Nikon Z 24-120 f/4 S

As a first lens to equip my Nikon Z8, I had to decide on an all-round zoom for my frequent needs. Nikon offers multiple models of medium-zoom lenses in the Z system, and specifically in the FX (full frame) series:

• 24-70 mm f/2.8 II S
• 24-70 mm f/4 S
• 24-120 mm f/4 S
• 24-50 mm f/4-6.3
• 24-200 mm f/4-6.3 VR (with in-lens image stabilization)
• 28-75 mm f/2.8
• 28-135 mm f/4 PZ (power zoom)

The first three lenses in the above list belong in the S (premium) line of Z lenses. These lenses have better optical performance and general features than non-S lenses, but not necessarily a prohibitively high price. They are said to exhibit a lesser amount of focus breathing (i.e., change in angle of view while focusing) than other lenses, which makes them better suited for video and for applications like focus bracketing. By and large, I think that the S line corresponds to the Olympus/OM System Pro series.

Some, albeit not all, S-line lenses carry a small LCD screen that displays some of the operating parameters of the lens, like focus distance and aperture. In my opinion, this feature is not so useful as to justify the increased cost of the lens. The physical controls of a typical S-line lens are discussed here.

The 24-70 f/2.8 II S is good but a bit expensive and bulky, so I decided to leave it out of my choices. With the Z8, it is not all-important to have f/2.8 lenses unless you plan to frequently shoot in poor light. An f/4 lens on full frame collects twice the number of photons of an f/2.8 lens on Micro 4/3, at the same exposure time, effective aperture and scene (it is only in photomacrography that this advantage becomes less than one stop). If you do plan to shoot frequently in poor light, it may make better sense to invest in two or three really fast prime lenses (f/0.95 to f/1.4), rather than f/2.8 zooms.

The efficiency of current camera sensors is so high that, in practice, any difference in efficiency of different sensors can only be minor. Nominal and effective aperture are the same, in practice, at subject magnification less than 0.05x, or 1:20. Therefore, during the same exposure time, with a distant subject, at the same light intensity and same lens aperture, the number of collected photons per unit of sensor area is the same. Thus, a full-frame sensor, being roughly four times the surface of a Micro 4/3 sensor, in the same conditions collects four times more photons than a Micro 4/3 sensor. An f/4 lens is one-stop dimmer than an f/2.8 lens, i.e., transmits half the amount of light. Hence, the full-frame sensor equipped with an f/4 lens still collects twice the amount of light of a Micro 4/3 sensor with an f/2.8 lens.

Things are different in photomacrography and photomicrography, where the goal is imaging a given area of the subject, and therefore the magnification must differ on different sensor sizes. In photomacrography (up to roughly 10x), effective aperture differs from nominal aperture, and is a function of magnification. Thus, a smaller sensor has a modest advantage in light-collection over a large one, because the smaller sensor allows shooting a given subject area at a lower magnification (and therefore at a lower effective aperture).

In photomicrography, the subject area is enlarged by a high magnification factor. With the same subject illumination intensity and objective NA, once the subject is enlarged just enough to fill the sensor (via projection optics), there is no significant difference in the total amount of light collected by sensors of different sizes. In fact, it is often more practical to use a Micro 4/3 sensor than a larger one, because in most cases Micro 4/3 is small enough to work in direct projection by the tube lens of an infinity-corrected microscope, while larger sensors need additional optics.

The ISO setting of the camera does not affect the number of collected photons, as long as the other factors described above remain the same. The ISO value is used to process the image brightness only after the exposure.

Dual Base ISO sensors slightly complicate the above statement about ISO. These sensors, in practice, have two built-in native ISO sensitivities instead of just one. When one gradually increases the ISO setting in the camera, at a certain point the sensor switches from it lower to its higher (boosted) built-in sensitivity. The difference between the two native ISO sensitivities is not great, but enough to allow the recording of satisfactory images in a moderately broader range of illumination intensities. The problem of Dual Base ISO is avoided by operating the sensors being compared at their lower Base-ISO.

The 24-200 mm f/4-6.3 VR offers a wide zoom range and is not an S-line lens. I am somewhat suspicious that it may trade some optical quality in return for a reasonable price. In addition, at the 200 mm end of the zoom range is has an f/6.3 speed, a bit low for general use. With a camera offering a very good sensor-based image stabilization like the Z8, in-lens VR is not so important at focal lengths below roughly 200 mm, so the advantage of VR in this lens probably becomes noticeable only around the 200 mm end. This is why Nikon did not add in-lens VR to the other medium zooms.

