Friday, October 29, 2021

More thoughts on Multi Image Capture: PART 1 (updated)

Anyone who has read my blog knows that I love playing with technology and especially creating in-camera scripts. In this post, although there is a link to CHDK and Magic Lantern scripting, the ideas I'm discussing can be adopted by anyone with a camera.

Multi Image Capture is used to gather data beyond what is possible in a single image, eg: achieving greater depth of field; covering a larger dynamic range than the sensor can capture in a single image; realising an artistic vision, for example a long exposure; for reducing noise; to eliminate people from your image...etc etc

In other words, multi image capture is an essential tool for all serious photographers.

The two most common multi image capture use cases are, of course, increasing the depth of field and/or covering a large dynamic range. Focus and exposure bracketing, where we change a camera or lens prosperity between captures, are well know techniques and I'm not going to repeat the basics here. A search of this blog will bring up previous posts on both subjects.

Before I develop my multi image capture ideas further, it is useful to remind ourselves what most are brainwashed to believe as they start their photography: The so-called Exposure Triangle.

Yes it's a triangle, but it doesn't really represent the 'photon-based' exposure, ie formed from the aperture size, that controls the 'flow' of light on the sensor, and a shutter time, that allows the flow to fall on the sensor for a fixed amount of time.

ISO is simply a magnification that gets used to increase the brightness of the captured scene, ie both the signal and some components of the noise.

Many modern cameras are ISO invariant, and applying an ISO setting above the base ISO in the camera, is broadly no different to applying it in post. Other cameras, like my Canon M or my Canon 5D3, only exhibit ISO invariant like characteristics above a certain (camera specific) ISO.

Another thing that follows from the above is that shutter time is coupled to capture time. For example, if I wish to create a long exposure artistic look, eg smooth out some flowing water, then I need to match my shutter speed to the capture time that I need for my artistic vision.

This could be achieved by closing down the aperture, but as we know, the downside of this is that diffraction blur will increase, to the potential detriment of the image quality we are seeking. 

To overcome such problems we usually introduce neutral density filters to reduce the amount of light we can capture, allowing us to run with longer shutter times at the ‘optimum’ aperture. But NDs come at a cost.

First, in our pockets, as one ND is not going to cover all our needs; and also in terms of image quality, ie that extra glass (or even plastic), especially with stacked NDs, will only degrade your image quality. Thus, the thought of buying an expensive camera and lens, and then putting a cheap ND in front of it, is something to think about.

It would be far better if we could decouple the aperture, which is our primary depth of field tool, from the shutter time, and our capture time needs.

This is where multi image photography comes in and, in particular, image or frame averaging, where we don’t change camera or lens properties between shots, which gives us an alternative 'exposure/capture triangle' - note this is just a visual alliteration to the original exposure triangle:

Here we see, on the left, our driving vision for the image, which, on the right, which we wish to create. 

We will set the aperture (N) to achieve the depth of field we wish to realise, and set focus appropriately, eg hyperfocally, non-hyperfocally or multiple times if focus bracketing.

We choose a total/integrated capture time, eg for: long exposure impact; or reducing noise; or extending the captured dynamic range. Plus set the camera to the base ISO, to ensure maximum dynamic range coverage.

If the shutter time, to achieve an acceptable exposure, is similar to the capture time we are seeking, including, if required, a small ISO lift from the base, then we are there: all in a single exposure.

We could also iterate a bit with aperture, needing to play off depth of field with shutter-induced motion blur: but that's detail.

If, on the other hand we don't want to change our aperture, and our shutter time falls short of the capture time we wish, then we may decide to try a low strength ND, ie trying to stay away from high strength NDs, with their colour casts and image degrading glass/plastic. 

Once again, this may do it. However, if it doesn't, then we take multiple images at a single shutter time to ensure the integrated shutter time matches the desired capture time. For example, with a 1/10s shutter and a 2 second total capture goal, we will need 20 images; which, in passing, will help us reduce the noise in post capture processing.

For now we will ignore motion-strobe effects etc and assume the images are captured in a seamless/gapless fashion.

Although ISO is still there to be used, ie as a moderator, for the kind of photography we are talking about in this post, eg tripod based city/landscapes etc, we will usually wish to remain at the base ISO.

With the above we thus only need to carry one or two (maybe) low strength NDs and/or a circular polariser, which we will use to reduce the number of images we take and help take 'near' seamless/gapless images. That is, we need to match/respect the camera's image buffer limitations.

Let's now finish this post and look at the best way to achieve the above. Simply go out and buy yourself a Phase One XT IQ4 150Mb camera, which does all the above in-camera and delivers you a single (gapless) RAW image of any capture time.

OK, you don't have over 50,000 pounds to spend: then we will need to create an in field and post processing multi image capture work flow, to achieve a very similar result: for free!

But that's a story for future parts of this post.

As usual I welcome any comments on the above or any of my posts.

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