GH4 LensRentals.com - Sensor Stack Thickness Part III: The Summary

[Bern Caughey]

SUMMARY

While I don’t have a mathematical formula to predict how well a given lens will work on a given camera, it should be apparent that wide-angle rangefinder lenses designed for film are going to struggle, especially on m4/3 cameras, but to some degree on other cameras. Longer focal length rangefinder lenses tend to have longer exit pupil distances and should do better. Stopping down to f/4 or f/8 should reduce the problem significantly, although it isn’t guaranteed to eliminate it near the edges of the frame.

SLR lenses may also have some troubles on m4/3 cameras, since there’s nearly a 2mm difference in stack thickness. They shouldn’t have as much of a problem as rangefinder lenses, though, since they are designed for a 2mm sensor stack and they tend to have longer exit pupil distances. Older lenses designed for film cameras will tend to have more issues, of course, since they were designed for no sensor stack.

Obviously, these are generalizations and suggestions. There are going to be exceptions. But, if nothing else, hopefully we can help a few people stop trying one adapter after another, hoping to make their 15mm rangefinder lens look great on the m4/3 camera.

-Roger Cicala and Brian Caldwell

 

[optitek]

what a bunch of nonsense. What stack thickness? The only problem is vignetting of range finder film lenses on digital cameras. But that's been well documented. But SLR lenses? really? have they even heard of retrofocus lenses- the ones Dr Angenieux invented in the mid 1950's?
but... since it's on the net it must be true....

 

[LyleGorch]

what a bunch of nonsense. What stack thickness? The only problem is vignetting of range finder film lenses on digital cameras. But that's been well documented. But SLR lenses? really? have they even heard of retrofocus lenses- the ones Dr Angenieux invented in the mid 1950's?
but... since it's on the net it must be true....

I suggest you wander over to the FM forum where Cicala is generally in attendance and express your feelings. It does seem to me that most of this stuff from lensrentals.com is aimed at still photographers and part of the eternal FF-format-superiority discourse. People post a photograph over at FM with a touch of CA in the oof bg and it will trigger debates on optical design for 30 or 40 pages.

 

[brianc1959]

what a bunch of nonsense. What stack thickness? The only problem is vignetting of range finder film lenses on digital cameras. But that's been well documented. But SLR lenses? really? have they even heard of retrofocus lenses- the ones Dr Angenieux invented in the mid 1950's?
but... since it's on the net it must be true....

Are you questioning the existence of the filter stack, or that actual measure-able aberrations are caused by it when using lenses optimized for film?

Aberrations caused by the filter stack are the reason we developed the Panavision Primo-V cine lenses. Differential spherical aberration created by the unusually thin filter stack in Blackmagic m4/3 mount cameras is one of the reasons I developed special Speed Booster designs for the Blackmagic cameras.

 

[optitek]

Really,
All these rental houses using Ultra Primes , Cooke S4s and Master Primes must have missed the memo.
And, as a result, all the films shot on them look awful haha.... (just kidding carry on your magic please)

 

[brianc1959]

Really,
All these rental houses using Ultra Primes , Cooke S4s and Master Primes must have missed the memo.
And, as a result, all the films shot on them look awful haha.... (just kidding carry on your magic please)

Surely you aren't denying established optical facts? Aberrations induced by a filter stack is most certainly a real effect - and not particularly subtle in many cases. Naturally, many in the industry would prefer to ignore the problem, but that doesn't make the problem go away.

Did you actually look at the *measured* data in the LensRental articles (particularly the first one)?

 

[optitek]

OK lets start from scratch here- what is "filter stack"?
Brian- with all due respect- are you saying that Master Primes- since they are designed for film- are not good for electronic acquisition?
Same goes for the UPs S4s, etc. Bench marks in electronic image acquisition today.
Obviously you found a problem with Primos but they have been the ultimate choice since "Genesis" camera 10+ years ago...

 

[brianc1959]

OK lets start from scratch here- what is "filter stack"?
Brian- with all due respect- are you saying that Master Primes- since they are designed for film- are not good for electronic aquisition?
Same goes for the UPs S4s, etc. Bench marks in electronic image aquisition today.

The filter stack is all of the flat glass between the lens and the actual sensor surface. Typically it consists of a coverglass, an anti-aliasing filter, an IR blocking filter, and sometimes a neutral density filter. For example, in the case of the Sony F55 camera the total thickness of glass in the filter stack is about 3.5mm, which is enough to cause noticeable issues with high speed lenses that aren't properly optimized to take account of the filter stack.

I'm saying that there will be issues, not necessarily that the lenses will be "not good". Of course, it all depends on exactly how the lenses were originally designed, which is something that manufacturers tend not to reveal. The only way that a high speed lens such as a Master Prime could possibly behave identically on both digital and film is if it were somehow designed with an intermediate thickness filter stack in mind, which seems very doubtful to me. It also depends on how thick the filter stack is on a particular camera. For instance, the Arri Alexa has a thinner stack than the F55, so it will function a little better with the old film lenses.

