SL for better focus?

[Jared]

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9 hours ago, Photoworks said:

what nobody is mentioning is the fact that with SL2 and EVF is it recommended opening the lens all the way to focus and then go back to you desired F-stop.

If you shoot most of the time wide open that is less of an issue.

There is one important exception to this rule.  Some M lenses, particularly those designed prior to the adoption of aspherical lens elements, have a significant amount of focus shift.  That is, the optimal focus varies with aperture. For these lenses, if you are below about f/5.6 you should probably focus at the shooting aperture not wide open.  

[BernardC]

10 hours ago, Jared said:

Some M lenses, particularly those designed prior to the adoption of aspherical lens elements, have a significant amount of focus shift.  That is, the optimal focus varies with aperture.

It's not just M lenses. Almost all lenses have focus shift away from center. Zooms are the worse for this, but it is noticeable in almost all primes as well. There was a series of posts on the Lensrentals blog about that recently.

The SL and SL2 make it extremely easy to focus at working aperture, so it's much less of an issue now than it was a few years ago.

[Schittra]

I feel like 30s again after I can focus 90/2 in such a low light. Thanks to all input. I finally brought SL back home. 

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[Jared]

2 hours ago, BernardC said:

It's not just M lenses. Almost all lenses have focus shift away from center. Zooms are the worse for this, but it is noticeable in almost all primes as well. There was a series of posts on the Lensrentals blog about that recently.

The SL and SL2 make it extremely easy to focus at working aperture, so it's much less of an issue now than it was a few years ago.

I think what you are referring to is field curvature, not focus shift.  That is, the focus distance varies with location in the frame.  Dealing with field curvature requires you to focus after framing your subject by moving your focus point (as opposed to focusing in the center of the frame and recomposing). That is what you were referring to, yes?  If so, I agree that’s an important habit even with many modern lenses especially if you want to shoot wide open. It should not require you to focus at anything other than the widest aperture, though, since the focus distance is not changing with aperture.

That being said, I agree with you that this is a significant advantage in using an SL or SL2 to focus an M lens as opposed to a rangefinder.  With an M camera you HAVE to use the center for focus then recompose (or use Liveview).  With a mirrorless you can just zoom into your subject regardless of where it is in the frame and focus by eye.

[Jared]

1 hour ago, Schittra said:

I feel like 30s again after I can focus 90/2 in such a low light. Thanks to all input. I finally brought SL back home. 

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Simply register for free here – We are always happy to welcome new members!

Congratulations!

[BernardC]

7 minutes ago, Jared said:

I think what you are referring to is field curvature, not focus shift.  That is, the focus distance varies with location in the frame.

Both. Optimal focus changes in different parts of the field, and at different apertures.

[Jared]

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57 minutes ago, BernardC said:

Both. Optimal focus changes in different parts of the field, and at different apertures.

Would love to see the discussion.  Do you have a link?  I couldn’t find it with a casual Google search.

[Jeff S]

I’m not a Lloyd fan, but this discussion might be of interest...

https://diglloyd.com/blog/2019/20190526_1119-FocusShift.html

Jeff

 

[Jared]

Thanks, Jeff. I read through the article, but it doesn’t describe any lenses where there is no focus shift at the center of the lens, but focus shift is present towards the edges.  Not saying it’s impossible, but Lloyd provided no examples or discussion.  That article just describes what focus shift is and how to detect it.

[Jeff S]

1 hour ago, Jared said:

Thanks, Jeff. I read through the article, but it doesn’t describe any lenses where there is no focus shift at the center of the lens, but focus shift is present towards the edges.  Not saying it’s impossible, but Lloyd provided no examples or discussion.  That article just describes what focus shift is and how to detect it.

No, he describes in parts where one needs to assess outer edges, not just the center, where it might not be apparent.

Jeff 

Edited April 11, 2020 by Jeff S

[chasdfg]

15 hours ago, Schittra said:

I feel like 30s again after I can focus 90/2 in such a low light. Thanks to all input. I finally brought SL back home. 

Welcome, dear visitor! As registered member you'd see an image here…

Simply register for free here – We are always happy to welcome new members!

Great move! I bought an SL myself early this year and sold the M240 I had for 5 years. Picked up an M mono for rangefinder shooting.

It's a different experience from shooting with a rangefinder for sure, but it works really well (nice colours, does well with M lenses). Much more enjoyable than shooting with Sonys and Fujis I tried to pair with M glass (both in terms of performance and user experience). It's my EVF M. ISO 50 (though not base iso?), 4 button layout, electronic shutter are all very welcome coming from an M, and in a lot of respects it is a very modern camera especially if you don't need resolution.

