Hi all,
In my search for information on the GH4 i came across this very informative (at lest to me) youtube video about sensor size, apertures, & focal length.
https://www.youtube.com/watch?v=DtDotqLx6nA#t=19
Certainly interesting to see how the lenses being marketed by Panasonic & others are marketed in a way that is very misleading. A long video, but well worth watching especially if you are not exactly up to speed on all the technicalities of this stuff.
Regards,
Bryce

Hi Bryce,

Thanks for the link, most informative.

Having been into photography for well over half a century, I thought I knew all this stuff but boy, was I wrong where ISO and crop factor are concerned! Like most (I imagine), I assumed that if I buy a lens or camera stated to be (say) f2.0, that's what I will get, whatever size the camera sensor is. Naughty marketing tactics by some? It would be interesting to know how manufacturers determine f-stop values and if they all use the same method.

I would like to know how this relates to fixed lens camcorders.

It's thought provoking stuff, I will certainly watch it a few more times until I memorise the maths.

Dave

Like most (I imagine), I assumed that if I buy a lens or camera stated to be (say) f2.0, that's what I will get, whatever size the camera sensor is.

That's exactly what you get regardless of the sensor size.

+Gary ....not entirely
excellent find Bryce
in watching the vid, one is exposed to a good deal of light on the subject :)
I'm hoping that the info in this very informative piece applies to video in a much less dramatic fashion given the slow shutter speed 1/60 necessary for smooth video and in particular to the lumix fz1000 which I'll be using as a full time camcorder rather than primarily a stills cam, 1/60th of a second allows a generous exposure incident for lighting the sensor.
The next 6 weeks of waiting for my purchased Lumix fz1000 (if you use paypal they charge it right away) will be a new experience. Usually getting a new cam produces gratification within the week

That video is more of a political statement than real information.

A couple of reasons for my opinion:

He doesn't talk about the real reason why DOF is different for large versus small sensors - circle of confusion. As the imagers get smaller the cross sectional area of the pixels get smaller. This changes how the light from the lens interacts with the pixels and with that changes the appearance of the DOF. The actual DOF for a 200mm f/2.8 lens doesn't change regardless of the imager it is resolving onto. What changes is how small the pixels are and in that the APPARENT DOF changes due to COF.

He also ignores a real issue with digital photography sensors which is that the manufacturers do not state signal-to-noise ratio in any relatable terms. That is what he should really be pointing out in the section about noise and ISO.

ISO makes sense when you understand that ISO is a numeric representation of the sensitivity of film. Films' sensitivity is determined by how much light is needed to expose the emulsion to a specific density of grey in the developed film. What makes this more or less irrelevant to digital photography is manufacturers don't tell you the S/N ratio for a specific illumination of a pixel to get some mid level output signal. That would be the equivalent way to think about it as compared to film. THAT is what he should be bringing to the conscienceless of the viewers instead of what he presented. How much light does it take to reach a specific S/N ratio with a specific electronic signal level (this is the equivalent of film density) would be the real way to calibrate ISO in a camera and would give you more comparable noise results between sensors and manufacturers. Don't hold your breath for that though.

Anyway, the information in the video has some crude relevance if you are looking to get similar looking images from different sensor sizes assuming a similar absolute resolution for each sensor. What is lacking is the true underlying reasons for the differences and disguised in its place is a rant about corporate dishonesty.

Yes for me though, it was very interesting to see just how much light i loose with a smaller sensor. Being able to put that into stops of light was very handy. My Nikon 80-400 f4.5-5.6 on my EX-3 for example has the field of view on a full frame sensor of a whopping 432-2160mm, BUT, it is far less sensitive on the 1/2" sensor of the EX-3 & is like using an f 24.3-30! Certainly that is how i understood it. Just made me think of how much harder you have to work to get the light you need onto the sensor to make the images we want.
Certainly sounds like we could use an overhaul of the system used to measure sensitivity, & it sounds like you are on the right track Chris. Something that represents the S/N ratio would be ideal. Apertures won't change, for a given lens, but how that relates to a specific sensor size does. This video for me at least, highlighted this.
Bryce

I think the entire video is serving a good purpose as there is a lot of misunderstanding as well as liberties being taking in the industry.

