Digital quality (vs 35mm): Any real answers?

Stephen H. Westin wrote:
Toralf writes:
[ snip ]



Be that as it may, some of the questions I'd like to have answered are
these:

1. What is the resolution of a 35mm film anyway?


It's hard to say, as the resolution limit is different from that of a
digital sensor. Rather than a hard limit, you get less information and
more blur and noise as you increase resolution in scanning a piece of
film.


I think I read
somewhere that a colour negative is at least 3000dpi. Is that
correct?


It's in the ballpark. [ ... ]
OK. Thanks


How about black&white?


Well, the data sheet on Kodak Technical Pan, which is an extreme upper
bound, has MTF at 50% at 180 c/mm or so. That's for ISO 25, developed
in Technidol. That's Kodak Tech Pub P-255, available at
http://www.kodak.com/global/en/professional/support/techPubs/p255/p255.jhtml?id=0.1.18.14.21.22.16&lc=en.
For Plus-X, at ISO 125, Tech Pub F-4018 (at
http://www.kodak.com/global/en/professional/support/techPubs/f4018/f4018.jhtml?id=0.1.18.14.21.20.16&lc=en)
rates it at 50 lp/mm at 1.6:1 contrast, 125 at 1000:1 (which you
aren't likely to reach it practice).

So I think we can say that scanning film at more than, say, 360
samples/mm (about 9,000 samples/inch) is pretty much useless. In most
circumstances, 100 samples/mm or 2,540 /inch is plenty. So we're in
the region of 9MP for a normal 24x36mm 35mm film frame.
I that would be 9MP without any sort of interpolation, though. But then,
like I said, you get the error introduced by the scanner instead...



(Yeah I know, a film doesn't have
pixels in exactly the same sense as a digital image, but it *is*
made up of discrete elements after all.)


But they aren't spaced on a regular grid, so things look a lot different.
Exactly. It would still be fun to know exactly how many "points" there
are, though.


2. What about the print? 300dpi?


Well, there are lots of variables involved there. One of the little
secrets that the "film-only" zealots don't seem to mention is that to
view the film, it must either be scanned, projected, or printed. I
have heard the claim that scanning digitally loses quality, but so do
the other two processes. How good are the optics in your enlarger or
slide projector? How perfectly is either one focused? I really don't
know what sort of degredation is involved, but it seems that it might
be on the same order as through the camera optics, or worse. And lens
MTF's usually aren't quoted beyond 40 cycles/mm! Check the Zeiss or
Canon Web sites if you don't believe me.
I believe you. I just wish that someone would show me the numbers and/or
do the maths so I could really compare the different "errors".

Of course, film has the advantage that most errors perhaps are
introduced later in the chain that the actual image capture. In other
words, if you get lousy results because of a bad scan or projection, you
still have the film to go back to.


3. I know that the most common sensors are made up of individual
elements for the red, green and blue channels, arranged in a
special pattern, whose data is somehow interpolated into RGB
pixels. But what exactly does e.g. 6 megapixels mean in that
context? Does it mean that the sensor has (just) 6 million
elements, or that data from a higher number (like 18 or 24 million)
is combined into 6 million RGB pixels?


The former. Except for the Foveon sensor in the Sigma SD9/SD10, which
has about 3.5 million sites, each of which detects all three channels.
Yeah, I know. I think the Foveon technology is really interesting, but
I'm not too convinced about the current implementation or, *cough*, the
camera producer that uses it.


The same question more bluntly put: When Canon/Nikon/Pentax is talking
about 6MP, is that just a big a lie as the one about 10MP on Sigma
cameras? (I'm hoping not, as I think the Sigma/Foveon way of counting
[ ... ]

Not really. The reality is that sensing all channels at each location
is a Good Thing, other factors being equal, but the color filter array
isn't as bad as you might think. First of all, the three color
channels aren't completely uncorrelated, so the information from a red
sensel can be used to help estimate the red and green values at that
point.
Fair enough. I still think they're stretching the reality a bit, though,
when they're talking about e.g. 6M pixels when all they have is 6M
*sensors*.

Second, images have some sort of spatial structure, and modern
demosaicing algorithms try to detect that to deduce missing
values.
Yes. I assume those algorithms do quite a good job, too. The mere
existance of any sort of "post processing" still makes me sceptical,
though; I think it can never beat getting all components directly from
the sensor like you usually do with a scanner.

Finally, the sampling rate is higher for the green channel, to
which the human visual system is most sensitive, both in luminance and
resolution.


