We’ve seen how SNR curves can help us analyze digital camera IQ:
In this post we will use them to help us compare digital cameras, independently of format size.
The graph above shows the noise performance in terms of SNR of the Nikon D610 at base ISO. Raising the ISO does not change the Exposure (the Signal) but because it may raise the gain amplifying the signal in before it is converted to ADUs it can result in effectively lower read noise figures (if you are interested see Emil Martinec’s pages for why). The Exposure at which saturation (or clipping) occurs generally drops about one stop for every stop increase in ISO, except sometimes near base ISO where manufacturers try to eek out all they can from a sensor’s dynamic range.
Here is an estimate of how average channel read noise and saturation vary with changing ISO in the D610, derived from DxOMark.com data:
And here is how they change in the Olympus E-M1
When we plot the relative SNR curves against absolute Exposure in lx-s this is what we get:
The solid lines (red and blue) represent the cameras’ noise (SNR) performance at base ISO. The E-M1’s is identical to DxO’s average Full SNR curve while the D610’s has been normalized to match the smaller format’s 16MP sensor resolution.
The dashed lines show how SNR varies with increasing ISO as a result of these changes: as amplification of the signal in is increased the output starts clipping at a lower Exposure – but at the same time input-referred read noise may be reduced, ameliorating performance in the deep shadows, thus Dynamic Range, as evidenced by the dashed lines in the picture. I haven’t shown ISOs higher than 400 for the D610’s because the change in read noise is minimal after that, as is apparent from the earlier chart.
The red colored band represents the noise performance advantage of the larger frame sensor versus the smaller format’s at base ISO, from highlights to shadows . The dashed lines show that in this case the larger format bests the smaller one throughout the ISO range as well, sporting better SNR and DR at all tonal levels at every ISO.
This graph is especially useful in situations where photographers are able to maximize image IQ by exposing the brightest desirable highlights in the scene just short of clipping (sometimes referred to as exposing to the right or ETTR) while remaining within their artistic constraints, which are typically dictated by DOF (controlled by the f-number) and motion blur (controlled by shutter speed). Together f-number and shutter speed are the main determinants of Exposure for a given scene. This is often the case in daylight captures and generally true of landscapes off a tripod because exposure time can then be chosen almost arbitrarily (hence the chart’s name).
If on the other hand, due to artistic constraints (say the need for longer DOF and/or shorter exposure time), the brightest desirable highlights result in an exposure a couple of stops short of clipping at base ISO, the top end of the SNR curve would not contain any useful image information and be wasted. In this case the image SNR envelope starting point would effectively be a couple of stops down the curve, as shown for the FF camera in the figure below. However, that would also mean that there are two stops of worthless highlight ‘headroom’ which could be used to the photographer’s advantage by raising ISO – if doing so resulted in effectively lower read noise. We know that for every stop we raise ISO we lose about a stop of (in this case unneeded) DR in the highlights – but we may also gain increased SNR in the deep shadows, hence better noise performance and extended DR. This combined effect is shown in the figure below: the sensor saturates at an Exposure two stops lower than before but by raising the ISO two stops from base, noise performance in the shadows has been improved.
As a result of the lower Exposure (say due to two stops less aperture with the same shutter speed) and the correspondingly higher ISO, top SNR for the FF camera has dropped to a value of 150 from 249 and eDR to 12.7 stops from 13.8.
The fact that Exposure is now two stops short of the ideal for IQ (ETTR) does not mean that the camera is limited to it. All photographers need to do is relax one of their constraints related to f-number and/or shutter speed and select the appropriate ISO to get back to ETTR and max IQ – artistic requirements permitting.
Incidentally the situation depicted above is classic Equivalence, where shutter speed is held firm assuming it provided just enough motion freezing at base ISO – and the f-number dialed into the full frame camera is doubled (say from f/4 to f/8) in order to provide the desired depth of field, equivalent to f/4 on a format with sensor diameter half its size like mFT. In this case you can see that the larger format still comes out on top both in terms of SNR and DR throughout the tonal range. In fact in practice (relative to Photographic Dynamic Range) it would still hold its own if constraints forced it to have 2 stops less maximum Exposure than the shown mFT camera at base ISO.
This is one of the reasons why a larger format will typically provide better IQ in landscape captures than a smaller one – and not just noise IQ, as we will see in future posts.