When talking about older/classic (pre-1950s) super wide angle lenses, people often say "they have hot spots". Could someone kindly explain to me what a "Hot Spot" is, in association with those super wide angle leses? Would a center filter help eliminating the hot spot (assuming a hot spot is uneven illumination on the ground glass caused by that lens, ie., corners get darker)? Thanks.

-- Geoffrey Chen (DB45TEK@AOL.COM), April 17, 2001

Answers

The basic idea is that the cone of light emanating from a wide lens has a pretty steep angle. These rays of light hit the GG and continue through. Obviously the image is brightest when your eye is placed in a direct line to the emanating light. So, when you have your eye near the center of the GG, the center looks very bright while the edges look less bright (since the light striking the edges is moving through at an angle and thus will not strike your eye). If you move your eye now to the edge to intercept these light rays, the center will look dark since the rays going through the center are not intercepted by the eye.

With a long lens, the angle of the cone is much smaller. The nett result is that a small movement (or no movement at all) of the head is enough to intercept the light rays. Also the light rays strike the GG at less extreme angles as compared to wide angles.

I've never used center filters and cannot comment on whether they would help (although I think they are meant to help with cos^4 falloff etc - so I wouldn't be surprised if they didn't).

Obviously the grain of the GG is meant to provide a partial diffusion surface to form the image on - the tradeoff is that greater diffusion reduces the hotspot but makes for a dimmer (and grainier) image. In other words, a fine ground glass will provide a sharp and bright image but with a pronounced hot spot, while a rough GG will produce a moree grainy and dim image but with less of a hot spot. Fresnel lenses are the typical solution, although they are beest optimised for each lens. The Bosscreen also gets rave reviews from users.

Ron Wisner has an interesting discussion of these points in his paper on fresnel lenses. Its available at www.wisner.com/viewing.

Cheers, DJ.

-- N Dhananjay (ndhanu@umich.edu), April 17, 2001.

Thanks, DJ! Sounds like a wide angle fresnel lens will reduce/eliminate hot spots. Am I correct? My widest lens for 8x10 is 210 mm and 90 mm for 4x5 (just say I have a narrow vision), so I can not experiment hot spots. Do those modern super wide angle lenses such as SA 121/8 and Nikkor SW 120/8, for 8x10, also have hot spots? If they do not have, then how do the lens makers eliminate it? My impression is that hot spots are ONLY associated with older super wide angle lenses. Of course, I could be deadly wrong.

-- Geoffrey Chen (DB45TEK@AOL.COM), April 18, 2001.

Current lenses have a hot spot. The rays reaching the edge of the film travel farther then the ones reaching the center. A center filter eliminates most of this effect from the film. A fresnel does nothing to the effect of the hot spot on the film. A frenel spreads the light over the entire viewing surface so the viewed image is bright and as even as possible. It will still show the hot spot caused by cosine failure.

-- Bob Salomon (bobsalomon@mindspring.com), April 18, 2001.

There are two different issues being discussed here, although they are both associated with wide-angle lenses.

The first is that all lenses have a cosine^4 fall-off, which is most noticable in wide-angle lenses because, well, the angle is so wide. This affects exposure and viewing. If this becomes objectionable, a center filter is used.

The second issue only involves ground glass viewing. Light rays come from the lens and hit the ground glass. Although the ground glass diffuses the light, the ray proceeding straight from the lens through the glass will still be the strongest. Therefore, when you look through the glass, when you look at it along an angle that points back to the lens, the center of your view will be brighter. Therefore, you may have to move around and tilt you loupe to see the whole image. A fresnel screen on your ground glass is intended to mitigate this problem. Again, this is most obvious with a wide angle lens since the rays are striking the glass at a not-so-perpendicular angle.

Now when someone mentions a "hot spot," you need to determine which phenomenon to which he is referring.

Interesting, about the only advantage to a reverse-telephoto design (or retrofocus, but Kingslake points out in The History of the Photographic Lens that retrofocus is a brand name, but has come to be used generically - like kleenex and velcro.), besides clearing the mirror in SLR's, is that it reduces these "hot spot" problems. But a reverse telephoto is apparently more difficult to compensate for distortion, so I consider one of the greatest advantages of a view camera or a rangefinder to be distortionless wide angles.

But I digress.

-- John H. Henderson (jhende03@harris.com), April 18, 2001.