5. And how about those other parameters I mentioned briefly above -
like different kinds of geometric distortions, noise, flat field
bias etc.? Can those be compared with the ones of plain old film?


Yup. Lots of them are only being noticed now with digital sensors
behind the same lenses that people have used for years with film.
I suspect that it's a matter of display more than anything else;
the magnification on screen at a 1:1 pixel magnification is huge
for most cameras, and people can use the little eyedropper to see
just how much the illumination falls off in the corner of the frame.
Possibly.

Maybe digital photos are actually a good way to test the quality of lenses?


6. And the chromic aberration effects? How serious are they these
days? And are the full-frame sensors that are actually found in
some high-end cameras now, in any way comparable to film in that
respect?


Chromatic aberration is basically in the lens. Doesn't matter
what sensor is behind it.
Well, maybe "chromatic aberration" wasn't the right expression to use.
(Right, I really ought to know more about this, as we see the same
things on our scanners...) The point was, I know that CCDs are quite
sensitive to various errors or inaccuracies introduced at the edges of
the lens, or just to the angle at which the beam hits it. The question
was really how important those effects are.



Well, maybe some people will say I have a somewhat critical or
conservative attitude towards digital cameras, but I actually think
you ought to be a bit sceptical when something "new and wonderful"
comes a long; new technology is too often introduced for technology's
own sake, IMO.


Oh, sure. But there are a number of photographers who are putting away
their 4.5x6 film cameras because they see better quality out of the
Kodak DCS 14 MP cameras. In a functional way, the best digitals are
pretty good, though their limitations are different from those of film
cameras.
Probably. I guess what's put me off so far is really the higher price
(I don't think that can be justified if they are "just as good as" or "a
little better than" film) combined with the shorter life-span and/or
limitations introduced by the fact that the "film" is glued to the the
camera body. As I've mentioned briefly earlier, I'm really dreaming
about consumer-level digital cameras with a more modular construction.
Imagine having the sensor in a socket with a standardised bus interface
- like the CPU, memory or card bus a computer - so you could replace it
when something better came along, or keep different sensors with
different caracteristics. Wouldn't that be nice?

Anyhow, some where good answers, there. Thanks.

- Toralf

Stephen H. Westin wrote:
Toralf writes:
[ snip ]



Be that as it may, some of the questions I'd like to have answered are
these:

1. What is the resolution of a 35mm film anyway?


It's hard to say, as the resolution limit is different from that of a
digital sensor. Rather than a hard limit, you get less information and
more blur and noise as you increase resolution in scanning a piece of
film.


I think I read
somewhere that a colour negative is at least 3000dpi. Is that
correct?


It's in the ballpark. [ ... ]
OK. Thanks


How about black&white?


Well, the data sheet on Kodak Technical Pan, which is an extreme upper
bound, has MTF at 50% at 180 c/mm or so. That's for ISO 25, developed
in Technidol. That's Kodak Tech Pub P-255, available at
http://www.kodak.com/global/en/professional/support/techPubs/p255/p255.jhtml?id=0.1.18.14.21.22.16&lc=en.
For Plus-X, at ISO 125, Tech Pub F-4018 (at
http://www.kodak.com/global/en/professional/support/techPubs/f4018/f4018.jhtml?id=0.1.18.14.21.20.16&lc=en)
rates it at 50 lp/mm at 1.6:1 contrast, 125 at 1000:1 (which you
aren't likely to reach it practice).

So I think we can say that scanning film at more than, say, 360
samples/mm (about 9,000 samples/inch) is pretty much useless. In most
circumstances, 100 samples/mm or 2,540 /inch is plenty. So we're in
the region of 9MP for a normal 24x36mm 35mm film frame.
I that would be 9MP without any sort of interpolation, though. But then,
like I said, you get the error introduced by the scanner instead...



(Yeah I know, a film doesn't have
pixels in exactly the same sense as a digital image, but it *is*
made up of discrete elements after all.)


But they aren't spaced on a regular grid, so things look a lot different.
Exactly. It would still be fun to know exactly how many "points" there
are, though.


2. What about the print? 300dpi?


Well, there are lots of variables involved there. One of the little
secrets that the "film-only" zealots don't seem to mention is that to
view the film, it must either be scanned, projected, or printed. I
have heard the claim that scanning digitally loses quality, but so do
the other two processes. How good are the optics in your enlarger or
slide projector? How perfectly is either one focused? I really don't
know what sort of degredation is involved, but it seems that it might
be on the same order as through the camera optics, or worse. And lens
MTF's usually aren't quoted beyond 40 cycles/mm! Check the Zeiss or
Canon Web sites if you don't believe me.
I believe you. I just wish that someone would show me the numbers and/or
do the maths so I could really compare the different "errors".

