Same settings, different exposure

<p>I noticed that pictures taken with my AF-S 24-120 f/4G at 85mm are 2/3 to one whole stop dimmer than those taken with my 85mm f/1.8G at the same exposure settings. The meter readings in the D700 are the same. If I set the camera on aperture priority and photograph the same scene in the same lighting, the camera will select the same shutter speed with the two lenses. But the picture will be much dimmer with the zoom, and you can obviously see it on the histograms too in Lightroom after importing and applying identical settings to the two pictures. I know that some lenses transmit more light than others at same aperture settings, but wouldn't the TTL metering compensate for that? And more than 2/3 of a stop difference? That's a little too much, isn't it? Any ideas? Thanks.</p>

<p>The question is which lens gives you the "correct" exposure.</p>

<p>Why don't you try some "sunny 16" situation and leave the lens wide open so that the aperture diaphragm doesn't play any role, and see which one is right (or both are off)?</p>

<p>If you want to test the two lenses you should use the same light, all settings (including WB) in manual and all should be on a sturdy surface such as tripod. Sort of similar to a lab. Having said that, the two lenses can still be off to some degree, but I wouldn't lose any sleep over it - it's OK for all intended purposes.</p>

<p>If you want lenses that have stricter tolerances, and you will pay for it, check out "T" type (stands for light transmission) such as Zeiss, Cook, Leica, etc. Many of these lenses are used in film/video and light transmission consistency between lenses is v. important.</p>

<p>Les</p>

 

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<p>T" type (stands for light transmission) such as Zeiss, Cook, Leica</p>

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<p>T is the trademark for Zeiss multicoating system</p>

<p>Mag, I see you`re kidding... :)<br /> For those confused, I`m afraid Leszek is refering to "T" stop values (a system that take into account any loss of light, because the value is about the <em>transmitted</em> light), in opposition to the more common "f" stops (which are just a measure of the lens <em>aperture</em>, despite of the light the lens loss due to the different factors).</p>

<p>This is helpful I think:<br>

<p>If it was due to the different T-stop, wouldn't the TTL metering compensate for the difference? Meaning if the lens transmits less light at the same aperture, the built in meter would select a slower shutter speed, wouldn't it?</p>

<p>Oliver, yes. My post above was simply to clarify the difference between T and F stops. I agree that this doesn't account for the difference that you are experiencing. I would suggest that you follow Shun's advice, i.e. investigate further.</p>

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<p >Oliver, although you say that "The meter readings in the D700 are the same" you only specifically refer to shutter speed. Are you certain that the Aperture and ISO setting are also the same? If indeed all three exposure parameters are the same it may indicate a malfunction with one of the lenses. I doubt that the F > T difference accounts for that much exposure imbalance. As Shun says "which lens gives you the "correct" exposure”?</p>

<p>compare at f/4 and see. There should be no error at full aperture. The only kind of error possible it the amount of stopping down.</p>

Thanks for the inputs. I'll make

further comparisons when I get

the chance, and maybe i will even

throw the older AF 85mm f/1.8 in

the mix, and compare the zoom with

all my other primes too. But how

would I know which one is

"correct"? Is there an objectve

way of measuring the accuracy of

the exposure when we are

photographing real life scenes as

opposed to gray cards?

<p>A balanced histogram without many blown highlights or lost detail in the blacks.</p>

<p>If you trust the sensitivity of your sensor then taking picture of a gray card should give you the RGB values of 117,117,117. </p>