Canon may be expensive but...

[neuroanatomist]

@ LetTheRightLensIn - Changing the ISO has no direct effect on the amount of light that hits the sensor. At ISO 400, enabling HTP does not result in a change in the amount of light hitting the sensor, no difference in the number of photons. Period. What the camera does is apply one stop less gain to the signal generated from those collected photons, then applies a tone curve to the jpg data to boost everything but the highlights back up that one stop.

Suggesting that at ISO 400, HTP reduces the light hitting the sensor is not just misleading, it's plain wrong. Neither aperture nor shutter speed are changed - and that's a damn good thing because aperture should be selected to give the desired DoF and shutter speed selected to control motion in the image as desired, and the camera shouldn't be changing those parameters if I don't want it to. I disagree with your statement, "...if I want to do HTP myself what do I need to do? To get the exact same result I shoot at my selected ISO, keep my selected aperture and then I raise the shutter speed 1 stop faster." What if you wanted motion blur of a fountain, but to preserve the highlights in the scene - would you sacrifice the motion blur you wanted, or stop down and change your DoF or lose sharpness to diffraction? I would do what the camera does with HTP - underexpose by lowering ISO as many stops as needed, and if that took me to ISO 100, it would be time for an ND filter.

HTP has limitations, foremost being it's limited to one stop of 'highlight recovery'. But if you understand the technical principles behind it, you can overcome those limitations to some extent, while still capturing the desired image in terms of DoF and motion control.

Like Mikael, you are confusing the actual collection of light by the photon wells of the sensor with what happens to the electronic information into which the energy of those photons is subsequently converted, and with the processing applied to the digitized form of that information even later in the image acquisition process. Those are discrete steps with their own characteristics, and if one is going to discuss the technical details of the image data generation, one should correctly describe and apply those details. You can think of it as a semantic issue if you like, but there was no semantic confusion about providing an incorrect answer to simple yes/no questions - questions which you answered correctly but Mikael did not.

At the outset, everyone deserves respect, appreciation of cultural and linguistic differences, patience, and the benefit of the doubt. If, over time, someone consistently displays rude and insulting behavior (to the self-admitted point of being placed under strict regulations by the mods), makes no significant effort to contribute in anything but a negative manner, is repetitious and combative, and offers neither apology nor any redeeming characteristics, that person deserves to lose the respect of the community...as Mikael has certainly lost mine.

 

[LetTheRightLensIn]

My problem with you [Michael] arose at the time of the "Half The Photons" discussion where, from your [Michael's] posts I got the impression that changing the ISO would somehow change the photons hitting the sensor. I'm sure you knew fully well that changing the ISO does not change the number of photons hitting the sensor then why the charade? What's the point of your expertise if you ultimately end up confusing people - Well you may very well say I'm a noob but there are a number of noobs on CR who log on only to look for some help?

Because is not just changing the camera back to ISO200. It keeps metering at ISO400 while changing the camera gain to ISO200. And if you meter at ISO400 you are getting less exposure than you would metering at ISO200 which the camera is actually working at. If anything, I think looking at it from his point of view seems less misleading. Effectively it is just an automatic and hidden EC -1 to the shutter speed, that is how you'd replicate it yourself in RAW without using the mode, with an automatic tone curve (applied in cam, and suggested to the RAW converter) making the JPGS and histograms look like EC0 with more highlights instead of a flat EC -1 across the board. How do you replicate ISO400 HTP in say Av mode for RAWs? You shoot ISO200 Av mode with EC -1.

Jisses, how many times has I said exposure/metering after a ISO NR
setting iso 1600 from 400 does not mean skit more than you are exposing 2 stops shorter/ less and raising the iso gain

I have to confess that I don't have a clue what you are trying to say here.

 

[LetTheRightLensIn]

no Im saying by exposure after 400 iso you have create a head room by 2 stops compare to 100iso
what is so difficult to understand Neuro! = exposure after 400 iso = halving the hitting lights twice compare to 100iso

Hypothetical example: I shoot jpg. I am taking a picture of a forest scene. I am in Av mode, and I set f/8 to obtain the desired DoF, and I chose ISO 400 to get a 1/100 s shutter speed at metered exposure to avoid camera shake, because I foolishly left my tripod at home. Please note, I couldn't care less about what exposure settings would be at ISO 100, 50, or 3200, that's tangential and irrelevant - I choose f/8 and ISO 400 for the reasons I stated. I take a shot, look at the review image, and see blinking highlight alerts where I want detail of the sun-dappled forest floor. I've read that HTP can preserve my highlights.

