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Should Nikon Make a 35mm "Full-Frame" Digital Sensor Chip?
© Darrell Young  
   

The 35mm "Full-Frame" vs. DX size digital sensor controversy is caused by a general lack of "digital sensor" knowledge on the part of many users, and strong marketing propaganda on the part of some sellers.

Many camera users haven't investigated the design of lenses and how light must fall on a digital sensor, compared to film. Sound physical evidence shows that, due to the microlenses over the sensor, a 35mm lens cannot fully cover current 35mm sized sensor chips! The light fall-off from 35mm sized lenses can be as much as two-stops on the frame edges, simply because a sensor chip requires light to hit it almost directly.


DX Sized Digital Sensor

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LIGHT FALLOFF EXPLAINED

Look at it this way. If the sensor's pixels are like a deep bucket, they need to be hit directly from the top to enter. If you tossed a ball into a bucket, it will go in easily if it's dropped in from overhead. But, if you toss it in from an angle there is a good chance it will bounce off the rim and not go into the bucket. (Which is why the basketball leagues pay so much to those who can get the ball in the hoop regularly) Light behaves the same way as the ball. 35mm lenses are not designed to bring light directly into the bucket (pixel), but instead cause the light to come in at a greater angle progressively as you move toward the edge of the lens. With 35mm film there are no pixel buckets, just random silver grains, so angle doesn't matter as much.

In Figures 1 & 2 below, I have attempted to explain how this works. The bubbles on the front of the sensor are microlenses that attempt to focus the incoming light rays on the best part of the pixel bucket. The pixel buckets are represented by the white rectangles in the blue sensor.

Notice in Figure 1 how the light rays (green) are coming from the back of the lens and are being focused correctly on the back of the pixel element by the microlenses on the sensor. No light is bouncing off the rim of the pixel bucket.

As you move from the center to the edge of a 35mm lens a higher and higher level of light is unable to enter the pixel bucket directly, so the light drops off and you have less light on your subject on the edges of the frame. As we all know, the less light that reaches the sensor the harder the "chip" must work and the more noise that is generated. So, a full-frame chip's image, with a 35mm lens, will have more noise on the edges, dark corners, color shifting, and image softness in the corners. Wide-angle lenses are worse than normal or telephoto, but it is often present with all lenses.

Notice in Figure 2 how the microlens is struggling to focus the light on the back of the pixel element. Since it cannot cause the light to go in directly, there is a certain amount of light lost.

This is called light "falloff," and causes the edge of the image to be dimmer. The larger the sensor is, the worse the angle the light hits the edges. So a full-frame sensor is destined by the laws of physics to have significantly more light falloff than a DX sized-sensor. In many cases the falloff can be as much as two full stops of light.

There are three ways to lessen this problem:

  1. Make the lens bigger so that it will have a larger image "circle" and the light will hit the sensor more directly.
  2. Make the sensor smaller so that the existing image circle can more completely cover the sensor.
  3. Redesign the lens so that it "collimates" the light, or makes the rays come out of the lens in a straighter path.

Nikon chose to do both 2 and 3. Normal 35mm-sized lenses are able to project an image circle large enough to completely cover the DX-sized sensor. So that means your older Nikkor lenses will still make great images with little to no light falloff. Plus, they have designed DX lenses with new lens elements that send collimated light directly into the sensor. DX lenses are made to put the light on the sensor in a straight line, so that much less light is bouncing off the pixel bucket rims, and going directly into the bucket instead.

As more DX lenses are released by Nikon we will have even better images than before. But, in meantime, all your old faithful expensive 35mm Nikkors can continue making their excellent images.

NOISE AND RESOLUTION

Nikon also chose to make the sensor resolve more than other sensors, with an excellent signal-to-noise ratio, so that your best lenses can make medium-format-like images. Some may complain that the full-frame sensor is able to make a less noisy image, since the pixel elements are larger and have a better signal-to-noise ratio. That is true! But, if you've ever seen an image from the Nikon D2x you will discover that there is simply NO noise in the images at 100 ISO. In higher ISO settings there will be more noise apparent, but it is so well controlled that even at 800 ISO it is not objectionable, and still less than ANY 800 ISO film. Noise is NOT a problem with Nikon's DX sized DSLR sensors.

Even if a full-frame sensor has less noise at higher ISOs, ask yourself a question. Which would I prefer...slightly more noise or considerable light falloff? For myself, I will take the little bit of extra noise in higher ISO settings. Everyone is used to seeing "grain" in film images, and so most would not even see the noise in the image anyway. The digital noise can also be reduced by adding extra noise reduction, if a person is really worried about it. Everything is a trade-off in lens and sensor design. Personally I will accept a bit of extra noise to get significantly higher resolution in small details, and no light falloff on the edges.

