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06-27-2012, 01:34 PM   #31
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Dear all,

I like this I have a knack for asking leading questions LOL!!

Once ya'll fight it out and figure out who's right and wrong I will go back and re-read each and every post (really) and quite possibly pick up some more valuable information...

Thanks!!

06-27-2012, 01:51 PM   #32
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06-27-2012, 03:10 PM   #33
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QuoteOriginally posted by Ben_Edict Quote
The AOV will only be the same, if the lens for the larger film/Sensor format has a longer focal length, thus the f-ratio will go down, if you keep the absolute aperture diameter constant. That is the reason for a dimming of the image on the sensor. If you forget the absolute diameter and keep instead the f-ratio constant, the illumination on the sensor plane will be the same, whatever be the sensor size. <-- Simply, because the longer fl lens will have bigger absolute diameters of the aperture iris at any given setting. ...
Ben
Exactly! If the F-ratio is kept constant, the absolute diameter of aperture will increase when the focal length increases. Therefore, it will let in more light in total, in order to achieve the same illumination per area unit on the sensor plane of the larger sensor (remember that the FOV is kept constant, and hence the total light-gathering area is bigger for the longer focal length). However, the two images will not be equivalent. The image on the larger sensor will have a shorter depth of field. To achieve the same depth of field, the absolute apertures have to be the same, implying a smaller F-number for the larger sensor (remember that the camera-to-subject position as well as FOV are kept constant).

Another approach: imagine having a piece of good old film, say ISO 100. Suppose that your light meter told you to choose F/8, 1/100, you did this, and got a good exposure. Now suppose that you put your film in a camera that exposes half-sized frames on the same roll of film (half-sized by area). Just like you say, you can use the same light meter, and set the camera in the same way, F/8, 1/100. Suppose that you wanted to take a picture from the same spot, with the same field of view. To do this, you would have to choose a lens with a shorter focal length to get the same FOV. With the same F-ratio, the absolute aperture diameter would decrease, just like you say. Since we use the same film in both cases, the total amount of light (number of photons) that it can receive per area unit without overexposing is the same. Lets say that the full-sized frame was correctly exposed and received 1000 billion photons in total. That means that about 500 billion photons exposed the right side, and 500 billion photons exposed the left side (if we assume an evenly lit scene) to give a correct exposure. Now consider the half-sized frame. It is exactly the same size as either side of the full-sized frame. If either side of a full-sized frame is correctly exposed at 500 billion photons, then the half-sized frame must also be correctly exposed by 500 billion photons. This means that at F/8, 1/100 and half the size of the same kind of ISO 100 film, half the amount of photons in total will be used to expose the film, so that the number of photons (amount of light) per area unit is the same (at the same F-number (aperture ratio) the absolute area of the aperture for the half-sized frame will be half of that for the full-sized frame because of the shorter focal length -- it all comes together). This is why we use relative apertures (F numbers), so that the light meter works for all film sizes and focal lengths -- F/8, 1/100 will always yield the same amount of light per area unit, and hence will expose ISO 100 film in the same way, no matter what focal length or film size we choose. However, the images will be different depending on those choices, and in the case of decreasing both the film size and the focal length to receive the same FOV, the depth of field will increase, as will noise because of the smaller amount of photons in total (even though it is the same ISO 100 film in both cases, it will be enlarged twice (area-wise) as much when a half-sized frame was exposed, and this will make noise more visible).

I encourage anyone who does not agree with the above to find a large-sensor and a small sensor camera with lenses that can be chosen for the same FOV on both cameras. Then set both of them to the largest F-number that both lenses allow, the same shutter time and the same ISO. Take the same picture with both cameras, on a scene where there is lots of depth to show out-of-focus areas. Focus on something that is not too far away, to get easy-to-see out-of-focus areas both in front of and behind the focal plane. Then compare the two images. The effect should be very visible, unless you use a very slow aperture.

About the review on the Q: Here is a quote from the review:
"... This relates to the size of the camera sensor which has a crop factor of 5.5x so in full format terms we are talking about "47mm" thus a standard lens. Now this is not the full story. The max. aperture of f/1.9 sound fast and it is terms of light gathering. However, if you apply the cropping factor (f/1.9x5.5 = ~f/10.4) things will appear not all that sexy anymore regarding the depth-of-field potential. ..."

