Equipment

Testing Lenses: Finding The Best Average Focus Point

You have been patient, my friends. You have made it through the first article, where we looked at how MTFs improve stopped down and saw it was a bit different for every lens. You enjoyed the second article, where we looked at the Best Individual Focus MTF (BIF MTF) which showed how good the MTF could be at different points from center to edge if you focus at that point.

By the end of this third article, you will be able to use terms like “the BIF-BAF point”, “the VLOR”, and the modestly named Roger’s Point to both amaze and intimidate your online forum of choice for months to come. It’s like an early Christmas.

In more concrete terms, we’re going to work with all that data we get with the MTF versus Field versus Focus test to show you something that may also be useful for certain types of imaging; the times when you want as much of the image in focus as possible. That might be a landscape, a group photograph, architectural shooting, or anytime you just want the best overall sharpness possible, and don’t want to shoot stopped-down very far.

Let’s go back to the MTF v Field v Focus curve I used in the last article, the Zeiss CP.2 50mm Super Speed lens at T5.6.

Olaf Optical Testing, 2017

In that article, I pointed out that the normal MTF, which is measured along the black line across the center of the image didn’t show how sharp the lens could be at each position, if we refocused the machine to the sharpest position for each point. That’s the difference between the standard MTF (left, below) and the Best Individual Focus MTF (BIF MTF; below right.)

Olaf Optical Testing, 2017

When we take the measurements for the BIF MTF, we also get the exact focus position used to get the sharpest MTF at each point. We can plot that out, too.

Olaf Optical Testing, 2017

If you scroll back up to the MTF v Field v Focus graph, you can see the best focus position is similar to the curve of one side of the lens (the red area is the area of maximum sharpness). We can run a few more calculations and determine the focusing distance gives the best mean MTF from one side of the field to the other, which we do in the graph below.

Olaf Optical Testing, 2017

 

To give a more intuitive picture, I’ll draw a second bar across the Field graph from above in green, at +0.052mm.

Olaf Optical Testing, 2017

As you can see, the image will be a bit softer in the center, but reasonably sharp from side-to-side. This Best Average Focus distance is different for every lens but is easily measurable.

Of course, if you’re thinking ahead, you’re probably saying, “Well, that’s cool sciencey stuff, but how the hell am I supposed to set my camera to focus the equivalent of 0.05mm in front of center focus?” It’s easy; we go back to that ‘best focus’ chart and find the point where ‘Best Individual Focus Distance’ (red curve) crosses the “Best Average Focus Position” (gray line). This, of course, is the soon-to-be-famous BIF-BAF point.

Here’s where things get real. If we drop a vertical line from the BIF-BAF point (we will call the Vertical Line of Roger {VLOR}) it intersects the image height. (The image height is the distance from the center of the image to the edge of the image.)

Olaf Optical Testing, 2017

The VLOR points to a position some distance between the center and the side edge of the image (we can call this, oh, I don’t know, Roger’s Point). If you focus your camera at this point, you will be focusing at the Best Average Focus Position, for edge-to-edge sharpness.

A full-frame camera is 17.5mm from center to side. In the case of the lens above, focusing your camera just over 1/3 of the way to either edge (+/- 7.5 mm from the center) will work. You can do it by choosing an AF point in that area or manually in live view. Obviously, you’ll estimate a bit; you might pick 7mm or 8mm but that will be pretty close.

The distance from center for Best Average Focus will vary for each lens. It may also vary a little bit in some lenses depending on aperture (because field curvature can change slightly with aperture) but that difference is probably too small to matter for this technique.

Does This Work?

Yep. Obviously, it isn’t magic. We’re trading a little bit of center sharpness to get a larger part of the side-to-side image reasonably sharp. Let’s see how it works.

We took a Zeiss CP.2 50mm SS off the shelf and ran its standard MTF, side to side. It’s really sharp for +/- 6mm from the center(about the middle 1/3), then it falls off pretty steeply.

Olaf Optical Testing, 2017

Then we focused 0.05mm further than the best center point focus and ran the MTF again.

Olaf Optical Testing, 2017

You can see now that the best MTF is at about +/- 6 to 8mm from the center, just as predicted. The center isn’t quite as sharp, particularly at the higher frequencies (fine detail) but an MTF of 0.5 at 50 lp/mm is still good. Out at 10 to 14mm away from the center, the image is now usably sharp (high-frequency MTF > 0.3).  With center focus, this area had almost lost the high-frequency MTF. With Best Average Focus, the center 2/3 of our image is now reasonably sharp, where with center focus only about 1/3 of the image was.

