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06-29-2011, 05:24 PM   #211
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QuoteOriginally posted by Class A Quote
Correct.

Note that he was only looking for the "same exposure" though. To achieve the same exposure (not considering that the images won't have the same DOF), none of the exposure parameters need to be changed. The 47mm f/1.9 lens on FF lets in a lot more light than the 8.5mm f/1.9 lens on the Q-system but this is counteracted by the FF-camera using a correspondingly lower ISO value (ISO 200 instead of ISO 1100 (<- the FF-equivalent of the ISO 200 on the Q-camera)).
Yes, I was ignoring depth of field in this case. What I was interested in was just exposure. So, to apply, if had the f value, the shutter speed, and the ISO all set at the same on a FF camera and a Q, the Q would produce a much darker exposure than the FF. That is, unless the ISO on the Q is rigged with "equivalent" ISO.

06-29-2011, 05:34 PM   #212
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QuoteOriginally posted by Designosophy Quote
Yes, I was ignoring depth of field in this case. What I was interested in was just exposure. So, to apply, if had the f value, the shutter speed, and the ISO all set at the same on a FF camera and a Q, the Q would produce a much darker exposure than the FF. That is, unless the ISO on the Q is rigged with "equivalent" ISO.
If you set the Q and the FF camera to the same ISO, same shutter speed, and same aperture, you'll get the same exposure value, but the DOF of the Q will be much greater. No "equivalent ISO" is needed to achieve proper exposure.
06-29-2011, 06:25 PM   #213
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QuoteOriginally posted by Designosophy Quote
So, to apply, if had the f value, the shutter speed, and the ISO all set at the same on a FF camera and a Q, the Q would produce a much darker exposure than the FF.
No, the exposure would be the same.

Note, however that "exposure" is not the same as "total light collected". To achieve the same exposure on a larger format, you need to collect more light. The reason for this is the way "exposure" is defined based on candela/sq.mm, i.e., the amount of light on a unit area, rather than the whole image area. This means that larger sensors need to collect more total light in order to achieve the same exposure.

Conversely, a 5.5 crop factor sensor receives a lot less photons for the same exposure (this is why I previously wrote the 47/1.9 lens will let in a lot more light than a 8.5/1.9 lens). This reduces the SNR (signal to noise ratio) and hence will increase the apparent noise when you look at the small sensor image at the same output size as the large sensor image.

Many know that small sensor cameras tend to produce more noise but a lot less know that this is not caused by the sensor size but by the (typically) slow lenses in front of them. A 8.5mm Q-lens at f1.9 has an aperture diameter of 4.5mm. A 47mm FF-lens at f/1.9 has an aperture diameter of 24.7mm. No wonder that in the same period of time (provided the same shutter speed is used), more photons flow throw the lens with an aperture area that is 30 times the size of the smaller one.

P.S.: Note that we changed sensor formats when changing focal lengths. This is the reason why we cannot state "f/1.9 yields the same exposure, independently of focal length". We needed to change the focal length when going from Q to FF in order to maintain the same AOV (angle of view). If you just change the focal length without changing the sensor format, you are also changing the angle of view. The narrowing down of the angle of view when going from 8.5mm to 47mm for a fixed sensor size deprives you from receiving exactly the amount of photons you gain by making the aperture diameter bigger.
06-29-2011, 07:43 PM   #214
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QuoteOriginally posted by Class A Quote
No, the exposure would be the same.

Note, however that "exposure" is not the same as "total light collected". To achieve the same exposure on a larger format, you need to collect more light. The reason for this is the way "exposure" is defined based on candela/sq.mm, i.e., the amount of light on a unit area, rather than the whole image area. This means that larger sensors need to collect more total light in order to achieve the same exposure.
I think the problem here is that this part is being expressed in a confusing fashion. The light flux (candela/sq mm) is *identical* regardless of format. It is true that a greater amount of light falls on a larger sensor for the same exposure value, just as a larger amount of sunlight falls on a sheet of letter paper than falls on an index card. The amount of light at any point, however, is the same.

QuoteQuote:
Conversely, a 5.5 crop factor sensor receives a lot less photons for the same exposure (this is why I previously wrote the 47/1.9 lens will let in a lot more light than a 8.5/1.9 lens). This reduces the SNR (signal to noise ratio) and hence will increase the apparent noise when you look at the small sensor image at the same output size as the large sensor image.
I completely agree here (except for the part about more light).

