Do the math - focal length and Format demystified

[k-hawinkler]

Hi Stefan,
First off, let's dial back the rhetoric a notch or two, and make sure we are all talking about the same thing, because from the vitriol in this thread we clearly are not.

You and your friend that made the video are looking at the glass half full. I'm not even looking at a glass, I am looking at a set of tools, if you get my drift. I hear your objections, and understand your possibly valid concerns about how the industry chose to express its arcane wisdom over time. Remember though before tossing the whole baby out with the bathwater, there is a lot of history behind those choices. History for creating beautiful, well crafted images, not animated pixel comparison graphs or charts.

You ask do I have concern about a LEICA DG NOCTICRON 1,2/42,5mm on my GH3 not looking like an f1,2/85mm on my Canon 5D Mark III? No, actually I am not. If it did, it wouldn't be any dang good to me either, because Canon already has that covered with the excellent Canon 85mm f/1.2. You've got it backwards.

I would buy the LEICA DG NOCTICRON 1,2/42,5mm for my GH3 to go the OTHER way. In other words, @f8 using the Nocticron, I get total depth of field EXTENDED by two stops, closeup to infinity. Don't forget, in a filmmakers bag of tricks, different sensor sizes are great creative tools as well. You may not like the laws of physics, but we all have to live by them, and to be better photographers, learn to use them to our creative advantage.

If I wanted an 85mm f/1.2 look, I would pull out a 5D Mark III with a Canon 85mm f/1.2. I don't horse around with all those "math computations" your talking about when I am out shooting, nor do I carry my Captain America secret decoder ring. I carry a good light meter, a complete set of primes, a vision of what I want, and the experience collected over the years to help guide me in choosing the right camera sensor size, correct angle, and correct lenses to capture it - at the correct exposure.

I don't have the luxury of the time to run an Excel spreadsheet to compute anything. My clients rightly expect I get familiar with my gear, learn how to use it and what it can do in my own time, and then show up ready to complete the work I contracted with them to produce.

As a DP, I am responsible for deciding I need a GH3 & 12mm @f/2.8 for the look I am after. Or alternatively, maybe it is an 85mm f/1.2 Canon 5D Mark III look that is called for. Both are equally valid options, and both equally exclusive each to the other. I can't get the GH3 to look like the 5D anymore than I can get the 5D to look like the GH3. Or the Fuji X-E1 or the Sony A7R or the tiny Zoom Q4. They all look different, even with the same lens and the best grading suite in the business. MF & LF, same thing. Formats are options too.

What you seem to be saying is a conspiracy by camera manufacturers to somehow cheat the population, I as an artist see as viable creative options I wouldn't want to loose. Hope that helps to understand where some of us are coming from, and what we consider is relevant.

Chuck, well said. Thank you.

I consider it absolutely wonderful to have all these choices and the competition between equipment makers to keep costs down. Besides the creative choices/reasons you list there are also other ones that make me prefer one tool or format over another one for a given situation. For example bulk or weight considerations. I use everything from my iPhone to iPad, E-M5, E-M1, NEX-5N, NEX-7, D40, D200, D300, D3, M9, A7R, and D800E. I even used a G3 and S95. What's not to like?

I find the technical progress that has been made in the last few years quite amazing. With an A7R as a back MF isn't out of reach for me anymore. Even light field cameras might get advanced enough to become interesting tools, not just novelties.

I suspect some who inject cheating into this discussion are driven by ulterior motives. Enough said!

 

[Shashin]

I just spoke about this with Dietmar Wüller of Image Engineering (probably one of the few experts worldwide besides People like Dr. Nasse from Zeiss maybe - which I will also try to ask about it....as I can ) for maybe 15minutes now
.
The problem is:
1) it is additionally scale dependent, macro will differ from infinity
2) there are some other formulas involved to make it scientifically stringent
and thus there is no SINGLE Formula where you can calculate this.

Fact is though: if you look at the Image appearance, this is definitely right as the video describes it. Or as Lloyd descibes it.
The part with the photons (differing exposure), amount of exposure is completely left out to trial and error, as there are no correllating data for any sensors available (at least not in easy Public access) that could be used to define this.

He gave me a link for the formulas he once descibed for depth of field on differing formats, which is kind of a mathematical proof for the part of the differing depth of field (in german only-sorry), but as there are so many superior math geniuses around here, they may easily understand the formulas without an english explanation....

» Blog Archiv » Schärfentiefe

Greetings from Germany
Stefan

Here is an English paper on DoF:

http://www.smt.zeiss.com/C12567A8003B8B6F/EmbedTitelIntern/CLN_35_Bokeh_EN/$File/CLN35_Bokeh_en.pdf

Yes, you can make similar, but not identical, images from difference systems. That is not new. But the video is saying so much more.

