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I own a Nikon D500 DSLR, with a Nikkor 16-80 mm 1:2,8-4E ED VR lens.

I noticed that this lens, when applying exactly the same settings (same ISO, same aperture, for instance F8.0, same Shutter speed, for instance 1/800, same white balance, etc...), generates darker photos than other DX lenses I own, such as Nikkor 18-105mm f/3.5-5.6G ED VR, of course using the same camera body.

In order to obtain the same exposure I need to increase the ISO sensitivity or change the applied aperture/shutter speed.

The fact that one lens seems to be "darker" is also reflected by the in-camera exposimeter when mounting these two different lenses and the same behaviour was observed when trying exactly the same lenses on a Nikon D3200 camera.

Why is this happening? Is this due to a different T-stop? How can I know which is the T-stop value for the two lenses? By the way, shouldn't the Nikkor 16-80 mm be better than Nikkor 18-105mm also in terms of T-stop (as it seems from different reviews I read)?


Edit: Here are two sample images I have just taken out of my window, using the settings mentioned before. They were taken under the same conditions and to the same scene, with sun illuminating it. I took the first picture with 16-80 mm, then changed the lens, and took the second picture with 18-105 mm, both at 35 mm. (The images has been reduced in size to allow the upload here)

16-80 mm vs 18-105 mm

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    How much darker? A little or a lot? – mattdm Apr 20 at 13:13
  • @mattdm Not a lot, in my opinion, but enough to be clearly noticeable, especially when comparing different pictures side-by-side. I just edited my question adding two sample images I just took using the two lenses. – es483 Apr 20 at 14:08
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    @scottbb Yes, I'm actually shooting at the same scene, setting both lenses to shoot at the same focal length. I just edited my question adding two sample images I just took, during a sunny day, both at 35 mm and with less than 2 minutes passing between them. Thank you very much in advance! – es483 Apr 20 at 14:09
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    You didn't mention if you are using the same ND/polarising filter on both lenses, if you're using one as protection. – Andrew Morton Apr 21 at 19:07
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Although the transmissivity of the lenses might explain this difference, part of it could also be due to the possibility that the 16-80's electronic aperture mechanism might be miscalibrated. I do not know if this lens's aperture mechanism has a greater or lesser tendency to be miscalibrated, but I assume the possibility is not zero.

I agree with you that the 16-80's T-stop should not be that much worse than the 18-105's. The difference in number of elements/groups is not huge, and if anything the pro lens should have better coatings. The difference in EV in your sample photos is something like 1/2 to 2/3 stop. That would indicate an unacceptably low T-stop for the pro lens.

If the lens is still under warranty, you could have Nikon take a look and adjust it, if necessary, for no charge.

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    Good instinct, but I'd be inclined to go the other way: rather than look for miscalibration of the electronic aperture of the 16-80mm lens, it's possible the accuracy and/or repeatability of the mechanically-linked aperture of the 18-105mm is questionable. It's well-known that Nikon's mechanical aperture linkage allows for a lot of exposure variability (try to make a time lapse using a non-E Nikon lens — the shot-to-shot exposure variation is much worse than any electronic aperture lens). Luckily, this is easy for the OP to test: just take several (10 or so) shots of the same scene... – scottbb Apr 20 at 17:08
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    ... with the same exposure, with both lenses, and compare the exposure variability between lenses. But nevertheless, very good answer, great thinking! =) – scottbb Apr 20 at 17:09
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    This is the better answer; although T-stops are a thing, there shouldn't be this much difference between those two lenses. It may be possible to see the difference in aperture by looking into the lenses -- given that the zoom is set to the same focal length, the iris formed by the aperture blades should be the same physical diameter if everything is working properly. The difference might be too small to notice, but your images have quite a large difference in exposure -- it would be worth checking, it could be pretty obvious. Nice question @es483 – jkf Apr 20 at 18:42
  • To check aperture calibration, use spot meter wide open and calculate values for stopped down apertures. Then use spot meter to compare with calculated values. – xiota Apr 21 at 0:24
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    I'd venture to guess that the 16-80mm lens' electronic aperture is probably the more accurate one, and that the mechanical aperture linkage may not be stopping the 18-105mm lens down as much as it is supposed to be doing. Mechanical aperture linkages for both the camera and lens must be periodically checked and calibrated as they are used, due to things like mechanical wear or adjustment screws becoming looser. – Michael C Apr 21 at 5:24
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We depend on the accuracy of our camera settings in anticipation that a “correct” exposure will result. In modern times, built-in metering and chip logic all but guarantee a good outcome. I think this is remarkable because “correct” exposure is a path laden with pitfalls. We place dependence on the f-number markings and shutter speed settings along with ISO values. We will be in luck if all these setting plus the meter reading deliver as promised. Sorry to report that often there is no joy in Mudville.

For most lenses, the f-number settings are derived using a modest math formula. We divide the focal length of the lens by the working diameter to compute the f-number. The f-number is supposed to be universal. In other words, we set our lens to f/8 in the belief that it will pass onto film or digital sensor, the same amount of light energy as any other lens set to this same aperture. Again, sorry to report that all too often, the resulting exposures will not match.

