Viewfinder brightness with 645 lenses

bc_the_path · 03-24-2009, 04:48 PM

I wonder if the viewfinder image becomes brighter if one uses a 645 lens.
In other words; will the brightness be different with two lenses each with a maximum aperture of, say, f/2.8?
I somehow think that the larger physical size of the 645 lens should allow more light to come in.
Nonsense?

*SOldBear* · 03-24-2009, 05:01 PM

I don't think so. I once use a huge Pentacon lens that was made for a 6X6 camera, didn't see any difference.

Have you used a Pentax 110? The lens is tiny, yet the viewfinder is just as bright as those of the big brothers.

*Wheatfield* · 03-24-2009, 05:23 PM

Originally Posted by bc_the_path

I wonder if the viewfinder image becomes brighter if one uses a 645 lens.
In other words; will the brightness be different with two lenses each with a maximum aperture of, say, f/2.8?
I somehow think that the larger physical size of the 645 lens should allow more light to come in.
Nonsense?

The f number is the measure of how much light a lens lets in. An f/2.8 lens for the 645 will let in the same amount of light (more or less) as an f/2.8 lens designed for the APS-C digital format.
The physical size of medium format glass is largely due to the increased image circle that the lens covers.
This extra light is not image forming, it just bounces around inside the mirror box, possibly causing flare.

Sean Nelson · 03-25-2009, 01:34 AM

Originally Posted by bc_the_path

I wonder if the viewfinder image becomes brighter if one uses a 645 lens.

No. Brightness is controlled by the f/number of the lens, it doesn't matter whether it's a 645 lens, a full-frame (FA) lens, or an APS-C (DA) lens - if the f/number is the same the image will be the same brightness.

What the 645 lens does is to illuminate a wider field - it means you could make the viewfinder (or sensor, of course) wider and taller and still see a clear image across the whole thing. Technically speaking that's more total light, but it's spread over a larger area so that the brightness per unit area is the same.

*christinelandon* · 03-25-2009, 01:43 AM

Well explained, Sean!

bc_the_path · 03-25-2009, 01:52 AM

I am illuminated

Thanks, friends!

RiceHigh · 03-25-2009, 07:29 AM

Originally Posted by bc_the_path

I wonder if the viewfinder image becomes brighter if one uses a 645 lens.
In other words; will the brightness be different with two lenses each with a maximum aperture of, say, f/2.8?
I somehow think that the larger physical size of the 645 lens should allow more light to come in.
Nonsense?

That depends on the finder magnification. For the same image magnification with the same Angle of View, the 645 lens will be brighter, even with the same aperture number. It is just because the image area is larger, if the same magnification is kept, more light are condensed into the same viewing area.

The same thing holds true if you have a 4/3 DSLR and viewing for the same magnification, the 4/3 will be dimmer.