 

[Shashin]

Chuck -

well it probably depends on what you are demanding for your money ?

Lets say you go to your Photodealer of your choice and want to buy a brand new LEICA DG NOCTICRON 1,2/42,5mm H-NS043 for whopping 1500 € (around 2000 $ US).
At home you shoot this lens and (it may or may not equal absolute exposure depending on the technology of your used MFT sensor, most likely it will NOT be the same as most new FF 24x36 sensors) but even worse - the image does not look like a f1,2/85mm (which is the actual used terminology in ads) but exactly like a 2.0/85mm on 35mm FF - both on open aperture (added to clarify even more !!!) ?

Wouldn´t you feel cheated ? Especially as a good 2,0/85mm from Nikon, Canon or SonyFF costs only about 1-/4th to 1/5th of this ?

Of course this lens may be the best of the best and you say you don´t care if it is so superior, but additionally you are limited to 16 Mpix resolution whereas the 24x36 FF rise to 36mpix ?

Totally out of relevance ?

Really ?

Actually, if that is what you think, then the problem is not with the lens nor the camera. We cannot fix the ignorance of the buyer. The best way is to educate yourself on optics and photography. Unfortunately, with the video you posted, you will not find help there.

 

[Jack]

Smaller stops (larger f numbers) produce a longer depth of field,

Only when ALL OTHER FACTORS REMAIN EQUAL. And that's not the case in the video description. As stated earlier, f-number is simply the ratio of focal length to aperture diameter as a determination of luminance. It is but one factor in determining DoF, and image magnification size as well as CoC are critical.

 

[Stefan Steib]

Shashin

Thanks for posting the link to Dr. Nasses´s article, that would have been the next one I would have linked too. See the table on page 10. There - I forgot - is even a larger format reference to 4,5/6 6/9 and 4/5". QED.

There are therefore equivalent f-numbers
for all formats, corresponding to the linear format
size.

Next: even if some people dislike it, but it is pretty safe to say that smaller sensors collect less light. Exception: when their pixels are the same size as of a bigger one and thus have a smaller resolution. - which is why we don´t see actual MFT´s go beyond 18 Mpix - which is around 36Mpix on the Sony A7R.

Next: on this level of equal pixel size and probably CMOS architecture, the photon count wil probably be similar. but then next:

By the lack of higher resolution the additional depth of field may not be of relevance at all because it may not even be recorded (depends on the subject and focusing distance - hence "may").

Next: it is pretty safe to say that a 21,7 Mpix FF24x36 like a Canon 5DIII will have a significant higher photon collection ability than a much smaller sensor with the same resolution on half of the chip-area.

I still wonder why this is disputable.

And lastly: if the industry decides to advertise their lenses for formats with other sizes with equivalents to 35mm, they probably also should change the given image result description by valueing the real comparable aperture equivalent and write f2,0 when this lens is an f.stop of 1,2. The other - probably a better - solution would be to introduce a clear description of a factor number to describe the actual depth of field value comparable to the absolute first opening.

Pretty easy, but why isn´t that done nor mentioned ?

Regards
Stefan

 

[Stefan Steib]

Chuck - You are kidding right ? You must be kidding.

Ok- you tell me (transferred to 35mm)instead of using an FA Zeiss 1,8/55mm and stop it down to f 16 when needed you would do the same preferrably with an Otus 1,4/55mm for 2500 € more, even if this technically doesn´t make any sense at all ?

or staying at the original example using a Nocticron 1,2/42,5mm for 1500€ even if e.g. the M.Zuiko Digital 1,8/45mm costs 329 € and there is (very likely - especially on Video!) not any visible difference at f8 ?


Congrats. You must have won in the lottery. All others should maybe calculate their spendings instead of paying for a myth.

Regards
Stefan

I would buy the LEICA DG NOCTICRON 1,2/42,5mm for my GH3 to go the OTHER way. In other words, @f8 using the Nocticron, I get total depth of field EXTENDED by two stops, closeup to infinity. Don't forget, in a filmmakers bag of tricks, different sensor sizes are great creative tools as well. You may not like the laws of physics, but we all have to live by them, and to be better photographers, learn to use them to our creative advantage.

If I wanted an 85mm f/1.2 look, I would pull out a 5D Mark III with a Canon 85mm f/1.2. I don't horse around with all those "math computations" your talking about when I am out shooting, nor do I carry my Captain America secret decoder ring. I carry a good light meter, a complete set of primes, a vision of what I want, and the experience collected over the years to help guide me in choosing the right camera sensor size, correct angle, and correct lenses to capture it - at the correct exposure.

.

 

[Lonnie Utah]

Is this pissing match still going on? oke:

 

[Ben Rubinstein]

I've given up cause maths are really not my thing but can I say that I like the guys goatee and wish mine looked as good?

