If you are thinking of same size prints (like 4x6) from each shot—not the original image on the sensor, then you have that backwards. The smaller sensor produces the larger image ( it magnifies the central portion) and the larger sensor produces a smaller image—but includes more surrounding area.

But you got the second part right. To match the image from the larger sensor you would put on a ‘wideangle’ lens (one with a shorter focal length) so that more of the scene can also be captured on the smaller sensor.

I am confused! I am looking to buy a Nikon DX 35mm f/1.8 lens. I have always been under the assumption that 35mm is 35mm...a mild wide angle lens. The exception is an FX lens on a DX camera, where the smaller APS C sensor size, using the central portion of the lens' coverage circle, gives an equivalent angle of view of about 52mm.

I see statements..."35mm is the new 50", and the information printed with the lens description;"52.5mm 35mm equivalent".
I've been of the opinion that nobody that knows anything uses a DX lens on a FX or 35mm film camera, due to extreme vignetting. It seems to me that a 35mm lens, designed for the FX sensor or film should give a 35mm angle of view, while a 35mm lens designed for the APS S sensor should do the same. Where is the error in my understanding?

First, congrats on the R6.May you enjoy it for many years.

The issue of comparing lens focal length to image size has been around since camera obscuras were made in the 16 th century—long before photosensitive film or electronics.

The movie makers had a similar situation 50 years ago. You could get regular 8 mm, super 8 mm, 16 mm, and 35 mm movie films. Each had a different frame size. Built-in zoom lenses were popular with the 8 mm cameras, but some had interchangeable lenses. These were frequently on a turret.

Adapters were available to allow using lenses from 16 mm cameras on 8 mm and super 8 mm cameras. There were (and still are!) adapters for using 35 mm movie camera lenses and also 35 mm still camera lenses on 16 mm cameras. Couple that to the first adapter and those lenses also worked on both 8 mm formats, too. Dr. Frankenstein would feel right at home with those tools !

The technique almost always works if you use a lens designed for a larger format on a smaller format, but it rarely works in the opposite direction.

This led to some interesting lens history in the development of 35 mm photography. A goodly amount of that history leads (like branches on a family tree) back to Oskar Barnack, and a few unknown people before him. Barnack and friends saw that the thin emulsion 35 mm movie film used around 1918 captured a much sharper image than the prevailing sheet film. It was so good that a film image just 18 x 24 mm could be enlarged to fill a 10-meter-wide screen in the movie theater.

So Barnack made a camera that used movie film— because it was the best film available.
To get double the advantage, he turned the film on its side in his camera and was able to get a 24 mm area between the sprockets. While movie film had exactly 4 sprocket holes for each frame, Barnack used 8 sprocket holes for each frame. That gave a frame 36 mm long and a small gap between frames.

The world would be a different place if he had done the ‘appropriate’ thing and chosen a spacing of 7 sprockets! The slightly shorter image would be much closer to the shape of 4x5”, or 9x12 cm films of the day. And a much better fit for 8x10 prints.

Once he had selected the format he couldn’t go to the store and buy a proper lens. He had to make a lens for it, because ithe format was bigger than standard movie film, and it was much smaller than roll film cameras.

Probably he should have designed a
43 mm lens, matching the diagonal, but instead he made a 50 mm lens. I suspect it was an easy compromise to maintain sharpness in the corners of that elongated frame. After that 50 mm became a standard, and every other company needed to match that to compete.

At the same time Barnack was tinkering with 35 mm movie film, Rolleiflex was building 6 cm film cameras, amateurs were using 6 cm and 7 cm roll films (120, 620, 116, 626, 122 sizes), and pros were using 4x5 press cameras in the field and view cameras in the studios.

The amateur bought a folding roll film camera with a 100 mm or 105 mm lens, or the press camera with a 135 mm lens. Amateurs aspiring to do professional work bought 4x5 press cameras, or their smaller (or metric) siblings. Then, as now, the quality of lens was important, and a lot of development went into designing 135 mm focal length lenses for 4x5 cameras.