In the case of the Panavision Primo-V lenses mentioned earlier, the aberrations introduced by a filter stack were eliminated by a rear corrector group, and so these lenses behave on digital exactly the way they did on film without the corrector.

 

[optitek]

OK so what you refer to as filter stack is commonly referred to as OLFP filter. Film lenses are designed to relay the light in a very accurate manner so white light coming in- white light coming out. At least in theory. OLFP filters are designed to cut off frequencies approaching pixel pitch so aliasing is minimized. They also are IR coated to cut off the IR spectrum. Properly designed OLFP filter does just that and does not alter the color of the light i.e does not disperse the light.
If it does that is the OLFP design issue not a lens issue. if you optimize the lens for certain aberration introduced by a particular OLFP (filter stack) that lens will be useless on a camera with a different filter stack- that's just logical. what if a camera has a replaceable OLFP - like RED Dragon?
Why don't we just leave the white in-white out convention alone and fix the real problem- screwed up OLFP design.... that if it's even a real problem....
BTW I thought BM cameras had no OLFP filter so what is the issue there?

 

[OldCorpse]

All that theory and optics and all is very valid and interesting in its own right. But. This is the piece of wisdom I've come to after all these years: use your eyes. You are the judge of the final effect. If there are distortions with a particular lens/cam combo, change things up - but use your eyes. Don't worry about the theory - worry about the observable effect, i.e., does it change the aesthetic experience for the viewing public. I cannot tell you how often I heard and/or read various "experts" discuss this or that technical aspect of this or that piece of gear - it doesn't mean a thing, unless it translates into an effect that is observable for the audience. And you, the artist, is in charge of determining if you want this or that effect. Use your eyes. Someone will have a neat theory about why something looks the way it does, but at the end of the day I don't worry about the technical explanations. I worry, always and only, about the final effect, which I always come to by trial and error - always. The sigma 18-35 f/1.8 works on my GH3 with a speed booster? Great! I don't particularly care that it's because the 18-35 has a "longer exit pupil distance".

I have however seen the opposite danger. People get so caught up in technical babble, that it actually distorts what they see, and they make decisions based on all that talk - one may call it a negative placebo effect. You stop using your eyes, and your judgment, and instead see thing that are not there.

I appreciate all the technical research and explanations, but I really only care about practical effects.

 

[Egg Born Son]

Aberrations caused by the filter stack are the reason we developed the Panavision Primo-V cine lenses. Differential spherical aberration created by the unusually thin filter stack in Blackmagic m4/3 mount cameras is one of the reasons I developed special Speed Booster designs for the Blackmagic cameras.

You're Brian Caldwell? I haven't got anything to contribute in this debate but I'd like to thank you for the BMPCC speed booster variant. It's fantastic, a game changer. Not only does it bring my existing lenses back into play it, they look better than with a dumb adapter. Better in fact than they have looked on previous cameras and that's not just the raw and the sensor itself. So thanks for that.

 

[brianc1959]

OK so what you refer to as filter stack is commonly referred to as OLFP filter. Film lenses are designed to relay the light in a very accurate manner so white light coming in- white light coming out. At least in theory. OLFP filters are designed to cut off frequencies approaching pixel pitch so aliasing is minimized. They also are IR coated to cut off the IR spectrum. Properly designed OLFP filter does just that and does not alter the color of the light i.e does not disperse the light.
If it does that is the OLFP design issue not a lens issue. if you optimize the lens for certain aberration introduced by a particular OLFP (filter stack) that lens will be useless on a camera with a different filter stack- that's just logical. what if a camera has a replaceable OLFP - like RED Dragon?
Why don't we just leave the white in-white out convention alone and fix the real problem- screwed up OLFP design.... that if it's even a real problem....
BTW I thought BM cameras had no OLFP filter so what is the issue there?

The filter stack is the sum of *all* the flat glass, including the anti-aliasing filter. From a lens design point of view the function of the flat glass can be ignored. All that really matters is the total thickness and its refractive index. Also, the color of the glass is irrelevant to this discussion - its only the aberrations (e.g., spherical aberration, coma, astigmatism, etc.) induced by putting flat plates in a converging light beam that count.

If a camera has a replaceable rear filter then you have to replace it with a piece of glass have equal optical thickness, otherwise the focus marks of your lenses will all be off, and you might not be able to focus to infinity.

Its true that Blackmagic cameras have no anti-aliasing filter. But they do have an IR blocking filter and a coverglass. The total thickness of those two pieces of glass is 2.4mm, which is 1.6mm less than the 4mm of glass found in standard m4/3 cameras. That 1.6mm difference is a problem, and causes high speed m4/3 lenses to perform less well on BM cameras than they do on regular m4/3 cameras.