[Jared]

1 hour ago, Jeff S said:

No, he describes in parts where one needs to assess outer edges, not just the center, where it might not be apparent.

Jeff 

OK, I see that comment.  Still, a blanket assertion from one tester that this can happen without any specifics on which lenses or where he has encountered it or whether it is common or rare and any possible details hidden behind a pay wall. 

Like you, I’m not one of Lloyd’s fans. In general he seems to encounter so many issues with so much equipment that it strains credulity.  Either he is extraordinarily unlucky or unbelievably finicky about issues that are swamped into technical insignificance in the real world or just making things sound dramatic than perhaps they are in order to entice subscribers.  Maybe some combination of all three.  

[BernardC]

22 hours ago, Jared said:

Would love to see the discussion.  Do you have a link?  I couldn’t find it with a casual Google search.

Among the recent posts, there is one called "Things You Don’t Know About Stopping Down Your Lens" that has graphs of field curvature for specific lenses at different apertures. It's easy to see that the point of optimal focus varies by aperture throughout the field, not just in the center. He later published "Stopping Down Some Bargain Primes and Zooms", which continues in the same vein with additional lenses. Many of the articles that were recently posted about field curvature also have aperture-based comparisons.

In a more general way, focus shift is not something that happens only in the center, or even predominantly in the center (the way that comma is only visible away from center). I am not sure where that assumption comes from, but it's probably based on a misunderstanding. Just like the assumption that focus shift only affects rangefinder lenses is a misunderstanding. Maybe it's because many discussions of focus shift mention rangefinder lenses and the center of the image (where the rangefinder patch is), so people naturally assume that the issue is somehow related to those two factors.

[Jared]

1 hour ago, BernardC said:

Among the recent posts, there is one called "Things You Don’t Know About Stopping Down Your Lens" that has graphs of field curvature for specific lenses at different apertures. It's easy to see that the point of optimal focus varies by aperture throughout the field, not just in the center. He later published "Stopping Down Some Bargain Primes and Zooms", which continues in the same vein with additional lenses. Many of the articles that were recently posted about field curvature also have aperture-based comparisons.

In a more general way, focus shift is not something that happens only in the center, or even predominantly in the center (the way that comma is only visible away from center). I am not sure where that assumption comes from, but it's probably based on a misunderstanding. Just like the assumption that focus shift only affects rangefinder lenses is a misunderstanding. Maybe it's because many discussions of focus shift mention rangefinder lenses and the center of the image (where the rangefinder patch is), so people naturally assume that the issue is somehow related to those two factors.

Thanks, that second article in particular was useful.  I like Roger’s work a lot.  I especially like the fact that he is in the luxurious position of being able to test multiple units of a given lens and so can eliminate the effects of sample variation.

It had never occurred to me that focus shift was a “rangefinder” issue as opposed to a “lens” issue.  I always assumed it was both.  I had also assumed that modern aspheres largely addressed the issue with modern primes.

What I hadn’t considered was that lenses that show minimal focus shift in the center of the frame (such that increased depth of field as you stop down would cover any small shifts) might have larger focus shift at the edges.  I’ll have to go through Roger’s graphs in more detail. I suspect there might be a further complicating variable which is distance.  Behavior at 2m might be very different from behavior at infinity.

The big challenge with auto stop-down lenses is what, if anything, one can/should do about it. I had already changed my habits from “focus and recompose” in critical situations to “compose and move the focus point” in order to address field curvature.  But with an auto stop-down lens I am not aware of any way to focus at anything other than full aperture.  I may be stuck with just trusting that as long as I have accounted for field curvature the increased depth of field from stopping down will cover any small focus shift.  That may not technically be true with a high resolution sensor, but it’s all we have.

To the original question of using an SL with M glass... This all reinforces the potential benefits of using a real time, through the lens, magnified view for focusing vs a rangefinder (at least for situations where critical focus is important).  Of course, all of these nuances will matter in only a small minority of picture taking situations.  Most of the time we are shooting three dimensional subjects where we need focus “reasonably good” and depth of field to cover any minor issues.  When we get to the point that an image looks “bad” because the eyelashes are in slightly sharper focus than the iris in an environmental portrait, we may have taken razor thin depth of field and pixel peeping a bit too far.

Edited April 11, 2020 by Jared
Clarification