Wednesday, 24 June 2015

Canon 7D mk 2 - High ISO and noise suppression.

This last week I came upon a situation where I had to use high ISO settings - actually very high ISO settings on my Canon 7D mk 2.
I had found a family of stoats - 5 pups of various sizes and an adult. Most of the time the pups were on a ledge that was deeply shaded - think of a ledge under a bridge and you will have a reasonable picture of the situation. I was pretty close - 7-8 yards max.

Normally I use ISO 400 but the shutter speeds I was getting was not allowing me to get sharp shots (subjects very fidgety) and the depth of field was very short. As a consequence I upped the ISO to 3200 and the F to 6.3.
However despite a shutter speed of 1/800th sec the shots still looked very soft on the back of the camera.....and even more disappointing they looked soft on my computer monitor.

As I was so close to the stoats the image after cropping was still ~4000 pixels so reducing the pixel count to 1800-2000 for posting on Flickr improved the apparent sharpness a little but overall I was pretty disappointed.

Then I noticed the "Threshold".

I shoot in raw and normal do the first part of the processing in DPP as my version of Photoshop Elements (version 10) does not recognise CR2 and anyway I have always found it easier to get the colour "right" using DPP than Photoshop.
Since I normally shoot at ISO 250-400 I have up to now ignored the "Threshold" setting DPP automatically applies in the "Unsharp mask" option as it ranges between 1 and 2.  However when using ISO 3200 the default setting is 6 so I checked what effect different "Thresholds" would have on the noise and sharpness/crispness of the shots. It was an eye opener.

The shot I experimented on was this one:

Whilst this looks reasonable you have to remember this is a 3500 pixel shot reduced to 800 for the blog so hardly a test of how sharp it is.

What I did was crop into a small portion of the above picture to produce an image around 1100 pixels wide. I then processed the image at different thresholds (everything else being constant) and finally reduced the image to 800 pixels (I can't imagine me using a 100% crop). Here are the results:-

Threshoold 1

Threshold 2

Threshold 4

Threshold 6

What is clear is that as the Threshold increases the noise decreases markedly (look at the background top right) but the shot becomes progressively softer (look at the eye) and at 6 it's so soft it looks out of focus.

I then went back to the original picture and after a bit of experimentation found that using a threshold of 2  plus the normal downstream processing and pixel count reduction (to 2000 pixels) gave the best (in my opinion) overall image in terns of sharpness and noise control.

I suppose it would be interesting to experiment with some specialist noise reduction software and see whether I could get away with threshold 1 (or even zero) but since I don't own any such  software that will remain an academic point.

So in conclusion if you are using high ISO on the Canon 7D mk 2 beware of the threshold the DPP software applies - your pictures may be a lot sharper than you think. I assume those of you who shoot in JPEG you will be stuck with the high threshold value DPP applies and consequently softer images though if someone out there knows differently please post a comment.

1 comment:

  1. Useful information Steve, and some lovely pictures of the Stoat kits.