Of course, film has the advantage that most errors perhaps are
introduced later in the chain that the actual image capture. In other
words, if you get lousy results because of a bad scan or projection, you
still have the film to go back to.


3. I know that the most common sensors are made up of individual
elements for the red, green and blue channels, arranged in a
special pattern, whose data is somehow interpolated into RGB
pixels. But what exactly does e.g. 6 megapixels mean in that
context? Does it mean that the sensor has (just) 6 million
elements, or that data from a higher number (like 18 or 24 million)
is combined into 6 million RGB pixels?


The former. Except for the Foveon sensor in the Sigma SD9/SD10, which
has about 3.5 million sites, each of which detects all three channels.
Yeah, I know. I think the Foveon technology is really interesting, but
I'm not too convinced about the current implementation or, *cough*, the
camera producer that uses it.


The same question more bluntly put: When Canon/Nikon/Pentax is talking
about 6MP, is that just a big a lie as the one about 10MP on Sigma
cameras? (I'm hoping not, as I think the Sigma/Foveon way of counting
[ ... ]

Not really. The reality is that sensing all channels at each location
is a Good Thing, other factors being equal, but the color filter array
isn't as bad as you might think. First of all, the three color
channels aren't completely uncorrelated, so the information from a red
sensel can be used to help estimate the red and green values at that
point.
Fair enough. I still think they're stretching the reality a bit, though,
when they're talking about e.g. 6M pixels when all they have is 6M
*sensors*.

Second, images have some sort of spatial structure, and modern
demosaicing algorithms try to detect that to deduce missing
values.
Yes. I assume those algorithms do quite a good job, too. The mere
existance of any sort of "post processing" still makes me sceptical,
though; I think it can never beat getting all components directly from
the sensor like you usually do with a scanner.

Finally, the sampling rate is higher for the green channel, to
which the human visual system is most sensitive, both in luminance and
resolution.


5. And how about those other parameters I mentioned briefly above -
like different kinds of geometric distortions, noise, flat field
bias etc.? Can those be compared with the ones of plain old film?


Yup. Lots of them are only being noticed now with digital sensors
behind the same lenses that people have used for years with film.
I suspect that it's a matter of display more than anything else;
the magnification on screen at a 1:1 pixel magnification is huge
for most cameras, and people can use the little eyedropper to see
just how much the illumination falls off in the corner of the frame.
Possibly.

Maybe digital photos are actually a good way to test the quality of lenses?


6. And the chromic aberration effects? How serious are they these
days? And are the full-frame sensors that are actually found in
some high-end cameras now, in any way comparable to film in that
respect?


Chromatic aberration is basically in the lens. Doesn't matter
what sensor is behind it.
Well, maybe "chromatic aberration" wasn't the right expression to use.
(Right, I really ought to know more about this, as we see the same
things on our scanners...) The point was, I know that CCDs are quite
sensitive to various errors or inaccuracies introduced at the edges of
the lens, or just to the angle at which the beam hits it. The question
was really how important those effects are.



Well, maybe some people will say I have a somewhat critical or
conservative attitude towards digital cameras, but I actually think
you ought to be a bit sceptical when something "new and wonderful"
comes a long; new technology is too often introduced for technology's
own sake, IMO.


Oh, sure. But there are a number of photographers who are putting away
their 4.5x6 film cameras because they see better quality out of the
Kodak DCS 14 MP cameras. In a functional way, the best digitals are
pretty good, though their limitations are different from those of film
cameras.
Probably. I guess what's put me off so far is really the higher price
(I don't think that can be justified if they are "just as good as" or "a
little better than" film) combined with the shorter life-span and/or
limitations introduced by the fact that the "film" is glued to the the
camera body. As I've mentioned briefly earlier, I'm really dreaming
about consumer-level digital cameras with a more modular construction.
Imagine having the sensor in a socket with a standardised bus interface
- like the CPU, memory or card bus a computer - so you could replace it
when something better came along, or keep different sensors with
different caracteristics. Wouldn't that be nice?

Anyhow, some where good answers, there. Thanks.

- Toralf

Sabineellen wrote:
And there's also a question of whether it looks *right* (based on
whatever you take a photo of), which is not the same thing as looking good.


