I feel like I've stirred up a hornet's nest with some of my comments about focal length. Especially in when discussed in conjunction with depth of field and perspective.
While focal length is part of the equation for figuring both depth of field and perspective, I think a lot of discussions miss the point and give too much emphasis on focal length. Let's look at each of these separately.
First I'll tackle perspective. I define perspective as the relative distance between objects in a scene. When you work close to your subject items appear spread out with a lot of room around them. You could say that the scene is very open and spacious. An example of this situation could be a portrait of a person holding an object in their hand with their hand stretched forward towards the camera with the camera only inches from their hand. That object, be it an apple, a cup of coffee, a tree ornament, or whatever will appear very large. The person holding their hand out will be smaller, and anything in the background will be tiny.
When you work far away from your subject the items appear to all close in on each other and stack up giving a flattened perspective. We are used to seeing this flattened perspective when we see photos of distant overlapping mountain ranges or a distant image of a city with all its skyscrapers appearing to touch each other.
Focal length gets "blamed" for both of the above scenarios. But the focal length of the lens does not cause these issues. In the first example with the person holding their hand out you have to use a short focal length lens to capture the scene. A longer lens would not have a wide enough angle of view or allow you to focus that closely, so you cannot take this image with a long lens. You could turn that around and say that a short focal length lens allows you to capture that apparently distorted scene. But the lens does not cause that distortion. It is a fine distinction, but a very important one. Consider the following passage from a book, Elementary Photography by C.B. Neblette, Frederick W Brehm, and Everett L Priest that was published in 1936...
"A lens should seldom be used with a film the diagonal of which is greater than the focal length of the lens. This is because with a lens of short focus the image is small. To make the image larger the photographer is tempted to get close to the object, with the result that he gets pictures the drawing or _perspective_ of which looks wrong. This effect is caused by the fact that the objects near the camera are drawn on a much larger scale than distant objects. If the picture is viewed at a distance equal to the distance of the film from the lens when it was taken (in the case of 35mm photography, about two inches), the perspective will not appear distorted."
The problem is, how often do we view photographs from two inches away?
The second scenario is easier to show, as you can take a photo of distant mountains with either a short or a long focal length lens. Try this some time. Put your camera on a tripod and point it towards some mountains or skyscrapers in the distance. Take a photo with a short lens (maybe a 28mm or a 35mm) and then _without moving the camera_ switch to a longer lens (maybe 200mm) and photograph the same distant objects. Afterwards make an 8x12 print from each frame. Make the print from the long lens a full-frame image and enlarge the image taken with the short lens so that the objects are the same size as in the long lens print. You will find that the distant objects have the exact same perspective in each print. The quality of the long lens print will be better because of less enlargement. But the perspective is the same. We tend to use a longer lens for this type of photo because it works out better quality-wise. Not because it gives a different perspective.
OK, that one was pretty easy. Now for the trickier subject. Depth of Field.
Everyone has been taught that a shorter focal length lens gives more depth of field than a longer focal length lens. I don't dispute that. But, that description is only half of the story (maybe 1/4 as the missing part is very important). Let's quote Elementary Photography again. "...the shorter the focal length of a lens, the greater its depth of field." OK, that's a given. But then it goes on to say, "However, if the short-focus lens is brought close enough to the object to give the same size image as the long-focus lens, they both with have approximately the same depth of field." That's the part often dropped off or ignored in discussions. So, what does that mean?
Let's take an example. You have a scene in front of you. There is a park bench in the foreground and a wall with a mural painted on it in the background. You want to photograph someone sitting on the bench to make it appear that they are interacting with something in the mural. Maybe they are picking an apple off a tree in the mural or something like that. Also consider that there is a telephone pole off to one side and the edge of the building on the other side and you do not want to include those elements in the photograph. You initially set up with a 100mm lens and find that by setting the tripod up at 15 feet from the bench you can get the framing you want between the person, the mural, and exclusion of the pole and edge of the building. But you also find out that the bench is far enough in front of the mural that you can't get them both in focus, even if you stop down to f/16. What are your options?
A. You might be able to stop down to f/22 or f/32 if the lens allows it.
B. You can switch to a shorter lens.
C. You can just go for it and hope for the best.
D. You can switch to a shorter lens and move in closer to get the subject to the size you wanted and to try to frame the background the way you wanted.
E. You can move further back.
What happens when you try each of these?
A. Not all lenses will stop down as far as f/32. Some may not go down to F/22. And even if they do, you are now well into having to deal with diffraction. The aperture is so small that the light rays have to bend considerably as they pass by the aperture blades that the quality of the image begins to be seriously degraded.
B. The shorter lens will give more depth of field because the image is smaller (less magnification). But, as noted, the image is smaller. Now that telephone pole and the edge of the building are in the image. You need to crop the image and enlarge it more to make the print. Enlarging it will reduce the quality and will also reduce some of the depth of field that you gained by switching to the shorter lens. Or you can go with it as is and be limited in how large a print you can make. Smaller prints will display more depth of field than larger prints of the same crop.
C. Nothing different happens. This is the original setup.
D. By moving closer to the subject you are increasing the magnification and your depth of field is diminishing. When the subject is the same size in the viewfinder (same magnification) the depth of field is going to be approximately the same, no matter which lens is used. And by moving closer you have altered the perspective relationship between the person and the background, changing your concept.
E. Moving back will give less magnification and greater depth of field. Like in B, you will then have to crop and enlarge the image, sacrificing some image quality. Plus, like in D, by moving the camera (back this time) you now have a different perspective on the scene. The person on the bench will not have the same relationship with the mural in the background. Your grand concept now has to be altered. You will get more depth of field. But you will be making a very different photograph.
Of these options, which is best? B is the only option that maintains your original concept. But you may find out that you have to limit the size that your image can be printed (maybe no larger than a 5x7 print). Or you have to accept that at 16x20 something in the image that you wanted to be in focus (either the person or the background) is going to be soft. The silver lining here is that with a larger print the viewer will tend to stand back and view from a greater distance. So it may all balance out. But you really haven't gained any additional depth of field, you will get about the same DOF results in a print from either lens because the final image magnification is the same.
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