Testing NEX-6 equivalent focal length

boggle · Oct 15, 2014

I have a NEX-6 camera with the 16-50 kit lens. I also have a Minolta 50mm legacy lens. I just thought I would check the relative focal lengths of the two lenses.

Using the 50mm, I adjusted the distance from a window until it just spanned one of the window panes.

I then replaced the lens with the zoom lens and, at the same difference, I adjusted the zoom until the pane was also spanned.

.I was surprised that the focal length on the screen of the NEX also showed 50mm. I would have expected there to be a difference. Isn't the "equivalent" focal length about 75mm when using the 50mm lens?

I am probably being very stupid here but I must be missing something. Could someone advise me?

boggle · Oct 16, 2014

Any ideas, anyone?

CatLikeFelix · Oct 16, 2014

50mm is 50mm

mikethelaserman · Oct 16, 2014

Exactly!

The field of view is not controlled by the lens, but by the dimensions of the sensor.

EinsteinsGhost · Oct 16, 2014

boggle said:
I have a NEX-6 camera with the 16-50 kit lens. I also have a Minolta 50mm legacy lens. I just thought I would check the relative focal lengths of the two lenses.

Using the 50mm, I adjusted the distance from a window until it just spanned one of the window panes.

I then replaced the lens with the zoom lens and, at the same difference, I adjusted the zoom until the pane was also spanned.

.I was surprised that the focal length on the screen of the NEX also showed 50mm. I would have expected there to be a difference. Isn't the "equivalent" focal length about 75mm when using the 50mm lens?

I am probably being very stupid here but I must be missing something. Could someone advise me?

The focal length is still 50mm. However, due to crop, the field of view compare to that of 75mm lens on a FF camera (note the distinction between focal length and field of view).

boggle · Oct 16, 2014

so, as far as field of view is concerned if I want to fit an equivalent to a 30mm on the Nex-6 I use a 30mm legacy lens?

uhligfd · Oct 16, 2014

Why do you think that a 50mm prime lens and a zoom extended to 50mm focal length should not give the same view on the same camera?

When I buy a 12 ounce bottle of beer, somehow it still has the same volume of brew inside when I get home.

Did you have different experiences somehow? Please tell!

chromnd · Oct 16, 2014

boggle said:
I was surprised that the focal length on the screen of the NEX also showed 50mm. I would have expected there to be a difference. Isn't the "equivalent" focal length about 75mm when using the 50mm lens?

The NEX does not show the equivalent focal length but only the real focal length. This is also true for all (E-mount) lenses: The focal length printed on the lens is not the equivalent focal length but the physical focal length.

The focal length for your Minolta 50mm and the SEL1650mm at the long end is therefore the same (50mm) or 75mm equivalent when used on the NEX-6

eques · Oct 16, 2014

http://www.dpreview.com/articles/2666934640/what-is-equivalence-and-why-should-i-care

... at least the first part about FLs. There are helpful pictues, too.

Peter

CatLikeFelix · Oct 16, 2014

boggle said:
so, as far as field of view is concerned if I want to fit an equivalent to a 30mm on the Nex-6 I use a 30mm legacy lens?

If you want to to get the same/equivalent FoV that you would on a '35mm film/FF camera using a 30mm lens' you would need to put something like an 18-20mm lens on your NEX system camera. In other words the smaller (APS-C) sensor crops the image down, in effect putting you closer to your subject

--
https://www.flickr.com/photos/catlikefelix/

boggle · Oct 17, 2014

so 50mm lenses (NEX and Legacy) look the same apart from depth of focus, etc, on the NEX camera? Why then do people so often talk about equivalent focal length and quote factors to do the conversion?

I assume with my NEX-6 when I buy another legacy lens I need essentially just to concentrate on the focal length. So using a 50mm NEX lens, if I decide I need a wider angle lens I just go for, say, a 30mm legacy one and not bother about equivalent focal length?

eques · Oct 18, 2014

Here a small overview:

3b79cb55c9144a6a81effdafc9d606e9.jpg.png

If you use a 35mm on the NEX6 it will give you the same picture as a 50mm on a FF camera. This applies to all legacy lenses and Sony FE lenses
A 50 mm on the NEX is a short tele lens.

The only thing is, if you use a Sony E 50mm on an A7 FF camera, the lens won't fill the wole sensor aerea and you will only be able to use the APSc sized part of the sensor. Because of this the Sony E 50mm will again be a short tele.

