Why are images from FujiFilm X-T20 darker than those from Canon 400D?

Question

I’ve just bought a mirrorless FujiFilm X-T20. In my first tests, it seem to ask for quite high ISO even in daytime indoor. So, as a test, I compared it with my previous camera, digital reflex Canon 400D.

With the same configuration, old Canon seems about +1 brighter than FujiFilm. How is it possibile?

First picture is from Canon, then FujiFilm. Both are shot with:

• f/4.0
• 1/15 sec
• ISO 1660
• Exposure Bias 0

About the cameras:

• Canon lens is ES 28mm f/2.8
• Canon raw size is 3888x2592
• FujiFilm lens is XF 18-55mm f/2.8-4 and the shot is made at 25.4mm
• FujiFilm raw size is 6000x4000

Canon

FujiFilm

Answer 1

The difference you are seeing is likely caused by a combination of effects from using different bodies and lenses. To compare lenses, use the same camera body. To compare camera bodies, use the same lens.

• Canon and FujiFilm cameras do produce images with slightly different exposures at the same settings. According to the Wikipedia article on Film Speed, camera manufacturers can choose from "five different techniques for determining the exposure index rating at each sensitivity setting". FujiFilm is reported to use the SOS method. Canon apparently uses the REI method.

  Here are some test images taken on Canon and FujiFilm bodies with the same lens and exposure settings. I estimate about a 2/3-stop difference.

  

• Lenses can transmit different amounts of light, despite using the same aperture setting. Here are two images taken with different lenses using the same camera body and settings. Again, there is about a 2/3-stop difference.

  

I don't have the Canon body anymore, so cannot test using the brighter lens on Canon vs the darker one on FujiFilm. But I'd expect a 1.3-stop difference. To determine the specific contributions of various factors to the results you're seeing, you'll have to further test your equipment.

If it matters enough to you to do such testing, an EF-FX adapter will allow you to use Canon lenses on the FujiFilm body. You'll have to focus manually and use a hack to set the aperture. STM lenses and some USM lenses won't be able to focus. You could also use manual lenses with adapters for both EF and FX mounts. Options include Nikon F, Pentax K, and M42.

Unlikely possibilities:

• Differences in lighting. You took the test shot after noticing the ISO difference between the cameras. It's unlikely that lighting just happened to decrease every time you used the FujiFilm camera, but not the Canon camera.

• Auto Lighting Optimizer (ALO). ALO works by reducing image contrast. A similar effect can be achieved on a FujiFilm X-Series camera by adjusting highlights and shadows settings. Your Canon test image does not look like an ALO-adjusted image. It does look like exposure was increased.

Answer 2

In addition to all the other great answers I'd like to add that the Fuji x-trans sensor used in your cameras has been known to not really meet the ISO standard. Here is an excerpt from DPReview's review of the Fuji X-T1 (which also uses an x-trans sensor):

By our tests, the X-T1's measured sensitivities are around 1/2 - 2/3EV lower than marked, which is unusual for a modern camera. This means that for any given light level, the X-T1 has to use a significantly slower shutter speed, brighter aperture or higher ISO to get an image of the same brightness as an accurately-rated camera.

It's unusual to see this sort of discrepancy and we're disappointed that Fujifilm persists with a system that, while technically compliant with the ISO standard, ends up appearing rather disingenuous.

DPReview Review of Fuji X-T1' ISO Accuracy

Answer 3

I'm not sure if this is actually the problem, but this is probably one thing that is affecting the brightness.

Try looking at the T-stops of the lenses. (T-stop is like f-stop, but, tells how much light is transmitted through the glass, f-stop only takes into account aperture size). Your zoom lens may let less light through at the same aperture as your prime solely due to the number of glass elements in the lens (the more glass, the more light is lost).

That is assuming that the ISO for each camera means the same thing. If the fuji sensor is less sensitive, all ISO means is how much gain you are applying to the sensor data so if you get less sensor data (like from a different or smaller sensor) it will be darker even at the same multiplication level.

Answer 4

Your Canon has not exactly the same configuration. I suspect it to have the auto lightning optimizer on. You have the possible magnitudes: off/low/standard/high. And by default it is on standard.

Try to set it to off and you should have the same results. Try also to set the white balance to manual and to the same tuning, so you can have closest colors. You might have still some differences due to how each manufacturer is processing colors, and how the lens is processing the color.

[1] http://cpn.canon-europe.com/content/education/infobank/digital_camera_features/auto_lighting_optimizer.do

[2] https://digital-photography-school.com/what-is-canons-auto-lighting-optimizer/

Answer 5

There are at least two likely sources that are contributing to the variation you see.

• The light coming in your window could be variable if the atmospheric conditions at your location are changing between the two shots. Clouds moving across the sky, for instance, can significantly influence both the brightness and the color of sunlight coming through a window.
• The different ways each camera is capturing and processing the light to which it is exposed. Each sensor has its own sensitivity curves. Actual vs, rated ISO settings can vary significantly from one model to the next. To a usually much lesser extent, so can actual vs. target aperture values and shutter times. The way each camera calculates automatic white balance, automatic contrast adjustment, saturation, etc. will all affect the final outcome. Depending on the contents of a scene and the nature of the light illuminating it, just changing the color temperature/WB of the same raw file can affect the resulting brightness of an image.

In the case of your Canon 400D, the differences between using the "Standard" (slightly oversaturated and slightly too contrasty) Picture style with 'Auto Lighting Optimizer' turned on compared to using the "Neutral" Picture Style with 'ALO' turned off will almost certainly result in greater differences than those seen in your example images comparing your result with the 400D vs. the Fuji X-T20. One could post process the raw data collected by either camera and get a final result very, very close to the what you got with the other camera above.

Another possible source of the difference is that even though both lenses are set at f/4, one may have a higher transmissivity than the other. The f-number is an expression of the ratio between the diameter of the entrance pupil and the focal length. This is very useful for calculating/predicting the effect of a specific aperture setting on depth of field. The f-ratio does not take into account how much of the light passing through the lens is lost due to reflection and/or absorbtion. What we call 'T-stop' is a measure of how much light is actually allowed to pass through the lens. This is more useful for calculating exposure, but sometimes doesn't tell us as much as we want to know regarding depth of field. Most conventional lenses have a relatively small difference between the f-number and T-stop at any particular aperture setting, but there are some notable edge cases where the f-number and T-stop can be several stops different.

Answer 6

Canon uses the Bayer matrix, which is 2x2 photosites per pixel, while Fujifilm X-T2, X-T20, X-E3 etc. use the X-Trans matrix, which is 3x3 photosites. The smaller the photosite, the less light the sensor gets, so pictures tend to be darker. That's why when I had to choose, I bought a X-T1 (16mpx) rather than the X-T2 (24mpx), for the larger photosites (and enough resolution for my needs, either).