View Full Version : Lens Confusion

21-Nov-2010, 15:57
I know Nikon DSLR lenses pretty well, and how they perform on a camera. I am a bit confused by large format lenses. I got the image circle thing, but not so much angle of view. I'm after a vintage lens for my Shen Hao 4x5 field camera. I want something that will perform roughly like a 35mm lens on a Nikon film camera. I.e., moderately wide angle. I currently have a Schneider SA 90mm f5.6 that seems to be just that. OK, so 90mm lens roughly equiv. to 35mm in "Nikon" format. The 90mm lens is roughly 3.5 to 4 inches. I got it. Here's what's confusing me. The B&H site says that a Nikon 35mm f1.4 lens has 63 degree angle of view. It also says a Schnieder SA 90mm f6.8 has 100 degree coverage. Angle of view and degree coverage are not the same thing? Moving on, if I see that the SA lens has 100 degree coverage, but I see that an 19th century vintage lens is also saying 100 degree coverage but is focal length 13 inches, in practical terms what does that mean?

Bottom line is if I'm looking for a c.1870 lens that will perfrom as a wide angle (e.g. like my SA 90mm) on my 4x5, should I pay attention to degree coverage or focal length? Should I be looking for a lens that is 4 or so inches and that's all I need to consider?

Kent in SD

Jan Pedersen
21-Nov-2010, 16:11
Image Circle is important, that is what will give you room for movements.
A 4x5 sheet of film needs about 154mm corner to corner so you always need at least that. If you also want to have a bit of movements 200mm IC is nice.

I am not sure there are any 100 degree lenses from 1870 at least not in short focal lenght like a 90mm. If you don't need anything shorter than 90mm 90 a degree lens will give you roughly 180mm IC which is tight.

A 90mm SA with 100 degree coverage will have around 214mm IC which is a little bit less than Nikons 90mm 4.5 which has a 105 degree coverage and 234mm IC

You small format 35mm lens does not need a large IC because the film is much smaller and a small format camera does not have any movements the exception being the few shift lenses. The IC for your 35mm is roughly 42mm.

Hope this helps.

John Koehrer
21-Nov-2010, 19:38
The AOV doesn't have a direct comparison because of the difference in ratios of format.
Using a horizontal AOV 65deg=28/93mm, 54deg=35/117mm, 39deg=50/167mm.
So your desire to have an equivalent of a 35mm on 35mm film means you would need(roughly) a 117mm lens on 4X5 for similar angle of view.

Mark Barendt
22-Nov-2010, 05:08
The angle of view can be "defined" geometrically as a 5 sided pyrimid.

The film makes up the 4 sided base "side" the lens defines the top point.

Focusing at infinity provides the widest "sharp" angle of view for a given film/lens combo.

Focusing on subjects closer than infinity with a LF camera means moving the lens further away from the film, that makes the pyramid get taller and the effective the angle of view get narrower.

The coverage of a LF lens may be much larger than the circle needed to cover the film.

The lens projects a "cone" of light. On a LF camera the lens is regularly tilted or swung on purpose in relation to the film for various reasons like scheimpflug.

This swings/tilts the cone of light.

You can actually tilt/shift/swing/lift/drop that cone of light far enough on many LF cameras so that the film starts going outside the cone of light, this creates a vignette.

"Good" coverage allows reasonable tilt/shift/swing/lift/drop of the lens, it does not change the angle of view.