Confusion over the 1.6x factor for my Canon 20D and 40D.

A 50mm lens is a 50mm lens is a 50mm lens, irrespective of what camera it is put on, so the perspective will be the same. However, due to the sensors (or film) being different size, you will see different amounts of the image. This is why a 50mm lens mounted on your 40D is said to be equivalent to an 80mm lens. ie you'd need an 80mm lens on your OM1 to see the same (reduced) field of view you see on your 40D.

/dl

Dave,

Surely, if the 50mm focal length on my 40D becomes, according to all the photographic press, an 80mm equivalent, then surely the scene in the viewfinder should show this equivalent?
If any photographers here could just check this out for themselves I would appreciate it.

Thanks

Tony

Tony,
A quick google on 'fovcf' - field of view crop factor will give you a huge amount of reading...

A 50mm lens on non full frame canon cameras (which happen to have a fovcf of 1.6 or thereabouts) only uses the central part of the 50mm image that falls upon its sensor - hence making it 'appear' that the lens is actually 80mm - imagine zooming into a standard image at its centre by 1.6 times - and losing the outer edges.

the digital picture.com is a very good resource for canon based equipment... and has an excellent feature on this.

Tony,
What you are experiencing may be due to some degree of magnification in the viewfinder. The angle of view, and therefore the perceived perspective*, of a 50mm lens with DX format will indeed be the same as an 80mm with 35mm format.

The basis for what is termed a 'normal' lens is based on the following observation: When a person views a photographic print (or any image for that matter) they will be most comfortable viewing the image from a distance equal to the diagonal of the print (this does not hold for very small prints where holding it so close strains the eyes, nor for very high aspect ratio 'panoramic' formats). A normal lens is one which will produce a print that, when held at this optimum viewing distance, will show the scene as it appeared to the naked eye. It turns out that this occurs when the taking lens is at a distance from the sensor/film plane equal to the diagonal of the sensor/film format i.e. when focused at or near infinity(true most of the time for most scenes) the format diagonal equals focal length. Therefore a true 'normal' lens for 35mm film is actually 43mm and not the 50mm conventionally supplied - which equates to 27mm for a DX sensor. I think what you are experiencing is a result of the viewfinders imposing non-optimal (in the above sense) viewing distances and each viewfinder being different in that respect.



*lenses do not actually alter perspective, only angle of view. The eye, however, interprets otherwise.

It's just strange that the view is pretty much the same as the view through the viewfinder of my OM1n with 50mm lens attached. I appreciate that the viewfinder tells a few porkies, but 60% of the magnification doesn't go missing!

Tony

quote:

It's just strange that the view is pretty much the same as the view through the viewfinder of my OM1n with 50mm lens attached. I appreciate that the viewfinder tells a few porkies, but 60% of the image doesn't go missing!

You still don't have the same view as the CCD though. It's as if the picture is bigger than the CCD .
What you need to do is look through the VF and then take the picture without moving and see how much of that view is taken, if all of it is there, then I'm as mystified as you.

The viewfinder shows about 95% of what the image will contain. However, this is always about the same at sub-pro SLR level, film or digital.



Tony

Tony

Does this help at all?

Jim

Jim,

Thanks for the link. However it just backs up what I'm saying. I 'should' see the equivalent of an 80mm lens through the viewfinder with a 50mm lens fitted but I'm not! I'm seeing the same thing as I do through my old film SLR with its 50mm fitted..........

Tony