QuoteQuote:
Many know that small sensor cameras tend to produce more noise but a lot less know that this is not caused by the sensor size but by the (typically) slow lenses in front of them. A 8.5mm Q-lens at f1.9 has an aperture diameter of 4.5mm. A 47mm FF-lens at f/1.9 has an aperture diameter of 24.7mm. No wonder that in the same period of time (provided the same shutter speed is used), more photons flow throw the lens with an aperture area that is 30 times the size of the smaller one.
This, however, is *not true*. At f1.9, the same "amount" of light flows through the lens in the same amount of time (in flux, or candela/sq mm). That's the DEFINITION of exposure value. A sensor, regardless of size, requires a some fixed amount of light for proper exposure. This is true of film, as well. Any lens you strap on the camera, focussed at infinity, will pass the same "amount" of light to the sensor in the same amount of time at f1.9.

If it were true that a 47mm lens at 1.9 passed more light than an 8.5mm lens at 1.9, you'd have to tell your hand-held meter what the focal length of your lens was in order to calculate proper exposure; but you don't.

To make this even more clear, this means that each lens - say, a 35mm f2, a 50mm f2, an 80mm f2, and a 135mm f2, would all have to have different exposure values for the same situation. But sunny 16 works with all of them.

I mean no offense to anyone, and I don't mean to make this thread unnecessarily long. I think this is an important piece of information. A 135mm f2 lens passes exactly the same amount of light to the sensor that a 50mm f2 lens does in the same amount of time. So does a 10mm lens at f2. Less light actually strikes a smaller sensor, but the flux at any point is the same, and the total light per sq mm is identical.

If we make the sensor larger than the image circle of the lens, it can be true that the total amount of light that flows through the lens is different, even though the light flux per sq mm is identical - but this is irrelevant to the subject at hand, and also dependent on image circle design, not focal length; the concepts are loosely coupled, but it's quite possible to make a 90mm lens that covers 4x5" and a 135mm lens that only covers 1x1.5", and the 90mm lens will pass LOTS more light, even though the aperture is physically smaller.

06-29-2011, 07:53 PM   #215
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Guys, please stop this nonsense with lens equivalence! I can't demonize effectively in such conditions! You're messing up my mojo!

QuoteOriginally posted by rparmar Quote
Laurentiu, we would probably agree more than we disagree, except that you have to demonise me for some reason. You assume I said things I never did.
I think you are making statements that are too generic and as a result they sound outrageous when taken at face value. If you would qualify your statements more carefully or if you would adjust them as a result of feedback, I wouldn't have to keep pointing the logical errors that you are making so that you end up with the feeling that I am demonizing you.

QuoteOriginally posted by rparmar Quote
And your primary device is to over-emphasise my points.
I don't overemphasize anything, but you keep ignoring facts I present and the context in which past statements were made. Which is why this will be the last attempt I'll make at responding to these accusations.

QuoteOriginally posted by rparmar Quote
I never said anything about people not needing compact cameras. YOU MADE THAT UP.
I didn't make up anything. It is the logical implication of saying things like:

QuoteOriginally posted by rparmar Quote
All this proves is that no-one needs the Q -- they can just use their phone.
If no one needs the Q because they can use their phone, nobody needs compact cameras either because they can use their phone, by the same line of argumentation you are making. The Q and other compact cameras (except a few recent ones) are having similar sensors and your main argument for dismissing the Q is that its sensor is small - but it is not smaller than that of standard compacts. And there is no indication that the IQ is lower than that of current "super" compacts with slightly larger sensors (S95, XZ-1, etc). So there is no proof of anything, just a false assertion.

QuoteOriginally posted by rparmar Quote
I have used no fallacies -- you have yet to name them
I did that already, you just forgot which one I was referring to:

QuoteOriginally posted by Laurentiu Cristofor Quote

QuoteOriginally posted by rparmar Quote
Hmmm... I wonder if all the fans would be out in such force defending an inferior sensor if this was a Canon or Sony camera?
Hmm, a loaded question. I'm sorry that you have to resort to such rhetoric.
Where is the loaded argument? It lies in the fact that you start your question from the assumption that the Q is an inferior camera and then you are wondering why it is defended by Pentax fans, by which you attempt to dismiss opposite arguments as being born from fan bias. If you think such rhetoric is not insulting to the person you are talking to, you should think again. This type of rhetoric is commonly used by politicians but it shouldn't have any place in a serious friendly conversation - because it's a dishonest tactic - it's like an under the belt hit in a friendly boxing match - it's immature too from a personal growth perspective. Which is why you have no right to feel offended and play the victim for me telling you to grow up and have a serious discussion.

