Back focus correction

Richard Cichon · 12-14-2022, 08:38 AM

Hello, trying to check the correct focal point on my K-70 paired with Sigma 30mm f1.4 Art. I have a tool to do this. My question is : do I use 30mm or 45mm (cropped sensor) distance from camera's sensor to the tool. Wanted to know if the lens is not back or front focusing. Thank you.

Last edited by Richard Cichon; 12-14-2022 at 08:45 AM.

AfterPentax Mark II · 12-14-2022, 09:23 AM - **1 Like**

The focal length is 30mm so you use 30mm. For a 45mm focal length you use 45mm and so on... Focal length does not change.

Richard Cichon · 12-14-2022, 10:44 AM

Originally posted by AfterPentax Mark II

The focal length is 30mm so you use 30mm. For a 45mm focal length you use 45mm and so on... Focal length does not change.

Thank you 😊

---------- Post added 12-14-22 at 10:52 AM ----------

Originally posted by AfterPentax Mark II

The focal length is 30mm so you use 30mm. For a 45mm focal length you use 45mm and so on... Focal length does not change.

The lens is 30mm but with that 1.5 cropped sensor factor it makes 45mm. So you say I'm still going with 30mm, right ?

kypfer · 12-14-2022, 11:03 AM - **1 Like**

Originally posted by Richard Cichon

The lens is 30mm but with that 1.5 cropped sensor factor it makes 45mm. So you say I'm still going with 30mm, right ?

30mm is 30mm … crop factors are sales pitch … it's only the coverage that changes, not the focal length.
Crop factors may have had some relevance 20 years ago when people were moving over from 35mm film (full frame) and they wanted to know what usage they might expect from their existing lenses … these days they're just bigger numbers used by salesmen to make their product seem better than the competition

Richard Cichon · 12-14-2022, 12:10 PM

Originally posted by kypfer

30mm is 30mm … crop factors are sales pitch … it's only the coverage that changes, not the focal length.
Crop factors may have had some relevance 20 years ago when people were moving over from 35mm film (full frame) and they wanted to know what usage they might expect from their existing lenses … these days they're just bigger numbers used by salesmen to make their product seem better than the competition

Thank you 😊

AfterPentax Mark II · 12-14-2022, 03:51 PM

Originally posted by Richard Cichon

The lens is 30mm but with that 1.5 cropped sensor factor it makes 45mm. So you say I'm still going with 30mm, right ?

Yes, because the crop does not make the focal length 45, the focal length is and remains 30mm.

pschlute · 12-14-2022, 04:26 PM - **1 Like**

Originally posted by Richard Cichon

The lens is 30mm but with that 1.5 cropped sensor factor it makes 45mm. So you say I'm still going with 30mm, right ?

You have had the correct advice above, but if I can elaborate it may help your understanding.

The focal length of a lens is a physical property of the lens. It describes the distance between the optical centre of the lens and the plane of the sensor. As long as you are using it on a format/mount for which it was intended (ie without teleconverter or mount converter), that cannot change.

The image magnification the lens produces at the sensor is identical for both aps-c and FF. So for the purposes you need the FL for ( AF adjustment), you need to input the FL of the lens which will always be 30mm in this case

Richard Cichon · 12-22-2022, 05:48 PM - **1 Like**

Originally posted by pschlute

You have had the correct advice above, but if I can elaborate it may help your understanding.

The focal length of a lens is a physical property of the lens. It describes the distance between the optical centre of the lens and the plane of the sensor. As long as you are using it on a format/mount for which it was intended (ie without teleconverter or mount converter), that cannot change.

The image magnification the lens produces at the sensor is identical for both aps-c and FF. So for the purposes you need the FL for ( AF adjustment), you need to input the FL of the lens which will always be 30mm in this case

Thank you 😊

12-14-2022, 08:38 AM	#1
Richard Cichon Junior Member Join Date: Nov 2019 Location: Manitouwadge, Ontario Posts: 33	Back focus correction Hello, trying to check the correct focal point on my K-70 paired with Sigma 30mm f1.4 Art. I have a tool to do this. My question is : do I use 30mm or 45mm (cropped sensor) distance from camera's sensor to the tool. Wanted to know if the lens is not back or front focusing. Thank you. Last edited by Richard Cichon; 12-14-2022 at 08:45 AM.