If you want to look at it another way, here are the Full Frame MTF Displays at 40 lp/mm, which is a pretty high resolution. Red we consider unacceptably soft, yellow is borderline.

Center Focus MTF

Olaf Optical Testing, 2017

Best Average Focus MTF

The FFD shows it nicely. We’ve sacrificed the center sharpness from excellent to good, but now have the entire image in the acceptable category. This was at T4; things would be a bit better at T5.6.

Olaf Optical Testing, 2017

Again, it’s not a tool you’d want to use for all photography. But when the goal is to get as much side-to-side sharpness as possible, it’s a nice trick to have. It could be useful for landscapes, group photos, architectural shots, and some types of street shooting at any rate.

Another Example

I tried the same test on the Sigma Cine 85mm T1.5. I wasn’t sure that this would make a difference since this lens stays pretty sharp so far from center. I was also interested in this lens because the Roger’s Point was way out at 11mm from center, more than half-way to the edge, which is fairly extreme. Because this lens is so good at T4, I dropped the aperture down to T2.8, so we had a narrower depth of field.

The difference is more subtle, but it’s there. Follow the 0.9, 0.8, and 0.7 MTF lines out, and you’ll lens has a higher MTF at each frequency away from the center. Here is the standard MTF on top, and the Best Average Focus MTF (BAF-MTF) below.

Olaf Optical Testing, 2017

Olaf Optical Testing, 2017

 

It’s more apparent if I use the Full Frame Display MTF. I’m doing this one at a higher frequency, 50 lp/mm, because the Sigma 85mm T1.5 can handle that, which many lenses can’t. Again, center focus MTF is on top, BAF MTF on the bottom.

Olaf Optical Testing, 2017

Olaf Optical Testing, 2017

BAF MTF Position for the Cine Lenses

I’m not going to (at least right now) show you the MTF curves for every single Cine lens we’ve tested. I’m just going to put up a table of the focus position that will give you the Best Average MTF. One of the questions I have is, do people really want the BAF MTF curves? I think the field curvature graph and BAF Position would be sufficient.

If people really want to see it, we could either just show the BAF Position and the VLOR graph (scroll way back up). And I guess we could show give the BAF MTF curve for each lens. Personally, I think the FFD MTF graphs (the colored circle showing the image field) are best for that purpose.

Until our software upgrades are finished, none of this is automated, though. So, for now, I’m just going to give you the BAF Positions for the Cinema Primes we’ve tested. (They are the same for the photo lens version of each lens.) Since a full frame sensor is 17.5 mm from center to side, it should be fairly easy to estimate where to focus to obtain BAF. You don’t need to count mm, we ran these a bit to either side, and there was no detectable change, so ‘just short of halfway to the edge’ or ‘about 1/4 of the way to the edge’ is accurate enough.


Lens

Focal Length

BAF (mm from center)
Canon CN-E14mm6
Canon CN-E24mm7
Canon CN-E35mm5
Canon CN-E50mm9
Canon CN-E85mm5
Sigma Cine24mm8
Sigma Cine35mm9
Sigma Cine
50mm
11
Sigma Cine85mm12
Xeen24mm11
Xeen35mm7
Xeen50mm9
Xeen85mm11
Zeiss CP.2 21mm21mm14
Zeiss CP.2 28mm28mm6
Zeiss CP.2 T2.135mm0
Zeiss CP.2 T2.150mm8
ZEiss CP.2 SS T1.550mm8
Zeiss CP.2 T2.185mm16
Zeiss CP.2 SS T1.585mm16

I’ve made the table sortable so you can look for a pattern, but I don’t see one myself.

So What Does This Mean?

I have no idea. I can see it being a sometimes useful tool. If I did a lot of weddings or group photos, I’d keep the BAF in the back of my mind. It’s kind of counterintuitive to focus on the 3rd lady from center, but it might keep everyone in even focus in the group photo. For landscape photographers it might help when just stopping down isn’t getting everything as sharp as you want. Or perhaps to let you get the shot at f/8 instead of f/16.

Whether that means there’s enough demand for us to publish either the BAF number or the focus curve graph when we do reviews, I don’t know. A lot of times you could probably eyeball it by looking at the MTF v Field v Focus graphs. So I’ll wait for your input on making that decision.

But I think it’s an excellent example of lenses are tools, and the more you know about the tool you’re using, the better you’ll be able to use it. Knowing how far from center the BIF-MTF stays sharp, and where the BAF MTF point is are useful things, no matter how geeky the technique we use to find them is.

