Suggested feature to fix focus shift

Started 5 months ago | Discussions
calin Forum Member • Posts: 97
Suggested feature to fix focus shift

A reminder that focus shift is an issue specific with certain fast lenses and results in the focus plane not been the same at all apertures.

As cameras are focusing wide open and change the aperture to the specified one when the picture is taken, the lenses suffering from focus shift will not focus well.   There are few Canon and 3rd party lenses that suffer from this issue.

I will follow my findings with my Tamron 24-70mm G2 @ 70mm-50mm focal range.

When focusing closer 0.3-3m with the EOS R5 the focus is acquired at F2.8.   If the picture is taken at F4-5.6 the result is worse than F2.8  and requires manual adjustment of the focus at the new aperture.

With my Canon 5D mark iv Tamron uses it's focusing processor which can be calibrated.  I've calibrated it such that images at 50-70 mm are clear at F4 (loosing the option to have clear F2.8 @ 50-70mm)

However, mounted on a mirrorless camera, the focusing processor is bypassed and the trick above can't be made.

The solution I'm suggesting Canon to add is a feature for focus shift lenses:

- Max affected focal range    xxx

- Min affected focal range   yyy

- Min focusing aperture F zzz

In the Tamron case:  xxx = 70,  yyy = 50 and zzz = F4

How it works:
If the focal range is outside the yyy to xxx

do nothing

else

If the picture aperture F is less (numerical ex 3.2) than F zzz

use the picture aperture

else

use F zzz

-- hide signature --

Having the feature above handled by Canon would help Canon will all its fast lenses.  Otherwise, Tamron and other 3rd party lens manufacturers may release another firmware for their lenses that will put Canon in disadvantage as Canon lenses can't be firmware updated.

 calin's gear list:calin's gear list
Canon EOS 70D Canon EOS 5D Mark IV Canon EOS R5 Canon EF 50mm F1.8 II Canon EF-S 17-55mm f/2.8 IS USM +5 more
Canon EOS R5 Panasonic Lumix DMC-G2
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PAntunes Contributing Member • Posts: 953
Re: Suggested feature to fix focus shift
1

Or canon could focus stop down.

Sittatunga Veteran Member • Posts: 3,660
Re: Suggested feature to fix focus shift
1

But Canon lens' firmware can be updated, through the camera, in a similar manner to updating the camera firmware.

As PAntunes says, stopped down focussing avoids focus shift if the spherical aberration correction isn't very good.  It does slow things up a bit and it's not as precise if the lens doesn't suffer from focus shift.  The match-triangles manual focus aid can work stopped down seven stops, depending on the lens, distance and subject contrast, though it can struggle stopped down only four stops with a low contrast subject.  That's usually ok though, as the worst case for focus shift is about two stops down; more than that and the depth of field should be big enough to cover the focus shift.  Focus shift was less of a problem with classical SLR prime lenses which were usually designed to be focussed at full aperture and stopped down at least a couple of stops.

OP calin Forum Member • Posts: 97
Re: Suggested feature to fix focus shift

There is an advantage to focus wide open.

I was proposing to allow a configuration per lens (remember the micro adjustments in DSLRs which are per lens adjustments) where the user will set the focal range where to focus stop down and the limit for the stop down.

For ex the focus shift doesn't happen on many fast zoom lenses on the wider portion of the zoom.  It also changes only in the first 1-2 stops from the wider aperture.  So for ex:

A picture taken at 70mm F4-F22 should focus with F4 but a picture taken at 70mm F2.8-3.2 should focus at the required aperture.

 calin's gear list:calin's gear list
Canon EOS 70D Canon EOS 5D Mark IV Canon EOS R5 Canon EF 50mm F1.8 II Canon EF-S 17-55mm f/2.8 IS USM +5 more
OP calin Forum Member • Posts: 97
Re: Suggested feature to fix focus shift

Sittatunga wrote:

But Canon lens' firmware can be updated, through the camera, in a similar manner to updating the camera firmware.

As PAntunes says, stopped down focussing avoids focus shift if the spherical aberration correction isn't very good. It does slow things up a bit and it's not as precise if the lens doesn't suffer from focus shift. The match-triangles manual focus aid can work stopped down seven stops, depending on the lens, distance and subject contrast, though it can struggle stopped down only four stops with a low contrast subject. That's usually ok though, as the worst case for focus shift is about two stops down; more than that and the depth of field should be big enough to cover the focus shift. Focus shift was less of a problem with classical SLR prime lenses which were usually designed to be focussed at full aperture and stopped down at least a couple of stops.

Agree Sittatunga manual focus is a way around the issue but even with the new R5 focus helpers it's harder to focus especially if you pass the camera to someone else.  I'm proposing a way to enable autofocus even throughout the lens range.

 calin's gear list:calin's gear list
Canon EOS 70D Canon EOS 5D Mark IV Canon EOS R5 Canon EF 50mm F1.8 II Canon EF-S 17-55mm f/2.8 IS USM +5 more
Sittatunga Veteran Member • Posts: 3,660
Re: Suggested feature to fix focus shift

calin wrote:

Sittatunga wrote:

But Canon lens' firmware can be updated, through the camera, in a similar manner to updating the camera firmware.

As PAntunes says, stopped down focussing avoids focus shift if the spherical aberration correction isn't very good. It does slow things up a bit and it's not as precise if the lens doesn't suffer from focus shift. The match-triangles manual focus aid can work stopped down seven stops, depending on the lens, distance and subject contrast, though it can struggle stopped down only four stops with a low contrast subject. That's usually ok though, as the worst case for focus shift is about two stops down; more than that and the depth of field should be big enough to cover the focus shift. Focus shift was less of a problem with classical SLR prime lenses which were usually designed to be focussed at full aperture and stopped down at least a couple of stops.

