Why doesn't a zoom lens at 25mm on a Micro Four Thirds camera have the "normal" 50mm lens field of view?

Question

I've acquired an Olympus PEN E-PL1 mounting the kit zoom lens and EV-2 electronic viewfinder. Somehow, setting the lens at 25 mm does not yield the field of view I expect from a 50 mm equivalent focal.

I remember aiming with both eyes open, one through the viewfinder of my Pentax ME Super with SMC Pentax 50/1.7, and seeing the two images as one. With the Olympus, the viewfinder shows smaller objects, so I'm not able superimpose the two images and see them as if I weren't looking through a viewfinder from one eye.

I should note that I'm able to do what I say by setting the zoom lens on the Olympus somewhere between 35 and 42 mm (70-84 equivalent): in that case I see the image with exposure data and other informations superimposed, which is my goal. The focal length at which this happens, which is 70-84 mm, is wrong: human eye has the same field of view of 50 mm on 35 mm film and since micro four third sensors have a crop factor of 2, this should happen at 25 mm.

Where does the problem lie?

The camera (Pentax ME Super) has a viewfinder magnification of 0.95x, so what I see in the viewfinder of the Pentax is slightly zoomed out. This can't justify such a big difference: on the E-PL1 I must set the focal length of the lens to 70mm (equivalent) at least, otherwise things are too small.

I discovered that viewfinder magnification (which should be 1) are given with a 50 mm lens mounted also with the micro 4/3 sensor, so my EV-2 has a magnification of 1.15x/50mm*25mm=0.57x when mounting a 25 mm lens. The 25 mm lens gives me the same FOV of human eye on the micro four thirds format (2x crop factor). The real problem here is the viewfinder and its misleading magnification rating. To get a 1x magnification I have to set the lens focal length to 25mm/0.57x=44mm, which is true because I tried and could perfectly superimpose the images from both eyes (one with viewfinder), but this makes the lens a telephoto. The problem is unsolvable: if I could set the viewfinder to show a 1/0.57x=1.75x version of the image, I would lose coverage. Picture composition would be approximate since I'd be viewing 57% of the final picture, but I'd have a 1x magnification. I suggest to read this nice article: (http://www.luminous-landscape.com/tutorials/understanding-series/viewfinders.shtml)

Answer 1

The difference is that the crop factor doesn't apply to your eyes.

A 50mm lens on an SLR appears to have the same "focal length" as the human eye, because the lens combined with a typical optical viewfinder magnification results in an overall magnification factor of 1.0 x, hence if you look through the viewfinder with one and whilst keeping the other open, objects appear the same size with both eyes.

A 25mm lens will give a wider absolute view, which when used with a smaller sensor gives the same field of view as a 50mm lens on a 35mm SLR. However this doesn't matter to your eye which just sees the absolute field of view.

The VF-2 electronic viewfinder on your Olympus has a magnification of 1.15x, so the 50mm lens on your ME has effective 47.5mm, whereas the 25mm lens has effective 28.75mm

This is why you have to zoom your lens to 35mm (70mm equivalent) to get close to how your 50mm looks.

Answer 2

Here's the thing about crop factor: The image from a crop body sensor is only magnified 1.5x or 2.0x more when it is displayed at the same viewing size as an image from a 35mm full frame camera.

If you take an image made by a 36x24 mm sensor and display it at 6x4 inches, you magnify the image 4.23X. If you take an image made by a 2.0x crop factor µ4/3 camera (assume it is an 18x12mm sensor with a 3:2 aspect ratio for ease of the math here), to print at 6x4 inches you must magnify it at 8.46X.

With that in mind, let's look at the respective viewfinders of a FF camera and a 2.0x crop body. If they both magnify at 0.95x magnification and show 100% of the FoV projected on their respective sensors, then the viewfinder will be twice as wide and twice as tall in the FF camera as it will be in the 2.0x crop body. So when you are observing the same FoV in the viewfinder of the 2.0x crop body with a 25mm lens that you would see with a 50mm lens attached to the FF camera, what you are seeing is still 1/2 the size in the 2.0x crop body's viewfinder than the same FoV in the FF camera's viewfinder. If on the other hand, you put a 50mm lens on the 2.0x crop body, you would see 1/2 the FoV in the 2.0x crop body's viewfinder as you would with a 50mm lens on the FF camera. But since the viewfinder of the 2.0x crop body is 1/2 the size, the objects you can see in the 2.0x crop body's viewfinder would be the same size as the objects in the middle of the FF camera's viewfinder, assuming you hold the camera in the same spot and aim it in the same direction.

In actuality, your old Pentax ME Super 35mm camera had a viewfinder magnification of around 0.95X and 93% coverage of a 36x24mm image size. Your Olympus E-PL1 has a magnification of around 1.15x and 100% field of view of a 17.3x13 mm image size. Do the math and you see that for the same object at the same distance from the camera to appear the same size in the viewfinder as it does with a 35mm ME Super and 50mm lens, the E-PL1 needs a lens of 43mm (87mm FF "equivalent") in focal length.

Answer 3

Field of view is one thing, magnification is another. A 25mm (or 35mm) lens doesn't stop being a 25mm (or 35mm) lens just because you put a smaller sensor behind it. Despite it yielding a "normal" FoV on a specific format (e.g., µ4/3, APS-C).

Try slapping the m.Zuiko 45/1.8 on your E-PL1, with both your eyes open, or any 50mm lens, and you'll doubtless get the same "it matches" phenomenon with both eyes open. There are times I use the 45/1.8 as a walkaround on my GX7, despite it nominally being a telephoto lens because of this.

A 50mm lens, on full-frame/135 format, was normal in two different ways: both FoV and magnification. But on a smaller/larger format, while a 50mm lens always retains the same magnification, the FoV is different.

Answer 4

You say, human eye has the same field of view of 50 mm or 35 mm film, where did you get that? See Human eye - FOV here. Probably that is the error in your calculation...

Please also see this link. There is a difference of opinion about eye focal length.