Size Matters

Very informative. Thank you Bob, for another of your great insights into digital imaging.

Well, this is a quantitative confirmation of the old rule that one mustn´t stop down a lot if using small format! The classic Minox sub-mini had a fixed aperture of f/3.5 (plus a curved film plane), and my Digital Ixus, at least, always shoots wide open (there is an ND filter that is used in strong light; the EXIF data reports its use as a smaller f stop). I don´t know if this is the case for all small-sensor digicams.

Also, this shows that there isn´t much sense in providing more modes than Program in a small digicam! The program keeps the aperture open as long as possible, shortening exposure time first, which is the best you can do in virtually all situations. Exposure compensation really gives all the control you need, preferrably combined with spot metering.

Bob,

Thank you for publishing this very valuable and timely article. I think anyone considering a digital camera would be very well advised to read it. Your professional contribution represents photo.net at its very best.

Regards,

Clear & concise article.
Have to agree with the gist of the article but for the sake of convenience, a "tiny" p+s digicam is invaluable for informal snapshot type of images.
On the other hand, if size is really what one is looking for in terms of printing a hardcopy that is 4ft wide or larger, then scanning in a 4"x5" negative/transparency using a Tango scanner seems like a better route to go.

True - but the goal of the article isn't to tell you how to make big prints!

The goal is to point out that pixel count isn't everything, and there are good, sound, scientific reasons why a 4MP full frame 35mm image may in fact be better than a 10MP image from a sensor the size of your fingernail.

It's not to say more pixels aren't better, but that there are pixles and then there are pixels. Not all pixels are equal and there may be good reasons to chose a camera with less pixels and a larger sensor than a camera with more pixles and a smaller sensor - all else being equal.

The fact that you CAN get pretty good images out of a tiny sensor is great, since it means you can make really tiny cameras, just realize those tiny cameras have their limitations, especially in terms of DOF, aperture restrictions and ultimate image quality - and they still would even if they were 25 megapixels!

One thing about this article that confuses me is how aperture relates visually across the format sizes. Yes, f16 is unusable on the smaller sensors. I might shoot at f45 on my 4x5 camera, but there's just no need for that on my 35mm.

If i'm shooting with a 150mm lens at f16 on a 4x5, what is the visually corresponding f-stop on 35mm?

There is an interesting aspect in this article:

The lowering in prices of silicon chips has always been made through 'shrink' - that is, make the silicon smaller.

If we have an optical barrier to it, it means high quality digital will be expensive for a long time to come...

One more thing to point out about sensor size is that it has a great effect on the noise present in the final digital file.

Comparing my S30 point and shoot to a Nikon D100, you find that the D100 is better at ISO 400 (or even 800) than the S30 is at ISO 100 or 200.

>> The lowering in prices of silicon chips has always been made through 'shrink' <<

Although many components do get less expensive because they can fit on smaller pieces of silicon as feature geometry shrinks, that isn't the only source of cost reduction. Although I have no "inside" information, given the size of the sensor I believe yield is probably the constraining factor in sensor cost. As feature geometry shrinks and manufacturing processes mature, yield improves substantially and costs come down. As an example, look at the cost curve for LCD displays. I expect we'll see more full-frame sensor cameras and a general erosion of the price premium for them over the next five years.

At that point, digital photographers will be in the same boat as film photographers - trying to afford the next great piece of glass in their collection.

Impressed, I am. Like Yoda, I speak.

There is extensive discussion of the implication of sensor (and pixel) size on image quality on Norman Koren's site -- see e.g.

http://www.normankoren.com/Tutorials/MTF7.html

Kaa

In regards to the cost factor for the electronic components of digital cameras. There are two factors that are involved.

The one that you've already touched on, the cost of production, drops very slowly, usually only with improvements in production technology and efficiency.

The other is the reseach and developments costs that are built up before the component ever goes into production. It is usually these costs that drive up the initial release cost of electronics, and after they are paid off in a year or two of sales you see the price start to drop to a level driven by the production costs.

Unfortunately in our current market new and improved products come out so fast that a better version comes out before the price on the older one has a chance to drop, and often enough they will discontinue the older model one the R&D costs are paid off in order to force people to upgrade and to keep their profits up.

Maybe I'll just buy a 35 mm compact camera for 150 bucks, instead of that 550 dollar compact digicamera . . . and make an 8x10 with crushing resolution that'll whip 'em all!