They say exactly what I am trying to explain! Yes, F/1.9 is fast in terms of light gathering, but remember that it describes how much light is gathered per area unit. If the sensor has a smaller area, it will receive less light in total. They even mention that the Q's 8.5 mm F/1.9 lens is equivalent to a 47 mm F/10.4 on FF (meaning that you will get the same FOV and depth of field, and hence the same amount of light in total, meaning less light per area unit and thus a higher ISO for an FF 47 mm F/10.4).

Again, since light meters, F-values and ISO numbers all relate to the amount of light per area unit, it works no matter what format or lens you use. The images will be exposed in the same way if those numbers stay the same, but the images will not be equivalent in terms of depth of field for different frame sizes.
06-27-2012, 06:12 PM   #34
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QuoteOriginally posted by Ben_Edict Quote
Have you ever seen a lightmeter that gives different reading, dependent on film format?
No, but that's because -- as hjb981 nicely explained -- lightmeters care about "exposure" and the latter is defined in terms of light intensity, not in terms of total light. In other words, exposure is defined in terms of photons per standard area unit, not in terms of total photons. Thus, exposure becomes independent of format size.

This definition of exposure implies that if you shoot larger formats, you can also print larger without increasing noise levels. That's a valid viewpoint, but note that this comes at a price. The DOF will be shallower compared to a smaller format, because you had to increase the aperture diameter in order to collect the respective amount of total light (which is higher than the total light for a smaller format).

Note that using larger formats makes sense even if you don't maintain the same exposure, but only the same total amount of light. For example, the enlargement factor to reach a given output size will be lower compared to a smaller format.

Keeping the exposure (i.e., here the f-ratio) constant while changing format size does not lead to equivalent images, because the larger the format, the more total light you are collecting. If you always print to the same output size -- independently of the capture format -- the larger format images will show less noise and shallower DOF. A fair comparison between different formats, however, must ensure that every capture receives the same total light (independently of the format size). If you ensure the latter, all images will show the same noise (at the same standard output size) and the same DOF (however, this requires changing the f-ratio according to sensor format).

I agree with everything hjb981 has written (thanks, BTW, for the insightful posts), perhaps with one small exception. I do not equate "light gathering ability" with "achieving a certain exposure". That's why I take issue with photozone's statement about the 8.5/1.9 having the same light gathering ability as any other f/1.9 lens.

Every f/1.9 lens achieves the same exposure, but an FF f/1.9 lens has to gather 30.25 (=5.5^2) times more photons to do that compared to the f/1.9 lens on the Q. Would you not agree that the FF f/1.9 gathers more (total) light than the Q f/1.9 lens?

If you convert the Q-f/1.9 into an FF-f/10.45 equivalent, both lenses now gather the same total amount of light and I'd be happy to call them both equal in terms of light gathering.

I concede that my viewpoint may not be standard and that a large proportion of the world equates "exposure" with "light gathering". However, I believe I have good reasons not to subscribe to this standard view.


Last edited by Class A; 06-27-2012 at 06:22 PM.
06-27-2012, 06:49 PM   #35
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QuoteOriginally posted by Class A Quote
a large proportion of the world equates "exposure" with "light gathering". However, I believe I have good reasons not to subscribe to this standard view.
OK, now I understand where you're coming from.
The problem is linguistic, the way words are used,
and change their meaning with time.

"Productivity" used to mean "number of widgets made per unit of labor,"
but has now shifted to "number of widgets made."

You are interpreting "light gathering" as "number of photons captured,"
where the Photozone usage is "number of photons per unit area."

This is a special problem in a forum like ours,
which is a mix of technical, precise scientific stuff,
and a more popular, everyday approach to photography.
06-27-2012, 07:28 PM   #36
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Just as a sidenote (maybe to obvious, thus not mentioned), but it is of course possible to stop down for example a medium-format camera and use a longer shutter time if one wants to attain a deep DOF. This is one reason why you might see a medium-format camera being used with a tripod even in bright light (another being usually higher pixel count, making smaller vibrations visible, and yet another being that anyone shooting medium format is likely to be more particular about perfection, and thus use a tripod when many other photographers would consider handheld good enough :P).

