Triplets Loc: Reading, MA

When I shoot star trails, I will "stack" images to produce a single photo.

When someone does HDR, he/she will "merge" images to produce a single photo.

Is there a difference between "stacking" and "merging" images? If so, what is it?

Regards,

Dennis

Jrhoffman75 Loc: Conway, New Hampshire

I believe merging involves some computations; stacking puts the images one on the other.

Triplets Loc: Reading, MA

Computation done by what -- software, photographer?

Jrhoffman75 Loc: Conway, New Hampshire

HDR software.

https://www.kadamsphoto.com/nightphotography/stacking-and-blending-and-hdr-oh-my/

Linda From Maine Loc: Yakima, Washington

Most simply put, stacking is done to extend depth of field: combining photos of differing focus points.

The purpose of HDR is to extend dynamic range: combining photos of differing exposures. Merging photos of different exposures does not change the depth of field.

If you are interested in in-depth discussion on the technical aspects, check in with UHH user selmslie.

Triplets Loc: Reading, MA

Thanks Hoff.

Triplets Loc: Reading, MA

Thank you Linda.

selmslie Loc: Fernandina Beach, FL, USA

Thanks for the referral, Linda.

With astro photography (which I don't do) images of the stars are usually against a black background. Merging simply adds the trail from one image to the one before it in order to continue the trail that might have gotten cut off when the camera reached a time limit like 30 seconds. With film this is not necessary since film cameras can use a Bulb or T setting and an arbitrarily long exposure does not build up any heat.

The question as to whether you are stacking or merging two digital images is not really crucial.

When you focus stack with something like Zerene Stacker you are actually doing a little bit of both. It selectively uses groups of pixels from the sharper of two or more images and merges them into a final image where the sharpest items from the various images in the set. You can manually override this to overcome any errors the camera makes when things are moving like grass or flowers in the foreground.

The merging that takes place when you do HDR combines the exposures from fully overlapping images into a single image that will fit into a range that can be displayed.

Panoramas are also merged using partially overlapping images to get things aligned and distorted to make them look like they were taken with a single capture. But once aligned, the program uses stuff from one image or the other rather than merging them at the pixel level. You can intervene to avoid conflicts from moving elements like people.

billnikon Loc: Pennsylvania/Ohio/Florida/Maui/Oregon/Vermont

HDR (high density range) refers to exposure balancing the highlights and shadows in a photo. In HDR the camera will MERGE several shots into one shot in an effort to balance the highlights and shadows.
Stacking refers to the camera COMBINING several of your shots into one shot that will give you a increased depth of field.
Best read your manual on these to make sure you are doing it correctly.

Blaster34 Loc: Florida Treasure Coast

PGHphoto Loc: Pittsburgh, PA

While I agree with Linda's assessment when it comes to focus stacking, I want to suggest that the depth of field is not always affected when stacking.
The best example as mentioned previously for stacking that does not involve depth of field is star trails.

I think the simplest way to understand the difference between stacking and merging is that stacking adds the contents of one picture to another without manipulating the exposure differences between the shots. Merging takes the optimal exposures from multiple shots and combines them together without manipulating the depth of field.

TriX Loc: Raleigh, NC

I think the best answer will probably come from someone in the astrophotography forum, but I’ll take a stab at it. In the astrophotography world, Stacking or combining multiple images of the same object averages out the noise since noise by definition is random and averages to zero, while the signal or image is non-random and averaging increases the value (and in so doing, increases the signal to noise ratio). If the camera or telescope is tracking correctly (and/or the alignment is corrected in SW), the images will stay aligned. We do exactly the same averaging in the electronic world when we average multiple acquisitions of a noisey signal. The improvement can be amazing, but the effectiveness begins to fall off as you increase the number of averages.just as in photography, aligning the images of the desired object is critical, whether done in HW or software. We typically reach the point of diminishing returns (the advantage of further averaging is outweighed by the efforts required to keep the images aligned) at 16-32 averages in the electronic world, but many stacked astrophotography images I see are composed of 70 or more averages.

HDR on the other hand, has a different purpose. In this case, you are “stacking” multiple images of different exposures of the same object to increase the dynamic range of the resulting “stack”

Edit (maybe I misunderstood the context of the term “stacking” by the OP - sorry if I provided extraneous information). To the OP’s initial application of creating star trails, then Scotty has already addressed that, and of course, as noted, you can stack images at different focus distances to achieve a larger depth of field.

Linda From Maine Loc: Yakima, Washington

Thank you, very easy to understand explanation (I shortened your comments to reply). I've only seen stacked photos of flowers and other close-ups

MauiMoto Loc: Hawaii

I thought this was high dynamic range.

MauiMoto Loc: Hawaii

I'm new to digital and still learning software, but either I'm not doing it right or HDR merge is not for astro. I like to further separate the shadows from the highlights for astro.