What does looking *righ* mean?
Maybe that's hard to define precisely for photos. For scans it's quite
easy, since the original subject is also 2D, and you're making a 1:1
representation of it - so you can measure the "correctness" based on the
distance between, and position of, details in the scan compared to the
original, and the same for other parameters like colour values.

But, the point is, if you take pictures of one black and one white cat
and they both turn out grey (to use an example I read somewhere in a
discussion about light metering limitations), the result may still look
good (if you like grey cats), but it's definitely not *right*.

- Toralf

Sabineellen wrote:
And there's also a question of whether it looks *right* (based on
whatever you take a photo of), which is not the same thing as looking good.


What does looking *righ* mean?
Maybe that's hard to define precisely for photos. For scans it's quite
easy, since the original subject is also 2D, and you're making a 1:1
representation of it - so you can measure the "correctness" based on the
distance between, and position of, details in the scan compared to the
original, and the same for other parameters like colour values.

But, the point is, if you take pictures of one black and one white cat
and they both turn out grey (to use an example I read somewhere in a
discussion about light metering limitations), the result may still look
good (if you like grey cats), but it's definitely not *right*.

- Toralf

David J. Littleboy wrote:


20x24 is a pitiful joke from 35mm B&W films, even Tech Pan. If one has any
sense of quality imaging at all, 11x14 is MF (645) territory. 20x24 from 6x7
would be OK, but would look better if you used LF.


I agree that 20x24 is stretching things a bit (no pun intended) using
35mm and that MF or LF would be far preferable, but with a good negative
the results are still acceptable, especially at real viewing distances.

I much more often do 30x40 cm (12x16) with fully satisfactory results
(given of course a decent negative). In a direct comparison with a MF
shot, there is a little less tonal smoothness & 3D look, but then I
wonder how much tonal smoothness & 3D look digital would have in this case.

Chris

David J. Littleboy wrote:


20x24 is a pitiful joke from 35mm B&W films, even Tech Pan. If one has any
sense of quality imaging at all, 11x14 is MF (645) territory. 20x24 from 6x7
would be OK, but would look better if you used LF.


I agree that 20x24 is stretching things a bit (no pun intended) using
35mm and that MF or LF would be far preferable, but with a good negative
the results are still acceptable, especially at real viewing distances.

I much more often do 30x40 cm (12x16) with fully satisfactory results
(given of course a decent negative). In a direct comparison with a MF
shot, there is a little less tonal smoothness & 3D look, but then I
wonder how much tonal smoothness & 3D look digital would have in this case.

Chris

"Toralf" wrote in message
...
Hi.

I'm still wondering about how good the image quality of modern digital
cameras (especially SLRs) really is, in particular how it compares with
35mm film. I've seen many articles on the subject on the Net, but few of
them seem to give you a lot of tangible information (I want to see the
numbers, please), and I can't help feeling that tests they refer to are
usually done to prove a point, i.e. that digital cameras are as good as
35mm, which is not the way you do proper research.
Check out http://clarkvision.com/imagedetail/f...digital.1.html
Seems to be not trying to prove a point. In some tests the film wins and in
some the digital wins.

To say a few words about myself, I'm working for a company that makes
high-accuracy, large-format scanners, so I'm not particularly impressed
when I hear e.g 6 million pixels (you need to talk about *billions* of
pixels if I'm really going to listen), and the word "interpolation"
leaves a bad taste in my mouth. But this also means I know that high
resolution isn't everything, of course; parameters like geometric
precision or signal-to-noise ratio also count a lot.
Actually, they mean SFA. I think too often we forget the purpose of
photography - to make images that look good. Who cares if the SN ratio is
crap if the image looks good. Some of my favorite digital photos are as
noisy as all heck, in these the noise added to the photo, not took away from
it. The proof isn't in the technical specifications, the proof is in
whether the photo looks good. People will tell you that 2MP is no good
above 8x10 - I have a 16x12 photo made from 2MP on the wall that looks fine.
If you walk right up to it you can see some pixelation, but you can't see it
from a normal viewing distance of about 2 feet. Likewise I have images made
from ISO400 consumer grade film that look great too.

Be that as it may, some of the questions I'd like to have answered are
these:

1. What is the resolution of a 35mm film anyway? I think I read
somewhere that a colour negative is at least 3000dpi. Is that correct?
How about black&white? (Yeah I know, a film doesn't have pixels in
exactly the same sense as a digital image, but it *is* made up of
discrete elements after all.)
The site I posted above indicates that Fuji Velvia is approximately
equivalent to 15MP in it's resolving power.

