Pixiq

There is more to choosing between the DX and FX formats of Nikon D-SLR cameras than you may have considered at first. Simon Stafford offers a few thoughts on the matter.

I am often asked questions about the relative merits of the Nikon DX (16 x 24 mm approx.) and FX (24 x 36 mm approx.) formats and how to choose between them. Although those questions may differ in form most distill down to a common denominator that amounts to which format is the best; in my book it is not a simple case of “best”, as each has its own particular merits, and as with any tool is it a matter of choosing the right one for the job.

Assuming use of top quality lenses and proper camera technique I would suggest that for most practical purposes spotting any difference between the D700 and D300s, when used at their base ISO levels up to ISO800, or even ISO1600 in some circumstances, when printing to sizes up to Super A3 (13 x 19-inches), becomes a matter of splitting the proverbial hair. In fact I believe the same applies to all the 12Mp Nikon D-SLR cameras (FX and DX formats); essentially what you gain as you ascend the product line-up from the D5000 to the D3s is feature set, build quality, speed of operation and high ISO performance. 

However, potential print quality is far from the only consideration you should give to choosing between the two formats, as there are many other aspects that should influence such a decision.

The resolution of detail will depend to some extent on the density of pixels on the sensor. When you compare the D3s/D700 with the D300s all three models share approximately the same number of pixels; however, in the case of the D300s DX-format sensor they are packed in to an area that is less than half of the D700 FX-format sensor. Now, assuming all other factors are equal the D300s has, potentially, the upper hand when it comes to recording very fine detail, although this will be dependent on faultless camera technique and pin point focusing accuracy, if any advantage is to be realised. How does this come about? Well, consider what happens when a picture is taken at the same camera-to-subject (focus) distance, using the same focal length on a DX-format (D300/D300s) camera and an FX-format (D700, or D3/D3s) camera; the image of the subject will occupy a greater area of the DX-format sensor compared with the FX-format sensor, due to the different linear dimensions of the two sensor formats, which has a direct effect on the field-of-view characteristics of the lens. 

In this case the pixel density (call it the input resolution if you prefer) is higher on the DX-format sensor of the D300/D300s than the sensor of the FX-format D700, or D3/D3s: therefore the D300/D300s sensor provides more detail, as it has a greater number of pixels in any area of the subject’s image you care to define, despite all four of these camera models having the same 12MP resolution.

Of course if all other things are not equal and the subject-to-camera distance is altered (i.e. the same focal length is used but the FX-format camera is positioned closer to the subject), so that the area of the image formed by the subject is the same on each of the two sensor formats then the pixel dimensions of any area within the subject will be the same.

In a situation where it is not possible to approach a subject as close as you would wish, the higher pixel density of the DX-format D300/D300s could, potentially, and assuming all other variables are the same, enable these camera models to resolve more detail in the subject compared with the FX-format D3/D3s. It is for this reason that many wildlife photographers prefer the DX-format cameras, particularly for certain subjects, such as small birds, especially if they expect to have to crop their images at a later stage.

 The depth-of-field (DoF) characteristics of the two formats is less easy to quantify, since DoF is a quality derived from our perception of what is sharp and what is not sharp. In an effort to enumerate DoF, the geometry of image projection by a lens and the viewing conditions under which the 2D image it forms, and will be observed, are considered; however, no DoF model is perfect and thus there is always a highly subjective aspect involved when it comes to the assessment of this image attribute. 

Common convention holds that on the DX format a lens with the equivalent effective focal length to the FX format, used at an aperture 1-stop larger, will achieve approximately the same DoF coverage as on the FX format. For example, a 200mm lens set at f/2 on a D300s will achieve a similar DoF as a 300mm lens set at f/2.8 on a D700, provided the camera to subject distance is the same in both instances. Of course in this case the angle-of-view produced by the two lenses will also be similar. As to whether this is an advantage, or disadvantage will depend on individual requirements. The FX user will claim better subject isolation when using equivalent aperture values, while the DX user can equally claim the wider aperture required to achieve a similar DoF will mean they can shoot at a proportionally faster shutter speed, since the lens is not stopped down as far. Of course the effect of the pixel density (as discussed above) can, and often does sway opinion as to whether an area within an image falls in to the sharp, or not sharp camp.