 

[optitek]

OK this is all mixed up now.
Glass thickness and refractive index only affect the flange depth. That's obviously taken care of with placing the flange in the right place.
And has nothing to do with aberrations.
Other than that the optical design of the multiple glass layers should not introduce any aberrations. If it does it is poorly designed.
FYI we've been putting glass blocks in lenses for years when we needed the extra room. I've never seen any effect on optical performance as long as the surfaces were adequately flat.

 

[brianc1959]

OK this is all mixed up now.
Glass thickness and refractive index only affect the flange depth. That's obviously taken care of with placing the flange in the right place.
And has nothing to do with aberrations.
Other than that the optical design of the multiple glass layers should not introduce any aberrations. If it does it is poorly designed.
FYI we've been putting glass blocks in lenses for years when we needed the extra room. I've never seen any effect on optical performance as long as the surfaces were adequately flat.

I'm really at a loss for what to say here, other than NO! Flat plates placed behind a lens (e.g., placed in a *converging* light beam) really do introduce aberrations that can be calculated, measured, and observed in actual footage on a movie screen.

 

[brianc1959]

You're Brian Caldwell? I haven't got anything to contribute in this debate but I'd like to thank you for the BMPCC speed booster variant. It's fantastic, a game changer. Not only does it bring my existing lenses back into play it, they look better than with a dumb adapter. Better in fact than they have looked on previous cameras and that's not just the raw and the sensor itself. So thanks for that.

Yes, its me. Glad you like your BMPCC SB!

 

[optitek]

Ok so how do you measure it?
Obviously you have mo lens prescription so you can't model it.
you have no sensor DSP code so you don't know how the image from the lens is processed for display.
For all you know the optical corrections may be performed by the camera DSP. Every sensor manufacturer does that. Sony, Canon , Panasonic all have optical corrections coded into their DSPs.

 

[brianc1959]

Ok so how do you measure it?
Obviously you have mo lens prescription so you can't model it.
you have no sensor DSP code so you don't know how the image from the lens is processed for display.
For all you know the optical corrections may be performed by the camera DSP. Every sensor manufacturer does that. Sony, Canon , Panasonic all have optical corrections coded into their DSPs.

You can measure the effect of a filter stack by doing an MTF measurement, as was done in the LensRentals articles. In case you missed that, its really worth reading them. Star testing is also a very sensitive method. You can also see it in projection

You can model the effects using any of the thousands of lens prescriptions available in patents, or you can simply model its effect on a perfect lens (the best method). In the case of the Primo-V series I did modelling based on the actual prescriptions, of course.

Cameras can do a good job with corrections involving re-mapping, like distortion and lateral color, but they cannot correct spherical aberration, coma, and astigmatism, which are the main aberrations caused by flat plates in a converging beam. In any case, since the degree of aberration depends strongly on relative aperture, a cine camera can't possibly make corrections to all-mechanical lenses like Masterprimes because the aperture isn't "known" by the camera.

 

[optitek]

OK how exactly do you do an MTF ?
Image processing DSP influence can not be ignored so MTF measurement of a processed image is not a reliable method. also every OLFP filter is specifically designed for a particular sensor so measuring just the OLFP (how do you even do that ?!!!) with a different sensor nullifies the result.
Sorry just too many holes in your theory (IMO of course)
And there is still a pending question you have not answered:
How come NOBODY noticed any issues with MPs, UPs, S4s, Primos for all these years and continue to produce sharp clear images with these optics designed for film and used almost exclusively in electronic high end image aquisition.
Believe me- I'v been involved in thousands of productions in Hollywood over a quarter century and there some extremely sharp eyes out there that spot issues I have to look for to find. I find it extremely hard to believe that lensrentals is so much more competent than the rest of the industry.
I've done a good deal of testing lenses on high end equipment myself and find their articles a bit.... naive. Of course they are very active on the web and that makes them experts in the public opinion but that's entirely different story...

 

[stoneinapond]

I've done a good deal of testing lenses on high end equipment myself and find their articles a bit.... naive. Of course they are very active on the web and that makes them experts in the public opinion but that's entirely different story...

Belittling the source is the beginning of a lost argument....

 

[brianc1959]

OK how exactly do you do an MTF ?
Image processing DSP influence can not be ignored so MTF measurement of a processed image is not a reliable method. also every OLFP filter is specifically designed for a particular sensor so measuring just the OLFP (how do you even do that ?!!!) with a different sensor nullifies the result.
Sorry just too many holes in your theory (IMO of course)
And there is still a pending question you have not answered:
How come NOBODY noticed any issues with MPs, UPs, S4s, Primos for all these years and continue to produce sharp clear images with these optics designed for film and used almost exclusively in electronic high end image aquisition.
Believe me- I'v been involved in thousands of productions in Hollywood over a quarter century and there some extremely sharp eyes out there that spot issues I have to look for to find. I find it extremely hard to believe that lensrentals is so much more competent than the rest of the industry.
I've done a good deal of testing lenses on high end equipment myself and find their articles a bit.... naive. Of course they are very active on the web and that makes them experts in the public opinion but that's entirely different story...

If you're sincerely curious about this subject, I'll keep trying to help.