D5200 and poor t-stop performance?

PaulWog · Aug 29, 2014

I was taking a look over at DXO mark, and I noticed that the D5200 gets about 0.2 T-stops less light transmission than a lot of other cameras. The D5300, D5000, D3200, D7100 and many other cameras with similar sensors all perform at a superior t-stop level. I was looking at the 18-35mm 1.8 Art Sigma lens, and I saw that it performs at a 2.2 Tstop level, whereas it's at 2Tstops with every other camera. I looked at the 50mm 1.8G, and it's reported at 2.3Tstop for the D5200, and 2.0Tstop for other cameras. The 24-70 f2.8 reports 3.7 Tstop for the D5200, and 3.4 for the D7100.

I would like to think that the D5200 doesn't have some light transmission issues. I hope that this is just due to DXO mark's copy of the D5200.

Does anyone have some insight into the matter?

Tests and reviews for the lens Sigma 18-35mm F1.8 DC HSM A Nikon mounted on Nikon D7100 - DxOMark

Derrel · Aug 29, 2014

Have they normalized things for the ACTUAL, real, genuine ISO levels for each camera, or is the number an absolute, and not 'normalized' or "corrected" number? I do not know the answer, but it appears to be 1/3 EV off; ISO levels are often deliberately fudged by the camera manufacturers, which can result in lower reported noise figures; if for example, ISO 6,400 is ACTUALLY ISO 5,000, and the noise performance is lower as a result, then a camera appears to be testing out better than a camera that has dead-on accurate, legit ISO 6,4000 at the 6,400 setting.

It could be due to better sensor technology, or better, more-efficient electronics in the more-expensive cameras. Even if the issue is real, is it actually "significant"? I mean, you can drive 65 miles per hour, or drive a bit slower or faster than 65 MPH, and the actual time needed to go 10 miles in infinitesimally different. Again, it might be a case of ISO miscalibration. But the thing is...if a lens's diaphragm is even the SLIGHTEST bit out of calibration, f/8 can easily be f/7.1. If the metering system on a camera is tuned ever so slightly differently, exposures can vary from one camera to another by one-third to one-half to maybe even one full EV...so...again, is this even real-world relevant?

PaulWog · Aug 29, 2014

Thanks Derrel. I understand what you're saying, and I am in complete agreement. Am I then ignorant for still wondering how they came to their numbers on the D5200? With everything you've stated taken into consideration, it's still baffling. I feel somewhat safe assuming something was out of alignment (user error or hardware on the D5200 just a little off), since the D5000, D5300, D7100, and D3200 all perform better than the D5200 across the board (consistently and somewhat significantly) according to their TStop readings.

I know it doesn't matter a whole lot in terms of real-world practicality. I love nitpicking through things though.

unpopular · Aug 30, 2014

If it's 1/3 off, that's a heck of a lot less than the optimism of film manufacturers.

But yes. The problem is you can't really "make" digital cameras ISO compliant since the standard is based on optical density.

I noticed on the pro video cameras I will be using this semester that gain (which is what ISO is essentially) is actually stated in decibels, which kind of makes more sense from a technical stand point.

KmH · Aug 30, 2014

T-stop only refers to how much light a lens transmits.

F-number - Wikipedia, the free encyclopedia
T-stop

A T-stop (for Transmission-stops) is an f-number adjusted to account for light transmission efficiency (transmittance). A lens with a T-stop of N projects an image of the same brightness as an ideal lens with 100% transmittance and an f-number of N. For example, an f/2.0 lens with transmittance of 75% has a T-stop of 2.3. Since real lenses have transmittances of less than 100%, a lens's T-stop is always greater than its f-number.[SUP][6][/SUP]
Lens transmittances of 60%–90% are typical,[SUP][7][/SUP] so T-stops are sometimes used instead of f-numbers to more accurately determine exposure, particularly when using external light meters.[SUP][8][/SUP] T-stops are often used in cinematography, where many images are seen in rapid succession and even small changes in exposure will be noticeable. Cinema camera lenses are typically calibrated in T-stops instead of F-numbers. In still photography, without the need for rigorous consistency of all lenses and cameras used, slight differences in exposure are less important.