A lens or sensor that can resolve more Lp/mm can record finer details than a sensor that resolves fewer Lp/mm. The unaided human eye can only resolve about 10 Lp/mm. The D2x resolves nine times that figure, at 90 Lp/mm, with less noise than some "Full-Frame" 35mm-sized sensors. By giving us a very high resolution, but low-noise sensor, Nikon is thinking of the most important things...not just what the market thinks it wants.

Other camera manufacturers chose to go with marketing pressure and design the 35mm-sized sensor as a standard even though their engineers knew that 35mm lenses, with their angled light transmission, couldn't cover the full-sized sensor. It almost requires a medium format sized lens circle to cover a 35mm sensor size with light that is moving straight enough to go in the pixel bucket directly. How many of us want to carry around medium format sized lenses on our DSLRs so that we can get full coverage of the sensor?

I honestly don't care if Nikon EVER makes a full-sized sensor. Wide angle lenses are MUCH cheaper than telephoto lenses. I LOVE the extra "reach" I get with my 400mm lens acting like a "big-gun" 600mm lens. Plus, the Nikon D2x's High-Speed Crop mode gives me even greater reach when I absolutely need it.


AF-S Nikkor 24-120mm f/3.5-5.6G ED-IF VR lens at 120mm

My 120mm lens in the above picture, with the Nikon D2x's CMOS sensor, acts like a 180mm lens. In high-speed crop mode, it performs like a 240mm lens. Since telephoto lenses are MUCH more expensive than wide-angles, this makes me very happy. The Nikkor 12-24mm lens costs about $1000.00 USD, where the 600mm Nikkor costs about $5000 USD. Which one do I want my camera to lend the best support to? The most expensive lens! I can now buy a nice DX lens for 20mm equivalent shooting, and I'm sure even wider DX lenses are coming.

So, unless noise was a real issue, which it isn't, I don't need a sensor any larger than the one in my Nikon D2x. And, I truly enjoy the extra "reach" my 1.5x lens factor gives me. See what I mean:


Taken with a Nikon® D2x digital camera and an AF Nikkor® 80-400mm f/4.5-5.6 VR Lens

I enjoy being able to use a 400mm lens for images like the above Cardinal. Normally this would require a 600mm or larger lens, but the extra reach of a 35mm lens on a DX-sized sensor allows one to take pictures too expensive to take before, due to the higher cost of longer lenses.

INCREASED DEPTH OF FIELD

In addition, many do not realize it, but the smaller sensor on the D2x provides for another very important thing in photography, significantly increased depth of field. Most of our images need as much depth of field as we can give them. This article shows just how much more depth of field the Nikon D2x has over its closest full-frame competitor: Depth of field--the shorter focal length advantage by Lloyd Chambers.

Did you know that the D2x gives you greater depth of field by one and one third stops? That is a nice depth of field advantage!

CONCLUSION

35mm "full-frame" is truly an arbitrary size in the digital world. There are pros and cons to all types of technology. With DX sized sensors the pros and cons are as follows.

DX sized sensor Pros:

  • increased depth of field
  • higher resolution
  • no light falloff
  • no corner image softness
  • less edge chromatic aberration
  • 1.5x lens factor (if you shoot a lot of telephoto)
  • lower cost

DX sized sensor Cons:

  • increased high ISO noise
  • 1.5x lens factor (if you shoot a lot of wide angle)

With Full-Frame sensors the pros and cons are as follows.

FF Sensor Pros:

  • no lens factors
  • high resolution
  • less high ISO noise

FF Sensor Cons:

  • corner image softness
  • light falloff
  • less depth of field
  • more edge chromatic aberration
  • higher cost

Which is best? YOU DECIDE!

I choose Nikon's way of doing it. They would rather create world-class cameras with extremely high-resolution, low-noise sensors and attempt to educate their users, instead of making a lesser quality full-frame sensor, and being forced to sell their best cameras for thousands more than they must.

Maybe someday digital sensors won't require microlenses, and will accept angled light more like film. When that happens I'm sure Nikon will come out with a full-frame sensor, and, as usual, will continue to overstep the competition in ergonomics, quality, and features. Until then, give me my DX-sized sensor with low-noise, no light fall-off, better edge color, greater depth of field, and higher resolution!

Keep on capturing time...

Update: August 24, 2007 - Well, Nikon did make a new FF sensor after all. They call it the FX format. Seems that they are using a new "gapless" microlens system that relieves the problems seen in other FF sensors on the market. Go Nikon!

   

Here is an UPDATE to this article with new information
(Less controversial - but, read it carefully!)

Click here for an interesting Photo.net thread about the vignetting found in some FF sensors.

   
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