Most of what has said is under the real world use umbrella. Pick up this camera you get this, pick up that camera you get that. All will not be equal even though it is implied it can be. You have to know your stuff and be an informed consumer.

I do agree with you Chris, noise or the signal to noise ratio is really at the heart of all of this. The S/N ratio is the area where camera manufacturers can cheat the science so to speak. If they can make a super efficient sensor it will break though the usual yardsticks and make up for the lower levels of light being let in with smaller lenses.

One example would be 1/3" camcorders. The most recent cameras have huge resolution and almost zero noise compared to the earlier generations. The lenses have stayed to the same size but they have made up the difference through sensor development.

I have just re-watched the first part of this video and it's giving me more questions than answers.

Taking the analogy of a sensor surface area being like a bucket in the rain, then surely the lens is like a funnel. If I put a funnel with the same size inlet as the large bucket into a smaller bucket, the smaller bucket must collect the same amount of water. So if the analogy works, a lens designed for a crop sensor should put as much light on it as the equivalent maximum f-stop lens designed for full frame does on the full frame sensor.

If the sensor can't handle all the light, maybe due to it having smaller photo receptors (the analogy being the small bucket overflowing), then it's simply the equivalent of having a slower film in the camera?

Please humour me, I am trying to understand what the man meant.

Dave

You have to think of the front element of the lens as the size of your bucket. What he is focusing on is just the amount of light allowed to hit the sensor. A lens with a larger opening will just let more light in than a lens with a smaller opening. Very simple concept.

Cameras with smaller sensors usually have lenses with smaller front elements. Even though these smaller lenses will say f2.8, they will not let the same amount of light pass as a full frame f2.8 lens. So this naturally inhibits the smaller sensor cameras due to light starvation, not sensor weakness.

If you put a Canon EF 24-70 f2,8 on a GH4 with an adapter that focused all of the light passing through (like a Metabones adapter does) the GH4 will then have sensitivity more like a larger sensor camera. Solely because more light is getting to the sensor.

Does this help?

One thing we need to make sure stays straight in the discussion.

A f/2 lens can be large or small. Don't think the f2 lens on your pocket camera isn't f2 because it is small. It is f/2 and small because the image circle is also small. The image on a pocket sized f/2 will be equally as bright as a huge f/2. What will be different is the size of the image circle. The bigger lens will transmit more TOTAL light but since it will be spread over a LARGER image circle resulting in a brightness in any one spot will be equal to the small lens. You need a large image circle for a large sensor and you can get away with a small image circle with a small imager.

What the SpeedBooster adapter does is change the size of the image circle for the lens. It takes the larger image circle of a full frame lens and compresses it into a smaller space. With that the same number of photons are concentrated over a smaller area and the spot brightness is increased. You didn't change the f-number of the lens. You only changed the size of its image circle.

Lets look at it this way - Remember back when we were kids and used a magnifying glass to burn things? You figure out pretty quickly that as you move the lens back and forth from the object the size of the light spot grew and contracted. Once it contracted to a tiny dot we concentrated the energy enough to get things really hot. This is a practical demonstration of image circle. As you shrink the size of the image circle the light is more and more concentrated. You didn't change the f-number of the magnifying glass. You only changed the size of the image circle.

So, we need to keep in mind that unless you are stating the size of the lens pupil, the focal length of the lens, and the size of the image circle you aren't able to directly compare lens efficiencies. Or we can just use the f-number. ;)

Thank you both for your replies.

My confusion arises from having known most of this stuff for years, but not being able to grasp the concept of modern lens labelling, or should I say mis-labelling?

I am picking up conflicting information about what appears to be the same thing and that is what does not make sense to me. Let me try to explain........... If I buy an f2.0 lens, it will be an f2.0 lens whatever the sensor size, as confirmed by Gary, right? Now, this is the bit that sounds wrong to me, if I buy a pocket camera with a lens that says it's f2.8 but it does not let as much light pass as a full frame f2.8 lens, then how can it be an f2.8 lens?

