When testing cameras, it is not unusual to see a situation when one camera can produce results a bit darker or brighter than another. In some cases, lenses are to blame for this variance, since most lenses cannot ideally transmit all of the incoming light. What this means, is that a lens with a maximum aperture of f/2.8 could potentially transmit less light, which could be equivalent to say f/3.5 in terms of brightness. The latter number is what is often referred to as a “T-stop”, or Transmission-stop, which is basically an adjusted f-stop that takes into account this light loss. In other cases, the camera itself can be the source of brightness variance. Although manufacturers are supposed to adhere to an ISO standard that guides the process of determining the right brightness level for each ISO, there is usually still some variance between not only brands, but also between specific camera models. We won’t get into the question of why there are such variances. Instead, we will concentrate on implications of such variances to camera sensor comparisons and ratings. Particularly, we will be looking at exposure variances in Fuji cameras, such as the Fuji X-T1. Many photographers, including myself, have been fond of the way Fuji sensors render images, outputting very clean and pleasant-looking images, even at high ISOs. But are those ISOs real? And is Fuji doing something shady to make its images look better? Let’s take a closer look…
When testing cameras, I am pretty open about showing what camera settings I use for a particular scene. Knowing that light can have a huge impact on the scene and also knowing that the same source of light is practically never perfectly constant in brightness, particularly over a long period of time, I always retest every camera that I provide in comparisons. This way, you are not looking at a scene captured a year ago from one camera and the same scene captured a week ago from another camera. This process takes a long time, but I don’t mind doing it, as I want to only show our readers a more accurate representation of true sensor performance. Due to the above-mentioned variances, making a fair comparison between cameras proved to be rather difficult. How do we compare cameras? Do we compare them at the same ISOs at identical exposure levels? Do we compare them at the same ISOs, but at different exposure levels to match the same brightness? Do we compare them at the same brightness at equivalent ISOs? Or perhaps we should compare them at the same ISOs, but make further adjustments in post-processing to match brightness levels? As you can see, making the proper decision on comparisons is no easy task. So far, I have been comparing sensors based on the same ISO level, but making exposure adjustments to match the same brightness across different cameras. While this method works fine to show differences in noise levels at the same ISOs, differences in exposure can yield to potential issues for such comparisons, as we are not necessarily looking at an apples-to-apples comparison, particularly when variances in exposure are very high.
Fuji X-T1 vs Sony A7 II
Let’s take a look at a rather typical situation involving a Fuji camera (in this case the Fuji X-T1) and the Sony A7 II (Left: Fuji X-T1, Right: Sony A7 II):
The Fuji X-T1 had the Fujinon 35mm f/1.4 lens mounted on it, while the Sony A7 II had the 24-70mm f/4 lens mounted on it. Granted the two might not be equivalent in light transmission, for now we are simply looking at equivalent brightness between these two cameras. At the same exposure settings of ISO 200, f/8 and 2 seconds, it is pretty clear here that the Fuji X-T1 on the left looks significantly darker than the image from the Sony A7 II on the right. This difference in brightness makes it difficult to compare the two cameras properly, as one simply shows no shadow detail.
Let’s take a look at what happens if we compare the two cameras at the same ISO and exposure settings, but after making exposure adjustments in post to see how much variance there is between the two. Here is the same image from Fuji X-T1, but this time adjusted by +0.85 EV in Lightroom:
Now the two look pretty comparable. I tried increasing the exposure on the Fuji X-T1 to higher values, but it resulted in over-exposure. Looking at the JPEG histogram in FastRawViewer and a comparable RAW histogram with exposure adjustments indicated that the above change was more or less accurate in most regions of the image. According to my estimates, the real difference in exposure between these two cameras is roughly 2/3 of a stop – the rest of the difference comes from lens light transmission levels. A similar conclusion can be made from looking at the DPReview Studio Test Scene. I downloaded RAW files from both cameras and compared them in Lightroom with 0.66 EV adjustment on Fuji X-T1. Here are two images compared at ISO 3200:
This result is interesting, because it shows that despite the 0.66 EV boost that I gave to the Fuji X-T1, its images at ISO 3200 actually still look a little cleaner than on the Sony A7 II (and that’s with Sony A7 II images down-sampled to 16 MP). If you look at the files, you can clearly see that the Sony A7 II contains more chroma noise than the X-T1. This essentially shows that even if we were to equalize the X-T1 sensor performance, it still can be a challenge for the A7 II in IQ at high ISOs – this confirms my findings from my Sony A7 II review, which as I have demonstrated, cannot match the Nikon D750’s sensor performance either, although the D750 also sports a 24 MP sensor. But we are not here to discuss the A7 II – we are here to see if Fuji has been cheating with their cameras when it comes to real ISO performance.
Fuji X-T1 vs Nikon D7100
Let’s take a look at another example and compare the Fuji X-T1 with another APS-C camera, the Nikon D7100. Here are images from both at ISO 3200, the X-T1 is again adjusted by +0.85 EV in Lightroom:
Although the X-T1 lacks the details of the D7100 (down-sampling from 24 MP to 16 MP did its job here), the image looks slightly cleaner in comparison. Again, this is because the X-T1 image is showing less chroma noise than the D7100. This is an interesting result, because we essentially have somewhat of an equivalent comparison here, considering that the X-T1 image saw a +0.85 boost in post.
Fuji Real ISO Comparison
Now that we know that Fuji is cheating by underexposing its images somewhere around 2/3 of a stop, let’s take a look at how the actual ISO of the Fuji cameras look like compared to the “simulated” ISOs. Here is a comparison of real ISO 3200 vs Fuji’s underexposed ISO 3200:
As expected, the real ISO 3200 does look noticeably worse than the simulated ISO – the 2/3 of a stop difference that I adjusted in Lightroom shows more noise. Here is ISO 6400 comparison:
Again, it is clear that the results are different – ISO 6400 with 0.66 EV dialed in Lightroom looks comparably worse, showing more chroma noise and introducing some artifacts to the image, particularly in the shadow area.
By now, it is a pretty known fact among the photography community that Fuji underexposes its images by around 2/3 of a stop to a full stop when compared to other cameras when shooting RAW and using most commercial RAW converters. There are all kinds of theories out there, with some people claiming that Fuji does this on purpose to look better, while others attribute this difference to Fuji’s X-Trans sensor and the demosaicing process. I have to say, that despite my attempts to normalize the RAW files by making adjustments, Fuji still looked pretty darn good compared to other cameras. The main source of this is reduced chroma noise, which is evident when you look at RAW files from both the above comparisons and other sources, such as DPReview’s studio comparison tool. That’s the main reason why Fuji looks so clean for an APS-C sensor. I do not know exactly how Fuji achieves this, but the X-Trans sensor and its demosaicing process are probably the reason for the reduced chroma noise we see in images.
Update: Thanks to some awesome people like Iliah Borg, we now know the reason why Fuji RAW files appear darker. Turns out that Fuji has a special tag (0x9650) in its RAW files that highlights the necessary midpoint compensation for RAW files to interpret and make necessary changes. Below are the values for the Fuji X-T1:
ISO 200 / 0.72EV
ISO 400 / 0.72EV
ISO 800 / 0.72EV
ISO 1600 / 0.72EV
ISO 3200 / 1.38EV
ISO 6400 / 2.38EV
So keep the above in mind when looking at Fuji RAW files and comparing them to other cameras. If you are using a RAW converter from Adobe (and potentially other RAW converters), make sure to look at the above table for adjustments needed to make images appear as they should. Big thanks to Iliah Borg and the LibRaw team for discovering the Fuji tags and letting us know!