The 24-50 mm f/4-6.3 is definitely a consumer lens, so it is also out.

The 28-75 mm f/2.8 is bulky, and not an S lens. The 24-70 f/2.8 II S would make a better choice.

A power zoom like the 28-135 mm f/4 PZ makes sense for video or for zooming with a remotely controlled camera, but this is not what I do.

This restricts my choice to the 24-70 mm f/4 S or 24-120 mm f/4 S. The 24-70 f/4 is relatively compact, but the 24-120 offers a significantly higher zoom ratio, and its price, bulk and weight are not much higher. Reviews of these two lenses indicate that the optical quality of both is very high for their price. The 24-120 extends significantly while zooming, but I have other lenses that do the same in the Olympus system, and I am used to it.

At this point, my choice of the 24-120 f/4 felt natural, and I purchased a second-hand one in like-new condition on eBay. It does not look too bulky on the Z8, and feels well balanced, even though its balance changes moderately as a result of the front extending with zooming.

Compared to an Olympus 12-40 mm f/2.8 Pro, the difference in size of the two lenses is obvious. The Olympus lens is made for Micro 4/3 and is the pro-level medium zoom of the Olympus/OM System lens line-up. It is very visibly the smaller of the two, in spite of being one stop faster than the Nikon lens. Olympus/OM System also has a 12-45 mm f/4 Pro, which is only a little smaller and lighter than the f/2.8 model.

The two lenses shown above produce an equivalent field of view at their lowest focal lengths on their respective formats, but the Nikon lens has a much higher zoom ratio (5 vs. 3.3 of the Olympus). This partly explains the larger lens size.

In the above figure, both lenses have been zoomed to their shortest focal length and are equipped with protector filters. The protector filter for the Olympus lens is significantly smaller than the one for the Nikon lens (62 vs. 77 mm), in spite of the Olympus lens being faster. When the two lenses are zoomed to their highest focal lengths, the difference in physical length is even higher (as expected from the higher zoom ratio of the Nikon lens).

Function ring

This lens has dedicated zoom and focus rings, plus a configurable function ring. I find it natural to use the function ring as an aperture ring, since this allows me to change the aperture without moving my right hand away from the shutter button.

On the Z8, the function ring configuration is lens-specific, i.e. it can be configured with a different function for each lens equipped with this ring. For example, theNikon Z 180-600 mm is also equipped with a function ring. However, this lens does not have a dedicated focus ring. On this lens, I chose to configure the function ring as a focus ring, since I cannot rely exclusively on AF in wildlife and bird photography and I frequently need to manually override AF by turning the focus ring.

Image quality

The following samples were shot with the 24-120 at 24, 50 and 120 mm focal lengths (as indicated on the zoom ring), with the lens fully open at f/4, 200 ISO and exposure varying between 1/1,500 and 1/3,000 s because of rapidly moving clouds. In each pair of images, the first image is the whole image reduced to 900 x 600 pixels for display, and the second a 1:1 pixel crop near the center of the image.

The above figures show that, already fully open, the 24-120 displays excellent resolution at all focal lengths in the center of the frame.

The above figure is a 1:1 pixel crop of the extreme lower left corner of the image at 24 mm focal length shown in Figure 4. Image resolution is slightly worse than in the center, but still fully acceptable. On the other hand, this part of the subject is about 30 m closer to the camera than the subject at the center of the image, so this part of subject at the extreme corner might be slightly out of focus. I do not show here the 1:1 pixel crops at 50 and 120 mm, but I can say that they are slightly better than at 24 mm (as expected).

Shortly after I wrote this page, the 24-120 was voted in a Digital Camera World poll the "best Nikon lens of all time".

Conclusions

The Nikkor Z 24-120 mm f/4 S zoom is a medium zoom with a focal length range that extends a bit more into the telephoto range than typical medium zooms like the various 24-70 and 28-75 mm S and non-S models.

This lens gives up one stop with respect to typical f/2.8 zooms in return for a zoom ratio of 5, excellent image quality at all focal lengths in the image center and only slightly worse in the extreme corners at 24 mm, a constant lens speed throughout the zoom range, a manageable size and weight, and a reasonable price.

This lens is commonly available on the second-hand market.

The function ring is configurable. On the Z8, the function ring can be configured for a different use on different lens models. On this lens, I use the function ring as an aperture ring.