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06-30-2011, 10:49 AM   #226
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QuoteOriginally posted by jstevewhite Quote
Sounds like you're getting it. Equivalency is about comparison, not actual photography.
Great! This whole discussion might have been annoying and/or trivial to some, but I have found it useful in understanding the way that a photo is made. For me, and many others, I suspect, understanding a process can lead to both a richer experience and improved results.

06-30-2011, 10:53 AM   #227
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QuoteOriginally posted by Designosophy Quote
And this doesn't even get into the signal-to-noise discussion. Smaller image sensor pixels, fewer photons, more scaling, etc.
Bigger sensor = more signal + more noise.

Starting with more signal is better to begin with. That applies to film as well as digital.
06-30-2011, 03:16 PM   #228
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QuoteOriginally posted by Aristophanes Quote
Bigger sensor = more signal + more noise.
Not quite.

Bigger pixel size -> more photons captured by that pixel -> better signal to noise ratio.

Since SNR is proportional to square root of the number of photons captured (Poisson behaviour).
06-30-2011, 06:06 PM   #229
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QuoteOriginally posted by froeschle Quote
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.
You got that almost right. The correct ISO value is 1344.

So the 8.5mm @ f/1.9 (ISO 100) on the Q-camera allows you to take the same images as a 31mm @ f/7 (ISO 1344) on APS-C. That is the reason why I call the 8.5mm f/1.9 lens on the Q-camera "slow". Because it takes the same images as a 31mm f/7 lens on APS-C. If anyone wants to call the latter "fast", well...

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).
If you multiply focal length, f-ratio and ISO setting when changing formats then you get an equivalent image. Everything is the same (including DOF and exposure).

QuoteOriginally posted by Designosophy Quote
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 combination these two statements don't make sense (if you include lenses for other formats in the comparison). It is physically impossible for a lens to produce deep DOF and let in a lot of light at the same time. If the lens is "fast" (lets in a lot of light), it will create a thin DOF as well (all relatively speaking of course).

QuoteOriginally posted by Designosophy Quote
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.
Yes. It also would have to have this smaller f-stop value to let the same total amount of light in (which is not the same as the "same exposure").


Last edited by Class A; 06-30-2011 at 10:19 PM.
06-30-2011, 08:25 PM   #230
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QuoteOriginally posted by Class A Quote
So the 8.5mm @ f/1.9 (ISO 100) on the Q-camera allows you to take the same images as a 31mm @ f/7 (ISO 367) on APS-C.
Lens equivalence needs to provide equivalent exposure, so what you say cannot be true: an increase in aperture along the line of f/1.9->f/2->f/2.8->f/4->f/5.6->f/7 would mean that the ISO would have to increase much more than from 100 to 367. Without getting into small units, stopping down from f2 to f/5.6 alone implies that an ISO adjustment is needed from ISO 100 to ISO 800. You do need something around ISO 1600 like froeschle mentioned if you decide to stop down to f/7 all the way from f/1.9.

Not that this makes the Q sensor look better according to this theory, but it is important to keep examples correct so other readers don't get confused.

Where I fail to see the equivalence is in noise - I don't remember from my experience with compact cameras that images were as noisy at ISO 100 as my images from APS cameras are at 1600.

I put this on the account that on the implementation road that leads from theory to practice, a few extra parameters need to be added to the model, but those make the model more complicated and only an optical engineer with experience building cameras would be able to come up with equivalence formulas that are of any relevance in the real world.
06-30-2011, 10:27 PM   #231
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QuoteOriginally posted by Class A Quote
In combination these two statements don't make sense (if you include lenses for other formats in the comparison). It is physically impossible for a lens to produce deep DOF and let in a lot of light at the same time. If the lens is "fast" (lets in a lot of light), it will create a thin DOF as well (all relatively speaking of course).
This is the only thing we disagree on that isn't a matter of opinion or context (in this discussion), but fact, and I think you've got it wrong. I don't know who you will accept as an authority if not your own reasoning, but this statement is *false*. I think it's important that it's understood that this is *false*.