Associated with the DoF argument is the issue of angle of view and image magnification. Using a 200mm lens on a DX camera and a 300mm lens on an FX camera, with both at the same distance from a subject, will produce pictures with approximately the same angle of view but the degree of subject magnification is greater with the 300mm lens than it is with the 200mm lens. If the 300mm lens is put on the DX camera the degree of subject magnification will then be equal to that on the FX camera; however, the angle of view will be reduced (all that using the smaller DX format does in this instance is crop in closer to the subject). The FX user could achieve the same image by shooting on the 300mm lens and then cropping it to the DX-format dimensions. The penalty of this approach is only the central section of the FX-sensor is used, so the pixel density and image file size will be reduced, which in turn will limit the degree of enlargement that can be applied.

One potential advantage of the DX format that is often overlooked comes with lenses designed to cover the FX format; by only using the central portion of the larger image circle projected by such lenses it is rare for the DX-format camera image to exhibit any significant adverse effects due to lens aberrations, such as vignetting (the fall off in the level of illumination at the periphery of the image), linear distortion, or lateral chromatic aberration.

A benefit of working with the FX format cameras is the far greater choice of short focal length lenses (prime, or zoom). The smaller DX-format sensor requires a proportionally shorter focal length to achieve an equivalent wide angle of view, which pushes at the current limits of optical design and engineering if the lens is to achieve a comparable performance to its FX-format counterpart. Another positive aspect of the FX-format cameras is that they have restored the expansive viewfinder view of professional grade film cameras such as the Nikon F5 and F6, making the use of ultra-wide angle lenses, viewfinder accessories and close-up equipment that much easier, while the smaller viewfinders of the DX-format cameras produce a rather tunnel like effect that are less conducive when focusing manually, or examining detail in a composition. 

Beyond the optical considerations of the FX/DX formats there is the matter of noise and dynamic range, particularly at elevated ISO levels. Larger pixels gather more light, more efficiently – it’s a fact – and the current FX cameras have a double micro lens layer makes sure that every last photon of light is recorded by the sensor. This is the reason the normal ISO range of the D700 and D3s extend to ISO6400 and ISO12,800 respectively, while by comparison the DX sensor of the D300s has an upper limit of ISO3200 in its normal ISO range. In general I find above around ISO800 the noise performance of the D700 is about 1EV ahead of the D300s, while that of the D3s is at least 2EV higher. Linked to noise performance when pushing the ISO level ever higher are the side effects on colour saturation, contrast and dynamic range, all of which become progressively reduced; however, the larger pixels on the FX format sensor means the impact of these unwanted side effects are less than they are with the DX format cameras at equivalent ISO levels.

Finally, there is the matter of AF point coverage. The D3-series, D700, D300 and D300s offer essentially the same 51-point pattern AF system but the area of the frame covered by these AF points is highly centralised in the FX models, while it is much broader on the DX format D300/D300s. To place the subject away from the centre of the frame when using the FX format cameras you will probably find that focusing and recomposing becomes common practice; by comparison auto focusing on a highly off-centre subject is that much easier on the DX format cameras, providing greater flexibility in composition.

The list of pros and cons for the DX and FX formats is lengthy, for example, when photographing wildlife, sport, or any subject where it is not always possible to approach it as closely as you would wish, the higher pixel density, broader frame coverage of the AF points, and reduced angle of view can all be considered advantages of the DX format cameras, over and above those of smaller size, reduced weight, and lower cost, when compared to the FX models. Conversely, if high-ISO performance coupled with very rapid firing rates are a specific requirement the FX-format D3s cannot be beaten. 

So what lies ahead? Well, for the Nikon Corporation to maintain their stated market share aspirations of 35 to 40%, they must continue to develop DX-format products, as they are key to volume sales, which generate the revenue to support their R&D operations and manufacture of the more specialist FX format products. Of course this strategy is nothing new, for example, the diminutive Nikon EM, FG and FG-20 models were the engines that drove general film camera development and allowed production of the professional F3 and F4 cameras during the 1980’s. 

No doubt arguments will run and run in the FX versus DX debate – but at the end of the day it comes down to picking the right tool for the job! The two Nikon digital formats are now well established and I am confident there will be plenty of new products in both, for example, the recently announced D3100 and D7000 that have 14MP and 16MP sensors respectively. The continued development of both formats by the Nikon Corporation is a prudent move, since it provides photographers with plenty of solutions, whatever their requirements might be.