Sensor Size and Noise

[joseph_albert]

It has been articulated here that sensor size places a lower bound on

the noise in a digitally captured image. My understanding of the

problem is that smaller sensors have smaller pixel sensors and smaller

pixel sensors produce a weaker photoelectric effect, and thus need

greater amplification to get a read of the image. The greater

amplification also amplifies dark noise more than less amplification

would. (If that is an imcomplete or incorrect understanding, I'm sure

someone will jump in to clarify :-)

<p>

However, smaller sensors also have more depth of field, which means

that you can get more DOF with a wider aperture, which in turn means

that at a given shutter speed you can shoot at a slower ASA setting.

This tradeoff is often well understood by medium and large format

users who are used to thinking in terms of losing shutter speed in

comparison to a smaller format after stopping down more to maintain

desired depth of field.

<p>

It seems to be universal that digital cameras have noisier images as

ISO speeds get faster. So there are tradeoffs here-- larger sensors

produce less noise at a given ISO speed, but smaller sensors can shoot

at slower ISO speeds while maintaining the same shutter speed and

depth of field as a larger sensor, and slower ISO speeds means less

noise.

<p>

This seems to suggest an interesting minimax problem to find the

optimal sensor size for handheld shooting. Of course, if you have a

tripod, you have the luxury of shutter speeds as slow as desired in

most cases (subjects that won't hold still being an exception in the

absence of flash to freeze the action).

<p>

To illuminate the problem further, let's compare a full-frame (ie 35mm

sized) DSLR with one of the non-interchangeable lens cameras with

sensors 1/4 as large in linear dimension as the full-frame 35mm

camera. Examples of the latter camera today would be Konica-Minolta

A200, Olympus C-8080, Nikon Coolpix 8700 etc., ie cameras with the

so-called 2/3" sensor. These are 8MP cameras, but I don't know if

there are any 8MP full-frame DSLRs around.

<p>

Now, if you increase format size by 4x in linear dimension, then to

frame the same shot you need a lens that is 4x longer in focal length,

and you need top stop down the aperture an additional 4 stops to get

the same depth of field. To compensate for the 4 stop closing of the

aperture, you need to regain 4 stops via shutter speed or ISO setting.

Let's suppose we are shooting handheld and shutter speed has to

remain fixed. If you are shooting at ISO 50 on the all-in-one

digicam, you are now shooting at ISO 800 on the full-frame DSLR.

<p>

This raises some interesting questions: is ISO 50 on a 2/3" sensor

noisier, less noisy of similar to ISO 800 on a full-frame DSLR? for

handheld shooting, is larger always better in terms of image quality

or is there a minimax point where the two tradeoffs give most

favorable results? if so, where is this minimax point?

<p>

Thoughts?

[david_clingingsmith]

Another thing to consider is that we often want less DOF rather than more, like when we

want to isolate a subject against a blurred background. Since the f/ratio of lenses on

DSLRs and fixed lens 2/3" cameras is never much less than 2, the 2/3" format camera will

sometimes not be able to deliver the shallowness of focus that a DSLR can. Additionally,

for a given DOF we will be able to stop the DSLR lens down more, which generally will

deliver better edge sharpness. So the performance of the lens has to be considered too.

[mark_man]

David is correct - we often want less DOF rather than more. In fact, I often find the DOF on a run-of-the-mill digicam to be excessive. For photographers who want the option of narrow DOF, there is no question that larger sensor sizes are better. Noise is generally better also at high ISOs. I find it frustrating that digicams and DSLRs with larger sensor sizes have not yet made the market as yet, but I'm sure they are not far off!

[phule]

<< However, smaller sensors also have more depth of field >>

 

Should this not be "smaller sensors require very wide-angle lenses for creating equivalent fields of view and thus the resulting image has greater DOF" ?

 

Sensors, by themselves, have nothing to do with DOF.

[peufeu]

You do have a point...

I had a Minolta Dimage A1 and now have an EOS 20D.

 

So, yes, to get the same depth of field I would shoot at f5.6 on the A1 and at like f16 on the Canon. Moreover the A1 has anti-shake.

 

However the images at ISO800 on the Canon look a lot better than at ISO100 on the A1. Not just noise, but about everything looks better. It's hard to explain, it's just there.

 

Also most shots don't really need large depths of field. The only situation I could imagine having a lot of DOF as an advantage is when you want a really close foreground over a far away background and everything in focus. It's pretty rare (but if you want that, a digital P&S is even better than a 2/3"). Digitall P&S work great for macro if they have the option, too.

 

There is probably an optimum point but I don't know where it is. What I know for sure is that there is no comparison in quality between the image from these two cameras...

[joseph_albert]

No, actually, it is the sensor size and not the lens. At fixed magnification, DOF is independent of focal length. With a smaller sensor, you are working at a lower magnification to get a given composition, and hence DOF is greater.

<p>

I do understand that a limitation of small sensors is their ability to get shallow DOF (unless you could build an f/0.1 lens or somesuch, which may even be technically impossible, aside from impractical). I shoot medium format when I want the freedom of shallow DOF.

<p>

However, the idea that a larger format requires stopping down more and hence gets better image quality from the lens is not clear. It is easier to build a lens with more modest coverage, which could translate into a sharper lens for the smaller format. It is MUCH easier to make an f/2.8 lens for a smaller format than a larger format (that's one reason lenses for larger formats are generally slower). Also, stopping down increases diffraction effects that limit resolution, so it is by no means clearcut that a lens for larger format is sharper because it is stopped down more. In fact, I don't know if I've ever taken a photo I considered flawed because of lack of resolution, despite the intense scrutiny photographers often place on this aspect of a photosystem's imaging capabilities. With film, grain, and not resolution is usually the limiting factor in enlargeability. When digital images are not acceptable from a quality point of view, it is color fringing, moire patterns and noise that is the problem.

