There is a Differance: DX & Full-Frame Lenses

[Jumpseat]

Ok, I am going to try this again because there have been a number of posts by people curious about the difference between “Full Frame” and “Digital” lenses. The lens manufacturers have been trying, for several years, to ignore some issues, and even spread urban myths.

Being a Nikon person I am going to use the term FX to represent full frame imaging media whether it is digital or film, and DX to represent the APS size sensor found in most DSLRs.

Every lens projects a round image into the back of your camera, and that round “Image Circle” is the center of this discussion.

Before digital there was 35mm film that was an FX size medium and required full frame size image circles, from full frame lenses, in order to completely cover the film. Within a camera the four corners and outer edges of the rectangular FX media (film) came right up to, or very near, the outer circumference of the image circle.

There was a problem though; the image at the outer edge of the image circle suffered from a number of maladies. Among these were vignetting, distortion, corner softness, and light fall-off. As you moved toward the center of the image circle, the image would dramatically improve.

The center of an image circle is always the sweet spot. The sharpest, clearest, best portion of the picture comes from the center of the image circle.

Then came the DSLR with its much smaller DX sensor. When a full frame lens was mounted on a DX camera something wonderful happened!! The DX sensor was so small that it never got near the outer edge of the image circle. Suddenly, most of the problems with vinetting, distortion, corner softness, and light fall-off were gone. The DX sensor only used the sweet inner area of the image circle. Now, even less expensive lenses gave outstanding performances.

Meanwhile, back in Japan, inscrutable minds were trying to create a better market for their lens product lines. There was discussion on the street (some fostered by second tier optics manufacturers) about circle of confusion, sensor glare and so on, making us believe that there was something incompatible between those hordes of existing auto-focus, full frame lenses and our new DSLRs. (Frankly, there was some credibility to the technical aspect of the street talk, but in practical application it was a non-issue.)

So, with the background of street (mis-)information and a ton of perfectly usable, inexpensive, full-frame lenses floating around on the used market, the inscrutables invented the DX lens. Now, there are some advantages to the DX lens; it’s smaller, it’s lighter, and it’s less expensive to manufacture (buy). Other than that, it has one major heartbreaking drawback. The image circle is a much smaller diameter and once again the corners and outer edges of our sensors protrude into the worst image areas. The return of vignetting, distortion, corner softness, and light fall-off.

These outer image circle problems are always there to some degree. Yes, you can buy lenses with a minimum of these maladies but they are usually extremely expensive; $1000, $1500, $2000, or more depending on how perfect you want your image. Some people think that aperture has a bearing on how well a lens performs (particularly on a DSLR). Well, an f2.8 lens usually costs $1000+, and wouldn’t you expect outstanding performance from any lens at that price, regardless of aperture? With the DX sensor even a reasonably inexpensive full frame lens will give you excellent results. Unfortunately, only the most expensive DX lenses will compare to it when considering corner and edge problems. And, if you mount one of the real expensive full frame lenses on a DX … well, nothing can touch that.

Now, we are confronted with the FX sensor and it appears there is nothing to alleviate corner and edge problems other than buying those real expensive lenses (If I am not mistaken all of the lenses Nikon announced with the D3 sell for over $1200 each). I now have much more sympathy for my Canon friends; they have been dealing with this longer than any Nikon users.

[deckcadet]

There are more ways to combat light falloff and edge softness than simply buying the new zooms Nikon has released.

For one, all of Nikon's pro zooms (17-35, 28-70) covering FX are relatively new designs. They did not have the same level of issues as say Canon's wide glass, which is why a great many Canon users look to third parties...even Nikon lenses...to get the best wides.

The Nikon 17-35 especially was designed with digital in mind- released alongside the D1 digital SLR.

Another way to combat this is within the camera. Part of the reason for light falloff being much more noticeable on digital than film is the way that the digital sensor is sensitive to light. Microlenses on the sensor's photosites gather light into the photosites, but they tend to have a limited capture area. The D3 has a new microlens array with a gapless surface to improve light collection.

A third way is with the camera's firmware. The D3 has advanced in-camera algorithms that provide correction for lens aberration.

DX lenses also aren't simply designed to have a reduced image circle, but many have different optical design strategies that improve results in the coverage area. I will admit though that many of them, being ultra-cheap consumer lenses, don't correct for corners as much.