Again, read the DPR article I linked for you 2 days ago. If you don't understand it, just try a few cheap legacy lenses and you will see yourself.

Peter.

EinsteinsGhost · Oct 18, 2014

boggle said:
so, as far as field of view is concerned if I want to fit an equivalent to a 30mm on the Nex-6 I use a 30mm legacy lens?

Equivalence is basically comparing the field of view (FOV) between formats. For example, to get a FOV of about 45-degrees on a FF camera, you'd use a 50mm lens. To get the same FOV on an APS-C camera, you'd use a 35mm lens. In other words, the same lens will give you a narrower FOV on a smaller format.

So, if you want a FOV that you get by using a 30mm lens on a FF camera, you would need a 20mm lens on APS-C camera (a crop factor of 1.5x). A 30mm lens on APS-C camera will give you a FOV which you will get by using a 45mm lens on a FF camera.

CameraCarl · Oct 18, 2014

If you put a 50mm lens on a NEX-6 the camera will show it as 50mm, but if you look in the exif data in Lightroom 5 it will show both the actual focal length of the lens as 50mm and the equivalent focal length ("35mm focal length') as 75mm.

EinsteinsGhost · Oct 18, 2014

CameraCarl said:
If you put a 50mm lens on a NEX-6 the camera will show it as 50mm, but if you look in the exif data in Lightroom 5 it will show both the actual focal length of the lens as 50mm and the equivalent focal length ("35mm focal length') as 75mm.

It is simply a lazy approach, when the more appropriate would be using field of view instead.

boggle · Oct 23, 2014

Hi Everyone,

Thanks for being so helpful but I STILL don't understand why if I look at the FOV when using a 50mm Legacy lens and adjust my distance from the subject (a window) until it fills a pane of the window and then switch to the Sony 16-50mm zoom supplied with the camera. I adjust this to 50mm and it also spans the pane, so it looks as though 50mm Legacy has the same FOV as the Sony NEX lens adjusted to 50mm

Brian

eques · Oct 23, 2014

boggle said:
Hi Everyone,

Thanks for being so helpful but I STILL don't understand why if I look at the FOV when using a 50mm Legacy lens and adjust my distance from the subject (a window) until it fills a pane of the window and then switch to the Sony 16-50mm zoom supplied with the camera. I adjust this to 50mm and it also spans the pane, so it looks as though 50mm Legacy has the same FOV as the Sony NEX lens adjusted to 50mm

Brian

You have got it! As someone said before - 50mm is 50mm:

If used with the same sensor size, the same FL will result in the same FOV.

However, given the same FL, if the sensor is larger, the FOV will be wider, and if the sensor is smaller, the FOV is smaller. Here the picture from the dpreview article, I recommended 6 days ago (p.2): read it from right to left:

You see the image circle on the right with the dotted line of a big sensor and the darker rectangle representing the smaller sensor.

On the left you see the resulting FOV: orange for the small sensor, yellowish for the bigger sensor. In both cases the FL is the same; the lens is represented by the light blue circle.

59cf3d33506b49f9b0ae4bdd7ee4168a.jpg.png

Peter.

CatLikeFelix · Oct 23, 2014

boggle said:
Hi Everyone,

Thanks for being so helpful but I STILL don't understand why if I look at the FOV when using a 50mm Legacy lens and adjust my distance from the subject (a window) until it fills a pane of the window and then switch to the Sony 16-50mm zoom supplied with the camera. I adjust this to 50mm and it also spans the pane, so it looks as though 50mm Legacy has the same FOV as the Sony NEX lens adjusted to 50mm

Brian

are you joking?

mikethelaserman · Oct 23, 2014

I think that diagram is MUCH too complicated for someone with little understanding of lenses, photography - or anything.

boggle · Oct 23, 2014

I can assure you that I DO understand the reasoning, that the focal length of the lens is not changed by fitting it with an adapter to a camera because it is a measure of the design of the lens and the relative position of the lenses within it. I also understand that the reason equivalent focal length is used is because different sized sensors are involved and the NEX sensor is smaller. As a result its field of view is changed so it appears to be created by a lens of greater focal length (approx 1.5 times the actual focal length).

What I can't understand is. assuming when the zoom lens is set to 50mm, it should be equivalent to a NEX 50mm lens. So if I fit a 50mm Minolta lens with an adapter to the camera we are comparing its affect to comparing a 50mm Minolta to a 50mm NEX and in practical terms this is not so. The pictures produced have the same FOV.

Testing NEX-6 equivalent focal length

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