QuoteOriginally posted by rparmar Quote
(except the straw man I did not use -- YOUR MISUNDERSTANDING of what I wrote).
No, it is your misunderstanding. I did not say you used a strawman argument. My statement was generic:

QuoteOriginally posted by Laurentiu Cristofor Quote
AFAICT, the only people that equated the Q with a DSLR were those that were criticizing it for not being one. This is called strawman argument.
Except by the time I made this reply you already expected that everything I said critically was meant for you in particular.

QuoteOriginally posted by rparmar Quote
And of course I will be offended that you resort to such phrases as "grow up" since it is DESIGNED TO BE OFFENSIVE.
Loaded questions are designed to fool people. That is as offensive as you can get in a written conversation. And growing up is something you should always do - it doesn't refer to physical growth or to reaching drinking age. You could have outgrown your arguments by adjusting them to new facts, but you insisted on stubbornly defending your position. I'll pass to you the challenge of finding a non-offensive way of describing that attitude.
06-29-2011, 08:01 PM   #216
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06-29-2011, 08:53 PM   #217
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QuoteOriginally posted by jstevewhite Quote
If it were true that a 47mm lens at 1.9 passed more light than an 8.5mm lens at 1.9, you'd have to tell your hand-held meter what the focal length of your lens was in order to calculate proper exposure; but you don't.
That's because the light meter only cares for candela / sq.mm. It doesn't care for the total amount of light required.

You are saying yourself:
QuoteOriginally posted by jstevewhite Quote
It is true that a greater amount of light falls on a larger sensor for the same exposure value, just as a larger amount of sunlight falls on a sheet of letter paper than falls on an index card.
Note that a lens on the larger sensor camera needs to be able to let the larger amount of sunlight through. That's why it has to be faster.

You cannot simply use the same aperture diameter and blow up the image circle. By doing so you would reduce the light flux (same amount of photons spread over a larger area). Hence, the aperture diameter needs to be larger.

AFAIC, the problem lies with the definition of exposure. It assumes that one wants the same candela / sq.mm amount for all formats. If you scale various images from different source sizes to the same output size, however, you not only get the effect that larger sources require less magnification and hence yield sharper images, but they also represent more total light being collected. Remember, they received the same exposure, i.e., same amount of light per unit area, but have a larger area, hence collected more total light. This means that images from the smaller sources will not only be less sharp but also noisier.

The reason for this is not that smaller sensors/film formats are noiser, it is because we assumed that "same exposure" provides a level playground. Well, it doesn't. In order to produce an image with the same SNR, a smaller sensor needs a higher exposure. That's why using the same f-ratio isn't enough, it needs to be smaller (which also brings the DOF into line).

Requiring "higher exposure" sounds as if more light is needed for the smaller sensor, but that's not the case. We just need to supply it with the same total amount of light. Only the, unit area based, definition of exposure causes this fact to mean that a higher exposure is needed.
06-29-2011, 10:24 PM   #218
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QuoteOriginally posted by Class A Quote
That's because the light meter only cares for candela / sq.mm. It doesn't care for the total amount of light required.
But this is *exactly* the value that represents exposure to both film and digital sensors.

QuoteQuote:
Note that a lens on the larger sensor camera needs to be able to let the larger amount of sunlight through. That's why it has to be faster.

You cannot simply use the same aperture diameter and blow up the image circle. By doing so you would reduce the light flux (same amount of photons spread over a larger area). Hence, the aperture diameter needs to be larger.
No, actually, you can. It's done in large format lenses all the time. There are 120mm lenses that cover 2x3", ones that cover 4x5", and ones that cover 8x10"; most are f5.6 or so. As you know, a 120mm lens @f5.6 is going to be ~22mm, regardless of the coverage of the image circle. You don't have to make the aperture diameter larger, you have to design the lens for a larger image circle. It then accepts light from a greater angle of incidence - allowing more total light through the SAME opening. Same thing applies to FL on small cameras. A 28mm lens produces exactly the same total light on the sensor as a 100mm lens, because it accepts a greater angle of incidence through the opening.

QuoteQuote:
AFAIC, the problem lies with the definition of exposure. It assumes that one wants the same candela / sq.mm amount for all formats. If you scale various images from different source sizes to the same output size, however, you not only get the effect that larger sources require less magnification and hence yield sharper images, but they also represent more total light being collected. Remember, they received the same exposure, i.e., same amount of light per unit area, but have a larger area, hence collected more total light. This means that images from the smaller sources will not only be less sharp but also noisier.
See, when I read this previous paragraph, I think we're very close together; the only objection I'd have is that the reason exposure is defined that way is that, for a given EV, you *do* want the same candela/sq.mm regardless of format; otherwise, metering wouldn't work. Other than that, I'm with you.