So, in the 50s and 60s, as 35 mm use grew, and 4x5 waned, there was a lot of good glass already ground to 135 mm focal length, and factories set up to make more. It was relatively easy to put that glass in a helicoid focussing tube, and sell it as a ‘telephoto’ lens for a 35 mm camera. And around the 1960s, 135 mm became the most popular ‘second’ lens for a 35 mm camera owner. You could get decent ones at relatively low prices. The 100 and 105 lens designs from roll film cameras were also remounted for 35 mm cameras, and saw new life.

Remember, in the days before computers designed lenses (roughly 1978) all lens designs were drawn by hand and confirmed by grinding the lens and testing it, then repeating the process. It could take years to create a significantly improved design.

The nice thing is that most of the time a lens gives a sharper image in the center than at the edges. Using these lenses with smaller formats used the best part of the image quality they produced. This concept still survives when using lenses designed for full frame on cameras with cropped sensors. And you can still get adapters so you can mount the old 135 mm press camera lens designs on mirrorless camera bodies!

But films improved, too. 120 and 4x5 films got thinner emulsions and produced sharper images in 1970s than in 1920s. It was possible to shoot an image on a larger format and use the entire frame, or crop it to use just a portion.

With 4x5 film and a 135 mm lens one can snap a shot that has a moderate wide view ( similar to about 38 mm on a 35 mm camera—close to a 35 mm lens), crop that to match the image seen by a ‘normal’ 50 mm lens on 35 mm. Then crop it more to get the effect of a 100 mm portrait lens on 35 mm. And finally crop it to the identical size of a 35 mm frame, where it is the same as using that lens on a 35 mm camera.

Where you have a high-resolution image medium you can play around a lot more.

My Sony mirrorless lets me select shooting full frame, or APS-C. It essentially is doing the cropping and just using a center portion of the sensor. But why should I flip that switch? I can do the same cropping in post processing. Or maybe I’ll use a slightly larger area, with a marginal improvement in quality.

With cell phone cameras packing more pixels in smaller spaces, the trend is simply continuing. The new formats are smaller, but have great sharpness, and old lenses are repurposed for use with them. And as that happens we see that those old lenses were really very good in the center—and that is the only thing that matters.

I got into the habit of using the full frame focal lengths as my reference. That's what the camera manufacturers do, even when they're describing small sensor cameras like compacts. Ideally they would always include "full frame equivalent" (or some such) when describing the capabilities of small sensor camera lenses.

Funnily enough, they tend to not bother when referring to APS-C or m4/3 lenses, preferring instead to quote the actual focal length. But in both of those cases the maths is quite simple, as long as you stick to using the FF equivalent as the reference. For a long time my only lens was a DX 16-85mm but I always thought of it as a 24-127.5mm lens because that's how I'd taught myself to think about focal lengths. I did that even when my only cameras were small sensor compacts. Doing so was made easier by the fact that the camera manufacturers referred to the lenses using FF equivalent focal lengths.

I got into the habit of using the full frame focal lengths as my reference. That's what the camera manufacturers do, even when they're describing small sensor cameras like compacts. Ideally they would always include "full frame equivalent" (or some such) when describing the capabilities of small sensor camera lenses.

Funnily enough, they tend to not bother when referring to APS-C or m4/3 lenses, preferring instead to quote the actual focal length. But in both of those cases the maths is quite simple, as long as you stick to using the FF equivalent as the reference. For a long time my only lens was a DX 16-85mm but I always thought of it as a 24-127.5mm lens because that's how I'd taught myself to think about focal lengths. I did that even when my only cameras were small sensor compacts. Doing so was made easier by the fact that the camera manufacturers referred to the lenses using FF equivalent focal lengths.

I am confused! I am looking to buy a Nikon DX 35mm f/1.8 lens. I have always been under the assumption that 35mm is 35mm...a mild wide angle lens. The exception is an FX lens on a DX camera, where the smaller APS C sensor size, using the central portion of the lens' coverage circle, gives an equivalent angle of view of about 52mm.

I see statements..."35mm is the new 50", and the information printed with the lens description;"52.5mm 35mm equivalent".
I've been of the opinion that nobody that knows anything uses a DX lens on a FX or 35mm film camera, due to extreme vignetting. It seems to me that a 35mm lens, designed for the FX sensor or film should give a 35mm angle of view, while a 35mm lens designed for the APS S sensor should do the same. Where is the error in my understanding?