Lens setting inaccuracy is too much for the cinematography industry. Shooting a single scene can cost millions so reputations are at stake. This industry chose to upgrade to the T-stop. This is a super accurate f-stop based on an actual measure of the light energy that traverses the lens.

Why would the f-stop be inaccurate? It is derived from the ratio of the focal length to the working diameter. It does not take into account: A. loss of light due to the fact that the glass lenses are not perfect as to transparency. B. Each lens surface is polished, thus some light is lost due to surface reflections. C. Light rays that just brush by the blades of the iris become misdirected. D. Stray rays due to uncorrected lens aberrations miss their mark. E. Other interfering not cited.

Some still camera lenses are calibrated via the T-stop method. It is a mystery to me, why all high-end camera lenses use the f-stop as opposed to the T-stop.

  • Depth of field is still related to the f-stop, not the T-stop (more specifically to the absolute diameter of the aperture). – chrylis -on strike- Apr 20 at 16:34
  • I read that in some bridge cameras with long zooms, small "real" apertures (f/16+) aren't possible due to diffraction, and small apertures are simulated with a built-in ND filter. So in essence they are using T-stops :) – xenoid Apr 20 at 17:42
  • @xenoid many camcorders also use built-in ND filters (sometimes a filter wheel or linearly-translating filter with 2 or more ND filter values for more light control) – scottbb Apr 20 at 20:58
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    @ chrylis -- The difference in working diameter of a lens based on f-stops or T-stops will be tiny. The difference this makes in the length of the zone of depth-of-field is also small. Depth-of-field is also very subjective, the way we calculate is base on lots of assumptions that may or may not be correct. Thus its a moot point. – Alan Marcus Apr 20 at 20:58
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(This answer is based on the assumption that you are not using different "protective" UV filters, ND filters, Polarizing filters, or any other type of filters on either lens. If you have different filters on each lens, it should be rather obvious where the differences are mostly coming from.)

Why is a lens darker than other ones when applying the same settings?

The most likely explanation is that the 18-105mm lens with mechanical aperture control is incorrectly exposing lighter than the 16-80mm lens with electronic aperture control.

The difference is subtle, but significant.

That is to say, the electronically controlled aperture of the 16-80mm lens is probably giving you more accurate exposure than the mechanically controlled aperture of the 18-105mm lens.

If this is happening with all of your DX lenses, then the issue is most probably in the camera's mechanical aperture linkage, rather than in the linkages of the DX lenses. If it is also happening with other camera bodies, then chalk it up to the general differences between mechanical aperture control and electronic aperture control. Or maybe the linkage on your friend's D3200 is as worn or has been bent by about the same amount as your D500.

A little background¹

When AF technology began emerging in the late 1980s, Nikon attempted to create a system that would allow old F mount lenses all the way back to the late 1950s to remain usable as manually focused lenses on the new AF capable bodies. They chose to place the focus motor in the camera where it drove the focus elements in the lens via a mechanical linkage, rather than place the focus motor in the lens. In addition, they chose to retain the mechanical linkage between the camera and lens to control the aperture and associated metering so that it would be backwards compatible with older F-mount lenses. Pentax took this approach as well.

A couple of other major camera manufacturers chose to make a clean break and create a new lens mount system with an all electronic connection between the camera and lens and to place the focus motor in the lens. Minolta introduced a new 'A-mount' with an all electronic system in 1985 (this eventually became the Sony A-mount after Sony bought Minolta). Canon introduced the similar EOS system in 1987. Neither system allowed users to use previous lenses in older mounts bought from Minolta or Canon, respectively, with the new cameras that used the new mounts. Early on, Nikon gained market share by making their new AF cameras and lenses backwards compatible with existing F-mount cameras and lenses.¹

For most of the period since Minolta (1985) and Canon (1987) introduced camera systems with an all electronic mount, Pentax and Nikon have gradually introduced electronic connections to their existing mount systems in several piecemeal stages. Pentax did so sooner and more aggressively than Nikon.

Soon, the new "Ultra-Sonic Motor" design Canon used on all but their low end lenses proved to be far superior in terms of autofocus speed and accuracy when compared to the mechanical linkage that Nikon, Pentax, and others used. Almost overnight Canon captured much of the professional 35mm market that Nikon had dominated for decades, particularly among those who shot sports/action. In order to remain competitive, in the middle 1990s Nikon added electrical contacts to their F-mount system and began creating AF-I lenses with motors inside them for large telephoto lenses that require heavier focus elements. AF-S lenses with AF motors that were designed very similarly to Canon's ring type USM didn't appear until 1998. Nikon continued to place AF motors in their bodies as well to drive the existing AF lenses that lacked their own motor. (Only with the introduction of entry level digital SLRs did Nikon introduce AF era F-mount bodies that did not have AF motors in the camera. The D3xxx and D5xxx bodies can only AF with AF-S lenses or the even newer AF-P lenses.)

But Nikon continued to offer only mechanically controlled apertures in all of their lenses until well into the 21st century.