The amount of light - the total amount of light - let in by an ideal lens is constant at f1.9 regardless of focal length. A 135mm lens @f2, an 85mm lens @f2, a 50mm lens @f2, a 35mm lens @f2, a 20mm lens @f2, and, yes, an 8.5mm lens at f2, all let in the same total amount of light/cm^2. That's why we use f1.9 instead of an actual measurement of the physical opening of the aperture.

Furthermore, DOF increases with shorter focal lengths at a fixed aperture, even though it lets in the same amount of light.

I know you put "relatively speaking" in there, but this line is just false: "It is physically impossible for a lens to produce deep DOF and let in a lot of light at the same time." A 200mm f2 lens is fast, and produces very narrow DOF; a 35mm f2 lens is fast, and produces a much wider DOF (at the same subject distance), even though it lets in the same amount of light.

True equivalency is not possible, of course, in your terms. Making the Q's lens an f0.3 would require a much, much faster shutter speed (because it would be a FASTER lens), which would in turn create a different image. Equivalency is still just an analogy, and a pointless one if someone didn't live in a 35mm world for a long damned time. Those of us that moved between formats frequently don't need equivalency to help us understand, and those who didn't weld 35mm film into their cranium will only be confused by it.
06-30-2011, 10:28 PM   #232
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QuoteOriginally posted by Laurentiu Cristofor Quote
ISO would have to increase much more than from 100 to 367.
True. Sorry for the mistake! I have corrected my post. Of course the increase in ISO has to compensate the loss of light due to the smaller aperture by the same number of stops.

My (poor) explanation is that I was rushed when I wrote the message and just quickly referenced Falk's message (which contains an error about the ISO value needing multiplication by the crop factor only).

QuoteOriginally posted by Laurentiu Cristofor Quote
Where I fail to see the equivalence is in noise - I don't remember from my experience with compact cameras that images were as noisy at ISO 100 as my images from APS cameras are at 1600.
The "equivalence" considerations are based on an "everything else being equal" assumption. In practice, "everything else" is rarely the same. Sensors differ in their noise floors, processing uses different levels of (hidden) noise reduction, etc.

These practical details (and maybe a non-rigorous comparison regime) are the source of deviations between theory and practise.
07-01-2011, 02:13 AM   #233
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QuoteQuote:
You got that almost right. The correct ISO value is 1344.
I have just tried to set my camera to ISO 1344.444 ... and failed ... that's one reason why I wrote "give a first impression" ...
QuoteQuote:
Where I fail to see the equivalence is in noise - I don't remember from my experience with compact cameras that images were as noisy at ISO 100 as my images from APS cameras are at 1600.
... is another reason for the wording. The sensor of the Q, e.g., uses a back illuminated image sensor, i.e. other technology. Also, most compacts are trained to give at least a reasonable JPG-output, where noise reduction already has been applied. It will be very interesting to see RAWs from the Q. However, here again, equivalence can give some indications, why only now a 1/2.3'' sensor was declared to be sufficient. If you look at the ISO 1600 output of the K10D/K20D and of the K-5 you can see a clear improvement (I use auto ISO <=400@K20D and <=1600@K-5!). I expect ISO 400 to be usable with the Pentax Q, maybe even ISO 800.
QuoteQuote:
Equivalency is still just an analogy, and a pointless one if someone didn't live in a 35mm world for a long damned time.
Imho, it is the best way to compare systems with different sensor sizes, where understanding that
Equivalence
QuoteQuote:
exposure is a meaningless measure in cross-format comparisons
is crucial.

07-01-2011, 02:26 AM   #234
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I'm not sure I get it. Some here are arguing that you have to take the ISO and the aperture times crop factor? Really? I just placed my 1/2.3" compact camera next to my APS-C DSLR, and took a photo with the same settings. The photos were both quite similar. And we don't speak of a F2 lens when we attach a 50/1.4 to a APS-C camera. The only thing that changes when we do that is that we get a 75mm lens and the DoF is not like a 75mm lens on a FF camera.

