Many slrs digital or not have what’s called a viewfinder magnification ratio. So if that is 0.8, a “normal” lens will show a reduced size of the object when compared to the naked eye. A slightly longer than normal lens will match up, but appear to give a slightly more compressed view. Keep in mind that perspective is SOLELY based on your shooting position however… You can shoot a 50 on a full frame, switch to a 100mm, crop the 50 image to match the 100, and you have the exact same perspective.

It why I’m a firm believer in zooming with my feet. Zooms just fine tune cropping in camera to allow more of the sensors pixels.

Where your equation goes wrong is here “35mm looks like 50mm on a FF because the narrower FOV”

Just no.

Optically 35mm = 35mm and 50mm = 50mm on both Crop and FF. It will look the same regardless of your sensor size.

A crop will give you the idea of a narrower FOV because your image is cropped, aka the outer edge of your image is cut off.

The focal length, and thus how “large” objects look through the viewfinder is the same on both systems.

Guys im sorry for this dumb post, i somehow didnt realize that looking through the viewfinder will look exactly the same regardless of my sensor size BECAUSE İ AM LOOKİNG THROUGH THE LENS NOT THE SENSOR İ AM SO DUMB SORRY

https://www.reddit.com/r/photography/wiki/technical#wiki_should_the_crop_factor_apply_to_lenses_made_for_crop_sensors.3F

people say that 50-55mm is the focal length of the human eye

There isn’t really a good answer to that. A camera takes photos of a single moment in a rectangle with hard edges. Your eyes see in a amorphous field with higher acuity in the center that gradually drops off towards the edges, and really when perceiving a scene they move around quickly to move the center around and build up an amorphous larger image over a short period of time. That can’t be directly compared with a rectangle.

A 50mm focal length on full frame is the “normal” focal length (about the corner-to-corner diagonal measurement of the rectangle, or diameter of the projected image circle) so it’s more or less the agreed-upon middle ground matching your vision. But it isn’t exactly what your eyes see, and really no conventional photo is.

Then what i do is i take my camera put it on 35mm and then look at the vizor. What i expect is no zoom at all but the objects look smaller in the vizor (so fov is higher)

Field of view is what you can see from the scene from edge to edge of your frame.

Magnification of the image is a different issue, and also depends on things like the optics of your viewfinder itself.

FYI, good luck with this question - there have been thousands of threads across countless forums arguing aggressively as to what the concept of equivalent focal length and aperture actually means, because a lot of it has to do with how pedantic you want to get and how detailed you want to get in the comparison. And lord help you if people start talking about the amount of light a lens gathers based on aperture, sensor size, pixel resolution, and so on. That’s an argument that never ends because it gets even deeper into pedantry.

Honestly it’s an interesting discussion and it’s worth reading about, but be aware that there will be a lot of people defending their opinions very strongly, whether or not they’re actually correct. Believe me, it gets spicy, especially with the people who are technically correct but in was that don’t actually matter.

The best way to look at it is simply that lower is wider, higher is narrower. A smaller aperture number means more light and faster shutter speed options, and narrower depth of field.

Multiplying the focal length by the crop factor does help as a shorthand to help you compare with other lenses but it’s basically a simple cheat sheet rather than anything meaningful.

The real best way to do it, for any camera, is to use the kit lens or some cheap equivalent, and actually take pictures with it and get used to what you actually see. Try it and see how you like it, and use that knowledge to guide your purchase decisions. People on the internet will tell you what they think you should think, but ultimately what matters is how it looks and feels to you and how it matches your eye and the physical spaces you have to shoot in.

Focal length is focal length. It is a property of the lens, not the camera.

A lens focuses light at a certain point (the film/sensor plane of the camera). It projects this light as a circle (the image circle).

Most cameras have sensors that are rectangular. To maximise your imaging you need a sensor that is smaller than this image circle. Imagine for 35mm/full frame format, a rectangle that has its points close to or on the line of the circle. Now draw a smaller rectangle inside your larger rectangle. This is the relationship between larger and smaller sensors, and why it is referred to as crop-factor - you are literally cropping the image supplied by the lens by using a smaller sensor. The physical properties of the lens remains the same (aperture, focal length etc) regardless of sensor used.

Where people get hung up is equivalency. If you want to use a lens on a smaller format to mimic the appearance of an image taken with a larger format, then you have to do the math for focal length and depth of field.

A normal lens makes pictures with perspective that feels natural. That is most of what people mean when they talk about the human eye. On a 35mm film camera or full frame digital that would be a 50mm arguably down to a 35mm. On an APS-C thats 35-27mm, on a medium format it’s around 80mm depending on film format. Outside of that and one will often look at a picture and notice the perspective imparted by the lens’s angle of view. Smartphone cameras with wide lenses have also generally moved this preference wider angle. People have peripheral vision and are constantly looking around, so it’s not really about what you can see, but how it looks. Print size, viewing distance, and the individual shot all play into how accurate this guideline is. Keep in mind that with a good enough lens and film/sensor one could get a shot at ultra wide and crop the exact image you would get all the way to super-telephoto.

As a side note, I have an Olympus OM1 that I used to shoot sports with using a 50mm f1.8 lens. That particular combination was an exact match for my vision. I could shoot vertical images with both eyes open, tracking action with my left eye and focus and compose through the lens in perfect stereo vision with my right eye. I’ve owned dozens of other cameras and no other systems did this, but it was great.

focal length should always be focal length (unless it’s labeled specifically as “equivalent focal length”)

… and people say that 50-55mm is the focal length of the human eye.

nit pick here: people say a 50mm equivalent is the focal length of the human eye (and only some people… others will say 35mm… others have very different answers because the eye and brain do not perceive the world the same way a camera does… it’s an age old debate and doesn’t have an exact answer, I digress… I’ll come back to this at the end)

So when people say field of view of a 50mm on a full frame is the same as your eye, my first thought is 50mm on a full frame = 35mm on my camera

Yes

What i expect is no zoom at all but the objects look smaller in the vizor (so fov is higher)

Your viewfinder itself has a magnification to it. Many cameras have a viewfinder with something like a 0.7x magnification, which shrinks the image to your eye. You’re not going to view the image on your viewfinder you’re going to view it on a monitor or as a print… keep that in mind.

Are the numbers of focal lengths on my lens already multiplier by 1.5x ? So do i have to subdivide the numbers to get the full frame equivalent ?

Nope. If you take a 35mm lens made for your APS-C or a 35mm lens made for a full frame camera and put it on your camera… they’d have pretty much the same field of view.

Now here’s the other thing about field of view: Look straight in front of you. Hold your hands out to either side with your fingers stretched out and slowly swing your arms forward in an arch to move them so they’re in front of you. But do so slowly while looking straight ahead and keep moving your arms forward until you just start to see your hands come into your field of view. Odds are your arms are spread very wide… you’d nee a 10mm or so on your camera to photograph that wide an angle. But (assuming you still have your eyes straight forward and don’t turn to look at your hands) your fingers are going to be a blurry mush. We have very wide peripheral vision but cannot really see that sharply out that far. Keep moving your arms forward until you can start to see your fingers individually… that’s going to be a lot closer to that 35 or 50mm equivalent focal length. So the question of which focal length is “normal” really can’t answer the way the human eye sees, but it’s more about the area we tend to focus on in our day-to-day lives…

So when people say field of view of a 50mm on a full frame is the same as your eye

​

This is not true and if someone says this, stop listening to anything they say, because they dont know what they are talking about. Human eye is closer to 43mm.