Batis 85 focus shift

Started Feb 23, 2016 | Discussions
JimKasson
JimKasson Forum Pro • Posts: 26,532
Batis 85 focus shift
12

At ten feet, the Batis 85/1.8 exhibits more than 4 inches of focus shift over the range from f/1.8 through f/8.

As the lens is stopped down, the focus point moves closer to it. Peak sharpness is achieved at f/2.8 through f/4. The total focus shift from f/1.8 to f/8 is about 110 mm, or a bit over four inches in 10 feet. Focusing wide open is definitely not the plan here.

Details:

http://blog.kasson.com/?p=13369

Jim

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JimKasson
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Sony 90/2.8 macro focus shift
6

Focus shift in the 90mm FE macro is about the same as the Batis 85.

Details:

http://blog.kasson.com/?p=13347

Jim

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JimKasson
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Otus 85 focus shift
2

Spending a pile of money on lens does not mean you don't have to worry about focus shift:

Jim

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rogerjosem
rogerjosem Senior Member • Posts: 2,376
Re: Batis 85 focus shift

Nice and laboruous work Jim, thanks for those findings (and I am sure it was very time consuming collecting so many data points).

Luckily I focus after I set my aperture, even when dark, after having many focus shift issues with lenses before (specially on DSLRs). It seems like the more reliable way to go.

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l_d_allan
l_d_allan Veteran Member • Posts: 5,087
Re: Batis 85 focus shift

JimKasson wrote:

At ten feet, the Batis 85/1.8 exhibits more than 4 inches of focus shift over the range from f/1.8 through f/8.

Thanks for doing these tests, and sharing the information.

On your blog, you also have info for other "short tele's" in the 85mm range. I appreciate the summary of "[fill in the blank] inches of focus shift". This "dim bulb" found the charts less than easy to interpret.

These values seems rather high. Do you find them of concern, or about "as expected"?

Have you tested other lenses to provide a basis of comparison, especially from CaNikon that are not designed to AF at the stopped-down aperture (which is what I think the Sony a7x family cameras do)?

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JimKasson
OP JimKasson Forum Pro • Posts: 26,532
Leica 90/2 Apo-Summicron-M ASPH focus shift
1

You'd think that rangefinder lenses, since they can't be focused at the focal plane at the taking aperture on rangefinder cameras, wouldn't have much focus shift. Sadly, that is not the case:

When mounted on a rangefinder camera with no live view, like an M8, M9, or one of the film M-series Leicas, that there is no way to set the rangefinder to deliver maximum sharpness over the range of apertures from wide open to f/8. A compromise might be to set up the rangefinder for best performance in the middle, at f/4, but that would mean that then lens would be delivering an MTF50 of around 600 when rangefinder focused wide open, instead over over 1000, which could be achieved if the lens were indeed focused on the subject at f/2.

Jim

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l_d_allan
l_d_allan Veteran Member • Posts: 5,087
Re: Batis 85 focus shift

rogerjosem wrote:

Nice and laboruous work Jim, thanks for those findings (and I am sure it was very time consuming collecting so many data points).

I've asked the developer of MtfMapper (Frans van den Bergh ... thx for developing and supporting) if it would be possible to consider a "Feature Request" to have his freeware auto-magically determine the best focus from a series of images that had "focus bracketing".

That might save a great deal of time, and his reply seemed encouraging for such a utility.

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JimKasson
OP JimKasson Forum Pro • Posts: 26,532
Re: Batis 85 focus shift

rogerjosem wrote:

Nice and laborious work Jim, thanks for those findings (and I am sure it was very time consuming collecting so many data points).

It does take a while. The Cognisys motorized rail does a lot of the work.

Luckily I focus after I set my aperture, even when dark, after having many focus shift issues with lenses before (specially on DSLRs). It seems like the more reliable way to go.

I'm going to start doing that as my default if I haven't tested the lens this way, even though repeatability of focus is much more difficult for me with the lens stopped down. This whole set of tests has been a revelation to me.

Jim

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JimKasson
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Re: Batis 85 focus shift

l_d_allan wrote:

rogerjosem wrote:

Nice and laboruous work Jim, thanks for those findings (and I am sure it was very time consuming collecting so many data points).