I totally agree that sensor size is important, pixel count is mostly for marketing purpose. But one point I have to make here is that shooting a film P&S doesn't guarantee good image quality either. I noticed that the prints made by cheap chain stores are not impressive, probably because everything was done automatically, and the machines are tuned for highest speed instead of highest quality. When I got the prints from my Olympus C4000, I was stunned by the good quality. Then I guess for the same reason, if you give them a reasonable quality digital image, there is no loss in their scanning process. Therefore, you can get better prints at a lower price.

If 8x10 is pretty much as big as you'll want to print, I'd have to say 3 Megapixel can be excellent. You might not be able to tell the difference between digital and film.

But this article truly answers what I've been asking many people and on many forums, photo.net included.

Obviously pixels do matter. But so does the size of the sensor. This means a 6.3 MP sensor at 35mm would be better than the 6.3MP APS-C size used by the EOS 10D. And a 10 MP sensor at the 10D size is better than the 10D. So things are getting clearer. Thanks Bob.

GREAT ARTICLE! I think one VERY practical finding from this is that if one is in the market for an easy to carry P&S digital, spending more $$$ for increased "megapixels" may not necessarily bring better pix.

Most of what we are talking about here applies to film as well as digital.

Another advantage for the larger frame is the amount of light actually reaching the sensor/film. A given aperture means larger diaphragm opening (physically) for a larger frame. Allowing more light (count photons) to reach the sensor/film. A larger sensor with the same number of pixels as a small one, will have more light (photons) hitting each pixel. This in turns means better dynamic range and less noise. A camera with a small sensor will need to amplify the signal from each pixel more increasing the noise level.

Ron

I didn't go into noise comparisions, but digital image noise has two fundamental components. Photon shot noise and thermal shot noise. Photon shot noise dominates at high signal levels (short exposure) and comes from the basic statistics of photon detection. Thermal shot noise comes from the dark current of a sensor and is strongly correlated with temperature (lower temperature give lower noise). This is the noise that dominates long exposures.

Noise from both sources gets worse with smaller pixels.

There are also other noise sources of course, which depend on the design of the sensor array and quality of components, but in principle these other sources can be reduced to levels below those of photon and thermal shot noise. A good array with large pixels will always show less noise than an equally good array with small pixles.

Thanks for the article Bob. How about a page 2 that compares the different sensor sizes when shooting an 8x10 to produce equivalent depth of field? For landscape shots that would seem to be a more fair comparison (eg. I need from here to infinity in focus).

Thanks - Greg

Very educative! But there's another reason why you don't want a small sensor: in fact pixels are so small with the small 1/1,8" CCD (about 3 µm) that noise is far greater (and more annoying) with this kind of sensors than with the CMOS the EOS 10D (for example) uses. Everyone wants more pixels, not really knowing why since they never have the intention to male 10x8 prints of their photo's. And wanting more pixels, they get more noise, thus less image quality.

I had the possibility of using 5 MPixel Point-and-Shooters. All but one failed in this aspect. A good and 2 times cheaper 4 MPixel (like the IXUS 400) delivered better images at 100 ISO setting (can you imagen how bad the 5 MP score at 400 ISO setting?). The one who actually wás better than the digital Ixus was... the Powershot G5 which had comparable noise at 400 ISO, but was by far better at 100 ISO setting. Nice!

Very informative article, and something I've been pondering wrt my "what next" decision. Isn't the logical conclusion that Foveon-type sensors are the only way to go (big pixels)?

However, Foveon aside, there appear to be real-world considerations which somewhat mitigate the advantage of larger sensors.

My compact (I'd like a DSLR but don't have one yet) has an f/1.8 zoom which is crisp wide open, and optimum by f/2.3. That's about 2 stops faster than DSLR lenses.

I compared the resoution (not overall image quality) of my 5mp compact with a 6mp dslr, using a manually-focused high quality prime. The compact peaked at f/2.3, the DSLR at f/5.6 : identical resolution.

Of course the compact was about 2 stops noisier at any iso, but again there's that 2-stops-faster, lens...

So far the only thing completely obvious to me is that one should use lenses designed specifically for the sensor size.

Dan

The tiny sensor sizes of “prosumer” digital cameras allows for relatively fast lenses with gigantic zoom range which is an intriguing feature for many amateurs not willing to carry multiple lenses, even less prime lenses. Remember the late 1970s super-8 cameras with 10x zoom and f1.2. In digital land we now have f2.0-2.8/7-50mm lenses, being equivalent to f2.0-2.8/28-200 for 24x26mm, with probably 30cm front lens diameter and 30kg weight.

I however understand from reviews that, even with a Zeiss badge, optical performance is not that great. In fact in 35mm land, Canon’s or Nikon’s f2.8/80-200s are still not up to the level of prime lenses and the f4.5-5.6/28-200 Tamron/Sigma/Tokina are even less.

