Conundrum: exposure compensation for rise?

[Leigh Perry]

I have made a couple of attempts to shoot an ultra-wide angle shot with lots of front rise. My first attempt was wildly underexposed -- perhaps by 2-3 stops. This wasn’t a metering error. On the second attempt, I bracketed +1 stop, +2 stops, and +2.5 stops. I got the shot with the +2.5 exposure, but don’t understand the reasons.

The lens is a Schneider 38XL using roughly a 6x9 crop out of a 6x12 frame. I used a centre filter. The lens just ran out of image circle at the top of the photograph.

I suspect the cause must be some exposure compensation requirement for rise-type movement, perhaps exacerbated by the extreme hot-spot characteristics of the 38mm and the use of the image circle extremity. I’ve never seen any details of such compensation though, while there is plenty of mention of bellows-extension-related compensation.

Can anyone shed any light on the cause? If it helps, the shot is here.

[Bruce Watson]

I've never used a center filter before, but it would seem to work by putting a neutral density filter over the center, and feathering this density out to the edges as required for the lens in question. IOW, I think you have to have exposure compensation for the center filter. I don't think it's a problem with using movements.

[David A. Goldfarb]

Front rise per se doesn't require compensation, but if there is falloff with an ultrawide lens, then you need to take account for it. If you're using a center ND filter, then you shouldn't have falloff, but you need to add the filter factor for the amount of density at the center of the image circle.

[Bob Salomon]

Yo use a center filter the lens must be stopped down at least 2 stops for the filter to have any effect. Then you need to add the filter factor of 1 1/3 to 1 1/2 stops. Was this done? Lastly 35 aand 38mm lenses do not allow much movement. The 35mm 120° Apo Grandagon allows 20mm of rise and 15mm of shift with a 6x9cm in horizontal orientation.The 38mm isn't much different in coverage.

[Brian Ellis]

Here's a highly untechnical explanation. When you use a wide angle lens the light from the outer portion of the image circle has a greater distance to travel and travels at a steeper angle to the film than the light from the center portion of the image circle, which can lead to under-exposure of the portions of the photograph at the outer edge of the image circle (which you already know). When you combine a wide angle lens with a lot of front rise you're exacerbating this situation, i.e. you're moving part of the image circle even farther from the film than it already was, thus leading to even greater potential underexposure. That's my understanding of the situation anyhow. If it's wrong please don't correct me because it makes sense to me and I'd like to keep it that way without being confused by the actual facts if they differ from my understanding.

[Henry Ambrose]

I think you are asking about the darkening at the very top edge of the frame, as the rest of the frame seems good? Assuming that you used the appropriate compensation for the center filter, it looks like to me that you simply ran out of image circle.

[Michael S. Briggs]

Both Schneider and Rodenstock publish curves of the illumination vs angle off-axis for their lenses. The curves typically follow closely the theoretically expected cosine to the third, or cosine to the fourth, functions expected. I've explained this theory in previous postings. From these curves, or from the theory, one could calculate exposure compensations when using rise. But this isn't applicable to your photo since you used a center filter.

I know of only one illumination curve published by a manufacturer for the case that a center filter IS used. The Schneider instruction brochure that accompanies their center filters shows the relative illumination vs distance from the axis for the 90 mm f5.6 Super-Angulon, both with and without center filter. The curves show that the center filter helps greatly, but does NOT result in uniform illumination. For example, by the corners of a 5x7 film, there is about 2 stops falloff without a center filter and 0.9 stops with the center filter.

Scheider explains this in the brochure: "In the interest of not extending the exposure time excessively and of fully exploiting the exposure latitude of the negative layers, no attempt was made at achieving full compensation of the brightness differential."

I think probably all manufacturers of center filters follow this approach of not full correcting the uneven illumination -- the recommended exposure corrections are about the same from all manufacturers.

Did you use Schneider's IIa center filter? Schneider recommends 2 stops exposure compenstation for this filter. I have a different Schneider center filter and find that I need 1/3 to 1/2 more compensation than recommended by Schneider -- perhaps their are manufacturing variations, or perhaps I just judge differently than Schneider.

As Bob stated, the Schneider brochure also recommends stopping the lens down at least two stops. This is to get past vignetting and to something close to the best relative illumination possible according to the lens design. In the case of the Super-Angulon, that would be cosine to the third performance.

There are several aspects that aren't clear from your question. When you say that you applied +1 to +2.5 stops, if that from your normal metering, or in addition to the +2 stops needed when you use the center filter. Are you using transparency or negative film?

Using the lens to, or just past, its coverage limits, means that parts of the film are still receiving substantially less light than the on-axis location. With a negative film, you might want to apply additional exposure to correctly expose the extreme off-axis areas, allowing overexposure of the center. A negative film will work better because it has much more exposure latitude than a transparency film, particularly on the over-exposure side.

The extreme contrast of the scence you photographed is exacerbating the effect.

Some additional good advice from Schneider: "It is just as well to remember, that, whenever wide-angle lenses are used, high contrast negative material, under-exposure and strong contrast should, as far as possible, be avoided." Transparency films are more to be avoided than high-contrast negative films.

[Bob Salomon]

"I think probably all manufacturers of center filters follow this approach of not full correcting the uneven illumination -- the recommended exposure corrections are about the same from all manufacturers."

Heliopan was making 2 different density center filters. The standard 0.34 density and a 0.9 density. The 0.9 did result in a more even image but required a 3 stop factor. Due to the loss of 3 stops and the need to stop down at least 2 stops this was not a very popular filter and was discontinued. However, depending on the size required, we still do have some left in stock.

[David A. Goldfarb]

You can sometimes use this falloff to your advantage by composing around it. Use rise, fall, and shift to put the hotspot over the subject of the image or to darken the sky in a landscape. I think Adams mentions this technique in passing in _The Camera_.

[Leigh Perry]

Thanks for the responses.

You use a center filter the lens must be stopped down at least 2 stops for the filter to have any effect. Then you need to add the filter factor of 1 1/3 to 1 1/2 stops. Was this done?
Yes. I used the 38XL's centre filter and added the requisite two stops. The exposure compensation was in addition to the centre filter's 2 stops. I don't believe it is a metering error.

Brian's non-technical explanation matches my theory regarding lens hot-spot / light fall-off.