I've asked the developer of MtfMapper (Frans van den Bergh ... thx for developing and supporting) if it would be possible to consider a "Feature Request" to have his freeware auto-magically determine the best focus from a series of images that had "focus bracketing".

That might save a great deal of time, and his reply seemed encouraging for such a utility.

Imatest has such a utility. It says that it will work for any color plane, but I've only gotten it to work for luminance. It returns your input file list in rank order. You have to tell it which edge to use.

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JimKasson
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Nikon 85/1.4G focus shift

l_d_allan wrote:

JimKasson wrote:

At ten feet, the Batis 85/1.8 exhibits more than 4 inches of focus shift over the range from f/1.8 through f/8.

Thanks for doing these tests, and sharing the information.

On your blog, you also have info for other "short tele's" in the 85mm range. I appreciate the summary of "[fill in the blank] inches of focus shift".

I'll be doing that. First I'm turning my attention to LoCA.

These values seems rather high. Do you find them of concern?

I find them downright scary.

Do you have other lenses to provide a basis of comparison, especially from CaNikon that are designed to AF at the stopped-down aperture?

Here are the results for the Nikon 85/1.4 G lens:

Note that the focus shift is not monotonic, which, if you're focusing wide open is actually A Good Thing.

Jim

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flbrit Veteran Member • Posts: 4,436
Thanks for all the work.

Look forward to the cliff notes.

Timely stuff as I'm considering a 85 90 lens for my A7RII. Right now its between the Batis and a used Leica f2 APO.

What is the Batis like to manual focus?

Thanks

Brian

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JimKasson
OP JimKasson Forum Pro • Posts: 26,532
Focusing ease
1

flbrit wrote:

Look forward to the cliff notes.

Timely stuff as I'm considering a 85 90 lens for my A7RII. Right now its between the Batis and a used Leica f2 APO.

What is the Batis like to manual focus?

The Batis is great to focus manually. Of the five lenses that I'm testing the lens that’s the hardest to focus is the Summilcron, because it’s got that short rangefinder throw and making small focus changes is a fiddly operation. You think you've got it dialled in, and then the mere act of taking your hand off the focusing ring makes it slip OOF again. The Otus is a joy to focus, but, tactile pleasure aside, the Batis is even easier to use to dial in that last little tweak to the focus thanks to the firmware going into a mode where it takes more than a little ring turning to make a modest change in focus.

What a great fit the Batis is for the a7x bodies. It’s not a big lens, but it’s not tiny. It doesn’t stick out in front of the camera as much as you’d expect, which is nice. But the greatest thing is that it is very light. It’s the opposite of the 90 ‘cron; when you pick that lens up, you’re surprise by how much it weighs, but when you pick up the Batis, you’re impressed by how light it is. And, as far as I’m concerned weight trumps size in photography. I don’t mind carrying a somewhat bigger bag if it’s lighter.

Jim

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l_d_allan
l_d_allan Veteran Member • Posts: 5,087
Re: Nikon 85/1.4G focus shift

JimKasson wrote:

l_d_allan wrote:
I appreciate the summary of "[fill in the blank] inches of focus shift".

I'll be doing that.

I'm struggling to figure out how to interpret the chart. Is it the difference between the peak of the wide-open aperture compared to the peak of the aperture being checked?

So ... the f1.4 peak is about 112mm, and the f4 peak is about 104mm? Or am I confused?

Note that the focus shift is not monotonic, which, if you're focusing wide open is actually A Good Thing.

Sorry, but I'm also unclear on what that means.

These values seems rather high. Do you find them of concern?

I find them downright scary.

And these are with the target 3.3 meters away? About 10'?

These values seem to swamp concerns about "focus and recompose" when stopped down even a stop or two.

Could there be something else going on? Your "experimental design" seems sound (as is typical). Have you gotten feedback from the "Science and Tech" forum members?

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JimKasson
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Re: Nikon 85/1.4G focus shift

l_d_allan wrote:

JimKasson wrote:

l_d_allan wrote:
I appreciate the summary of "[fill in the blank] inches of focus shift".

I'll be doing that.

I'm struggling to figure out how to interpret the chart. Is it the difference between the peak of the wide-open aperture compared to the peak of the aperture being checked?

That's the focus shift. The rest of the curves tell you how much sharpness you lose at each f-stop if you misfocus by any amount from the best focus, which is at the peak.

