Exposure Averaging: Looking like FF at modest enlargements

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kenw Veteran Member • Posts: 5,620
Exposure Averaging: Looking like FF at modest enlargements

Yet another demonstration of a relatively old concept: reducing base ISO noise by averaging multiple exposures. In this case showing the difference is visible even at modest resolutions and enlargements (3MP or about 8x10 in this case) especially when contrast or saturation is boosted.

It is common to say that at "web sizes" or 8x10 that it is impossible to distinguish (insert advocated sensor size) from (some larger sensor size).  This proposition is based on the fact that at some reasonable screen or print resolution that you are significantly reducing both images so the resolution difference between the formats doesn't matter anymore.

This is a bit specious because there is more to it than just resolution.  There is also noise.

As discussed exhaustively over the years in a large fraction of scenarios we end up at "equivalent" exposures between formats and in this case the argument that at modest enlargements where we are down sizing both formats they are indistinguishable from each other is quite sound.

Outside this "equivalent" case, for instance when you can arbitrarily extend exposure time to use the lowest ISO possible, that is not necessarily so.

Landscape shooting is one of the common cases in which this is often true (but not always of course).  Usually tripod mounted without the subject moving and shooting at base ISO with long exposure times.

Note that this is not in any way restricted to cases of high dynamic range.  People often obsess over DR performance and use it as a figure of merit for landscape shots.  Indeed scenes can be high DR but the reality is even in low DR shots the base ISO noise in the midtones and highlights is an issue.  You don't have to push shadows to have visible noise - even at modest display sizes.

If you do a lot of landscape shooting or look at a lot of landscape photos in print and online you will note you can actually pretty quickly tell the difference between smaller and larger formats even at modest resolutions (few MP).  A lot of this is to do with the fact that many landscape photos get a lot of contrast and saturation enhancement in post processing which effectively amplifies noise quite a bit.  Sure we are shooting at base ISO but if you hammer on the contrast, clarity and saturation sliders as landscape photographers often do you are amplifying the noise quite a bit and making it effectively like a significantly higher ISO in the output image.

Here are two shots of the same scene resized to about 3MP:

Obviously these need to be opened to a larger resolution to see anything of consequence.  At full resolution they are only 3MP and akin to at most an 8x10 enlargement.

It doesn't take that much effort to see a difference between these two shots.  Especially in the clouds and some patches of sand one image has more noise.  The lower noise image is able to better show some fine tonal gradations.

What's the difference between them?  One of them is the GM1 shot at base ISO and carefully ETTR'd.  It is basically about the best that the 16MP m43 sensor can do in a single exposure.  The other one is seven similar exposures averaged together effectively lowering the base ISO by a factor of seven (or almost three stops).

Here is a similar pair with B&W processing:

Again it is not hard to see the difference even downsized to 3MP.  In this case the averaged shot is using five exposures (or a bit more than two stops).

In both of the above cases the averaged m43 exposures are collecting about as much light as a base ISO exposure in a FF system.  And indeed there is a visible difference in the final processed photos even though they have been downsized quite a bit.  So a reasonable demonstration that in fact it is possible for a larger format to be distinguishable from a smaller format at lower resolutions and enlargements.  Or similarly that us m43 landscape shooters could certainly make good use of an even lower base ISO if it were available.

The good news here is that if you don't want to spend thousands of dollars or carry more weight even the tiny little GM1 can give you a "FF look" at typical display sizes just by doing multiple exposure averaging.  A bit more hassle in post processing to be sure but so is carrying a larger system miles into the desert...

Note also that High Res modes effectively provide the same noise advantage and in theory (haven't tested this myself) the LiveND modes of the E-M1X and E-M1III should as well though I think there are some exposure restrictions to LiveND that might not always make it workable.  You can also use some multi-exposure options in some cameras but in my experience the implementation in camera is cumbersome and I prefer to do the combining in post.

Doing this kind of averaging has been steadily getting easier over the years as processing speeds increase and image processing tools advance.  The above were done fairly quickly in LR and PS.  Select the exposures to combine, open as layers in PS, select Auto-Align for the layers, convert them to a smart object and select stacking mode as Mean.  If you shoot on a tripod you can skip the Auto-Align step.

What you do in the field is even easier - just take a few more exposures.  Given how effective auto-alignment is in post you don't even need to use a tripod if the individual exposures are short enough.  (That's what I did for the above two examples which were shot at midday).

If we get to truly huge enlargements then the native resolution of the sensor would begin to matter and it isn't just about noise anymore.  There the potential resolution advantage of a larger format would begin to matter.  But for more typical enlargements (like A3) you can save your wallet and your back by just taking more m43 exposures and averaging them in post.  And yes your effort will typically be visible even at modest print sizes and full screen web presentation.

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Ken W
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