If that scenario is confusing, I'll summarize - with HTP off, I set the camera in Av mode, f/8, ISO 400, and the metered exposure gave a 1/100 s shutter speed.

Answer these questions about what happens when I set HTP to Enable:

1) Does my selected aperture of f/8 change?
2) Does the camera-selected shutter speed of 1/100 s change?
3) Does the amount of light hitting the sensor change?

Please, no hand-waving, no 'please read my earlier posts', no repeating what you've posted before, no referring to what may happen at some other ISO setting that I didn't select and don't care about - just answer those three, simple questions with a yes or a no.

1,2,3,
no, no, no

BUT you forget that when you hit the HTP button you are no longer at ISO400 any more but you are at ISO200 only the camera is still metering at as if it were at ISO400. You are not getting to the point of what HTP really is, it is NOT some magic hardware where a new set of photosensors are enabled to capture extra highlights or something, it isn't a 'real' mode, if you want to replicate the mode yourself you can even with a camera that has no HTP button. If you want HTPISO400 and were shooting scenes at say EC0 then you just use ISO200, lock your aperture and then EC -1 or roll up the shutter speed yourself, if in M mode say, 1 stop.

What the camera is actually really doing is just exposing 1 stop less, collecting less photons for any given auto-metering or placement of suggested metering mark.

I can see where your way of looking at it comes from too, but I think it kinda is a less natural perspective.

It doesn't really matter what the camera labels and calls things but what matters in the end is what the sensor is doing, what the shutter speed is, what you get in the RAW file. I think it makes more sense to look at it from if I want to do HTP myself what do I need to do? To get the exact same result I shoot at my selected ISO, keep my selected aperture and then I raise the shutter speed 1 stop faster.

I have newer said any different, you gays are mixed up with something we call head-room, a 400 iso HTP motive are exposed after 400iso with the less gain. TO CREATE A HEAD ROOM and 400iso in its self means 2 stops head room from 100iso = halving the signal twice from base iso.

By you guys are you referring to me too or just the others here???

 

[LetTheRightLensIn]

@ LetTheRightLensIn - Changing the ISO has no direct effect on the amount of light that hits the sensor. At ISO 400, enabling HTP does not result in a change in the amount of light hitting the sensor, no difference in the number of photons. Period. What the camera does is apply one stop less gain to the signal generated from those collected photons, then applies a tone curve to the jpg data to boost everything but the highlights back up that one stop.

this may be a mess since i typed this out quickly with no thought but:

Yes hitting the HTP button doesn't change the total light now hitting the sensor, but HTP mode both secretly changes the gain 1 stop less AND the metering normally used for that gain by to meter 1 stop down from normal.

If you want some HTP thing then you have some sort of scene with extra amounts of highlight stuff say a full stop more than typical so you have some gain that manages to let you maintain a high enough shutter speed to stop motion or to handhold. You then go one gain down and that gain is now getting shot letting in 1 stop less light than you'd normally let in for that gain for a scene that had less highlights.

Or you can go tripod and then you do ISO100 and just raise shutter more and more or you add on more and more ND filters and you are saving highlights by exposing the scene to less and less light.

ISO400 HTP EC0 is no longer ISO400 EC0 it is ISO200 EC0 metered to expose (or suggested to be so in M mode meter readings provided) 1 stop less than the camera normally would do at ISO200 EC0, i.e. it is effectively as if you were to shoot ISO200 EC -1.

You are not gaining a stop of highlights at a given gain and keeping the same light coming in, that can't be done of course.

Yeah it is at the face of it just applying 1 stop less gain but normally when you apply 1 stop less gain you'd also let in 1 stop more light too and in this case you are not so you are basically letting in 1 stop less light than you'd normally do for the gain. ISO400 HTP the camera isn't doing ISO400 at all it is doing ISO200 and it decides to do it at EC -1 metering instead of EC 0 metering and because of the latter part you might look at it that is letting in a stop less light. In an M mode scenario where you end up needing in some case to fix both aperture and shutter exactly it might be weird to think of it in terms of letting less light in since in this scenario you always want to let the same light in, but it is still metering in way that is compatible with thinking about it that way. And your scenario below where it adjusts many stops to match your light sounds more like AutoISO button than HTP button to me.