The reason why small sensors are so noisy is that the buckets which collect light are smaller. And not the whole surface can collect light, between each bucket there is some space (though it is getting smaller every year). So more MP means more spaces, less MP less -> less noisy. Though we are probably getting close to very small spaces.
07-01-2011, 03:51 AM   #235
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QuoteOriginally posted by jstevewhite Quote
The amount of light - the total amount of light - let in by an ideal lens is constant at f1.9 regardless of focal length.
That's true only if you don't change the sensor format.
While an f/1.9 lens for a smaller sensor format will achieve the same exposure it will not let the same amount of total light in.

QuoteOriginally posted by jstevewhite Quote
A 135mm lens @f2, an 85mm lens @f2, a 50mm lens @f2, a 35mm lens @f2, a 20mm lens @f2, and, yes, an 8.5mm lens at f2, all let in the same total amount of light/cm^2.
Two lenses @ f/2 on different formats will produce the same amount of light / cm^2 (note that I deleted the "total"), but not the same amount of total light.

Let's take this to the extreme and think of a "silly sensor" sensor of the size of a single sensor sensel from the K100D. Say a lens on the K100D lets in 6,016,000 photons (that's 3008 x 2000, i.e., one photon per sensor sensel). To achieve the same exposure a "silly sensor" lens only has to let through one (1) photon. In other words, the aperture diameter of the K100D lens has to be ~2450 times bigger in diameter.

Both lenses will be specified with the same f-ratio, because they can achieve the exposure on their respective sensors. What you are trying to tell me is that the "silly sensor" lens is as fast as the K100D lens. That it will have deeper DOF but will be the same in terms of gathering light. Technically, you are correct in stating that it achieves the same exposure but with this extreme example it becomes pretty clear how inappropriate it is to ask for "same exposure" rather than for "same total amount of light".

Say the "silly sensor" lens is an f/1.0 (Wow, that's fast! ). An equivalent lens on the K100D will have to be an f/2453. It will create the same DOF and will collect the same total amount of light. It won't achieve the same exposure, of course. In an "apples vs apples" comparison you cannot increase the exposure by stopping down less nor by increasing the shutter speed, because doing so would produce a different image (different DOF or motion blur). So the only chance is to increase the ISO.

QuoteOriginally posted by jstevewhite Quote
True equivalency is not possible, of course, in your terms. Making the Q's lens an f0.3 would require a much, much faster shutter speed (because it would be a FASTER lens), which would in turn create a different image.
Increasing the shutter speed is not allowed when talking about equivalent images. You'd have to lower the ISO. The fact that certain lenses (f/0.3) don't exist and certain ISO values don't exist either demonstrate limitations of the Q-system.

QuoteOriginally posted by jstevewhite Quote
Equivalency is still just an analogy, and a pointless one if someone didn't live in a 35mm world for a long damned time.
I disagree. It is a useful concept to predict the kind of images you can expect from a Q-camera if you have experience with another format camera (say FF or APS-C). Take your APS-C camera, set it to ISO 1344 (or as close as you can get), put a 31mm lens on it, use f/7 and shoot away. The resulting images will look like images taken on a Q-camera set to ISO 100, with the 8.5mm lens set to f/1.9.

QuoteOriginally posted by jstevewhite Quote
Those of us that moved between formats frequently don't need equivalency to help us understand, and those who didn't weld 35mm film into their cranium will only be confused by it.
Maybe in order to understand that tiny lenses for small sensors do not collect the same total light as bigger ones for big sensors, it is useful to understand equivalence.

Equivalence doesn't depend on experience with 35mm film. It is a useful concept to convert specifications between all sorts of different formats.

Last edited by Class A; 07-01-2011 at 03:59 AM.
07-01-2011, 04:02 AM   #236
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QuoteOriginally posted by froeschle Quote
I have just tried to set my camera to ISO 1344.444 ... and failed
Try harder!
07-01-2011, 06:37 AM   #237
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pentaxq-compare: godfrog: Galleries: Digital Photography Review
I wonder where they got the Q from, but this doesn't look too good. The NEX-5 obviously just kills the Q, the S95 looks a bit better too, though the difference isn't as big. The S95 is a bit more naturalistic to me, and captures more detail. And of course both cameras are cheaper than the Q.