Agree Sittatunga manual focus is a way around the issue but even with the new R5 focus helpers it's harder to focus especially if you pass the camera to someone else. I'm proposing a way to enable autofocus even throughout the lens range.

Focus shift is not a problem I've noticed with any of my lenses; that could be because most of them are DSLR lenses and my mirrorless lenses are APS-C slow zooms or fast manual primes.  I find the match-triangles manual focus aid of the EOS R quick and magically easy to use, but I've used cross-prism screens or coincident image rangefinders for decades.

I know Sony have avoided the problem by using stopped-down AF. While that simplifies lens design for nice bokeh, it causes problems in very low and bad lighting, unless you are using it at full aperture which eliminates both problems. I can't help feeling that it would be simpler for Canon just to allow AF with the stop-down button pressed.

PAntunes Contributing Member • Posts: 953
Re: Suggested feature to fix focus shift

Sittatunga wrote:

calin wrote:

Sittatunga wrote:

But Canon lens' firmware can be updated, through the camera, in a similar manner to updating the camera firmware.

As PAntunes says, stopped down focussing avoids focus shift if the spherical aberration correction isn't very good. It does slow things up a bit and it's not as precise if the lens doesn't suffer from focus shift. The match-triangles manual focus aid can work stopped down seven stops, depending on the lens, distance and subject contrast, though it can struggle stopped down only four stops with a low contrast subject. That's usually ok though, as the worst case for focus shift is about two stops down; more than that and the depth of field should be big enough to cover the focus shift. Focus shift was less of a problem with classical SLR prime lenses which were usually designed to be focussed at full aperture and stopped down at least a couple of stops.

Agree Sittatunga manual focus is a way around the issue but even with the new R5 focus helpers it's harder to focus especially if you pass the camera to someone else. I'm proposing a way to enable autofocus even throughout the lens range.

Focus shift is not a problem I've noticed with any of my lenses; that could be because most of them are DSLR lenses and my mirrorless lenses are APS-C slow zooms or fast manual primes. I find the match-triangles manual focus aid of the EOS R quick and magically easy to use, but I've used cross-prism screens or coincident image rangefinders for decades.

I know Sony have avoided the problem by using stopped-down AF. While that simplifies lens design for nice bokeh, it causes problems in very low and bad lighting, unless you are using it at full aperture which eliminates both problems. I can't help feeling that it would be simpler for Canon just to allow AF with the stop-down button pressed.

I think you can choose how the camera focus on the new Sony bodies. (Stop down or wide open)

OP calin Forum Member • Posts: 97
Re: Suggested feature to fix focus shift

PAntunes wrote:

Sittatunga wrote:

calin wrote:

Sittatunga wrote:

But Canon lens' firmware can be updated, through the camera, in a similar manner to updating the camera firmware.

As PAntunes says, stopped down focussing avoids focus shift if the spherical aberration correction isn't very good. It does slow things up a bit and it's not as precise if the lens doesn't suffer from focus shift. The match-triangles manual focus aid can work stopped down seven stops, depending on the lens, distance and subject contrast, though it can struggle stopped down only four stops with a low contrast subject. That's usually ok though, as the worst case for focus shift is about two stops down; more than that and the depth of field should be big enough to cover the focus shift. Focus shift was less of a problem with classical SLR prime lenses which were usually designed to be focussed at full aperture and stopped down at least a couple of stops.

Agree Sittatunga manual focus is a way around the issue but even with the new R5 focus helpers it's harder to focus especially if you pass the camera to someone else. I'm proposing a way to enable autofocus even throughout the lens range.

Focus shift is not a problem I've noticed with any of my lenses; that could be because most of them are DSLR lenses and my mirrorless lenses are APS-C slow zooms or fast manual primes. I find the match-triangles manual focus aid of the EOS R quick and magically easy to use, but I've used cross-prism screens or coincident image rangefinders for decades.

I know Sony have avoided the problem by using stopped-down AF. While that simplifies lens design for nice bokeh, it causes problems in very low and bad lighting, unless you are using it at full aperture which eliminates both problems. I can't help feeling that it would be simpler for Canon just to allow AF with the stop-down button pressed.

I think you can choose how the camera focus on the new Sony bodies. (Stop down or wide open)

Agree, it would be nice if Canon implements a feature like on Sony: "camera focus on the new Sony bodies. (Stop down or wide open)" enabled per lens and with a limit on the aperture.  Ex not slower than 5.6.

 calin's gear list:calin's gear list
Canon EOS 70D Canon EOS 5D Mark IV Canon EOS R5 Canon EF 50mm F1.8 II Canon EF-S 17-55mm f/2.8 IS USM +5 more
OP calin Forum Member • Posts: 97
Re: Suggested feature to fix focus shift

Based on everyone's suggestions I think a good solution to this would be a per lens configuration that allows limited stopped down focusing. It would have:

feature enabled/disabled  and

limit for the step-down aperture.

Example

Feature is enabled with F4:

For F2.8 aperture, focus a F2.8

For F3.5 focus at 3.5

For F4 focus at F4

For F5.6 and higher focus at F4

 calin's gear list:calin's gear list
Canon EOS 70D Canon EOS 5D Mark IV Canon EOS R5 Canon EF 50mm F1.8 II Canon EF-S 17-55mm f/2.8 IS USM +5 more
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