Anyway, you can only save 1 stop more highlights than the prior shot by either now letting 1 stop less light come in at the current gain (set EC -1) or by keeping same light coming in and lowering the gain 1 stop (by either dialing gain down 1 stop and then setting EC -1 or swapping on HTP because that is HTP). HTP lowers the gain one stop BUT to keep the same light coming in it must be set to meter 1 stop less than normal at the new gain that HTP selected under the hood (or simply fail to report the new actual gain being applied). So HTP is metering to let a given gain get 1 stop less light than it would normally be metered for.

So in the sense that it meters 1 stop darker than it normally would for the gain that it is actually using you might say it is letting in one stop less light than normal. If not it would no different than shooting 1 stop lower with normal metering. When you go to replicate it yourself, that is what you'd do set ISO 1 stop lower than what you had it in with HTP and then set EC -1.

If you set your camera to ISO100 EC -1 and then shoot all day in P,Av,Tv you'll get RAWs that can be made to give same results as ones from ISO200 HTP EC0. In M mode you could get files that can be made to deliver the same results using either ISO100 EC -1 as ISO200 HTP EC0 and meterings suggest to you to use would be the same in either case.

It might make more sense to think of ISO200HTP not as any sort of ISO200 at all but as ISO100 and as an ISO100 that gets exposed to 1 stop less light than typical.

You could think of it as exposed to the same light and then has 1 stop less gain applied which it is but since the basis for that decision was based on the meter thinking it had been getting 1 stop more gain....

So you shoot your fountain at f/4.5, 1/15th, ISO200HTP or you shoot it at f/4.5, 1/15th, ISO100 HTP-off and get the same thing, same shutter, same aperture,same SNR,same DR,RAW files are store a touch differently but are basically 1:1.

Suggesting that at ISO 400, HTP reduces the light hitting the sensor is not just misleading, it's plain wrong.

Yes, swapping ISO400 to ISO400HTP doesn't change the light hitting the sensor. Hit the button and the total light hitting the sensor stays the same.

But saving a stop of highlights for a given amount of sensor gain does mean 1 stop less total light hitting the sensor.

Under the hood, HTP it is doing 1 stop less gain and then setting the metering to expose 1 stop less than normal for that gain. If you have an ISO400 HTP shot and then want to do that on a camera without HTP you would set ISO200 and EC -1. If you shot at ISO400 and EC 0 and wanted to save a stop more of highlights while shooting at ISO 400 what would you do but EC -1 and let 1 stop less total light hit the sensor.

Although it is probably simplest to not talk about getting less photons.

What if you wanted motion blur of a fountain, but to preserve the highlights in the scene - would you sacrifice the motion blur you wanted, or stop down and change your DoF or lose sharpness to diffraction? I would do what the camera does with HTP - underexpose by lowering ISO as many stops as needed, and if that took me to ISO 100, it would be time for an ND filter.

HTP doesn't lower as many stops as needed. AutoISO does.
HTP just shoots one stop lower and then meters that ISO one stop under.

In this particular scenario of M mode, we are not talking about an at any given gain scenario any more. You just set the gain to balance how much shadows detail you want vs highlights saved. Once you get it ballpark you can then +/- 1 it to save more or less highlights. (although you might want finer tuning than whole stops and might want to tweak aperture or shutter 1/3 of a stop unless you MUST have it left as is exactly).

In the ap AND shutter MUST be exactly locked sub-case of M mode scenario deciding to lower gain a stop might be a bit odd to think of in terms of shooting a given ISO with 1 stop less light coming in since you are in a we decided to keep that fixed scenario, granted so yeah a bit weird. It is still true though that if ISO200 HTP is what ended up working for the particular scenario then so would ISO100 instead and if you cared what the meter told you, then setting that EC -1 same as always though.

I disagree with your statement, "...if I want to do HTP myself what do I need to do? To get the exact same result I shoot at my selected ISO, keep my selected aperture and then I raise the shutter speed 1 stop faster."

It is probably best to just say that shooting HTP at a given gain an EC 0 is 1:1 in RAW to shooting without HTP but at 1 stop less gain and metering set to EC -1. That is normally what I say. Of course in the latter case the histogram and image review may be a bit tougher to judge at first.