2. What about the print? 300dpi?
Depends on the viewing distance. If you are viewing close-up then 300dpi is
ideal, but 150dpi will still look quite good. You can get away with 75dpi
for a poster. Roadside billboards are only a couple of dpi (i've seen some
that are about 1dpi), and still look ok.

3. I know that the most common sensors are made up of individual
elements for the read, green and blue channels, arranged in a special
pattern, whose data is somehow interpolated into RGB pixels. But what
exactly does e.g. 6 megapixels mean in that context? Does it mean that
the sensor has (just) 6 million elements, or that data from a higher
number (like 18 or 24 million) is combined into 6 million RGB pixels?
6 million sensors. 1/4 are red, 1/2 are green, and 1/4 are blue. The colour
of each pixel is made up by looking at it's value and those of it's
neighbours. This 1:2:1 ratio roughly corresponds to the eye's sensitivity
to luminosity in these colours.

The same question more bluntly put: When Canon/Nikon/Pentax is talking
about 6MP, is that just a big a lie as the one about 10MP on Sigma
cameras? (I'm hoping not, as I think the Sigma/Foveon way of counting
really takes the cake.)
Canon/Nikon/Pentax's Bayer sensors have 6 million sensors, RGB in the 1:2:1
ratio. The foveon as used in the Sigma has 3.4 million sensor positions,
each position records all 3 colours - that is why they come up with the
10.2MP lie. Note that Bayer sensors still record luminosity at 6 Million
locations, whereas foveon only records luminosity at 3 Million locations.
Your eye is more sensitive to luminosity than it is to colour, and is more
sensitive to the green channel. In this respect the Bayer's response is
closer to that of the eye, even if it does have to interpolate colours to
some extent.

"nitzsche" wrote in message
. ..
6. And the chromic aberration effects?
- As far as I know, all digicams are prone to purple fringing, which is
something you don't see on any cheap slr.
You don't see it on all digicams - it is directly proportional to the
quality of the lens on the digicam.

Here in Australia a cheap film SLR is about $400-$500, with a 28-80 lens. Of
that, about $250 is the lens and the remainder is the body. Considering
that a compact digital with a similar zoom range can be picked up for $300 -
6x zoom for $400 and 10x for $500, it is pretty clear to see the difference
in quality of lens used. Camera's with a decent quality lens (eg mid to
high end olympus, canon, nikon) don't suffer from purple fringing as bad as
the cheap cameras. I dunno what the prices in the USA would be, but I
suspect the ratio of the prices would be pretty similar

"nitzsche" wrote in message
. ..
6. And the chromic aberration effects?
- As far as I know, all digicams are prone to purple fringing, which is
something you don't see on any cheap slr.
You don't see it on all digicams - it is directly proportional to the
quality of the lens on the digicam.

Here in Australia a cheap film SLR is about $400-$500, with a 28-80 lens. Of
that, about $250 is the lens and the remainder is the body. Considering
that a compact digital with a similar zoom range can be picked up for $300 -
6x zoom for $400 and 10x for $500, it is pretty clear to see the difference
in quality of lens used. Camera's with a decent quality lens (eg mid to
high end olympus, canon, nikon) don't suffer from purple fringing as bad as
the cheap cameras. I dunno what the prices in the USA would be, but I
suspect the ratio of the prices would be pretty similar

Zebedee wrote:
I spent ages working out all the details. I came to the conclusion that if
you measured the area of a perfect print from a 35mm negative or 35mm slide
in inches and then divided the dimensions by 150 you'd be able to work out
the vertical/horizontal pixels of the image. Then simply multiply the two
together to get megapixels. That's the theory anyway.

In practice, how big is the biggest print you normally make? If your normal
biggest print is 10x8 then 3 megapixels is all you ever need (two at a
pinch).

But the biggest limitation is your printer. How many of us can afford an A3
or greater printer or even the ink to make A3 prints?
I don't think its so much about the prints you normally make, as the
ones you *can* make. Even if you don't make a lot of A3 prints, or can
afford an A3 printer, today, who is to say you won't tomorrow? Or in
10-years time, when you have a different camera entirely, but wish to
make something based on those old shots.

Essentially what I'm saying is that I'd like to keep my options open, or
at the very least, I want to know in advance what my options are.


How many A4 prints can you hang on your wall?

I decided to settle on 3 megapixels. It's adequate for my needs and as with
slides, I ensure my photos are perfect before I squeeze the button. I claim
3 megapixels is the perfect equivalent of 35mm for most purposes. 6mp just
eats up storage space for no visible advantage.