I agree with Class A about the use of the word light-gathering, even though I can see that it could be interpreted both ways. These relative numbers (ISO, aperture) are very useful, but coming from them (like most if not all photographers do), some of the implications can be a bit elusive (like for example that my K-5 sensor gets about the same amount of light in total at ISO 800 as my LX3 sensor does at ISO 80, if the same F-number and shutter speed are used).

I think that make a good point, lytrytyr -- having text as the only medium for discussing makes it much more challenging, and often two people will interpret the same sentence or even word differently.
06-28-2012, 12:52 AM   #37
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It's interesting to look at what would be required for a Pentax Q type camera to be equal in all respects to something like a K-5:

Assuming equal pixel-counts, in order for the resulting images to be identical, the following would need to be equal: depth of field, angle of view and dynamic range.

The Q's sensor is roughly 1/4 the length of APS-C (6mm vs 24mm), yielding a "crop factor" of 4, and we'll assume a 3:2 aspect ratio for easy comparison. We'll also assume identical sensor technologies (apart from ISO), merely scaled.

Assuming a 40mm f2.8 lens on the K-5, the Q would have a 10mm lens for the same AOV, and its aperture would need to be f0.7 for the same DOF (with of course the same size of front element). In order to achieve the same noise performance (i.e. dynamic range), the sensor sensitivity would need to be 1/16 that of the K-5's. Assuming the native ISO for the K-5 is 200, this implies ISO 12.5 for the Q. (Exposure time would therefore be the same for both cameras.)

I don't think either f0.7 or ISO 12.5 are particularly feasible, so small sensor cameras are always going to be what they are now: limited in dynamic range and the ability to achieve background blur. (That's not to mention sharpness limitations due to lens resolution.)
06-28-2012, 02:10 AM   #38
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06-28-2012, 05:39 AM   #39
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QuoteOriginally posted by beholder3 Quote
And: for most people on the world money is a linear factor.
It's logarithmic for everyone,
but most people are on the low end, which looks linear.

An FA31/1.8 at $1000 is only 25% more useful than a DA35/2.4 at $250.
This is the law of diminishing returns.
06-28-2012, 11:59 AM   #40
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QuoteOriginally posted by lytrytyr Quote
It's logarithmic for everyone,
but most people are on the low end, which looks linear.

An FA31/1.8 at $1000 is only 25% more useful than a DA35/2.4 at $250.
This is the law of diminishing returns.
A 31/1.8 might be absolutely useful, whereas the 35/2.4 might be worthless in the same sitution, because the resulting image is blurred... A difference of slightly above 1/2 f-stop can be decisive (rarely, but possibly).

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06-28-2012, 12:05 PM   #41
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QuoteOriginally posted by Class A Quote

I agree with everything hjb981 has written (thanks, BTW, for the insightful posts), perhaps with one small exception. I do not equate "light gathering ability" with "achieving a certain exposure". That's why I take issue with photozone's statement about the 8.5/1.9 having the same light gathering ability as any other f/1.9 lens.

Every f/1.9 lens achieves the same exposure, but an FF f/1.9 lens has to gather 30.25 (=5.5^2) times more photons to do that compared to the f/1.9 lens on the Q. Would you not agree that the FF f/1.9 gathers more (total) light than the Q f/1.9 lens?

If you convert the Q-f/1.9 into an FF-f/10.45 equivalent, both lenses now gather the same total amount of light and I'd be happy to call them both equal in terms of light gathering.

I concede that my viewpoint may not be standard and that a large proportion of the world equates "exposure" with "light gathering". However, I believe I have good reasons not to subscribe to this standard view.
The FF lens at f/1.9 will NOT gather more light in absolute terms, if you keep the AOV equal with the f/1.9 lens for the smaller format!

Because your FF lens needs to have a much longer focal length. And if you have identical AOVs, the amount of light emitted (better: reflected) from the scene, call it the number of photons, is constant. There is no increase in photons. Physicists would need to rewrite our basic laws of physics, if that would be the case.-

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06-28-2012, 02:03 PM   #42
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QuoteOriginally posted by Ben_Edict Quote
And if you have identical AOVs, the amount of light emitted (better: reflected) from the scene, call it the number of photons, is constant. There is no increase in photons.
Yes, the amount of light emitted from the scene is constant, but if both lenses are set to f/1.9 then the aperture in the lens for the smaller format will block many more of the scene photons than the lens for the bigger format.