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12-14-2022, 09:23 AM - 1 Like	#2
AfterPentax Mark II Pentaxian Join Date: Mar 2021 Posts: 1,467	The focal length is 30mm so you use 30mm. For a 45mm focal length you use 45mm and so on... Focal length does not change.

12-14-2022, 10:44 AM	#3
Richard Cichon Junior Member Join Date: Nov 2019 Location: Manitouwadge, Ontario Posts: 33 Original Poster	Originally posted by AfterPentax Mark II The focal length is 30mm so you use 30mm. For a 45mm focal length you use 45mm and so on... Focal length does not change. Thank you 😊 ---------- Post added 12-14-22 at 10:52 AM ---------- Originally posted by AfterPentax Mark II The focal length is 30mm so you use 30mm. For a 45mm focal length you use 45mm and so on... Focal length does not change. The lens is 30mm but with that 1.5 cropped sensor factor it makes 45mm. So you say I'm still going with 30mm, right ?

12-14-2022, 11:03 AM - 1 Like	#4
kypfer Pentaxian Join Date: Jan 2018 Location: Jersey C.I. Posts: 3,597	Originally posted by Richard Cichon The lens is 30mm but with that 1.5 cropped sensor factor it makes 45mm. So you say I'm still going with 30mm, right ? 30mm is 30mm … crop factors are sales pitch … it's only the coverage that changes, not the focal length. Crop factors may have had some relevance 20 years ago when people were moving over from 35mm film (full frame) and they wanted to know what usage they might expect from their existing lenses … these days they're just bigger numbers used by salesmen to make their product seem better than the competition

12-14-2022, 12:10 PM	#5
Richard Cichon Junior Member Join Date: Nov 2019 Location: Manitouwadge, Ontario Posts: 33 Original Poster	Originally posted by kypfer 30mm is 30mm … crop factors are sales pitch … it's only the coverage that changes, not the focal length. Crop factors may have had some relevance 20 years ago when people were moving over from 35mm film (full frame) and they wanted to know what usage they might expect from their existing lenses … these days they're just bigger numbers used by salesmen to make their product seem better than the competition Thank you 😊

12-14-2022, 03:51 PM	#6
AfterPentax Mark II Pentaxian Join Date: Mar 2021 Posts: 1,467	Originally posted by Richard Cichon The lens is 30mm but with that 1.5 cropped sensor factor it makes 45mm. So you say I'm still going with 30mm, right ? Yes, because the crop does not make the focal length 45, the focal length is and remains 30mm.

12-14-2022, 04:26 PM - 1 Like	#7
pschlute Moderator Loyal Site Supporter Join Date: Mar 2007 Location: Surrey, UK Photos: Gallery Posts: 8,216	Originally posted by Richard Cichon The lens is 30mm but with that 1.5 cropped sensor factor it makes 45mm. So you say I'm still going with 30mm, right ? You have had the correct advice above, but if I can elaborate it may help your understanding. The focal length of a lens is a physical property of the lens. It describes the distance between the optical centre of the lens and the plane of the sensor. As long as you are using it on a format/mount for which it was intended (ie without teleconverter or mount converter), that cannot change. The image magnification the lens produces at the sensor is identical for both aps-c and FF. So for the purposes you need the FL for ( AF adjustment), you need to input the FL of the lens which will always be 30mm in this case

12-22-2022, 05:48 PM - 1 Like	#8
Richard Cichon Junior Member Join Date: Nov 2019 Location: Manitouwadge, Ontario Posts: 33 Original Poster	Originally posted by pschlute You have had the correct advice above, but if I can elaborate it may help your understanding. The focal length of a lens is a physical property of the lens. It describes the distance between the optical centre of the lens and the plane of the sensor. As long as you are using it on a format/mount for which it was intended (ie without teleconverter or mount converter), that cannot change. The image magnification the lens produces at the sensor is identical for both aps-c and FF. So for the purposes you need the FL for ( AF adjustment), you need to input the FL of the lens which will always be 30mm in this case Thank you 😊