Mmmm........you're right Chris, I think I will just stick with familiar f-numbers and forget all this high fallutin' stuff, once I have found the answer to this. I have to because it will bug me until I do!

Dave

Dave,
The way i understood it, & i may be completely wrong here, but the f-stop of a lens is determined by the focal length divided by the diameter of the aperture. So for a 70mm full frame lens, if with its iris opened up fully, is 25mm in diameter, then 70 divided by 25 = 2.8 & that is the f-stop of the lens.
So what i think he is saying, is that if you have for example the Panasonic 12-35mm f2.8 lens, the lens is always going to be a 12-35mm f2.8 lens, but on the micro four thirds sensor, that same lens has a field of view half that of what it would be on a full frame sensor, so effectively like having a 24-70mm lens. Now this is where he is saying, that the manufacturers are being misleading, because in many cases they are saying that a given focal length lens, with a given f-stop gives a longer focal range in terms of 35mm equivalent, but also that the light gathering abilities of the lens is the same. He is saying no it's not. For a micro 4/3" sensor for example, the light would be half that of the f2.8 so, f5.6.
I think Chris has it nailed when he talks about the magnifying glass & moving it in & out to concentrate the light. It's the same amount of light hitting the magnifying glass's lens, but depending on where you have it, the image circle is increased or decreased. The smaller sensor size only gathers the light from the middle portion of the light coming through the lens, therefore, for a 1920x1080 image, we have half the light hitting the sensor than we do for a 1920x1080 image on a full 35mm sensor.
Metabones seems to have fixed this problem by using another lens element to focus all the light from the lens, onto the smaller sensor for any given camera, hence the need for all the different models for different cameras with different sensor sizes.
As for small pocketable cameras. If you look at the Sony RX10, the focal length on that is 8.8-73.3mm f 2.8 lens. Now with the 1" sensor size of this camera, if we think of focal length in full frame terns, that would be the equivalent of a 24-200mm lens. The iris diameter however, doesn't change. So if we divide the 73.3mm focal length of the lens by the f-stop of 2.8, we can see that the iris diameter when fully opened up, is 26.18mm. This sounds about right judging by the size of the lens on that camera. But if we say that the lens is equivalent to a 24-200mm f 2.8 lens, then the iris opening diameter would need to be a whopping 71.43mm! This i would have to agree is simply not possible.
Regards,
Bryce

David, how about this approach: An f2.8 lens is always an f2.8 lens. If you have a beam of light that is 70mm wide passing through a lens it will provide a certain amount of illumination (photons) hitting the sensor. The issue with different camera/sensor sizes is how much of that beam will make it to the sensor.

So a FF lens with a 72mm front element would let all of the beam pass through to hit the sensor. A M4/3rds lens with a 35mm front element would only let half of the light beam hit the sensor. The lens is still a constant f2.8 on both cameras but the M4/3rds camera is receiving half of the light. So the M4/3rds camera would need to be set at a higher ISO to keep exposure constant. Which leads to more noise and less Dynamic Range.

So what the guy was trying to point out is the very question you are asking about. F-stops alone are not the only indicator in this scenario. ISO sensitivity has to play a part as well.

The advantage of FF cameras is being able to operate the camera at lower ISO levels which results in lower noise and possibly greater dynamic range images. The disadvantage of smaller sensors is being forced to use higher ISOs compared to larger sensor cameras. Image quality is not at issue given enough light in the scene.

This is the kind of confusion I was concerned about with how the information was presented in that video.

The short answer is that video does not provide real information. It only gives the appearance of real information. It is more confusing that it is valuable. The only thing I saw of value was the charts that can help you get similar looks from cameras with differing sized imagers. Ignore the rest and you will be much better for it.

I think Chris’s short answer has nailed it – to me the video has more style than substance.

However I am not happy with Chris’s magnifying glass analogy, by moving the lens in and out you are focusing the image of the sun and the sharper focused it is the smaller and hotter is the image.