 

[Lonnie Utah]

but can I say that I like the guys goatee and wish mine looked as good?

I thought he kinda looked like this guy...

 

[Shashin]

Shashin

Thanks for posting the link to Dr. Nasses´s article, that would have been the next one I would have linked too. See the table on page 10. There - I forgot - is even a larger format reference to 4,5/6 6/9 and 4/5". QED.

There are therefore equivalent f-numbers
for all formats, corresponding to the linear format
size.

No one said you cannot correlate DoF among formats, but to get there, it requires additional variables beyond f-number. The video claims that f-number is a measure of DoF. That is not true. Also, this is a very limited statement--true at infinity, not so at short object distances.

This simple statement, and it is incomplete, is simply an easy way to make comparisons between formats. It is much harder to take this and make universal statements about photography. And it is pretty useless beyond buy a new system. When you take a photograph, do you think in terms of a format you are not using or do you think in terms of the camera you have in your hand?

Next: even if some people dislike it, but it is pretty safe to say that smaller sensors collect less light. Exception: when their pixels are the same size as of a bigger one and thus have a smaller resolution. - which is why we don´t see actual MFT´s go beyond 18 Mpix - which is around 36Mpix on the Sony A7R.

So, by this statement, you are saying that smaller sensors may or may not collect less light. Not a useful statement.

Next: on this level of equal pixel size and probably CMOS architecture, the photon count wil probably be similar. but then next:

By the lack of higher resolution the additional depth of field may not be of relevance at all because it may not even be recorded (depends on the subject and focusing distance - hence "may").

DoF is not dependent on pixel resolution.

Next: it is pretty safe to say that a 21,7 Mpix FF24x36 like a Canon 5DIII will have a significant higher photon collection ability than a much smaller sensor with the same resolution on half of the chip-area.

I still wonder why this is disputable.

First, pixel area is important in relation to exposure and S/N, not sensor area. Or are you saying you are losing light if you crop you image later? Pixels are the smallest indivisible element of a picture. How many photons one pixel gathers is independent of all the other pixels around it. The fact that a larger sensor has more photon strikes is irrelevant to the image in regards to S/N or the image in itself. It is the photon strikes per pixel.

And lastly: if the industry decides to advertise their lenses for formats with other sizes with equivalents to 35mm, they probably also should change the given image result description by valueing the real comparable aperture equivalent and write f2,0 when this lens is an f.stop of 1,2. The other - probably a better - solution would be to introduce a clear description of a factor number to describe the actual depth of field value comparable to the absolute first opening.

Pretty easy, but why isn´t that done nor mentioned ?

Regards
Stefan

First, the industry understands the people don't get how format size changes field of view. They use the fudge of crop factor and equivalent focal length to communicate to consumers. So, when you understand that this is a method to counter people's ignorance, then you would want to question the consumer knowledge, not photo science.

Focal length is focal length. Sensor size does not change that. That is why the lens description is given in the actual focal length. For some consumer cameras, manufacturer have used equivalent focal lengths (Dimage 7, for example), but there again, it is for consumer ignorance (and you cannot remove the lens). And if you use any equations in photography, plugging in equivalent focal lengths is going to give wrong answers. Focal length is a real optic property independent of field of view.

And the same with f-number. The f-number given with the lenses is absolutely correct. f-number simply defines the illumination at the image plane. That is it. It is used for exposure.

What you are proposing is the f-number is a measurement of DoF. That is absolutely false. Folks have a hard enough time with crop factors now, but equivalent DoF given if f-number are really going to confuse these poor saps. How the hell are they going to figure out exposure? Is it the first f-number or the second? Then when you realize that your f-number/DoF scheme is only going to work at infinity and to calculate an effective f-number at some other distance, the whole problem becomes a mess. And you cannot plug in equivalent f-numbers into equations either and get the right result.

This is what I propose. Why not just use focal length and f-numbers to mean what they actually do mean and let photographers educate themselves instead of inventing crazy schemes?

 

[Stefan Steib]

@ Will:

Or as I said before:

" The other - probably a better - solution would be to introduce a clear description of a factor number to describe the actual depth of field value comparable to the absolute first opening."

Otherwise we mostly agree. What is creating additional confusion is the terminology. By demanding a terminology to be correct and allowed to criticise, but on the other hand tolerate that the focal lenght and the valid behaviour of the lens are spelled different you introduce Orwellian Zwiedenk here. I get your point about not unneccessary confusing people.

But then you see how confused many people are now, even if they call themselves Pros !

Greetings from Germany
Stefan

 

[Guy Mancuso]

One of the biggest problems that was created years ago was using the term focal length and putting a number on it like 24mm lens or 50mm. Back than everything was full frame in photography given the format. Today with APS , m3/4 and so on we are stuck with these multiplication factors to deal with to find the equivalent to FF. What they should have done back in yesteryear was called lenses by there angle of view. Than any lens made in any format change the consumer would go by angle of view. Than all this conversion stuff would have been eliminated. Okay that ship passed a long time ago

 

[Stefan Steib]

Guy

EXACTLY !!! I was just thinking to type that when you did !