Other than a few Perspective Control (tilt/shift) lenses introduced in 2008, Nikon did not offer an F-mount lens with an electronically controlled aperture until the AF-S 800mm f/5.6E VR in 2012. Several other high end (and expensive) 'E' lenses followed.

The AF-S 16-80mm f/2.8-4E Dx VR was the first 'E' lens from Nikon that did not cost upwards of around $2,000. It was rolled out in the second half of 2016, around thirty years after the first mass-consumer lenses with electronically controlled apertures. In the intervening years several other new mounts/systems had also been introduced that use only electronic, rather than mechanical, communication between the camera and lens. Among them: the Four Thirds and Micro FourThirds system from a consortium formed by Olympus and Panasonic, Sony's E-mount, Fuji's X-mount, Samsung's NX mount (now defunct), and even the compact Nikon 1/CX mount (also now defunct) announced in 2011.

As cameras that utilize all electronic camera/lens communication began being used for purposed not even dreamed of in the mid-1980s, the advantages of electronically controlled apertures became more and more apparent over the three decades between the mid-1980s and the mid-2010s:

  • Faster actuation. The servos used in electronic lenses are more compact and there is significantly less total slack in the system. With no return springs, the servos can also open the aperture after the exposure as fast as it was stopped down.
  • Less susceptibility from very cold temperatures slowing stop-down immediately before an image is captured.
  • Better shot-to-shot accuracy when both systems are new and properly adjusted.
  • No need to periodically test and adjust the linkage mechanisms on both the camera and each lens as they wear and/or as adjustment screws loosen.
  • Lack of susceptibility to the mechanical linkage being bent when the lens is attached to the camera. If the camera's lever is bent, it will be inaccurate with all mechanically controlled lenses used with the camera. This usually manifests itself with overexposure.

T-Stop Differences

There's also the possibility that 35mm, which seems to be the sweet spot for the 18-105mm lens' f-stop to T-stop ratio when wide open, is also a focal length where the 16-80mm lens may have a larger difference between f-number and T-stop. Even though you are using both lenses at f/8, most lenses tend to "preserve" differences between the specified f-number and the actual amount of light transmitted by a lens as it is stopped down. Lensmakers do this to maintain the distance between each stop in the range of aperture settings. With zoom lenses, it's more common to see differences between f-number and T-stop when the lens is wide open and the focal length is changed.

Here's the transmission profile for the AF-S DX 18-105mm f/3.5-5.6 G ED VR (Orange) and two other Nikon lenses published by DxO Mark (unfortunately, neither DxO nor Imaging Resources have published measurements for the AF-S 16-80mm f/28-4E ED VR):

enter image description here

What we would expect in the upper chart for a "theoretical" 18-105mm f/3.5-5.6 is a line with a more or less constant slope from somewhere slightly darker than T-3.5 on the left to about the same amount of slightly darker than T-5.6 on the right. That's what we do see with the AF-S 24-120mm f/3.5-5.6G IF-ED VR (Blue). There's very little difference between rated f-number and measured T-stop across the entire zoom range for the 24-120mm f/3.5-5.6. But that's not what we get with the 18-105mm.

Note that a few other Nikon DX zoom lenses, such as the AF-S 18-135mm f/3.5-5.6G IF ED (not shown) and the AF-S DX 18-70mm f/3.5-4.5G IF ED (Red) have an almost identical profile compared to the 18-105mm. It seems that with some of the lower cost DX lenses, Nikon is closing the wide open aperture down just a bit at the wider angle focal lengths, perhaps to limit aberrations on the edge of the image field?

Without T-stop measurements for the AF-S DX 16-80mm f/2.8-4E ED VR, it's hard to say if the difference you are experiencing can be attributable to that lens having a higher T-stop value when zoomed to 35mm. It might be interesting to try a similar test using 16-18mm, 50mm, and 70-80mm with each lens to see if the results are the same as at 35mm.

¹ For an even more extensive look at the history of the Nikon F-mount and how it compared to competitors' mounts since the introduction of AF in the 1980s, please see this answer to another question.

² The digital revolution made small increments of exposure variation more of an issue than with film. As time lapse photography and video using cameras primarily designed to make still images became more common, this proved more and more significant.

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As you've noted, the lenses likely allow different amounts of light to pass through, which is related to T-stops. This can be explained by containing a greater number of larger, thicker elements to correct for defects and to allow for the max F2.8 aperture at the wide end.

  • The Nikon AF-S NIKKOR 16-80mm f/2.8-4E DX ED VR SWM IF has 17 elements in 13 groups.

  • The Nikon AF-S DX NIKKOR 18-105mm f/3.5-5.6G ED VR has 15 elements in 11 groups.

There are different ways for lenses to be better than others. Although the 16-80/2.8-4 allows less light through than the 18-105/3.5-5.6 at a given aperture, it has a larger max aperture and can let more light through overall.

If you just want to know the difference between the lenses, you can use the spot meter on your camera. After measuring settings for several light sources and apertures, do some calculations to determine about how many stops difference there are between the lenses.

If you want to calculate T-stops, you can compare with a lens with known T-stop values.

See What is T-number / T-stop?

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