I thought ‘image circle’ referred to the size of sensor or film the image can usefully cover. For example can it cover a 1/3” sensor, the full 35mm frame or a full plate? The image circle is a function of the lens design.

My minor issue with ‘f’ numbers is that if you actually measure the iris diameter and divide it into the stated focal length you come up with a different ‘f’ number than stated on the aperture ring. I have always assumed that the discrepancy is related to the iris’s position in the lens and the properties of the various elements.

Chris is the voice of reason here. A lens has no idea what sensor it is attached to. A lens with a F2.8 maximum aperture transmits more light than a F5.6 lens on any given sensor. If I am shooting M43 I want to have a 12 to 35mm F2,8. If I am shooting a 5D MK3 want a 24 to 70 2.8. However you want to twist it around these are the choices for a given format,

I think you guys are missing the video's point. Yes you want an f2.8 lens on a 5D and a GH4 but the end result will NOT be the same from an exposure/ISO point of view. The GH4 due to the laws of physics will not be able to match the 5D in ISO settings because the 12-35mm lens does not let the same amount of light through as the 24-70. Even though they are both operating at f2.8. Duct tape the larger element 24-70 to the GH4 and focus all of the light to hit the sensor and it will be on par with the 5D. It is the lens size which is matched to the sensor size that is holding you back.

That is pretty much what the guys was after. f2.8 is not "the same" across different sensor sized cameras from a results point of view.

I think it is valuable for everybody to know this info. My compact camera shooting at f2.8 will need a ton more light than my 5D will...

I think you guys are missing the video's point. Yes you want an f2.8 lens on a 5D and a GH4 but the end result will NOT be the same from an exposure/ISO point of view. The GH4 due to the laws of physics will not be able to match the 5D in ISO settings because the 12-35mm lens does not let the same amount of light through as the 24-70. Even though they are both operating at f2.8.

Please don't take this as argumentative. I do quite well get the point. You are right that the reason is due to physics. Quantum Physics to be more specific.

I'm not disagreeing with the relative observation regarding differing noise and sensitives between a FF35 and a M4T imager. Where I do not agree with the video is with the conclusions as to the source of the differences. The source of the differences is plain and simple pixel size and not lens sensitivity. If the pixels of a GH4 (3.7 micron) were the same size as the pixels of a 5d miii (6.25 micron) the GH4 would exhibit similar noise and sensitivity results with the same efficiency lens. The difference would then become resolution. You would have to give up about half of the resolution of the GH4's imager to bring its sensitivity and noise characteristics up to a 5d miii. I am not condemning the GH4 and praising the 5d. I am only trying to explain why you are seeing these differences with actual information instead of blaming the lenses which is completely misdirected.

I also disagree with this specific statement in the video. - Small pixels are not more noisy than large pixels (if fed enough light).

This is completely false. Smaller pixels have less dynamic range and higher noise coefficients. This is unavoidable due to quantum effects of making the P/N junction material thinner and thinner to accommodate the smaller pixels. Adding more light does not fix this problem. Electronically reducing the sensitivity of the sensor as you have to do when adding more light will only further reduce dynamic range. He shot the noise test and may have gotten things to look similar at that one light density but I suspect if he had done a full battery of tests for dynamic range with those same settings it would reveal compromised DR.

As sensor technology improves this situation will also improve. I will not say it will always be this way. Using the current CMOS technology it is a real limitation and the true source of the problem. Not the lens.

I think Chris’s short answer has nailed it – to me the video has more style than substance.

However I am not happy with Chris’s magnifying glass analogy, by moving the lens in and out you are focusing the image of the sun and the sharper focused it is the smaller and hotter is the image.

I thought ‘image circle’ referred to the size of sensor or film the image can usefully cover. For example can it cover a 1/3” sensor, the full 35mm frame or a full plate? The image circle is a function of the lens design.


You are absolutely right that moving a magnifying lens closer and further from an object is NOT the same thing as changing the image circle of a compound lens. What I was attempting to do is describe the principal of light concentration in a way that would be more relatable. I should have made that more clear in my explanation.

Once again, thanks to everyone for your replies.