So ... the f1.4 peak is about 112mm, and the f4 peak is about 104mm? Or am I confused?

You'e got it.

Note that the focus shift is not monotonic, which, if you're focusing wide open is actually A Good Thing.

Sorry, but I'm also unclear on what that means.

That means that the focus shift isn't always in the same direction as you stop down. In this lens, it starts out getting closer as you stop down, and then, as you go from f/2.8 to f/4, it starts getting further away. The right side of the graph is closer to the target, and the left side is further away.

Jim

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vett93
vett93 Senior Member • Posts: 4,600
Re: Batis 85 focus shift

Good work, Jim. I noticed focus shift with my A-mount and E-mount lenses. So I turn on the Live-View on A7x for E-mount lenses. This is so that I can do AF at the aperture I plan to use for that image. However, from my experience, this does not work with A-mount lenses. They all focus with aperture wide open. Do you know if there is a way to get around that?

I think this focus shift issue would impact your recommendation of ISO 640 as well. If I set the ISO at 640 and then do exposure increase during PP, I would set the camera with Live View off. Then focus shift would impact the AF accuracy too.

Thought?

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Seeky Senior Member • Posts: 1,065
Re: Batis 85 focus shift

Very good information provided. However, luckily, FE lenses and all E-mount lenses for that matter focus at set aperture, or at least you have the option. I'm more worried about my adapted Minolta AF lenses such as the 85 f1.4 and other A-mount lenses such as the Sigma Art 35 f1.4.

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JimKasson
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Re: Batis 85 focus shift

vett93 wrote:

Good work, Jim. I noticed focus shift with my A-mount and E-mount lenses. So I turn on the Live-View on A7x for E-mount lenses. This is so that I can do AF at the aperture I plan to use for that image. However, from my experience, this does not work with A-mount lenses. They all focus with aperture wide open. Do you know if there is a way to get around that?

No, I don't. Sorry. I don't have any A-mount lenses.

I think this focus shift issue would impact your recommendation of ISO 640 as well. If I set the ISO at 640 and then do exposure increase during PP, I would set the camera with Live View off. Then focus shift would impact the AF accuracy too.

I leave setting effect on and never push in post more than three stops. I do it so the histogram is meaningful, but it turns out to help with this. However, I'm not sure that that setting keeps the lens from opening up to focus under all conditions. This was an academic concern for me before I ran these tests, but that is no longer the case.

Jim

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JimKasson
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Re: Batis 85 focus shift

Seeky wrote:

Very good information provided. However, luckily, FE lenses and all E-mount lenses for that matter focus at set aperture, or at least you have the option.

I am interested in why the aperture blades move sometimes on the a7RII even with setting effect on. I will be running some AF tests...

Jim

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vett93
vett93 Senior Member • Posts: 4,600
Re: Batis 85 focus shift

JimKasson wrote:

I think this focus shift issue would impact your recommendation of ISO 640 as well. If I set the ISO at 640 and then do exposure increase during PP, I would set the camera with Live View off. Then focus shift would impact the AF accuracy too.

I leave setting effect on and never push in post more than three stops. I do it so the histogram is meaningful, but it turns out to help with this.

Interesting idea, but why do you care about the histogram if you are going to under expose it anyway? I am curious.

However, I'm not sure that that setting keeps the lens from opening up to focus under all conditions. This was an academic concern for me before I ran these tests, but that is no longer the case.

My statement was not clear. Sorry about that. What I mean is that if I am going to under expose the images (and so I can use ISO 640), I would like to turn off Live View so that the EVF won't be too dark. This then would cause the focus shift.

Jim

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vett93
vett93 Senior Member • Posts: 4,600
Re: Batis 85 focus shift

JimKasson wrote:

Seeky wrote:

Very good information provided. However, luckily, FE lenses and all E-mount lenses for that matter focus at set aperture, or at least you have the option.

I am interested in why the aperture blades move sometimes on the a7RII even with setting effect on. I will be running some AF tests...

I think Sony changed the FW design in the last few updates. If I remember correctly, when A7RII first came out, it would open up aperture for AF to improve the AF speed. This was one complaint for A7II; i.e., slow AF speed under low light.

However, Sony has changed that design, because some folks complained about focus shift issues. I haven't tested the latest FW release. But I think it has improved from the first A7RII AF implementation.

Jim

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