I’ve had a few opportunities lately to help people edit their photographs where they wanted to combine two photos into a composite and were worried about the relative sizes being proper, especially when the camera settings and/or the scene were not identical.  Based on these experiences, I’ve created a couple of scenarios to introduce certain concepts.

## A Safe Selfie

Suppose you need to add part of a wild animal behind you – to make a safe selfie, if you will.  Most of your composite shots, where two objects are moved around in an image with plenty of other size reference points, fall in this category.   Generally, combining subjects is a two-part process:

### Resize One Picture To Match Pixels Per Inch For The Two Subjects

First, you must know the physical size of the two objects.  In the selfie case, you probably already know your own size, but suppose you want to place the head of an animal (whose picture you took from a safe distance) right behind you.  In one case, I Googled an animal to get size information but could not find the size of the head of an adult male of that species.  They did list shoulder height, however.   So then I found a picture of this type of animal online that showed the head and enough of the animal to measure its shoulder height (since my client’s picture of the animal did not have all of these features), and by comparing the two measurements on the picture, found the size of the head.  Fortunately, it’s not always that hard.  Now, measure the subjects in your two pictures in pixels.  Divide the number of pixels by their length in inches.  Resize  one of the images so that the pixels per inch that you just calculated are the same in both pictures.  For example, let’s say your 6-foot height (72″) measures 792 pixels in the first picture.  That’s 11 pixels/inch.  The alligator  or bear’s head, which you found to be 24″ long, measures 192 pixels in the second picture, for 8 pixels per inch.  You can either enlarge the ferocious animal or downsize your likeness.  If you want to reduce your size, open the first picture in Photoshop.  Click on “Image” in the menu, and then “Image Size…”.  Make sure the “Resample” box is checked.  Multiply the pictures existing resolution (say 300 Pixels/Inch) by the target pixels per inch calculated above (in this case 8 to match your animal) and divide by your starting pixels per inch (11).  That gives you 218.182, which is what replaces the existing 300 in “Resolution”.  Hit “OK”.  Now, you and your animal head are the appropriate sizes, if you plan to put them side-by-side in your picture.  If you want to move one in front of the other, its size will change.

### Use A Vanishing Point To Resize One Subject For Changing Distance

Now you can use vanishing points to maintain the correct sizes as you move your objects into place.  We’ve already explained that process in Using The Vanishing Point To Keep
The Size Right When Moving Wildlife Around
.  I would like to point out that as long as your object stays the same distance from the camera, or in the same focal plane, you can move it up, down, and all around without changing size.  If you move it closer to the camera, it should get larger.  When you move it away, make it smaller.  Once you resize it for its new distance, you can again move it up, down, and all around within that new focal plane at no extra cost.  Also, once you find the horizon in your picture, it doesn’t matter which point along that horizon line you use as the vanishing point; all of them will resize your object correctly.  Pick a point that is conveniently off to one side far enough to make long enough construction lines to give you some precision when changing size.

## A Beach Scene

I also helped somebody with a beach scene that invoked two simpler special cases of the resize problem.  The base or background image was a wide-angle beach scene and the photographer wanted to add objects that they took with a zoom lens at the same scene that same day.

### Floating Objects

The photographer’s intent, in this case, was to shoot objects floating on the water near the horizon with a strong zoom lens and add them to the picture so that they looked closer.  An object floating in the water is restricted to a specific plane in such a way that its distance from the horizon is directly related to its distance from the camera (within a camera’s normal field of view), which is the determining factor in that object’s relative size.  As long as the horizon is in the picture, this is no problem.  Whether you add that object at its original pixel size (as magnified by a telephoto lens) or even if you scale it further in Photoshop (by holding down the shift key to preserve the aspect ratio as you move a corner of the selected border while using the Move tool, for example), as long as you keep the horizon of the added object directly on the same line as the horizon of the background, the invisible construction lines from an invisible vanishing point will ensure the size and placement of your object are in agreement.  If the horizon is not in the picture, then you need to look for other size references and handle as in the first general case discussed above.

### Flying Birds

Birds (or other airborne objects) are even easier.  Their position is unrestricted and, more importantly, there are no other size references in view so there is really no way of knowing how large the object really is or how far away, meaning that if it were a ball, there would be no way for you to tell if it is a large ball far away or a smaller ball up close.  If you are familiar with the object and know how large cooper hawks are, for example, then your brain will automatically assign the hawk an appropriate distance based on its size when trying to make sense of the picture.  You can put that hawk just about anywhere and the viewer won’t know the difference.  Obviously, if you put a pigeon in a hawk’s talons then each would act as a size reference for the other and at least their relative sizes would have to match.  If they were not touching (or near enough to imply an interaction), there would be no such restriction.

## Other Considerations

### Lighting

There are other positional clues besides size to think about.  On a sunny day, an object’s shadow provides positional information, namely the object’s relationship to the sun, which must be consistent throughout the image (for best results).

### Perspective Changes

When you shoot someone’s face with a wide-angle lens from a close distance, it will not look the same as when you shoot the same face from far away with a telephoto lens.  An example of this is shown in the fourth image from the top at Choose the Right Lens to Make Flattering Portraits (the only image that’s in color). I’ve seen some experts blame this on lens distortion (as the guy in this otherwise great video at Focal Length for Storytelling – How Lens Choice Affects Your Images, but I don’t consider that lens distortion.  There is such a thing as lens distortion, but in this case, the subject’s nose really does look bigger and the ears really do disappear behind the cheeks if you were to close one eye and look at that person from 3″ in front of their face.  I call that a perspective shift and it is strictly a matter of angles and geometry, not lens issues. The “distortion” occurs when you take that image out of context by changing the perspective, which happens quite noticeably when you move an object from very far away to very close (or vice versa) in your image or if you take a 180° panorama, for example, and print/display it small enough to cover only 15°.  This perspective shift is virtually impossible to correct in Photoshop, so don’t go too wild while moving things around in your picture.  (Interestingly, it is by a lack of any perspective shift that you can catch somebody who created a reflection in their picture by just adding a flipped subject in post-processing.  The explanation of this tangent to today’s topic would require a separate article, however.)