I found this on Mirrorlessrumors.com (I hope I'm not breaking forum rules by pasting it here!). It, together with your comments, tells me that what I have always believed to be true, is.

"UPDATE: This is an important update by Ale (Mirrorlessrumors administrator)

There are a couple of logical errors made by Tony Northrup.

First: No one cheats. All company aperture lens info are correct! The Panasonic 12-35mm f/2.8 has a f/2.8 aperture and not a f/5.6 aperture. You don't have to make the equivalence he says has to be done! Use the Sony A7r and Sony A6000 for the same shot. To get same result the camera automatically sets on both the same ISO and same lens aperture despite the different sensor size. Just to say that the aperture remains CONSTANT and is not relative! There is no equivalence to make on that!

Please read that article why you cannot apply the aperture equivalence you mention on no other than the "depth of field" and "field of view" only:
"Full Frame Equivalence" and Why It Doesn't Matter - Admiring Light (http://admiringlight.com/blog/full-frame-equivalence-and-why-it-doesnt-matter/)

The focal length and aperture do remain constant!!!!

Second: He says Olympus, Panasonic and Sony do cheat. Nope. All companies use the same kind of measuring aperture for all lens formats (medium format, MFT format, APS-C and so on).

---

UPDATE of the UPDATE: After re-watching the video I am sure Tony confused "Focal length" with "Field of view" ! It's the field of view where you can make the equivalence and not the focal length. Tony has been misleaded by the companies because they do that (small) error too. That's why when he did the math in the video to explain how aperture gets calculated he made the mistake to change the focal length variable. But actually that variable doesn't change at all! What changes is the "field of view" which has no influence on aperture. Hope you got the message :)

LIke Admiringlight says:

"I’ve heard many times “Yeah, your 75mm f/1.8 is crap – it’s like a 150mm f/3.6.” No, it’s not, it’s a 75mm lens with an f/1.8 aperture and a field of view that is the same as a 150mm lens on full frame."

I know Tony had good intentions and I wrote him and hope to will remove the video soon. But please guys, spread the word that what he tells about the equivalence is plain wrong. Don't worry your f/2.8 MFT or APS-C lens is really a f/2.8 lens!"

I shall stick with good old f-numbers and only worry about the ISO setting if it is likely to cause noise.

Dave

If he had opened up the video stating something more like:

In my xxx years experience as a photographer I've used a lot of cameras with different sensors. It can be frustrating to figure out how to get the image you want from the array of different cameras and sensor sizes. In this video I'm going to share what I've learned about making images that have the same look and feel with DOF and noise regardless of the size imager in your camera.

From there if he had stuck to his experience as the angle for what he was presenting and left out the undertone of dishonesty from manufacturer's I would have given it two thumbs up.

I did find the setting information for how to get similar looking shots with different imager sizes interesting. He could have covered that info in 5 min and in the end had a much better video.

Obviously this has a bunch of moving parts. I am not defending Tony the video maker as being all knowing. I just think he hit upon an area that is misunderstood.

Dave, I respectfully disagree with what you just posted. The Panasonic 12-35 f2.8 lens is an f2.8 lens but from a noise characteristic, on a M4/3rds camera, it will perform like an f5.6 lens on a FF camera. Yes, you will get the same exposure, but if you care about noise they are going to be different.

The ISO/ F-stop system is designed to have a constant photographers can rely upon. And it is achieved through varying the noise/sensitivity through cameras. So a 5D set at f4 ISO 100 will get you the same exposure as a GH4 at f4 ISO 100. But that does not mean the images will be "the same". The 5D will have less DOF and less noise.

This may not even be an issue given sensors are very low noise today, but it does exist.

After further thought, I almost think this new world needs a baseline ISO system. A universal constant that all cameras could be compared to. That way a compact camera and a FF camera would not be classified in the same category.

I couldn't agree more Chris.

What threw me was, I learned what I know about photography the hard way, magazines, clubs and much trial and error. I also started a very long time ago, so I am quite prepared to believe that I have been happily snapping and, lately, shooting away and getting good results, in total ignorance of some fact or other.

