Optics question regarding LF vs 35mm ground glass

[marshallarts]

I've been doing some testing and am confused, hopefully someone can explain this to me.

I noticed when using a GG looking through the back of a 35mm film camera, at any given f/stop it has much less of a hot spot and is overall much brighter and covers edge to edge of frame than when I look at my medium density GG through my 4x5 Graflex.

Having the same f/stop means the same amount of light relative to the size of the image plane should be let in. I understand the inverse square law of light and the fact that the 4x5 is further away from the GG, but f/stop should account for this. If I stop down to a f/8 on my 35mm camera without the GG the aperture is small and yet when I place a GG over where the film will be the image still covers the GG edge to edge and is fairly brighter than when looking through the same GG on my 4x5 at that f/stop--and in this case the hot spot is severe and vignetting makes it very hard to see the edges when looking straight on.

The only thing I'm not accounting for that I can think of is grain size. With the same GG in 35mm the grains will be relatively bigger. But I would assume the fact it's brighter is opposite that of what I would expect.

I'm trying to find a way to capture images digitally from the back of my 4x5 without the hot spot. I can easily do this with a 35mm camera because the image on the GG is very good, of course I don't want to do that. I'm trying to figure out why I can't do it with a 4x5 camera.

Is it lens design? If so why? (Since both end images look correct at the film plane -- once you develop the negative). Hope this makes sense. thanks!

[Drew Wiley]

Your eye and the film plane see luminance a little differently, both with 35mm and 4x5. You'd have to make a critical test with your particular lens on film and with a densitometer. There are various factors, but wide angle lenses tend to have much more illumination falloff than longer lenses. Ground glass can be made in several different ways to improve viewing, and some people add fresnel lenses to help remove the "hot spot" with wide angle lenses, though I don't like them myself. Even most pro 35mm cameras have interchangeable screens to accommodate different kinds of viewing.

[Jim Galli]

I think you'll find the 'so-called' hot spot on the 4X5 glass is an illusion. The film doesn't see it that way. No one talks about center filters that can effectively balance a slightly hotter center except when lenses get well beyond the 90 degree angle of light, where the path to the corner is significantly longer than the path to the center.

[Struan Gray]

LF and SLR 35 mm wide angle lenses are designed differently. Those for 35 mm cameras have to be made so that the rear of the lens does not foul the mirror when it flips up, which is why they tend to have 'retrofocus' designs. Crucially, these make the light appear to be coming from further away, which means that light hitting the corners does not make such a steep angle compared to in a LF camera.

The shallower range of angles means that the intensity of the light drops off more gradually. So the vignetting even on film is, in principle, less severe, but this depends on several other aspects of the optical design so the correspondence is not simple or exact. More important for LF hot spots is the fact that the ground glass only partially scatters the light, allowing a significant proportion to pass directly through. At the corners, that straight-through light is moving off at an angle and never meets your eye - you only get to see the light which is scattered back towards the eye, and that is only a small part of the whole. If you put your eye at the corner and look towards where the aperture is, you should see a brighter image, but now some other part of the ground glass will appear dim.

With a ground glass on the film rails of a 35 mm camera the range of angles through which light must be scattered to reach your eye is smaller, and the ground glass image appears to be more even.

You can grind the glass so that it scatters more strongly, but then the whole ground glass becomes darker, and you need a coarser grain which makes focussing with a loupe more difficult. A Fresnel lens in an LF camera bends the light rays so that it is easier for them to enter your eye, and makes the ground glass image more even, but some people find the lines of the Fresnel disturbing too. A loupe which can tilt is useful for focussing the corners and edges, as it can be aligned with the brightest, unscattered, rays of light from the aperture.

[Bob Salomon]

Does your smaller gg include a fresnel? Does your 45 have a fresnel?

[marshallarts]

I can understand how the hot-spot in LF is really an illusion, seeing as I move my head the hot spot moves too -- what I'm really seeing is the lens opening in relation to what part of the GG I'm looking through and seeing it. Since my eye is only in one position it appears a hot-spot is there, but when there is film loaded all the points along that plane will absorb that hot spot evenly and there is no hot spot in the final image (vignetting on wide angle aside considering inverse square distance to the corners).

But why with the Exact Same GG would a 35mm "projection" appear even and edge to edge, and yet a LF "projection" much less so? How would lens design create this anomaly?

[Drew Wiley]

It might help if you were a little more specific about which specific LF lens are camera you are referring to. But until quite recently, 35mm SLR's were designed for straight-on lens axis only. A view camera is designed for movements which make the image circles of many hypothetical lenses behave in different manners, so it's difficult to take a one-shoe-fits-all approach to viewing. Anything serious is inspected through a loupe anyway. You also don't specify how it is you intend to do digital
capture of the film plane. Are you simply trying to take a shot of the groundglass itself with a DLSR, or are you contemplating an actual digital back?

[Vaughn]

If your eye is on the same line that the light is traveling thru the lens, then you will have the max brightness on the GG. Basically, look from the spot on the GG you are interested in (corner, edge or middle) and look from that spot towards the center of the lens. With 35mm, it will all seem bright since the light coming from the lens is all coming at a much shorter angle towards the edges of the film plane.

Hope that makes sense.

Edited to add: How big in the average 'hot spot' on a GG? Somewhere around the size of a 35mm negative. So a 35mm GG is really all one big 'hot spot' -- the area of the 35mm GG is small enough that all the light hitting it from the lens is traveling on a tight enough path that the eye sees little difference between the center of the GG and its corners.

Don't know if that is any clearer...lol!

[Bill_1856]

You need a good Fresnel screen to spread the light out evenly. The best is Maxwell, but they ain't cheap. If your Graflex is a Super D model it should have had a Kodak Ektalite screen already installed. You can also do the job cheaply, with one of those screens at Barnes and Noble which makes it easier to read books.

[Struan Gray]

Take a 90 mm for 4x5 and a 28 mm for 35 mm. For both lenses, light appears to emerge from the back of the aperture and travel to the film plane. The apparent position of the aperture as seen through the rear glass of the lens defines what optics engineers call the 'exit pupil', and it is the size and placement of the exit pupil which determines most of the relevant angles.

Roughly, a 90 mm lens for 4x5 will have its exit pupil 90 mm from the ground glass when focussed on infinity. Stopped well down, light coming from the small aperture meets the corners at an angle of 42° or so.

A 28 mm lens for a 35 mm SLR has an aperture which appears to be, let's say, 50 mm from the film plane. The angle to the corners is now around 24°.

Most ground glass scatters more light through small angles than through large ones. The smaller angle by which the light needs to be diverted in the case of 35 mm is the major reason why you don't see hot spots with those systems.

There is a further factor. Assuming that you are not using a loupe, your eye can only focus down to some minimum distance. In young adults without long or short sighted vision this is typically 20-30 cm. Long-standing convention says a 'standard observer' can focus down to 25 cm. You can do the math for this one, but it is clear that an eye 25 cm from a 24x36 mm patch of ground glass is going to collect light from a smaller range of angles than one 25 cm from a 4x5" piece. The brightness of the 35 mm ground glass will vary as you move your head off axis, but the eveness across the ground glass will always be better than in the case of 4x5.

FWIW, long lenses on 4x5 don't have any hot spot. It's one reason I like using them so much :-)