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Diopters for aesthetics not close focus


Pete Raynell
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I recently worked on a shoot (shooting alexa mini with rehoused zeiss contax glass) where the DP was always using a diopter in front of the lens, even though the shots and required focus range was well within minimums, I did not get a chance to ask what the reason for using the diopters was, but from what I could tell on the monitor they where shifting the focal range of the lens to make the bg appear softer as we where shooting in a small room.

 

Am I correct in assuming this is another use for diopters besides achieving closer focus? or is there another aesthetic use I might have been missing?

Pete.

 

 

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This is an interesting question. 

I just had a quick play around with an old Sigma Achromatic close up diopter. I have no idea of its power. It was bargain bin find some years ago. I set up a little still life, and took two pics. First was with the lens (50mm f1.4) focused at infinity, with the achromat, which brought the focus to 2' 6". Second frame was without the achromat and focused at 2' 6". There was tiny shift in image magnification between the two, with the diopter shot being slightly larger in frame. DoF appeared very slightly shallower on the diopter shot.There was a marginal difference in sharpness, but that was more than likely down to focusing error on my part. The differences were small enough to make no difference in my opinion, but of course, others might say otherwise.

As to why your DP was doing it, there may be a good reason, or it could just be down to a personal shooting quirk.

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Edited by Stuart Brereton
Incorrectly stated that diopter shot was smaller in frame, not larger
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While 'personal shooting quirk' satisfies Occam's razor, it's definitely interesting to speculate what the reasoning behind the decision is.  

For the most part, lenses designed before the 1990s had their best performance when focused at infinity - aberrations, especially away from the center of the frame, increased at the closer distances where filming normally takes place.  (Optimal performance at closer distances was a big selling point of the Ultra Primes.)  So perhaps the idea was to use the lenses closer to their 'sweet spot,' ignoring the effect that adding a diopter has on lens performance?  Of course, the minor side effects of adding an extra, presumably uncoated, lens to the optical path could be the entire point, too.

 

8 hours ago, Stuart Brereton said:

There was tiny shift in image magnification between the two, with the diopter shot being slightly smaller in frame.

This is the other possibility I can think of.  Like many older lens designs, most of the Zeiss Contax lenses focus by moving the entire lens away from the focal plane; and the closer you need to focus, the further the lens has to be moved (which is why the closer distances are spaced further apart on the ring).  So when the diopter is added, perhaps the optics don't need to move as far to change focus, which could reduce breathing.

Edited by Daniel Klockenkemper
typo
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33 minutes ago, Daniel Klockenkemper said:

So perhaps the idea was to use the lenses closer to their 'sweet spot,' ignoring the effect that adding a diopter has on lens performance?  Of course, the minor side effects of adding an extra, presumably uncoated, lens to the optical path could be the entire point, too.

This is a reasonable explanation, although I'm not sure how much of a net gain in sharpness there would be after adding the diopter. Also, a lot of lens designs (even older ones) incorporate floating elements to improve close focus, so there may not be much to be gained from this approach at all.

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38 minutes ago, David Mullen ASC said:

I thought diopters did not actually change DOF 

If they do, it's marginal. If you look at the word Canon in the pictures, you can see that it is very slightly less blurred in the second one, which was taken without the diopter. It's such a slight difference that it's hardly noticeable.

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  • 2 weeks later...

So I worked with the same DP again last week, same lenses, and same technique with the diopters.

I paid a little more attention to the monitor this time. Heres what I noticed...

Shooting on Mini LF with 50mm rehoused zeiss contax at around T2, focus was at about 4.5ft on talent,  we where shooting in a park with extras in the deep BG out of focus, maybe 50 ft away from camera, when a 1/2 diopter was placed in the matte box the out of focus extras in the BG became far more blurry and out of focus.

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Very interesting!

I've been shooting with a lot of DIY anamorphic lenses and they utilize variable diopters to pull focus. If I'm not mistaken, I believe a lot of higher and mid end anamorphic lenses do the same (ie Cooke, Atlas, but please correct me if I'm wrong).

I would think adding a diopter is another way to distort the lens, albeit a very subtle way. It's another piece of glass that is most likely not as refined like the glass in the lens, so its aberrations could be...very interesting.

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On 3/9/2021 at 1:02 AM, Pete Raynell said:

Shooting on Mini LF with 50mm rehoused zeiss contax at around T2, focus was at about 4.5ft on talent,  we where shooting in a park with extras in the deep BG out of focus, maybe 50 ft away from camera, when a 1/2 diopter was placed in the matte box the out of focus extras in the BG became far more blurry and out of focus.

Where was the focus set after the diopter was added? I'm curious because I can't replicate this softening of the BG.

I took another couple of frames. Fuji APS-C camera, Nikon 50mm at f2. Background is about 100' away. First frame is focused at inf. with a diopter that brings the focus to 2' 4". Second frame is without the diopter, focused at 2' 4". Aside from the slight image magnification caused by the diopter that I saw in my original frames above, I can't see a whole lot of difference.