Oh, btw., seems like dpreview got their hands on a Q.
Pentax Q Compact System Camera Samples - Hands-On Preview The photos are gone, but they may be the source of the above photo.
07-01-2011, 06:43 AM   #238
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QuoteOriginally posted by Class A Quote
You got that almost right. The correct ISO value is 1344.

So the 8.5mm @ f/1.9 (ISO 100) on the Q-camera allows you to take the same images as a 31mm @ f/7 (ISO 1344) on APS-C. That is the reason why I call the 8.5mm f/1.9 lens on the Q-camera "slow". Because it takes the same images as a 31mm f/7 lens on APS-C. If anyone wants to call the latter "fast", well...


If you multiply focal length, f-ratio and ISO setting when changing formats then you get an equivalent image. Everything is the same (including DOF and exposure).


In combination these two statements don't make sense (if you include lenses for other formats in the comparison). It is physically impossible for a lens to produce deep DOF and let in a lot of light at the same time. If the lens is "fast" (lets in a lot of light), it will create a thin DOF as well (all relatively speaking of course).


Yes. It also would have to have this smaller f-stop value to let the same total amount of light in (which is not the same as the "same exposure").
Great. I'm with you. I think we just have a different definition for the word, "fast." This seems to be at the heart of a large portion of the discussion in this thread. Is there some sort of official reference for this term, or is it just a convention? If it's just a convention, then I think the definition I am using is more conventional than the one you are using.

I don't think the conventional definition of "fast" includes depth of field. DOF is a corollary, but it's not part of the commonly understood definition. In other words, if the conventional understanding of a fast lens is that with a fast lens you can use a fast shutter speed at a given ISO in given lighting conditions to get the same exposure because you can use a smaller f-stop value, then by this convention, the 8.5mm f/1.9 lens of the Q is faster than a 31mm f/7 lens.

What I'm saying is it might be impeding understanding to use the terms slow and fast when talking about depth of field (I know it impeded my understanding). Again, this might be due to my lack of experience. If the conventional understanding is that a fast lens has a shallow depth of field and lets in a lot of light, then I'm off base.
07-01-2011, 07:28 AM   #239
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QuoteQuote:
QuoteQuote:
I have just tried to set my camera to ISO 1344.444 ... and failed
Try harder!
Didn't help

QuoteOriginally posted by kadajawi Quote
pentaxq-compare: godfrog: Galleries: Digital Photography Review
I wonder where they got the Q from, but this doesn't look too good. [...]
Enlarging to full size reveals that this is a ISO 3200 comparison. Coming back to
Q: ISO 100 <-> APS-C: ISO 1344
Q: ISO 3200 <-> APS-C: ISO 43008 (!)
So, the result should not be too surprising
07-01-2011, 08:17 AM   #240
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QuoteOriginally posted by Class A Quote
That's true only if you don't change the sensor format.
While an f/1.9 lens for a smaller sensor format will achieve the same exposure it will not let the same amount of total light in.

Two lenses @ f/2 on different formats will produce the same amount of light / cm^2 (note that I deleted the "total"), but not the same amount of total light.

Let's take this to the extreme and think of a "silly sensor" sensor of the size of a single sensor sensel from the K100D. Say a lens on the K100D lets in 6,016,000 photons (that's 3008 x 2000, i.e., one photon per sensor sensel). To achieve the same exposure a "silly sensor" lens only has to let through one (1) photon. In other words, the aperture diameter of the K100D lens has to be ~2450 times bigger in diameter.

Both lenses will be specified with the same f-ratio, because they can achieve the exposure on their respective sensors. What you are trying to tell me is that the "silly sensor" lens is as fast as the K100D lens. That it will have deeper DOF but will be the same in terms of gathering light. Technically, you are correct in stating that it achieves the same exposure but with this extreme example it becomes pretty clear how inappropriate it is to ask for "same exposure" rather than for "same total amount of light".
Here's the problem with that whole explanation AND with the argument here: I can put the same lens on both sensors. It's OBVIOUS that the same lens allows the SAME LIGHT through, yes? Yet your explanation about "total light" remains "true". This entire argument is claiming that I can change the total light a window passes by changing the size of an index card lying on the floor receiving the light from the window. The smaller sensor collects less light, yes, but the LENS passes the same amount, period. Thus, the Q's lens isn't slow; the sensor is small, and smaller sensors collect less light. There's no problem with the definition of exposure, because that's how the world WORKS. Film and digital sensors receive light in the distributed fashion described by light flux/cm^2. The redefinition you're attempting is pointless and leads to false conclusions.