[Jumpseat]

Thanks Harrison,

From a technical stand-point you are absolutely correct. But review what you wrote, you will find that the only area that can be addressed by the photographer is to buy a more expensive lens.

You brought up the Nikkor 17-35mm zoom as a good example of how to address the problem, and you are right. However, the photographer has no control over micro lenses or firmware and, you say that the cheap, consumer DX lenses don’t correct much. So the only alternative is to buy a lens in the category of the 17-35mm at a cost of over $1500. And, the 28-70mm zoom you mentioned is over $1400. As I said, “if you mount one of the real expensive full frame lenses on a DX … well, nothing can touch that.”

[deckcadet]

One could buy a more expensive camera instead with the features that correct for this.

But I don't understand why you're harping on the DX lenses having falloff etc.... even some of the cheaper FF lenses have it on DX. This is nothing new, and it is to be expected for consumer grade lenses. Even some pro-grade lenses have an expectation of falloff.

All the lenses announced for the D3 are over $1500. The 17-35mm, however, has a booming used market, and can be had for well under $1000.

[Jumpseat]

Again, as I said, “These outer image circle problems are always there to some degree.” If one takes a run of the mill full frame lens like Nikon’s 18-35mm zoom on which format will it give the fewest edge and corner anomalies, FX or DX? The answer is, obviously, DX because the sensor is so far removed form the extreme edge of the image circle. This is a good thing! If one mounts a DX lens say, the Nikkor 18-135mm zoom on a DX camera you have once again moved the corners and outer edges of the sensor back to the extreme edges of the image circle where all sorts of bad things can happen. I’m repeating myself.

I am not down on the DX lens, this is a simple matter of physical reality.

Yes, one can always spend more and more money on cameras and lenses, but the questions on this forum seem to be coming from those of us that photograph on a budget. And, all I am doing is trying to help members get better results without dipping into their children's college fund. The zoom you metioned is a good example, yet the 18-35mm still sells for much less new than the 17-35mm does used and other than build, it is right up there with it's expensive sibling.

[Franglais]

I won't try to enter into the theory. This is just what I have observed. I shot 24x36 (FX) film for years, first with primes then with zooms. It was always a fact of life that - at best - image quality decreased in the corners of the frame and - at worst - there was light fall-off at the corners. To get the best results you need to stop down two stops.

Now I have two DX zooms (18-70 and 18-200) and they break the rules. I have never observed a loss in picture quality anywhere in the frame at any aperture or focal length. If I do a test and go pixel peeping then yes you can see it but in real life it doesn't bother me. My DX format Tamron wide-angle does show loss of sharpness towards the edges on a 30x40 blow-up so I guess it depends on the lens.

The 24x36 f2.8 constant zooms are still the best but there's not much in it.

[Jumpseat]

Hello Charles,

Due to the time difference I was waiting to see your take on this. And, I think your first comment is right at the heart of the matter; technical details (theory). Over the time I have been prowling through these forums many posts have asked for guidance concerning the differences between FX and DX lenses. What I wrote in the first post of this thread is, shall we say, a historically technical view. Is it true? Yes, but as you so well point out, there are always exceptions to any rule.

The Nikon 18-70mm zoom has, overall, impressed me more than any other DX lens. Most “amateur lenses” (a rather arrogant term), as you know, can be very good within a rather limited range of aperture setting, but what makes this lens so remarkable is it’s even, sharp field across the entire frame at such a wide range of settings. And, while it may technically not be the sharpest lens among its peers, it is sharp enough that most of us will never be concerned. An excellent lens I would recommend to anyone.

Nikon’s 18-200mm is a must have lens. Is it technically perfect? Good lord no! Will it do things for the photographer that almost no other lens can? Yes, on a daily basis. I don’t know if it would retain its appeal with the loss of any one feature (i.e. VR, focal range, max aperture range, size, or weight), but as a whole, it’s an outstanding lens. Anyone who carries a Nikon DxSLR further than 100 feet from home should own one.

While what I presented, in post #1, were technical details, it is often that kind of information that someone new to this end of photography can use to help make a decision or simply as a foundation to build further knowledge. Too often we simply rely on information provided by equipment manufacturers, forgetting they have a vested interest in convincing us that their technology is correct, new, trendy, and required.