 

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Same thing again, this time with a 28mm lens at f2

 

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I believe the effect would be much more noticeable if the background was closer, the distance to the background should be another factor besides diopter strength. Higher strength diopters are great for enlarging bokeh balls. I am  big fan of DIY anamorphic lenses (= bad close focus), so getting diopters was essential. But whenever we used them, it was for an effect - I remember we needed geometric bokeh shapes from Christmas lights in the background around 2 metres away, and they were barely noticeable until we installed a +2 diopter (sorry we did not record "before" frames):

 

 

Cube-(0-00-04-13)-.jpg

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36 minutes ago, Alissa Alexina said:

I believe the effect would be much more noticeable if the background was closer

How can things be softer if they are closer to the plane of focus?

In the stills I posted above, the lamppost in the center of frame is only about 10-12 feet away. Both appear equally soft. In my first post, I had two frames where the background was only 7ft back from the camera. Again, there was virtually no difference between the diopter and non diopter shot. I would love to be able to replicate this effect, as it's a useful trick if it works, but so far, I can't see a difference.

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I meant relatively softer. Because the faraway background is already out of focus.. So, if a close background is almost in focus, the defocus effect from the diopter looks really noticeable. There are really too many factors in play, but the topic is interesting and worth some experimentation, will probably try when time allows.

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1 minute ago, Alissa Alexina said:

So, if a close background is almost in focus, the defocus effect from the diopter looks really noticeable.

But it doesn't, that's the problem. In my first two stills, the Canon camera is just a few inches behind the in-focus lens. The wall behind it is 7 feet back. The camera appears equally soft in both frames, as does the wall.

In Pete's second post, he clearly states that the background was 50' away

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OK with no time for  proper tests, I just brought the camera to the window at night and grabbed some quick shots. Close focus (1 m),  distant background (streetlights some 200 m away), 50mm DIY anamorphic setup at f1.8. There is no subject in the frame, so the results are not very accurate, but the effect on the defocused background is spectacular:

1) no diopter

2) +1 diopter

3) +2 diopter

4) split diopter, just for fun -  there are no markings but I believe the strength is +2. This one is certainly notorious for being used for aesthetics 🙂

 

 

streetlightjpg.jpg

streetlights1.jpg

streetlights2.jpg

streetlights-split.jpg

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I also decided to do a test to get to the bottom of this.  I actually have the same lens as the one in the original post, a Zeiss Contax 50mm (not rehoused, but the glass is what matters), and combined it with a "full-frame" camera and some Lindsey Optics diopters. 

The first test was set up to replicate the interior conditions mentioned above - the slate was 4.5 feet away, and the background 15 feet away.  The lens was set to f/2.  Focus was sharp on the slate for each take.  I only used the +1/4 and +1/2 filters on this test, because the farthest focus with a +1 diopter is 1 meter.  

Diopters always went into the same tray in the matte box.  I had read or been told at some point that putting a diopter in the wrong way can increase aberrations, so I tested that, too:  https://vimeo.com/522484255/02c168ba97


As Stuart already demonstrated, I don't think there's a meaningful difference.  Feel free to download the original files, pixel peep, and come to your own conclusion.  

I also wanted to see if the diopters were introducing any field curvature, which would affect focus on the background.  I set the focus to the surface of the coin for each take.  Skip to the second half if you want to see the differences made obvious:  https://vimeo.com/522487048/887642358c


Without the filter, the Zeiss 50mm has a very flat plane of focus (which is why Zeiss called it a 'Planar' lens after all).  There is some mild field curvature added by the +1/4 and +1/2, but not enough to notice if you aren't looking for it.  The +1 does noticeably distort the field of focus away from the background, especially when turned the 'wrong' way.  But given that a +1 filter is confined to 1m or closer distances, its utility in normal photography is limited.  

My final thought is that it's basically a magic trick for the people at the monitor.  The focus on the lens is set for the background; "Now watch as I put in the special filter for the talent's close up!"  Everyone watches the background suddenly go out of focus before their eyes as the filter slides into place.

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4 minutes ago, Alissa Alexina said:

OK with no time for  proper tests, I just brought the camera to the window at night and grabbed some quick shots. Close focus (1 m),  distant background (streetlights some 200 m away), 50mm DIY anamorphic setup at f1.8. There is no subject in the frame, so the results are not very accurate, but the effect on the defocused background is spectacular:

1) no diopter

2) +1 diopter

3) +2 diopter

4) split diopter, just for fun -  there are no markings but I believe the strength is +2. This one is certainly notorious for being used for aesthetics 🙂

 

 

streetlightjpg.jpg

streetlights1.jpg

streetlights2.jpg

streetlights-split.jpg

I think what Stuart was pointing out is that if you attempt to keep the plane of focus in the same place for each image, compensating for the difference in close focus with the focus ring of the lens, then the images are the same. 