QuoteQuote:
Say the "silly sensor" lens is an f/1.0 (Wow, that's fast! ). An equivalent lens on the K100D will have to be an f/2453. It will create the same DOF and will collect the same total amount of light. It won't achieve the same exposure, of course. In an "apples vs apples" comparison you cannot increase the exposure by stopping down less nor by increasing the shutter speed, because doing so would produce a different image (different DOF or motion blur). So the only chance is to increase the ISO.

Increasing the shutter speed is not allowed when talking about equivalent images. You'd have to lower the ISO. The fact that certain lenses (f/0.3) don't exist and certain ISO values don't exist either demonstrate limitations of the Q-system.
Again, we should stop using the term 'fast' in this way. Just google "What is a fast lens?" and everyone I can find uses it as I do, meaning "A lens is called fast because it allows a fast shutter speed". Since @ISO 100, the Q will use the same shutter speed as, say, the 43 f1.9 on my K-5, it is just as *fast*. You can pretend that equivalency makes it slow if you want, but that's not how anybody else uses the term, and it's an unnecessarily confusing definition.

It's completely counter-productive to attempt to re-define f-stops and exposure and the like - concepts that have stood comfortably for over a hundred years regardless of format, and still accurately describe reality and the physics of photography - in the service of an *analogy* like "equivalence".

QuoteQuote:
I disagree. It is a useful concept to predict the kind of images you can expect from a Q-camera if you have experience with another format camera (say FF or APS-C). Take your APS-C camera, set it to ISO 1344 (or as close as you can get), put a 31mm lens on it, use f/7 and shoot away. The resulting images will look like images taken on a Q-camera set to ISO 100, with the 8.5mm lens set to f/1.9.
No, it won't. It uses a different sensor and different electronics and a different lens and all of these things add up to differences that will not be captured by "equivalence". You even said as much: "The "equivalence" considerations are based on an "everything else being equal" assumption. In practice, "everything else" is rarely the same. Sensors differ in their noise floors, processing uses different levels of (hidden) noise reduction, etc."

QuoteQuote:
Maybe in order to understand that tiny lenses for small sensors do not collect the same total light as bigger ones for big sensors, it is useful to understand equivalence.
It's NOT THE LENSES. It's the SENSORS.

QuoteQuote:
Equivalence doesn't depend on experience with 35mm film. It is a useful concept to convert specifications between all sorts of different formats.
Here's a concept that's missing from your equivalence equation: Final magnification. Final magnification may be the single highest determiner of IQ of those discussed here so far, and your equivalence model ignores it completely. if the Q sensor is 8mm across, and the ff sensor is 36mm across, to get true equivalence, you'd need to use different sized prints to match the final magnification ratio, right? But when you do that it knocks all of the equivalence numbers into a cocked hat - because they aren't REAL numbers; they're an analogy between two different formats. All analogies break down eventually.

I can obtain all the information I need to know about "equivalence" by understanding just a few concepts.

1) The higher the final magnification, all other things being equal, the lower the image quality.
2) The shorter the focal length, all other things being equal, the greater the DOF.
3) Diffraction limits resolution as a lens is stopped down, and this effect becomes more pronounced as final magnification increases.

Knowing those three things, I can intuit that the image quality of the Q will be lower than that of my APS-C camera, everything else being equal. I can intuit that the DOF of images will be higher. I can intuit that I will run into resolution problems when I stop down in bright sunlight.

And none of these require me to say things that will confuse people into thinking they have to use different settings to get the same exposure. Which is the only reason I've continued this discussion; I mean you no offense, but it's obvious to me that your "equivalence" statements have confused more than one person in this thread - because they're only analogies that don't describe real-world photographic usage.
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