At high ISO where they disallow HTP is would actually make sense to always have it on since the digital gain just lops off the top stop each ISO you go up.

 

[neuroanatomist]

Yes hitting the HTP button doesn't change the total light now hitting the sensor

Thank you, you can stop there. I fully understand how HTP works 'under the hood', and how to emulate or 'improve' on it when shooting RAW, as do you - I don't think we need to explain it to each other. My point was that someone who states the opposite of the quoted statement above (repeatedly) is wrong, and does not understand the general mechanism of HTP. Incorrect statements like that can confuse people who are trying to understand the concept, and which is unfortunate and should be corrected.

 

[Admin US West]

Users are now reporting this thread because of the personal accusations. Calling another names is not professional and degrades the forum for everyone.

Please agree to disagree and move on, arguing back and forth solves nothing.

 

[sanj]

Come on!

 

[Neutral]

Was reading this thread and could not help but putting some comments.
Let's look at the root of the discussed HTP issue
What is the blown highlights means for the sensor?
Really blown highlights ??
This means that photodiode of the photocell got saturated and any increase in number of captured photons cannot increase current via this photodiode .
Anybody is going to argue about that ?
Then what could be the remedy for this?
Answer is simple - reduce the number of photons.
How this could be done?
Use EC to -1 and then later in processing change gamma in tone curve to raise mid-tones and shadows to the level which would be produced if camera was used with EC=0.
This could be done manually or camera could do that automatically using HTP mode but reporting that all was done with EC=0 , actually cheating camera user.
This all simple physics - anyone could see response curve of the photodiode and understand what it is all about.
It does not matter how you name what is the happening but essence of the process does not change with this.
If photodiode is saturated no further on chip electrical post processing could recover highlights - they already blown.Full stop here.
This is ABC of electical engineering.

 

[neuroanatomist]

@ Neutral - Thanks, and of course, that's absolutely true if the blown highlights are due to saturation of wells with photons prior to application of the user-selected amount of gain (but note that some gain is applied even at ISO 100, since the 'base ISO' of most current sensors is actually a bit less than 100).

But highlights can also be blown by less-than-full photon wells being subjected to too much analog gain as they are read out. That can happen at higher ISO settings (which are often needed for action-stopping shutter speeds or narrower apertures chosen for sufficiently deep DoF at handholding shutter speeds). In that case, simply reducing the gain (i.e. ISO) by one stop will preserve one stop of highlights, and that's what HTP does at ISO 400, for example. No change to aperture or shutter speed, so no change in the amount of light hitting the sensor, but rather, a (clandestine) reduction in the analog gain.

 

[TheSuede]

Actually, what Neutral wrote isn't really true.
A well constructed sensor isn't even close to saturating the charge well when the raw ADU reaches maximum, at base ISO. Constructing a sensor that way would mean some serious linearity problems.

The reason why sensors are made to behave like this is that the charge well capacitance behaves like any other capacitance, the potential difference has to be above a certain threshold for the transfer from cell to well to be linear. If you map the ADU/photometric exposure relationship at base ISO you'll see a slowly decaying rate as you approach blown values in the raw file. This rate of decay can tell you how much bigger than the maximum allowed translated digital value the well really is, and it's usually bigger by about a factor of 2.
So no, no modern sensor "blows to white" due to oversaturation in the charge well. The point where that happens is more than 1Ev past blown white in the raw file at the lowest analog gain ISO ("real" base ISO).

The sentence or meaning of "blown white" always relates to the base data you're looking at, no matter what it is. In a jpg, something blown is pegged at 255. In a raw file something blown is pegged at some value close to 15-16,000 depending on ISO and what channel you're looking at. And no, that value isn't 16383 as it "should be" in a 14-bit file, most cameras don't "fill" the raw file - Canon usually use a real raw value range ADU of either 1024-15,000 or 2048-15,000.

 

[Neutral]

@neuroanatomist & @TheSuede :

I think you are both correct but there are two aspects here.
Hopefully modern sensors are designed in a way to prevent photocell saturation in normal conditions and at base ISO saturation point of photocell (or point just below) should match upper level of input voltage range of correspondent ADC. This is required to obtain maximum possible DR from the sensor at base ISO.