 

Roger Cicala, Aaron Closz, Brandon Dube, Max Bruggeman, and Markus Rothacker

Lensrentals.com

November, 2017

Author: Roger Cicala

I’m Roger and I am the founder of Lensrentals.com. Hailed as one of the optic nerds here, I enjoy shooting collimated light through 30X microscope objectives in my spare time. When I do take real pictures I like using something different: a Medium format, or Pentax K1, or a Sony RX1R.

Posted in Equipment
  • Anthony New

    When I was a child, school pictures were taken with the children in a 180-degree arc and the lens rotated in clockwork with a curved film plane.

  • Anthony New

    I would add that in my experience (mostly with cheaper lenses) field curvature can vary a lot with focus distance. I have little faith in simplistic chart measurements. A flat brick building at 50m can be a useful DIY test of corner behaviour, but more information can often gained by photographing bare tree branches at 50-100m, where both field curvature and axial & lateral CA can be distinguished in branches at different distances in the same part of the visual field – one image doing the work of many, and can indicate issues that chart testing usually doesn’t. Of course, we don’t all have Roger’s equipment available!

  • Andrew Garrard

    Coming to this very belatedly, I’m afraid, but this seems very useful – thanks for all the hard work, Roger.

    Sorry if this is a dumb question, because it’s too long since I read the background to your test process, but my concern is the extent to which field curvature varies with focus distance. I’m reasonably sure that at least some of my lenses have variation (where they have CRC elements, at least). I’m forever fighting the field curvature on my 14-24 Nikkor, usually by hoping f/7.1 will hide it. It’d be nice to do better that “hyperfocal and hope for the best”, especially at large apertures.

    I suspect asking for a graph of BAF-point vs focus distance will just result in it not happening, but it’d be nice to know whether I’m worrying about nothing.

  • Devil’s Advocate

    Perhaps the best thing is to give us the raw data so we can do it ourselves and reduce Roger’s workload? Effectively crowdsourcing the site for a better ‘product’.

  • Brandon Dube

    You can’t use a %, because the relationship between image position and object position is nonlinear (but there is an equation).

    Something we could do is pick a nominal object distance (or ensemble of them…) and translate the y axis to object distances. So the “0” would be “5m” and you might have an axis that goes “6m..5m..4m,” but it could also be “inf..1mi..30ft,” for example.

    I suppose we could do the near and far as a %, but that is not so intuitive a unit, to me.

    Another issue is that in wide angle lenses in particular, the astigmatism changes dramatically as you focus closer, so these surfaces would not be the same at, say, 1.5m focusing distance.

  • Zak McKracken

    I second the usefulness of applying this to tilt/shift lenses (or regular lenses on Scheimpflug adapters). I know some people working on PIV* methods who’d be really happy to know exactly how to get the most planar focus plane with a tilted lens (and which lens to use).

    * https://en.wikipedia.org/wiki/Particle_image_velocimetry

  • Zak McKracken

    Actually … assuming this can be automated, and OLAF has enough time to go through it:
    Run an MTF vs focus test –> adjust aperture one bit –> repeat

    Once you’ve gone through all the aperture settings, you can work out at which setting the maximum sharpness can be achieved, at which point in the image. You could then make a map of that: “best” aperture over distance from image center. Of course not all frequencies behave the same way, but if you use one from the middle of the range, they should correlate reasonably well. So, although sharpness isn’t just one number, most measures of sharpness will be optimal at the optimal aperture.

    Or! You could slice that map of MTF as function of focus, location and aperture, and generate curves for sample distances from center, showing (say) saggital and tangential MTF as function of aperture, at ideal focussing distance. A user could then pick an aperture from that graph to balance out sharpness on the parts of a particular photo that matters to them.

    Or! Define depth-of-field for different frequencies, then use that instead of MTF in the previous plots…

    …really, there’s a lot of useful things one could do with such data. I’m sure there would need to be some experimentation, though.

    I’m definitely unsure if you guys would have the time to actually generate that data for a significant number of lenses… Especially as I’m suspecting these plots could be doubly useful for cheaper lenses, to find strategies to work around their limitations, and there a lot more cheaper lenses then the nice expensive primes you’ve measured for this article.

  • Zak McKracken

    To me at least, the use of the graphs in this post is mostly for getting the most out of “flat-wall” pictures*, whereas the graphs in the previous were much more useful for understanding when not to focus and recompose (and rather use the off-center focus points), and comparing lenses in terms of achievable sharpness (which in many cases is the more realistic scenario than results from center-focussed tests)

    * not trying to diminish those, as I do shoot murals quite a bit, as well as more scientific applications — but in general, most scenes are 3D, and in that case planarity does not matter.

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