DSLR Sensor Contrast Range Test

Next, return to the camera suggested exposure (recheck this in case the light levels on the target have changed) and make a second series of exposures but this time increasing the exposure (making the towel lighter each time) by one stop for each shot. Make about five exposures; this sequence will make the towel lighter until it goes into pure white.

Step 3

Open the images in your editing software without making any changes!

If shooting in RAW, do not do anything during the conversion as you want basic info; you want to see the 'pure' images. Once the image files are open carefully check the first exposed image that used the camera indicated exposure. Make sure this is very close to a middle tone value of 128 or %50% opacity. This is to ensure the initial exposure was correct. Now zoom each image to 100% and carefully examine the TEXTURE of the subject. Due to possible unevenness of lighting and camera lens issues just examine the centre portion of the images for greater consistency.

Find the darkest image that just shows the minimum texture. This is your lower limit of exposure for detail. Note down how many exposure stops less this is from the original meter reading, i.e. 3 stops less, or whatever.

Examine the lighter images and again find the one that just retains a little detail. This is your upper limit of exposure for detail. Note the difference in stops from the original reading.

And so...

...at the end of this simple test you will KNOW exactly how dark a subject can be allowed to go before it loses all detail and goes completely black. Also, you will know how light a subject can go before it turns completely white. Note down or remember how many stops there are from the darkest to the lightest textured images; this is the maximum ‘textured’ tone range of your camera. We can refer to this as the ‘texture range’.

OK, let’s say your results show that the last detail in the dark images is the image that received three stops LESS exposure than the camera recommended. Also, that the lightest image to retain minimal detail was the one that received two stops more exposure. You can safely say that the camera has a texture range of 5 stops (which is pretty usual for a DSLR); this is the difference in stops not the number of stops of exposure which would be 6!

Using this knowledge

If you conduct both of the above calibration tests properly you now have enough information about your camera system to accurately and confidently assess the SBR of any subject or scene. Once the SBR has been established, you can decide on your ‘desired exposure’ by asking yourself two questions:

1. How dark or light do I want this subject to be.

and

2. How much detail do I want to have in it?

When I say 'subject' I am referring to a selected area of the whole scene; the area from which you take a meter reading e.g. a shadow or a light area.

Additional possibilities

This method gives you the basic range of your camera sensor without any RAW tweaking in a conversion program. The next step would be to produce exposures above the zone IX or X i.e. to say zone XII or XIII and then use your RAW conversion to try and ‘bring back’ detail into these ‘over-exposed’ zones. This may produce additional zones at the upper end (don’t be surprised if you disprove the idea that you can rescue two or three stops of overexposure though). Bear in mind that any tweaks you make in the RAW converter may affect lower zones too so do check those lower zones.

If all goes well, you may have an extended zone range to work with over and above the basic range of the camera sensor. When this range still isn’t enough for the subject, you will need to use separate but different exposures of the same scene to combine in Photoshop or use a HDR (High Dynamic Range) process.