QuoteQuote:
The reason for this is not that smaller sensors/film formats are noiser, it is because we assumed that "same exposure" provides a level playground. Well, it doesn't. In order to produce an image with the same SNR, a smaller sensor needs a higher exposure. That's why using the same f-ratio isn't enough, it needs to be smaller (which also brings the DOF into line).

Requiring "higher exposure" sounds as if more light is needed for the smaller sensor, but that's not the case. We just need to supply it with the same total amount of light. Only the, unit area based, definition of exposure causes this fact to mean that a higher exposure is needed.
Ok. I see what you're saying here, but it's important to note that these things are only true when we're describing "equivalency", *not* when discussing standard photography. ISO100 is ISO100 regardless of the format, and has the same SNR at the source (the sensor); since we have to magnify it more to get the same size print, the output has a lower SNR. f1.9 is f1.9 regardless of the focal length and sensor size in terms of actual exposure at the film plane, because sensors and films WORK on the candela/sq.mm principles described above. It's not the definition of exposure that's at fault; it's the concept of equivalency, which is an analogy, supposedly to make it easier to understand the trade-offs as we change formats.

Any time we increase magnification of the source, we reduce output quality in measurable ways; smaller sensors must, then, have lower quality over the whole image area (with an ideal reproduction medium and all other things being equal).

But the thing I wanted to get at is that there's no need to re-write photography books to tell people they are incorrectly exposing their film because of the equivalency numbers you've spoken of. Light flux, or candela/sq.mm, or other measure of light power per area is the way sensors and film see light, and f-stops are the way lenses deal with light, and everyone is happy again. I can safely use the same settings for aperture, shutter speed, and ISO, across every film and digital body with impunity, and only be concerned about equivalency when I feel the need for a cheerful argument.

Some of the other stuff we've argued about is related to context; for instance, the discussion about pixel size and noise. Your statements were true, but only if you make the same size print regardless of MP; if I take a FF sensor of 6MP and one of 24MP, and I make an 8x10 from the 6MP (300 dpi) and make a 16x20 from the 24MP (300dpi), the larger print will show more noise; we can say it's because of the smaller pixels, or we can say it's because of the greater magnification, and *both* are true. But if we make both prints 8x10, we'll see exactly the same noise profile, as you said.

On the definition of "fast", I'm afraid we'll have to agree to disagree. It seems nonsensical to me to suggest that a lens is called 'fast' because it gives low DOF rather than because it allows fast shutter speeds.

06-30-2011, 01:59 AM   #219
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This discussion shows how futile it would be to try to explain and flog the Q to any photographers except those eager for exoensive novelties, trinkets or accessories.

Pentax are being criticised not for the Q itself - which might be very good for what it is - but for allocating precious resources to producing something that does not provide Pentax dSLR users with the lightweight alternative that so many have been waiting for... something pocketable but packing performance. That 'something' almost certainly has to be an APS-C format mirrorless/rangefinder, if it is to be convincing. Meanwhile the Pentax faithful have to look elsewhere.

And on the subject of dSLRs, while I'm very pleased with the capabilities of my K-7 it is still significantly larger and heavier than the film MX, which was my front-line camera for 20-odd years, and its auto back-up Ricoh XR7, and not as sweet to use as my Contax Aria (which again is smaller and lighter in spite of everything that's packed into it).

What I'm saying is that dSLRs are getting out of hand - they have to be 'lugged' around, whereas it was a pleasure to carry the MX, Aria or Olympus OM4Ti all day. They were true companions. If it wasn't for the fact that there's still nothing to rival the optical pentaprism viewfinder, the SLR would probably be extinct by now. All the more reason for Pentax to get stuck into the serious alternative market.
06-30-2011, 04:24 AM   #220
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QuoteQuote:
an ISO setting multiplied by ( the crop-factor )^2
QuoteQuote:
ISO 200 instead of ISO 1100 (<- the ff-equivalent of the ISO 200 on the q-camera
ISO 6050

So, shooting with a Pentax-DSLR, the 31@f7 and ISO 1600 should give a first impression, how pictures of the Q with the 8.5@f1.9 and ISO 100 could look like.
06-30-2011, 07:10 AM   #221
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I would like to say thank you to everyone for trying to explain the relationship between focal length, sensor size, aperture, and sensitivity. I have to admit that I'm just not getting it. Every time I think I understand, someone says something that seems to contradict my understanding. I think I'm going to have to do some independent research to figure out the principles behind the process.

