I know little about photography but I know that often, the image that goes into the lens isn’t the same as the image that gets printed. The process of taking the light from outside and eventually turning it into an image is influenced.
What I’d like to know is what is liable to be added, removed or modified in that process? Are there artefacts, distortions, omissions? What creates them?
I am particularly interested in how this can work for film cameras, especially early 20th century black & white cameras but also color cameras which existed back then.
A very good book* by that title exists. Also, that search will bring up lots of sites that explain from the basics to very deep.
(not “let me Google that for you” snark, a real answer, you will be amazed how much info is easy to find out there)
*by Carl Shipman. I used that when I taught photography in the '80s
There are two major lens distortions, Pincushion (squares look “deflated”) and Barrel (squares look “inflated”).
Lenses can also have wavy distortion, where horizontal (or vertical) lines are not straight.
Points should be rendered at zero-size objects, but any real camera will render then as discs, and often with a halo of color around the disc (chromatic aberration). If the lens has astigmatism, then the discs will be ellipsoidal. Lenses also can render points as tear-dropped shaped - this is called “coma.”
Lenses should have perfect image fill from edge to edge, but often, the light will “fall off” towards the edge of the frame, resulting in “Vignetting” (or more properly, brightness fall-off).
The photography process can also introduce distortions that are not related to the lens - one of the most common these days is “Moiré” - where repeating patterns in the image interact with the repeating pattern of the sensor and create a “woven” texture on the image.
There are many, many other distortions involved, but these are the ones that come to mind first.
There are also color and brightness distortions, due to the film’s limited response relative to the eye. Or expanded response, if they responded to UV light, for example.
Also remember that different focal lengths of lenses for their respective formats will introduce a specific perspective. From the enlongating of short focal length (wide angle) for close up objects to the foreshortening of distant objects for telephoto (or long focal length) lenses. And everything in between. This is an over simplification of the issue, but it will get you started.
Don’t get too sidetracked by the actual numbers of the lenses being compared. At or near the turn of the last century, there were numerous film sizes and various formats within those film sizes. A 75mm lens for a 4x5" format is different than a 75mm for a half frame 35mm, for instance.
^ IMHO, that is the big one–the way perspective varies by distance-to-subject (and therefore, lens selection plays into it.)
This is best illustrated with a photo, of course.
Here is an example of how various focal lengths affect the appearance of a subject in a standard facial portrait. Remember that it’s not just focal length that is at issue here, it’s really distance to subject. With the wider lenses, in order to get the face to fill up the frame, you have to get closer to your subject. Once you get lower than 50mm, facial features start becoming pointy and mousy. Look at the relationship of the nose to the ear (or, rather, in these pictures, where the ear would be.) As you get wider and wider, that distance appears to get stretched out. As you get more telephoto (higher focal lengths and farther from the subject), the distance is compressed, resulting in more pleasing features.
One of the most important creative choices as a photographer is knowing what focal length to choose for an image, and it’s not (just) based on how wide or narrow a field of view you want–it’s as or more important to consider the relationship between foreground and background different lens choices give you.
In the second picture: Is it me or does the 18mm lens almost give a 3D look to the boy? He sees to jump out of the picture.
Higher focal length means a lower number of mm, right? 200mm is a shorter focal length than 35mm, right?
Any idea why 35mm is so common?
No, bigger mm is longer focal length (the focal length *is *the measurement).
35mm actually refers to the film size - 35mm across the length of the frame (35 x 24 mm). The “standard” lens for 35mm cameras was 50mm, chosen because it was close to the same field-of-view as the human eye.
It’s a film size, the common name for the common format for that film size (still in use today even), and a popular lens focal length for cameras of that format. It can be confusing to someone unfamiliar with old school photography.
Also, there is the perspective distortion (converging and diverging verticals/parallels) that is caused by having your focal plane other than perpendicular to the ground. If you tilt your camera back in order to get the entirety of a building in the frame, your vertical lines will converge from the bottom to the top of the frame. If you tilt the camera down (like say you’re shooting down from a high vantage point), vertical lines will similarly be non-parallel. There are lenses that correct for this sort of distortion, called perspective correction lenses or shift lenses.
You might be thinking of aperture, where a “larger aperture” is a smaller f-stop number and a “smaller aperture” is a larger f-stop number.
Usually, for focal lengths, photographers refer to “longer” and “shorter” or “wider” focal lengths (although the term “wider” is typically only used when you’re already shooting a wide angle). I used “higher” just to refer to the number, but the usually shop talk would be to say “longer” as in, “man, I need something longer than a 300, my framing is much too loose.” or “Damn, it’s tight in here. Did you bring anything wider than a 24?”
My Kodak Pony (late 50s vintage) had a 43.5mm lens. Loved that perspective. Also, at f/8.0 and a hyperfocal focus distance, it became the perfect street camera.
All of the responses so far have been about effects from the lenses & sensors in the camera.
Besides all of that, there are many possible distortions that take place when the image is actually printed onto paper.
Everything from inaccurate ink colors to old printing paper or improper humidity in the paper, etc. And that’s before age & exposure start fading out the print.
The issue of focal length has been covered well - it is critical. Perhaps the remaining point to make is that there is one, and only one “correct” viewing distance for the final image. If you place your eye at the same distance as the scaled focal length of the original lens, you will see the correct perspective. The effect can be startling, and can lead to some level of 3D perception kicking in. By scaled I mean that you take the original film/sensor size and work out the scaling factor of that to the image you have - and then multiply the focal length by the same factor. You will find that most photos need you to view them from very close quarters for this to be correct. Small prints are awful in this respect.
wrt 40mm versus 50mm lenses on 35mm cameras - there are two terms. A “normal” lens is a lens that has a focal length that matches the image size - so 36mm matches the long dimension of a 35mm film frame - or 43mm matches the diagonal. Lenses in this range would be considered as “normal”. Where as a “standard” lens is simply the lens that most manufacturers put on a camera as default. Standard came to mean 50mm in many people’s minds.
Because the typical 35mm camera was a SLR, the lens to film distance was restricted by the need to clear the mirror as it flipped out of the way. The extra distance complicates short focal length lenses (they must retro-focus, meaning the location of the centre of the optics is actually behind the middle of the lens, and may even be in the air behind the lens) and thus they become more costly to make. A simple (non reflex) camera can use a much simpler short focal length lens, and typically will. So you will see rangefinder 35mm cameras with 30/35mm lenses, and SLRs with 50mm lenses as standard.
It is only that the background is in focus whereas in the others it is not. In reality it is the opposite, a blurred background gives more of a sense of depth.
What made this sensible to me when I was starting out was realizing that f numbers are really fractions (the diameter of the aperture as a fraction of the focal length of the lens) which is why they’re represented like a fraction: f/2. At f/2, the diameter of the aperture is 1/2 the focal length of the lens. At f/4, it’s 1/4 the focal length (and hence, smaller than f/2) and so on.
Actually, I thought of a more dramatic example to show how focal length/distance-to-subject changes the relationship of foreground to background. It’s a film effect called the dolly zoom or the “Hitchcock effect.” Basically, what you’re doing here is starting with a wide shot and then zooming IN while moving back (on a camera dolly) so that the size of your subject remains relatively constant in the frame.