Sony’s/Zeiss f2.0-2.8/7-50mm lens is not greatly smaller than a f4.5-5.6/28-200 from Tamron/Sigma/Tokina, but I understand a Canon 1Ds and even a 300D set to ASA 400 exhibit less noise than a Sony F828 set to ASA 100. In this regard the lens specs are misleading. A side-by-side comparison of small sensor, fast lens low ISO setting against large sensor, slow lens, high ISO setting would be interesting.

Apart from the sensor size issue, e.g. Olympus claims that their lenses for the new E-1 are somewhat optimised for the digital sensor. To some extend I can follow the arguments, but this approach seems not to yield in better image quality (www.dpreview.com), compared to DSLRs connected to conventional lenses designed for 35mm film.

I'm still unclear. I figure that I want print resolution at about 300 pixels per inch in PS, and therefore, unless I resample up, I am restricted in the size of photo by the number of my original pixels. If the bigger size CCD (but with the same pixel size) leaves me with the same number of pixels, can you explain how or why I get better pictures with a bigger CCD, or why I get a file that allows me to print larger pics? Is it because, with the larger CCD, the accuracy of the capture (each pixel) is better, and therefore the original unsampled pic is better and an up-sampled pic would therefore be better too. I'm guess I'm looking at this too simplistically.

>>In digital land we now have f2.0-2.8/7-50mm lenses, being equivalent to f2.0-2.8/28-200 for 24x26mm, with probably 30cm front lens diameter and 30kg weight.<<

I think there is a general misconception of the aperture values when comparing different sensor sizes, and Matt Kime above already gave the correct hint.

An f2.0-2.8/7-50mm lens on a small sensor is NOT equivalent to a f2.0-2.8/28-200mm lens on 35mm format! We are all used to apply a "cropping factor" to calculate the equivalent focal length, but we should do this also to the aperture value, in order to get an "equivalent f-stop". Then, the lens would be an equivalent f8.0-11.2/28-200mm lens. It's just that no manufacturer would like to name it this way.

But think about it:

1) For an equivalent depth of field, it is the actual diameter of the lens aperture that counts. Since the f-number N is defined as the focal length f divided by the aperture diameter D, we get D=f/N. If we multiply f by the crop factor, then we have to multiply N by the same factor in order for D to be constant. Take an online DoF-calculator to test yourself. If you have 2 cameras with different sensor sizes and lenses so that the Field of View is the same, then to obtain the same DoF, you need to stop down the larger camera by a factor equal to the cropping factor.

2) For the same noise level, we need the same amount of photons entering the lens. In other words, it is again the actual diameter of the lens aperture that counts. Smaller sensors show higher noise, because at the same ISO value (which is defined by the real f-value, not the equivalent f-value) and according exposure time, there is less light collected. (This assumes the underlying technique to be identical, just scaled in size, e.g. the number of pixels being identical). We see, we have to multiply the ISO value by the square of the crop factor to get an equivalent ISO! (We need the square, because the incoming light depends quadratically on the aperture diameter). We then could even use the equivalent f-number together with the equivalent ISO value in order to calculate the correct exposure, just as a valid alternative to using the real values.

CONCLUSION: if we want to compare cameras with different sensor sizes (crop factor C) but with lenses to obtain an equivalent Field of View, it is not sufficient to use the equivalent focal length (f2=f1/C), but we also have to calculate equivalent aperture values N and ISO values: N2=N1*C and ISO2=ISO1*C*C.

Now, an f8.0-11.2/28-200mm lens won't autofocus on an SLR! This is no contradiction, because the AF-system on a SLR is independent from the sensor size, and relies on the light flux coming in, which is dependent on the real, not the equivalent f-number.

To Bob's comparison: You would not want to stop down a 1/1.8" sensor camera (C=5) to f/16, because this is equivalent to f/80 on a 35mm size camera, which would also yield to an almost zero MTF, I guess.

I wonder why up to now not a single digital camera reviewer found out about this equivalent f-stops and ISOs and bases his reviews on these values. It would help compare cameras and understand the differences.

Thank you for an informative article. I now understand why my Panasonic FZ-20 has a maximum of an f8 setting.

I must say that line 4 of the chart doesn't seem very useful. Since a pixel is the smallest level of granularity, its color will be uniform, and its value will be some 24 (or 32) bit number that can be painted into as large an area as required with no loss of fidelity.

Thank you so much. Lovely information. It has always bothered why f8 is the maximum I could ever find on a digi cam. and lot more I know now, why my D SLR is a better machine

Very useful. Thanx!