24-70 vs 70-200 - is the 70-200 darker?

I'm sort of embarrassed to be asking this given the length of time I've been shooting, but I've been having a problem I haven't experienced before with my 24-70 f2.8, think I've resolved it, but just want to confirm one thing. My 24-70 has always been a bit flakey and has been serviced a couple of times, but not for this particular issue.

 

Assuming I shoot at the same f-stop, exposure, ISO in identical lighting conditions, and shoot set at manual (Nikon D4), should the 24-70 image be brighter than the 70-200 f 2.8? The reason I'm asking is because when shooting an event yesterday - again in a consistently lit space - the 24-70 was significantly brighter, to the point of epic overexposure. Same settings for the 70-200 were fine. I really had to crank the settings down way more than I had in the past to get correct exposure, and then could only tell by chimping as the meter still showed I was off the scale underexposed. I cleaned the contacts on the 24-70 and that seems to have resolved the issue, so now the same metering produces a minimal difference between the two lenses, the 24-70 being slightly brighter. It seems to me, the 70-200 being a longer lens barrel, that this would be the case, but just wanted to confirm this.

 

Thanks for your help!

Chris

Theoretically, 70mm f2.8 is 70mm f2.8; you are supposed to get the same exposure from both lenses.

 

I suggest find yourself a "sunny 16" situation and set your D4 to the all-manual M mode. Try both lenses at 70mm f2.8 as well as 70mm f5.6 and perhaps f8, and see which lens gives you better exposure.

 

Since you cannot open those lenses wider than f2.8, I would imagine that it cannot overexpose at wide open.

Ahhh OK, the one thing I didn't do was set them both to the same MM. Will try that. Thanks for this. Was shooting at manual all along - will continue with that. Just seems odd, I'm sure I would've noticed it, though I typically shoot theatrical/stage work in a darkened venue so it may not have been as noticeable as in a more uniformly lit space.

Theoretically, 70mm f2.8 is 70mm f2.8; you are supposed to get the same exposure from both lenses.

Not necessarily since the transmission of the two lenses could be (and most likely is) different. DxoMark gives the transmission of the 24-70/2.8G as 3.2 stops and the 70-200/2.8 VR (first version) as 3.5 stops; this difference should be reflected in the exposure metering and probably be visible in a direct comparison of viewfinder brightness as well. The transmission of the 70-200/2.8 VR II is given as 3.3 stops and hence the difference to the 24-70 is negligible. Transmission can depend on focal length and there is no report as to the focal length that the above numbers refer to. Since 70mm is at the long end of one lens and the short and of the other, it is conceivable that the transmission properties are more different than the numbers suggest. Transmission is measured in the center of the lens so vignetting differences between the lenses can also affect the metering results and the brightness impressions in the viewfinder.

That isn't an uncommon issue but a lot easier to fix these days in post processing than back in the days when colour transparency film was often used. You would think that in theory if you know the correct exposure and are using manual settings that images taken with different lenses at these same exposure settings should be fairly similar in appearance if the light is fairly constant but back in my medium format days I had one lens in particular which always seemed to produce over exposed images compared to the other lenses that I used, taking images of the same scene in the same light. That lens was my 105mm lens on a Pentax 67II, both bought new, the 45mm, 55mm and 200mm lenses all produced images with similar appearance in terms of colour saturation and exposure but unless I remembered to underexpose with 105mm lens its images were always that bit too light for my taste.

While the actual transmission indeed can be different, the OP states epic overexposure. The T/3.2 of the 24-70 f/2.8 may vary across the focal range, but even so, that would in my view still not be in the range of severe overexposure, compared to the 70-200 f/2.8 at t/3.5 (just 1/3rd stop difference all in all).

But the original post is also missing some more concrete figures of the shots, so it's hard to understand whether it's an apples to apples comparison. Testing them side by side with Sunny 16 or a handheld light meter, at identical exposure, using both wide open as well as stopped down shots to compare as Shun suggested is the way to go. Another thing that could be a cause is the aperture of the 24-70 not stopping down well, and this way you would find out for sure.

Vignetting at wider angle settings could easily influence the metering, and is another thing to take into consideration.

There can be factors such as vignetting, transmission differences that lead to slightly different image brightness at equal exposure and ISO settings but these should be really small, certainly not "epic". It would be nice to see a pair of shots which illustrates this difference and the exposure and ISO settings which were used to reach them.

 

It is possible that the aperture control mechanism in the lens is damaged or faulty.

Could it be that softness of one lense compared to the other at same F-stop make it looks brighter ?

The T stop of the 2 lenses could be different. But why not making a test to confirm? If a lens is brighter but not used at maximum aperture it could be the aperture doesn't close fast enough. Sticky diaphram problem?

It is possible that the aperture control mechanism in the lens is damaged or faulty.

Unless the aperture diaphragm fails to open up, comparing them wide open at f2.8 should eliminate that concern. That was why I mentioned testing at f2.8 as well as other apertures. If the aperture doesn't open, it should be quite obvious.

 

Also use the depth of field preview feature to check the aperture opening, especially if those are E lenses that don't have the mechanical aperture control.

And if the concern is that the aperture diaphragms are not closing down properly, you can always set the shutter speed to M, set to various apertures, hold down the shutter release button and then look from the front to see whether the aperture is actually closing down.

I found that T stop and F stop are different on all lenses but not by much. I don't think they are even 1/3 stop