ISO Review: Capture the Night Sky with the ISO-less Nikon D750

The clear night sky under a new moon is almost always the same brightness in the same location. I have blindly put my camera on ISO 6400 as a result. But after having read somewhere that Sony supposedly builds sensors that are ISO invariant, I wanted to test this claim with my own Sony-equipped Nikon D750.

The Nikon D750 is already my favorite camera to date for landscape astrophotography (I like to call them nightscapes). A full-frame DSLR that sports a tilting screen? Check. Large enough pixels (photosites) that help gather enough light? Check. But what exactly is ISO-invariance and how does it help to create better images?

The Sony sensor and some disclosure

The Sony-built full-frame CMOS sensor of the Nikon D750 scores 932 points on Senscore and 93 on DxOMark, taking in things such as bit depth (14), sensor size, photosite size (6 μm) and noise.

According to this Ukrainian website, the actual sensor type is the Sony IMX-128-(L)-AQP, the same one as in both the D600 and D610. I’ve owned both of those camera bodies. I traded in the oil-spot ridden D600 for a D610, and since the step up to the D750 meant a higher score and potentially less noisy nightscape images due to a newer design, I didn’t hesitate to trade that D610 in for the swivel screened bigger brother. It’s actually smaller and lighter.

I’m not affiliated with Nikon or Sony. I have shot with Canon cameras before Nikon started to produce the D600.

That's the International Space Station hurtling across the sky.

ISO-invariance

It basically means that it doesn’t matter if you shoot at ISO 6400 or ISO 200 and boost the image +5EV in post. That’s sliding the exposure slider in Lightroom or Camera Raw all the way to the right on a way underexposed image and getting the same results. A pretty hefty claim. In this regard, photographers refer to cameras with these type of sensors as being ISO-less.

Do be aware that the amount of ISO invariance is not a measure of how good a particular camera or sensor is. I just means that a lot of the exposure is generated with the image processor rather than in the sensor. That being said, an ISO-less camera does have some benefits and some drawbacks. But we will get to that in a surprising conclusion.

But let’s put that to the test and see some differences if there are any, shall we? Warning. Pixel peeping ahead.

The Test

This following test shot isn’t pretending to be an example of outstanding photography. I chose the following composition in order to capture both harsh highlights and clipped shadows, along with a light polluted part of the Milky Way. Let’s see how much can be recovered at six ISO speeds, ranging from 200 to 6400.

Nikon D750 – Samyang 12mm f/2.8 Fisheye - 30 seconds at f/2.8

And at different ISO's. You might think that it was shot at different white balance settings, but that isn't the case.

Here are the basic settings at which I post-processed all six images: (Save for the exposure setting)

Unsharpened and without noise reduction, a crop of the Milky Way looks like this:

Notice how different the color is being rendered. This is the same white balance setting. At ISO 800, there’s a clear green cast to the image, while ISO 6400 suffers from an extreme magenta cast. To me, ISO 1600 is the winner when it comes to a more or less neutral white balance.

Also, I personally like to render the night sky blue. But the color of the night isn’t blue at all. The Milky Way is way more brown than blue. In fact, most of the stars in the observable universe are tinged some sort of red as opposed to blue. If you want to be true to the night sky, make sure you select daylight as a white balance setting.

Let’s see how they compare when we dial in these detail settings in Lightroom.

A tiny bit of sharpening gets rid of the loss in sharpness after raw conversion.

For this, I set the radius to an amount of 0,5 and slide detail up to the point where it still is acceptable. Note that this isn’t the final sharpening I apply to my night sky images. When I export the images to Photoshop, that’s where the magic truly happens. Everything I do in Lightroom is just to give me enough to work with in Photoshop.

At this stage it becomes much more apparent that ISO 1600 is also the setting that generates less color contrast between individual stars. Some red stars look bland and desaturated. Either ISO 200, 400, or 6400 appear to contain more color information. If that seems weird to you, we can shake hands. I think it has to do with the math involved, but I already have trouble dividing 6 by 3. Ok it isn’t that bad, but just to illustrate I’m no mathematician.

Shadow detail recovery

In landscape astrophotography, it’s not only important to have a stunning night sky, but detail in the shadows of the foreground can tell much more of where the shot was composed. This next comparison therefore shows the bottom left corner of the frame, in which there’s pure black that gradates into the dark midtones.

In the above image we can see that ISO 400 and 6400 are more magenta than the other four. ISO 200 contains lots of color mottle. It’s like the image processor had a hard time figuring out which color a certain pixel would need to be. While there isn’t a lot of detail in ISO 1600, this setting does look pleasing to the eye as the details look finer. It’s also a bit darker than the others. ISO 3200 and 6400 look pretty similar here, with 3200 containing more detail than the highest ISO setting I’ve tested with.

Highlight recovery

This is the most surprising aspect of the test. Let’s me show and tell you why.

If we disregard the water rippling across the different ISOs for a minute, we can start to see that the recovered highlight detail is pretty similar on ISO 200, 400, and 800. But from ISO 1600 onwards, the distant sodium lamps get truly blown. The highlights are unrecoverable here. That also means that there’s less information to work with in your Milky Way crops, although that isn’t apparent right off the bat.

Summary of Findings

After all this pixel peeping, I’m failing to see the bigger picture. It’s a good idea to take a step back and summarize what we found in the tests before.

• I like the white balance of ISO 1600, despite the WB-setting being the same on all ISOs.
• There’s more color detail in ISO 200, 400, 3200, and 6400 than in the others.
• The dynamic range in the lower three ISO shots is much higher than in the higher three shots. This becomes particularly visible when comparing very bright subjects.
• At ISO 1600, the shadows are darker. But there’s more fine detail in the darkest of shadows than at other ISOs.

Once the data isn’t there to begin with, you cannot recover it. That goes for both shadows and highlights. As with all modern Nikon and Sony cameras, it’s vitally important that highlights are kept in check; not just in night photography. There’s an insane amount of detail that can be recovered in the shadows.

This is ISO 200 (before) (+5EV in Lightroom) compared to ISO 6400 (after).

Comparing ISO 200 to 6400 again. Here we can see how the highlights are blown in the latter.

Conclusion: At Which ISO Should you Shoot Nightscapes with your D750?

I have this custom function set on my camera that’s designed to let as much light in as possible, without overexposing a light polluted night sky. When I turn the dial to U1, the camera sets the shutter speed to 25 seconds (perfect for shots at 14mm), the aperture to f/2.8, and the ISO to 6400. But after this test, that’s going to change.

Is the Nikon D750 ISO invariant? Mostly so, yes. However, there are important differences between photos shot at different ISO settings. Considering all of the images, I’m certainly not continuing to shoot at ISO 6400. I’m still on the fence deciding if ISO 200 or ISO 1600 are the best to retain the most luminosity and color detail from my night images.

Tell me what you think. Could you help me decide?