<p>

Thus, my question really just concerns relative noise of, say, a 2/3" sensor at, say, ISO 50, and a full-frame sensor at, say, ISO 800. Anyone have any actual data or subjective observations concerning such a comparison?

[david_clingingsmith]

If you look at the reviews of the Oly C8080WZ and Canon Digital Rebel XT at

dpreview.com, you will see a noise vs. ISO graph on the "photographic tests" page for each

camera. I believe dpreview tests are pretty well standardized.

 

At ISO 50, the 2/3" Oly has a standard deviation of luminance (a measure of noise) of

about 2.5. This compares to ISO 400 for the Digital Rebel XT. At ISO 800, the Digital Rebel

XT has SD of luminance of about 3.5. So it would appear that for a given shutter speed

(say the slowest you can handhold), the Oly has gives a better DOF for noise tradeoff, if we

assume we want more DOF.

[joseph_albert]

Thanks, that is helpful. It seems that the optimal point will vary with the DOF requirements so there won't be 1 minimax point for every image.

The content I posted originally was something I noted over the weekend when I was using an Olympus C8080 to shoot a miniature golf birthday party in a photojournalistic style and I concluded that the niche for these 2/3" sensor cameras is shooting handheld with high DOF requirements. With a DSLR with larger sensor, shooting handheld and needing DOF means a faster ISO setting, and the image quality difference is at least narrowed if not nullified.

[david_kieltyka]

I'm impressed by the high-res, deep DOF look I get from the Panasonic FZ20 at the wide end of its focal length range (6mm, 36mm "equivalent") at f/5.6 or so. It's a look I can't duplicate with my Canon 20D. With the D-SLR I can get higher resolution with less DOF or lower resolution with close-to-equal DOF. Chalk up one for the smaller format. Of course knocking backgrounds or foregrounds out of focus with the FZ20 is difficult except at the long end of the lens' range or up close, whereas this is easy with the 20D and any reasonably fast lens. Thus IMO both sensor formats are valid.

 

-Dave-

[conrad_hoffman]

I'm sure it depends on what you like to shoot. I've achieved some closeup results with the FZ-20 that I probably couldn't have done with a larger format. I needed huge DOF, and every other issue of speed and noise was secondary- the customer had a sketch of exactly what he wanted, so there was no freedom to move things around or compress the shot. Large format movements wouldn't have worked either because just tilting the plane of sharp focus wasn't sufficient. This just happened to be a case where the tool turned out to be perfect for the job.

[oskar_ojala]

It was already noted that APS-sized sensors have far less noise than 2/3" sensors, which evens the situation somewhat. In practice, I seldom need huge amounts of DOF, with f5.6 to f11 on an APS sized sensor being entirely sufficient. Also, many cases could be covered by lens tilting, if that is available. I don't think diffraction limited resolution is a huge problem with APS-sized sensors and high-quality lenses. So the question that remains is that what is the optimum sensor size for attaining maximum DOF with a given angle of view and resolution? (the actual resolution should be fixed to make the comparison valid) Frankly, there's no clear answer, as we should consider what is the most limiting aspect of image quality and agree on a methodology to compare it. It should also be considered that post-processing software can be used to improve image quality in many ways.

 

However, it would be interesting to see a decent attempt at testing this...I might try it someday...

[scott_eaton]

<I>Thus IMO both sensor formats are valid. </i><P>Provided you have optics designed explicitly for the sensor format, which the FZ20 does and Canon doesn't. Like you said, chalk one up for the little Panasonic.

[joseph_albert]

<i>

With the D-SLR I can get higher resolution with less DOF or lower resolution with close-to-equal DOF.

</i>

<p>

I assume you mean just when shooting handheld, or maybe I'm missing something and you find this to be the case even with a tripod available?

[roger krueger]

Your premise only works if you are DOF constrained. If you aren't (as is the case in many,

many shots), f2 on the big sensor will smoke f2 on the small sensor.

 

For a given field of view there is NO FORMAT DIFFERENCE in maximum DOF possible on

equivalent sized prints assuming you choose aperture based on maximum diffraction

allowed in the print. But the aperture chosen will vary from format to format. A good DOF

calculator will bear this out, although it is often obscured by the use film/sensor CoCs that

do NOT translate into a consistent equal-print CoC.

 

Re: the FZ20 example, if it needs a 6x focal length multiplier, it needs a 6x fstop multiplier

too. Then divide by 1.6 for the 20d giving a difference of 3.75x. This means you're at f21

on the 20d to equal the DOF of the FZ20 at 5.6.

[joseph_albert]

Absolutely, you can just stop down more with a larger format to get the same DOF on a print. That's why I was suggesting it was a speed tradeoff not a DOF tradeoff. Normally with larger film formats, camera is on a tripod (for shots where you need DOF) and you just accept the slower shutter speeds. That's why the above ONLY applies to handheld shots (or other shots that require speeds fast enough to freeze action).

<p>

I'm not trying to claim that the smaller sensor digicams are as good as a DSLR across the board by any means, just that there do exist some applications where they may be preferable. When you are shooting handheld near the limits for shutter speed and DOF requirements, a larger sensor may not help much if at all.

<p>

This seems to be in contrast to the situation with film, at least C41 where there are in the last 5-10 years, the appearance of very good 400 and 800 speed C-41 films. You can shoot NHG 400 or 800 in medium format and get better quality than shooting Reala in 35mm, as an example. I suspect that DSLRs are getting close to their MP resolution limits and future enhancements will be in reducing noise, particularly at faster speeds.