I will be honest with you; I have always thought that DX lenses were a solution in search of a problem. They are smaller and lighter but otherwise (from a user’s standpoint) what technological progress have they presented that couldn’t have been built into an FX lens line? I suspect that they were more of a marketing strategy than a photographic necessity. Ironically, over the next few years we will see the use of FX sensors find their way into more and more consumer level cameras. Nikon has to ask themselves, who wouldn’t buy a D40FX? And, we thought the “nikonians” were upset by what they perceived as planned obsolescence before!

Is there a difference between FX and DX lenses? Yes. Is one better than the other? That’s the reader’s decision. Can FX lenses be used on DX cameras? Absolutely. Can a less expensive FX lens give results as good as a more expensive DX? Quite possibly.

You, Harrison, and I have been fortunate to find rationale for spending thousands of dollars on photo equipment. My post was to help generate ideas how a person may improve their photography without taking a second mortgage on his/her home. Besides, I’m retired now and have to start thinking of these things too.

Thanks for diving in. -Bill

[Franglais]

I think that there is a place for DX format cameras forever. They will always be cheaper to make than FX format, and that goes for the lenses as well. The D40+18-55 has now dropped in price to 545 Euros. The D300 is at 1749 Euros. The D3 is at 5000 Euros. For the price of the D3 I could have a D300, a D400 and a D500 (looking into the future).

We are used to the idea that computer components regularly drop in price. The price of a FX format will drop but maybe not as fast as people hope. Computer chips drop in price because manufacturers are packing more and more components into a smaller space. This just won't happen for FX sensors. They have to be big (24x36), they don't really fit into the regular manufacturing process (takes 3 exposures to make a FX chip vs only 1 for DX) and the level of wastage is high.

As for DX lenses - the lens designer has a smaller image circle to cover. He doesn't have to have the image sharp right to the edges of a 24x36 frame. He can be more daring and do things that aren't possible with FX lenses. Hence a succession of remarkable lenses from Nikon for the DX format.

[Jumpseat]

You could be right with your assessment of future DX sensor proliferation. However, notice how Canon, without pressure of competition, brought the FX much closer to the consumer’s price point. And, now we have the highly competitive nature of Nikon vs. Canon with Nikon, for the first time, offering FX. Once again competition may prove to the benefit of us all. I, for one, am somewhat hesitant to buy the new D300 because I suspect the D400 will be FX. And, at a very similar price. It’s not a matured industry yet; it has only been a little over 7 years since the D1 started all this and about 5 years since the D100 made it affordable. The D100’s third generation is not even on the street yet. It’ll be interesting to watch.

I have always thought that one of the primary reasons for going to DX lenses was, as previously stated, the economics of manufacturing. Smaller lens glass, smaller barrels, and so on make for lower manufacturing costs. In the end, like completion, it can only be beneficial for the consumer. One has to be amazed at the low price of a great lens like Nikon’s 18-70mm DX zoom. It’s better than some high priced primes I owned just a decade ago.

As for the ease of design with the DX, you may very well have a point. I will admit that I can’t see an 18-200 VR FX in the near future that would cost less than my Jeep. Though on the other hand, I’m not sure we would need an 18-200 in FX because we wouldn’t have the wide end battle. A 24-200 FX VR would be outstanding and not much of a stretch. But, I don’t ever want to see that Nikon’s 28-200mm again, even with VR. Ugh! -Bill

[Franglais]

You could be right with your assessment of future DX sensor proliferation. However, notice how Canon, without pressure of competition, brought the FX much closer to the consumer’s price point. And, now we have the highly competitive nature of Nikon vs. Canon with Nikon, for the first time, offering FX. Once again competition may prove to the benefit of us all. I, for one, am somewhat hesitant to buy the new D300 because I suspect the D400 will be FX. And, at a very similar price. ...

l

1. Canon and Nikon don't compete directly most of the time. Look at the price and features of their DSLR's and they are usually interleaved D40 - D40x/400D (the exception) - D80 - 40D - D300 - 5D - 1DMk3 - D3- 1DsMk3

2. I hestitate to comment on Canon's "affordable" FX sensor offering (the 5D) because there are a lot of users out there who are very satisfied with the camera (even passionate about it). I see the $2500 5D as an improved 20D (less than $1000) with a very expensive sensor, which just proves my point about how expensive an FX sensor is. But if I say that then people are just going to scream at me.