wider field of view then you need to step farther away or change to a different wider lens then if you were using it on a full frame camera.

08-22-2021, 02:07 AM	#5
handlep New Member Join Date: Jan 2011 Posts: 11 Original Poster	Originally posted by Oldbayrunner Your best bet is to forget that effective focal length garbage for Aps-c. It only effects the field of view which of course makes the subject appear closer even though the lens focal distance remains the same. Where it comes to play in thinking is if your trying to capture a wider field of view then you need to step farther away or change to a different wider lens then if you were using it on a full frame camera. Thank you.

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