I am still learning. For example, I only recently learned enough about diffraction limiting to understand its effects, but this video turned what I have learned over the years on its head. When I started to think about it, I realised something was wrong and started asking questions.

I shall stick with what I am happy with - shutter speed is shutter speed, f-stop is f-stop, ISO is another name for film speed (and therefore sensitivity) and crop factor doesn't matter until I mount a full frame lens on my APS-C DSLR. By the way, over here film was marked with both ASA and ISO for many years.

Thanks for helping.

Dave

Dave, I respectfully disagree with what you just posted.
Tim, feel free to disagree as much as you like. I didn't say that, Ale did!

I appreciate your help.

Dave

The manufacturers are correct in their assertion that, for example, a MFT 12 to 35 f2.8 will equal a FF 24 to 70 f2.8, but only in two of three criteria - field of view and aperture speed (f2.8). What they omit to say, and perhaps because it's obvious, is that an f2.8 12 to 35mm lens will NOT give the same depth of field as a 24 to 70mm f2.8 lens at any given f stop. No surprise there. So Tony would be correct in applying math to the aperture when it comes to depth of field performance between the MFT lens and the Full Frame one: they are not the same, and when the manufacturer goes to great lengths to strongly imply that (by some miracle) a MFT 12 to 35 f2.8 equals a FF 24 to 70mm f2.8 in all regards, then that is just silly.

Edit: clarity

As a (now somewhat less proud) GH3 owner, I have done a DoF test a few months back. I mounted my camera on a tripod and took a photo of a close subject using the Panasonic Leica 25mm f/1.4, wide open. I then took the GH3 off the tripod and put a Canon 6D instead, that had a Canon 50mm f/1.4 lens attached to it.

I took the same picture.

There was an obvious difference in DoF between the 2 pictures. Meaning the picture taken with my GH3 had more depth of field, so noticeably more stuff was IN FOCUS.

I have also talked with a fellow videographer who films with a 5D, and when I told him that with my GH3 and that same Leica lens I can film at f/1.4, he raised an eyebrow and said that with his 50mm lens he needs to be at least at f/2.8 to avoid things getting out of focus fast.

It pains me to admit this, but this whole Panasonic GH-hype and the tiny "miracle" lenses for it are not actually what they advertise themselves to be. At almost every event I film, I always proudly show the photographers my Olympus 45mm f/1.8 and make fun of their excessively huge and heavy telephoto lenses. Not in a mean way of course, just to tease.

But the truth is, their Canon 100m f/2.8 has better looking bokeh than my f/1.8 AND has image stabilization which allows you to shoot handheld, something I can NEVER do with my Olympus and not even my Panasonic 14mm f/2.5, unless I am being very careful. Its Canon counterpart, the 28mm f/2.8 actually has stabilization and it works beautifully for video! I tried it 2 weeks ago. In addition, unless I'm being somewhat close to my subject, my 14mm lens even at said f/2.5 produces an almost camcorder-like DoF which kind of defeats the purpose of filming with a DSLR. I'm not surprised now, considering my f/2.5 is actually f/5.

I guess the only "take that!" argument that I actually have for the full frame videographers is, perhaps, that I paid somewhere around 1/3 - 1/4 of the price for almost all of my lenses.

A bit disappointing for us GHx users, to finally have to admit this.

PS: The GH4 is a bit of a different animal with quite a few extra features, chief among which is the 4k-capability, so I still consider that particular camera to be a very good buy and I would buy it myself if I could.

DPReview has a comprehensive article on this issue. Basically saying the same info as in the YouTube video. What is equivalence and why should I care?: Digital Photography Review (http://www.dpreview.com/articles/2666934640/what-is-equivalence-and-why-should-i-care)

I really dislike these discussions. They basically breakdown to my sensor is bigger than your sensor. We all know the pros and cons of FF vs MFT vs APSC. A lens with a diameter to cover a MFT sensor designed with a 2.8 aperture is a lens with a 2.8 aperture. If you want to talk about equivalency in framing a shot in the same lighting conditions with different size sensors accommodations will have to made in subject distance and exposure settings. So what.