Both lenses, at f/1.9, produce the same exposure (same light per square unit), but the FF lens produces a much bigger image circle. It follows that the FF lens must gather more photons.

It really is that simple.
06-28-2012, 11:48 PM   #43
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QuoteOriginally posted by beholder3 Quote
That is why there in reality is no "noise" advantage for FF sensors if you go for "equivalent" pictures with the same AOV and DoF. You need to stop down and so you have to turn up the ISOs.
Well, if you want the same shutter speed that's true. But if exposure time isn't a factor, then the FF sensor will have a noise advantage - you operate at the same ISO but need a longer exposure.

QuoteOriginally posted by beholder3 Quote
That is simply a major misunderstanding. "Dynamic range" describes the maximum difference between the lightest and the darkest bit of a picture.
And what do you think defines the darkest part of the picture? It's the noise floor. For a given sensor technology, the noise floor is set by the size of each pixel. For a 16x reduction in pixel area (as in my example), you get 4x the noise. This equates to raising the noise floor by 2EV, so the dynamic range is reduced by 2EV. (Headroom at the lightest end is the same regardless of pixel size.)


QuoteOriginally posted by beholder3 Quote
Well, "sharpness" is just a combination of resolution and contrast and resolution is pretty much only contrast on detail level.
My assertion was that a camera with a tiny pixel-pitch will in practice have its images compromised by lack of sharpness, due to limited lens resolution. For a Pentax Q type of camera we're talking of maybe 600-700 pixels per mm. A FF/APS-C lens does well to adequately resolve 50 line-pairs per mm (and that will be in the lens's centre, and at the optimum aperture). I doubt the 10mm f0.7 lens I mentioned would even get close to even that level of resolution, so the camera will be well and truly lens-limited. Stopping it down a bit would certainly help, but then you don't have the DOF you were aiming for, and you have to increase either the ISO (thereby reducing dynamic range) or exposure time.

Last edited by m42man; 06-29-2012 at 12:16 AM. Reason: Said "dB" when I meant "EV"
06-29-2012, 03:32 AM   #44
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06-29-2012, 09:24 AM   #45
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QuoteOriginally posted by beholder3 Quote
Well, sorry, but that is just irrelevant nonsense as it completely leaves the boundaries of the comparison.
I'm not sure you're very sorry, but I don't think it's very irrelevant or nonsensense to say that you'll end up with less noisy pictures with FF than with smaller formats (but of course you may certainly end up with longer exposure times).

QuoteOriginally posted by beholder3 Quote
Sorry, again your are just plain wrong. Dynamic range and noise are not directly related. Dynamic range is about a range that can be positioned anywhere on the scale. It has nothing to do with the "noise floor" as you call it as it does not care about a "floor" or a "ceiling". If you focus on capturing a very bright area correctly the sensor is unable to capture darker areas a lot sooner than where it's "noise floor" might be.

And also, just read again, what I wrote about the underperforming Canon sensors. 5d1, 5d2 and 5D2 sensor as well as 1D3 and 1D4 sensors will definitely created much more easily clipping / blown highlights than a K-5, D7000 sensor even though they are larger. Their dynamic range is substantially worse even though only the 5D1 also is clearly worse noise-wise.
Again, I doubt you're very sorry. You're talking rubbish - dynamic range is dependent on the noise floor. Regarding other sensors, I've already stated that the discussion is centred on "given sensor technology". Don't introduce others.

QuoteOriginally posted by beholder3 Quote
As I said, the smaller lenses do resolve better than the larger ones on average, because they simply have to. And also there are currently enough FF lenses (such as the DA300) that can cope well enough with resolutions far beyond what a D800 can produce.
APS-C lenses have difficulty keeping in touch with APS-C pixel-counts, and a Q-sized sensor with the same pixel count is going to embarrass any lens - especially if it has to have to a huge aperture (i.e low f-number) in order to compete. Why you should you want to bother with a camera which is so compromised?

Last edited by m42man; 06-29-2012 at 10:23 AM.
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