Of course, this is not the typical way that diopters are used - your example of achieving closer focus is the most common use. But Peter’s original question was, why use a diopter if the desired focus can be reached on the lens without it? 

Personally, I think some piece of information is missing in Peter’s question. If you put a +1/2 Diopter in front of the lens and don’t touch the focus ring, your plane of focus will be much closer and your subject will be out of focus at the same distance.

To keep the subject in focus at 4’6”, Peter’s DP would have had to shift the focus ring closer to infinity (focus would be 2m or 6’6” at infinity). In which case, as Stuart has pointed out, the image should not look that different. 

So it seems more likely that the DP then moved the camera closer or moved the subject closer to camera to achieve the softer background. But if the lens could actually achieve the same focus without the diopter, then the image probably wouldn’t look that different.

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9 minutes ago, Daniel Klockenkemper said:

I also decided to do a test to get to the bottom of this.  I actually have the same lens as the one in the original post, a Zeiss Contax 50mm (not rehoused, but the glass is what matters), and combined it with a "full-frame" camera and some Lindsey Optics diopters. 

The first test was set up to replicate the interior conditions mentioned above - the slate was 4.5 feet away, and the background 15 feet away.  The lens was set to f/2.  Focus was sharp on the slate for each take.  I only used the +1/4 and +1/2 filters on this test, because the farthest focus with a +1 diopter is 1 meter.  

Diopters always went into the same tray in the matte box.  I had read or been told at some point that putting a diopter in the wrong way can increase aberrations, so I tested that, too:  https://vimeo.com/522484255/02c168ba97


As Stuart already demonstrated, I don't think there's a meaningful difference.  Feel free to download the original files, pixel peep, and come to your own conclusion.  

I also wanted to see if the diopters were introducing any field curvature, which would affect focus on the background.  I set the focus to the surface of the coin for each take.  Skip to the second half if you want to see the differences made obvious:  https://vimeo.com/522487048/887642358c


Without the filter, the Zeiss 50mm has a very flat plane of focus (which is why Zeiss called it a 'Planar' lens after all).  There is some mild field curvature added by the +1/4 and +1/2, but not enough to notice if you aren't looking for it.  The +1 does noticeably distort the field of focus away from the background, especially when turned the 'wrong' way.  But given that a +1 filter is confined to 1m or closer distances, its utility in normal photography is limited.  

My final thought is that it's basically a magic trick for the people at the monitor.  The focus on the lens is set for the background; "Now watch as I put in the special filter for the talent's close up!"  Everyone watches the background suddenly go out of focus before their eyes as the filter slides into place.

Great test, Daniel!

I think your ‘magic trick’ point is a good one. I wouldn’t begrudge a DP for their showmanship, if that’s what it is. You have to justify your day rate somehow! Magic is fun, and when the cinematographic craft was more mysterious, it was more fun too. 

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38 minutes ago, Alissa Alexina said:

Great staff, Daniel! Does this all mean that low-power diopters are generally used for technical reasons such as improving close focus distance, and higher-power diopters for effects?

Thank you!  I think technical reasons are the original purpose for all strengths of diopter, and the extra artefacts of the higher strengths are probably an unintended side effect.  But using technical flaws in unexpected creative ways is part of the innovation and fun of cinematography. 

In your case, anamorphic lenses are a special scenario - especially classic/traditional lens designs - because they're known for having different behavior depending on the focus distance.  I think they might behave differently, I wish I could try that! 

Thinking about all this reminds me of a tilt-shift style diopter holder I saw at the last Cine Gear Expo before the pandemic:  https://www.vocas.com/vocas-5-axis-diopter-holder.html  I can imagine a lot of creative ways to use something like that, especially with split or selective diopters. 

 

50 minutes ago, Satsuki Murashige said:

I think your ‘magic trick’ point is a good one. I wouldn’t begrudge a DP for their showmanship, if that’s what it is. You have to justify your day rate somehow! Magic is fun, and when the cinematographic craft was more mysterious, it was more fun too. 

Neither would I, and I hope I didn't come across as dismissive in my previous post.  If people want to believe a little bit of magic is real, why spoil the fun? 

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2 hours ago, Satsuki Murashige said:

I think what Stuart was pointing out is that if you attempt to keep the plane of focus in the same place for each image, compensating for the difference in close focus with the focus ring of the lens, then the images are the same. 

That's exactly my point 🙂 The diopter is not being used to compensate for a lack of close focus ability in the lens. That's why I asked Peter where the lens was focused after the diopter was added.

I tend to agree with Daniel that this is most likely something of a magic trick designed to wow the people at video village. Either that, or just one of those weird myths that gets passed around without ever being properly tested. I've actually heard someone fairly high up in sales at Zeiss telling people this "trick" as well.

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3 hours ago, Daniel Klockenkemper said:

If people want to believe a little bit of magic is real, why spoil the fun? 

I think this would be a good basis for a new thread - share your ‘magic’ tricks!

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