DX to FX and resolution

*Note that most DX lenses mentioned below I have  replaced with FX lenses. I have briefly commented on current gear on another page

A decision to move from DX (Nikon d7100) to FX (Nikon d800) is not as easy as first appears, causing self-evaluation and re-evaluation of previous techniques, coupled to questioning what was assumed to be secure technique. This article is not about learning the substantive needs in using a new camera, but shows a sequence of learning what lenses to use in order to make best use of the very different sensor. It is not just about basic resolution.

For most, it may be best to keep the DX to FX migration in mind over time, and keeping this in mind when making lens choices. When actually migrating from a DX to FX camera I first thought I needed to decide which lenses to keep in the short term, and whether those lenses performed better as straightforward DX lenses (autocropped in camera or software) or optically adjusted by using a 1.4x teleconverter. As an example, a 50-150mm f2.8 DX lens operates as a 70-210mm f4 FX lens with the teleconverter attached (as opposed to the 75-225mm f2.8 when considering the crop factor). I undertook some initial tests of the new options the FX camera offered.

There were some surprising outcomes. A caveat is that these tests applied to the gear I already owned – I would not buy a DX lens for an FX camera!

There is some dispute whether it is best to use a DX lens cropped in-camera (auto DX recognition), capture with a DX lens in FX mode and crop later, or use a 1.4x converter with the DX lens. The latter will create the largest files, but the debate is whether there is any gain from using DX crop mode.

DX lenses I used were: Samyang 8mm f3.5, Sigma 10-20mm f4-5.6 EX, Tokina 11-16mm f2.8 ATX Pro, Sigma 17-50mm f2.8 EX, Sigma 50-150mm f2.8 EX, Sigma 30mm f1.4 EX.

I also owned at the point of migrating to FX a Nikkor 20-35mm f2.8 AF-D, PC-Nikkor 28mm f3.5, Nikkor 50mm f1.4 AF-D, Nikon 80-400 f4-5.6 AF-D, Micro-Nikkor 60mm f2.8G, manual Nikkor 50mm f1.8 and manual Sigma 24mm f2.8 SuperWideII, three Teleconverters – Sigma 1.4x and 2x EX, and a Tamron 1.4x.

The superb Nikon d800 camera gives normal FX mode (36MP), 1.5x crop mode (15MP, DX crop factor), and also a 1.2x crop mode of 25MP using an area of sensor larger than DX. The comparisons I made were with DX, but many DX lenses just about cover the 1.2x area. The d800 in 1.5x mode closely equates to the D7000 in resolution and IQ terms, the 1.2x mode is a marginally higher resolution but covering a larger sensor area (and potentially less noisy and with better signal to noise and dynamic range) compared to the D7100/D7200.

In my testing I did not apply any sharpening in camera or picture control settings, and tested in FX, DX, and FX using either of the 1.4x teleconverters and similar apertures. I examined the resulting images cropped just off-centre to the same size of subject, minor adjustments made and then images resized to make a fair comparison for real life use. I also shot images with the d7100.

I do not think my limited expertise makes it worthwhile to review each lens but here are my conclusions as to preferred options by FX focal length.

(Current thoughts on current equipment is on the Phil and his gear page. These comments applied on migration and before acquiring a Sigma 12-24 and Nikkor 24-85 replacements. Using DX lenses was never satisfactory except in the very short term)

11/12mm: This focal length is only achieveable by the Samyang DX lens. Marginally, the best shot was with the Tamron TC shot in FX. Over time I have become happy with this combination and produces images that are reasonably sharp when the lens is stopped down to say f5.6 or so.

15/16mm: The Tokina at 11mm was not good and the DX version has more ‘accutance’. The d7100 shot seemed sharper but tonally, and better overall, was by using the Tokina at 16mm in FX mode. I would say the jury is out whether it is worth using the 1.4x TC, some more use is needed to make a better judgement for a lens that now becomes c15.5-22mm f4.

20-35mm: Undoubtedly the FX lens wins out. The D7100 with Sigma 17-50 is far better than the Tokina at 16mm. On the d800 a better tonality is offered using the Sigma/Tamron TC combination. The Nikkor 18-105 on d7100 combination was much fuzzier.  On the d800 it is possible to use the Sigma 17-50 with Sigma TC gave a good result but only from c23mm to the long end of this range. With the Tamron 1.4 TC it becomes a c24-70mm f4… and could have a future.

28mm: Lots of detail with the PC-Nikkor.

50mm: The Nikkor 50mm is an excellent lens. Using the Nikkor 20-35 at 35mm in DX mode is acceptable, the Sigma 17-50 works better with the Sigma TC rather than Tamron TC, but in DX mode the lens seems to ‘over-sharpen’. In practice I now seem to carry the 50mm Nikkor, and forego a standard zoom and use my feet more. I have an old Nikkor 35-70mm which isn’t anything like as bad as some reviews make it, and is light.

70-80mm: There is not much between the Sigma 17-50 at 50mm in DX mode and the version with Sigma TC. Surprisingly the best performer was the Sigma 50-150 with Sigma TC combination, producing a good result seemingly equaling the Nikkor 80-400 at this focal length!

120-150mm: Another dead heat between the Nikkor 80-400 and Sigma 50-150/TC combination, but maybe the image is less harsh with the Nikkor. The Sigma/TC combination is lighter and more portable – thus is the preferred option unless focal lengths beyond 200mm are required.

210-225mm: likewise at this focal length.

200-400mm: the Nikkor 80-400 is the only option.

Some lenses were also shot with the DX D7100. Interestingly, by comparison the resulting adjusted resized images often seemed over-sharpened, sometimes making a ‘jarring’ impression when severely cropped. The older AF-D FX lenses generally are good glass, tonally smooth, though perhaps not always with the ‘accutance’ of modern counterparts.

Each focal length seems to warrant a different lens option, at least until some changes can be made. (For instance, trading in the Sigma and Tokina wide angles in favour of a Sigma 12-24 FX lens would seem desirable.)

What I see no need to do is always revert to the auto=DX crop mode – simple maths seems to work against this.

Most of the time my normal kit was taken from these lens combinations depending on the task:

Nikkor 20-35, (+ PC-Nikkor 28 if shooting architecture), Nikkor 50mm, Nikkor 80-400 if longer focal lengths required, Samyang 8 + 1.4x TC if wider focal length required.

In DX days I loved the Sigma 10-20, and latterly Tokina 11-16 with occasional forays with the Sigma 17-50. Now I have started to really like the Nikkor 20-35, coupled to the Nikkor 50mm prime. When I have time I enjoy using the PC-Nikkor 28.

Interestingly, I think shooting with the DX lenses gives greater ‘accutance’, but this does not mean they have greater resolution. These older Nikkor lenses seem to have a better tonal response though they are slower in operation. I have concluded that some reviews misconstrue the abilities of the format – downsizing larger files to more typical sizes also gives sharper results, and the d800’s resolution is so high that there is plenty of latitude to crop and then downsize. The larger image size also allows the ability to manipulate the image more without compromising IQ.

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