With lower light input (so less input photons) higher digital ISO should increase analog circuit gain so less photocell output should be amplified to the same upper input voltage of ADC.
This also explain why sensor DR is reducing 3db with ISO going up one step (for properly designed up do date modern sensors - e.g. from Sony you see that starting from base ISO).
So for HTP at ISO higher than base ISO negative exposure compensation could be done just by reducing analog circuit gain.
But at base ISO this might not work - we still would need to reduce number of photons by reducing exposure time or using ND filters to prevent photocell saturation.
Unfortunately there is no freely available information for sensor cell full path gain distribution (from photocell up to ADC) to see actual sensor performance in this respect (for different sensors). This would be interesting to see - actually this is one of the most important parts of the system design.

 

[neuroanatomist]

So for HTP at ISO higher than base ISO negative exposure compensation could be done just by reducing analog circuit gain.
But at base ISO this might not work...

In fact, that is precisely the reason that with the HTP function enabled, the user cannot set the camera to ISO 100 (although technically, that's not really base ISO, which is in the ISO 70-80 range). When HTP is enabled, ISO 200 is the lowest user-selectable ISO, so the camera is able to reduce analog gain by one stop at any user-selected ISO setting (the expansion ISOs where digital gain is applied post-ADC are also unavailable).

 

[dlleno]

If I may make an observation of the way my mind naturally imposes a context to the HTP discussion:

this forum is about Canon cameras and the way they work. With few exceptions the reason people come here is to exchange information about Canon cameras, present and future to be sure. Therefore, when I read the expression "the mechanism of HTP" the primary context is, quite naturally and automatically, "what happens inside my camera when I enable HTP". It means "how are the principles of headroom and ISO gain applied inside my camera". It means "what effect does enabling HTP have on the way my camera behaves".

So with the behavior of Canon Cameras paramount to the discussion, I don't see why it is so difficult to think of Canons implimentation of HTP in these terms:

Set your Canon camera up to take a picture at ISO 400.
pause.
apply cyanoacrylate to the ISO dial -- just not to the electronic gain structure inside the camera .
take the picture
pause again. look at the dial. whew it is still set to 400.
enable HTP
look at the ISO dial again. yep, the cyanoacrylate is still there. ISO still set to 400
take another picture. how did the camera behave differently under the influence of the HTP setting?

THAT, to me, is the "mechanism of HTP". Reverting back to the general application of base ISO headroom, the special case of ISO 100, the benefits or characteristics of non-Canon cameras or sensors -- none of that helps describe what Canon cameras do under the vast majority of use cases when HTP is enabled.

Furthermore, a discussion on the differences between Canon and Sony sensors (and the supporting electronics), as regards their respective behaviors at high levels of light (near saturation) and how to advantage each sensor to the given situation is engaging to be sure ,but doesn't help us understand how Canon Cameras behave differently when HTP is enabled, compared to the way they behave when HTP is not enabled.

 

[neuroanatomist]

Quite the price, the Canon counterpart is priced better true.

My heart sinks. Canon may price their next 800 to match this...

Back to the actual topic of this thread...

If/when Canon releases an updated 800mm 5/f.6L IS, it may very well be in the same price range as the Nikon 800/5.6, if not higher.

Consider the MkII versions of the 500/4 and 600/4 lenses. Look at the selling price of the MkI versions at the time the MkII's were announced, add in the price increase that Canon applied during the long delay between announcement and availability, and you see that the MkII lenses were a 45-50% increase in price. Based on the current 800/5.6 price, that gives a range of $19-20K for an 800/5.6 II.

And don't expect Canon to throw a TC into the deal...

 

[wickidwombat]

Quite the price, the Canon counterpart is priced better true.

My heart sinks. Canon may price their next 800 to match this...

Back to the actual topic of this thread...

If/when Canon releases an updated 800mm 5/f.6L IS, it may very well be in the same price range as the Nikon 800/5.6, if not higher.

Consider the MkII versions of the 500/4 and 600/4 lenses. Look at the selling price of the MkI versions at the time the MkII's were announced, add in the price increase that Canon applied during the long delay between announcement and availability, and you see that the MkII lenses were a 45-50% increase in price. Based on the current 800/5.6 price, that gives a range of $19-20K for an 800/5.6 II.

And don't expect Canon to throw a TC into the deal...

I bet higher, and made from titanium and carbon fibre...