front/back focus and non macro sharpness

Hiroshi · 03-30-2014, 09:30 AM

Hi,

Does a lens that front or back focuses by 3 to 5 camera adjustments, also impact the sharpness of non macro pictures ( e.g. normal portrait or landscape)?

thanks,
Kevin

**jatrax** · 03-30-2014, 10:37 AM

Not sure I understand your question but I think you are asking if focus adjustment applies to all focal ranges? With a prime lens AF fine tuning is easy because you only have one focal length to worry about. With a zoom it is possible for the lens to need different adjustments at different focal lengths but since that is not possible an average or best solution must be used.

If I missed your question please clarify.

Hiroshi · 03-30-2014, 01:42 PM

Thanks for trying to help out. Let's say we have a lens that front focuses at f2.8. It's noticeable because the depth of field is shallow. Will this hold true for the lens at f4.0 or f5.6, or will the increased depth of field at the higher f stops mitigate the front focus error?

**jatrax** · 03-30-2014, 02:02 PM

Ahh, I see. The answer is that all other things being equal the error will remain but will be masked by the greater depth of focus. This is why many people have no problem with the kit lens but suddenly have FF/BF problems when they buy a new fast lens. The kit lens has enough margin in it that you rarely notice any FF/BF problems. f/1.4 lenses on the other hand have to be bang on or it is easy to see the issue.

Hiroshi · 03-30-2014, 05:45 PM

Thanks for the help, I really appreciate it.

vonBaloney · 03-30-2014, 06:44 PM

Older telephoto lenses (especially 300mm+) that were not high-end with special glass, will often have a "focus shift" because of spherical aberration (which is better corrected in newer lenses, in general), and so the focal plane will actually change somewhat as you stop down from wide-open (worse at close distances).

03-30-2014, 09:30 AM	#1
Hiroshi Forum Member Join Date: Nov 2008 Posts: 72	front/back focus and non macro sharpness Hi, Does a lens that front or back focuses by 3 to 5 camera adjustments, also impact the sharpness of non macro pictures ( e.g. normal portrait or landscape)? thanks, Kevin

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03-30-2014, 10:37 AM	#2
jatrax Site Supporter Join Date: May 2010 Location: Washington Cascades Photos: Gallery \| Albums Posts: 12,992	Not sure I understand your question but I think you are asking if focus adjustment applies to all focal ranges? With a prime lens AF fine tuning is easy because you only have one focal length to worry about. With a zoom it is possible for the lens to need different adjustments at different focal lengths but since that is not possible an average or best solution must be used. If I missed your question please clarify.

03-30-2014, 01:42 PM	#3
Hiroshi Forum Member Join Date: Nov 2008 Posts: 72 Original Poster	Thanks for trying to help out. Let's say we have a lens that front focuses at f2.8. It's noticeable because the depth of field is shallow. Will this hold true for the lens at f4.0 or f5.6, or will the increased depth of field at the higher f stops mitigate the front focus error?

03-30-2014, 02:02 PM	#4
jatrax Site Supporter Join Date: May 2010 Location: Washington Cascades Photos: Gallery \| Albums Posts: 12,992	Ahh, I see. The answer is that all other things being equal the error will remain but will be masked by the greater depth of focus. This is why many people have no problem with the kit lens but suddenly have FF/BF problems when they buy a new fast lens. The kit lens has enough margin in it that you rarely notice any FF/BF problems. f/1.4 lenses on the other hand have to be bang on or it is easy to see the issue.

03-30-2014, 05:45 PM	#5
Hiroshi Forum Member Join Date: Nov 2008 Posts: 72 Original Poster	Thanks for the help, I really appreciate it.

03-30-2014, 06:44 PM	#6
vonBaloney Pentaxian Join Date: Oct 2011 Location: Albuquerque, NM Posts: 6,029	Older telephoto lenses (especially 300mm+) that were not high-end with special glass, will often have a "focus shift" because of spherical aberration (which is better corrected in newer lenses, in general), and so the focal plane will actually change somewhat as you stop down from wide-open (worse at close distances).