A hint for people who are somewhat lost with all this:

There is a very nice app (well there are more but this is the best I know) called "Angle of View" which can be used on iOS.

https://itunes.apple.com/us/app/angle-of-view/id452946973

if you want to spend a bit more and you are into cinema/Video too get PCam

https://itunes.apple.com/us/app/pcam-film+digital-calculator/id295456485?mt=8

This is maybe the best tool to easily understand the differing formats and that is very helpful. Makes it much less abstract.

Greetings from Germany
Stefan

 

[Guy Mancuso]

Now if they did this if you wanted a 74 degree angle of view in any format than you buy that lens that is in that format system. Easy Peasy.

I think this is actually a 24mm in FF. But if they created that system a lot of confusion would have been eliminated. Not all but some

Im still having a issue with loss of light on format change, not sure I am buying into that one yet. I may have to put the A6000 24mpx APS sensor up against the A7 24mpx FF sensor with exactly the same lens and than change they magnification on the A6000 by backing up the camera to the same angle of view. Im just not thinking there is any loss of light with this. The same amount of light is hitting the same area of view, that does not change. Why would my effective aperture number change is my real question. The answer given is bigger photo sites , correct. Well is that not compensated in the design of the smaller formats???????????????????????????????????

I need a better understanding of this because i am not sure i have really seen it in action. Now I am somewhat of APS newbie but if memory serves me well which it does sometimes the same dang sunny 16 rule applied to both MF and 35mm equally. ISO 100 is 1/125th at f16 regardless of system

 

[stephengilbert]

:dh2:

:dh2:

:dh2:

:wtf:

 

[johnnygoesdigital]

No one said you cannot correlate DoF among formats, but to get there, it requires additional variables beyond f-number. The video claims that f-number is a measure of DoF. That is not true. Also, this is a very limited statement--true at infinity, not so at short object distances.

This simple statement, and it is incomplete, is simply an easy way to make comparisons between formats. It is much harder to take this and make universal statements about photography. And it is pretty useless beyond buy a new system. When you take a photograph, do you think in terms of a format you are not using or do you think in terms of the camera you have in your hand?



So, by this statement, you are saying that smaller sensors may or may not collect less light. Not a useful statement.



DoF is not dependent on pixel resolution.



First, pixel area is important in relation to exposure and S/N, not sensor area. Or are you saying you are losing light if you crop you image later? Pixels are the smallest indivisible element of a picture. How many photons one pixel gathers is independent of all the other pixels around it. The fact that a larger sensor has more photon strikes is irrelevant to the image in regards to S/N or the image in itself. It is the photon strikes per pixel.



First, the industry understands the people don't get how format size changes field of view. They use the fudge of crop factor and equivalent focal length to communicate to consumers. So, when you understand that this is a method to counter people's ignorance, then you would want to question the consumer knowledge, not photo science.

Focal length is focal length. Sensor size does not change that. That is why the lens description is given in the actual focal length. For some consumer cameras, manufacturer have used equivalent focal lengths (Dimage 7, for example), but there again, it is for consumer ignorance (and you cannot remove the lens). And if you use any equations in photography, plugging in equivalent focal lengths is going to give wrong answers. Focal length is a real optic property independent of field of view.

And the same with f-number. The f-number given with the lenses is absolutely correct. f-number simply defines the illumination at the image plane. That is it. It is used for exposure.

What you are proposing is the f-number is a measurement of DoF. That is absolutely false. Folks have a hard enough time with crop factors now, but equivalent DoF given if f-number are really going to confuse these poor saps. How the hell are they going to figure out exposure? Is it the first f-number or the second? Then when you realize that your f-number/DoF scheme is only going to work at infinity and to calculate an effective f-number at some other distance, the whole problem becomes a mess. And you cannot plug in equivalent f-numbers into equations either and get the right result.

This is what I propose. Why not just use focal length and f-numbers to mean what they actually do mean and let photographers educate themselves instead of inventing crazy schemes?

get a room!

 

[Stefan Steib]

Johnny - it´s so cosy here....

for those who don´t want to read the full Zeiss document- here is the important table that we were speaking about before

Regards
Stefan

 

[jlm]

gotta give stefan his due for keeping cool with all the angst flowing

 

[Shashin]

get a room!

:wtf:

 

[Stefan Steib]

Guy - this is already done correctly in the pro range- as a sample here from Rodenstock´s Digital lens catalogue - does say nearly all you need. Their advantage is that the MF sensor sizes right now do only vary in a narrow range.

Regards
Stefan