Thanks again, everyone. And I can't wait to see some photos from the Q! Has anyone seen any posted anywhere? If so, please share a link.
06-30-2011, 07:31 AM   #222
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QuoteOriginally posted by Designosophy Quote
I would like to say thank you to everyone for trying to explain the relationship between focal length, sensor size, aperture, and sensitivity. I have to admit that I'm just not getting it. Every time I think I understand, someone says something that seems to contradict my understanding. I think I'm going to have to do some independent research to figure out the principles behind the process.

Thanks again, everyone. And I can't wait to see some photos from the Q! Has anyone seen any posted anywhere? If so, please share a link.

It's damn confusing, and it really started because of the people making the flip from film to digital in the early days and all the salespeople (myself included) were looking for a simple way to explain the differences. now it just muddies things. If like a lot of people you've only shot apsc dslrs then it really should be ignored IMO, you're lenses perform the way they have always for you, and what the FOV or DOF on a FF or 35mm film camera would be has zero relevance for you.
the same will hold true for most people who buy a Q (in fact they may adapt better to the Q because when they take a picture a large area will be in focus and the common refrain of why are my pictures out of focus (DOF) won't occur, and they'll probably be very happy with their purchase. for any DSLR user thinking of it then think of it as performing like any point/shoot you've had but with some DSLR functions. Small and Fun but not your DSLR.
Christ we work hard at making what can be a fun hobby/obsession into something way to difficult sometimes
06-30-2011, 08:07 AM   #223
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QuoteOriginally posted by Designosophy Quote
I would like to say thank you to everyone for trying to explain the relationship between focal length, sensor size, aperture, and sensitivity. I have to admit that I'm just not getting it. Every time I think I understand, someone says something that seems to contradict my understanding. I think I'm going to have to do some independent research to figure out the principles behind the process.

Thanks again, everyone. And I can't wait to see some photos from the Q! Has anyone seen any posted anywhere? If so, please share a link.
Yeah, Eddie's right. Equivalency is working too hard unless you've spent years with a 35mm film camera or a full frame camera and need some way to compare a different format to your experience.

The long and the short of it: 1) exposure is exposure is exposure; that's why one meter works for all formats and focal lengths. This discussion only applies when we're talking about trying to create an identical (in every way) image with two different formats - "equivalency". 2) As you increase magnification, all else being equal, absolute image quality degrades. So smaller sensors, which need more magnification to reach a given print size, will have lower IQ, all else being equal. Whether or not that IQ difference matters - or is visible at the reproduction size - is another question that requires *actual samples* to know - all this stuff is a means of trying to predict that answer before the fact.
06-30-2011, 09:18 AM   #224
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QuoteOriginally posted by DragonLord Quote
A 35mm equivalent camera will have: * A focal length multiplied by the crop-factor * An f-stop value multiplied by the crop-factor * An ISO setting multiplied by the crop-factor !! (forgotten by most people)(Originally from Falconeye)
Okay, I think I'm starting to get it. In practical terms, to get the same exposure, the ISO setting would need to be multiplied only if the f-stop value is multiplied. It won't be an equivalence because the DOF would be different, but it would be the same exposure (darkest to lightest point in the resulting image).

So, to apply (and here's where I seem to trip), The 8.5mm f/1.9 lens is as fast as any other f/1.9 lens. However, it provides the DOF of a lens with a much higher f-stop value on full frame. In order to get the same depth of field on the Q sensor as a lens with the same field of view on a FF sensor, the Q lens would have to have a much smaller f-stop value.

And this doesn't even get into the signal-to-noise discussion. Smaller image sensor pixels, fewer photons, more scaling, etc.

Last edited by Designosophy; 06-30-2011 at 09:55 AM.
06-30-2011, 10:29 AM   #225
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QuoteOriginally posted by Designosophy Quote
Okay, I think I'm starting to get it. In practical terms, to get the same exposure, the ISO setting would need to be multiplied only if the f-stop value is multiplied. It won't be an equivalence because the DOF would be different, but it would be the same exposure (darkest to lightest point in the resulting image).

So, to apply (and here's where I seem to trip), The 8.5mm f/1.9 lens is as fast as any other f/1.9 lens. However, it provides the DOF of a lens with a much higher f-stop value on full frame. In order to get the same depth of field on the Q sensor as a lens with the same field of view on a FF sensor, the Q lens would have to have a much smaller f-stop value.

And this doesn't even get into the signal-to-noise discussion. Smaller image sensor pixels, fewer photons, more scaling, etc.
Sounds like you're getting it. Equivalency is about comparison, not actual photography.
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