Krueger Photography

Perspective Correction in the Darkroom

This article defines perspective control in photography and how perspective corrections can be applied in the darkroom.

What is Perspective Control in Photography

Anyone who shoots architecture knows about how to ensure that images containing a lot of straight lines and square corners are aligned with the edges of a picture. For example, when taking a picture of the front of a building, try to hold the camera as close to horizontal as possible without tipping it up or down. That technique, as illustrated here, eliminates or minimizes the convergence of vertical lines. Likewise, pointing the camera perfectly perpendicular to the face of the building eliminates converging horizontal lines. These two techniques can be used together or separately. The avoidance of converging vertical lines is far more widely practiced than avoiding convergence of horizonal lines. Generally, straight-on pictures aren’t the most interesting, so art photographers tend to look for more interesting and unusual angles. But, there are times when a straight-on approach is needed for illustrative purposes. Not only that, but architectural photographers are sensitized to subject matter lines that lie close to the edge of the photo that are almost, but not quite parallel to the edge of the frame, especially when tilting the camera would eliminate the aesthetic annoyance without adversely impacting the composition.

The above principle can be used with any geometric subject matter. Generally, you’ll notice adherence to the principles of avoiding converging vertical lines in architectural photography, both indoors and out.

Correcting Perspective After the Fact

This article is for someone who has a great negative and wants to make a nice darkroom print of it to hang on the wall, but didn’t get the framing quite right when the photo was taken. There’s just no way to frame the picture where the subject matter lines don’t look slightly “crooked” relative to the edge of the frame.

Most architectural photographers have a tool that makes it relatively easy to maintain correct perspective even when they can’t put themselves in a perfect position to do so. The tool is a lens that can shift and/or tilt relative to the film plane. View cameras are designed to have those movements and there are now many lenses that can add that functionality to more mobile film and digital cameras.

But, that doesn’t help with a less than perfect negative that you already have. The quick solution would be to scan the negative or print, fix the perspective in Photoshop and then make a digital print. An alternate solution would be to duplicate the shift and tilt capability with your enlarger.

Perspective control using an enlarger consists simply of tilting the easel to create a gradient to the amount of enlargement the negative is subject to. The end of the easel closer to the enlarger lens gets less enlargement. If your picture is of a building where you tilted the camera up slightly to include the top of the building in the frame, you would want the top of the building on the part of the easel further from the lens and the bottom of the building on the part of the easel closer to the enlarger lens.

Once you have a way tilting the easel, you with then need to tilt the lens to allow the entire image to be in focus on the tilted easel. Some enlargers have accessories or built in mechanisms to tilt the lens board, negative stage, or easel to achieve the desired effect. A few examples can be found in this Photrio thread. I purchased a lens tilting accessory for my Beseler 67SC that had graduated markings for tilting and shifting the lens, but that enlarger is currently in storage.

In 2018 I made a DIY tiltable lens board for my Omega D5 enlargers. It was intended to be an adjustable lens board to allow easy enlarger alignment corrections, but I intentionally made the adjustment range quite large to allow perspective corrections.

I only recently decided to test it to see if it could really be used to correct perspective. While I didn’t pick the best negative for the test, the results did confirm that it would work.

The negative to be corrected is a picture of a storefront taken in New Orleans. As you can see from the full frame negative scan below, the camera was close to horizontal, so there wasn’t a lot of vertical correction needed, but it would benefit from some horizontal perspective correction. The horizontal lines converge to the right. That part of the image will therefore be placed at the low side of the tilted easel.

One of the reasons this is a poor choice of negatives for demonstrating perspective correction is that this lens exhibited substantial barrel distortion which means the lines will not be straight and parallel even if the perspective adjustment is perfect. The results will show that I didn’t tilt the easel enough, making the result less than perfect.

The two following images show the cropped area from the negative scan followed by a scan of the perspective-corrected print. I used a negative scan simply because I never made a straight full frame uncorrected print from this negative.

Lens Distortion

The barrel distortion really interferes with a clear demonstration of the correction. Notably, the distortion correction actually reduces the apparent barrel distortion on the right side of the image. This negative dates back to 1993 when I was using a Sigma 28-70mm lens. At the time, I just lived with whatever distortion my lenses served up. I now have newer Nikon lenses with greater zoom ranges and plenty of distortion. To borrow from Ken Rockwell, some newer Nikon lenses have “heinous” amounts of distortion. Why? Because consumers prioritize low cost and greater zoom ranges over less distortion especially since modern digital cameras will automatically correct for most of it. Film cameras won’t.

For the last few years, I have been carrying a small folding pocket chart that lists the focal lengths at which the distortion is lowest for all of my zoom lenses . That’s the point at which the distortion crosses over from barrel to pincushion. That’s another contribution from Ken Rockwell who lists distortion characteristics for the lenses he reviews and which I have found to be quite accurate. Most of the pictures I take don’t demand low distortion simply because I often shoot from unusual angles, but when I want low distortion, I will find a way to use a lens (I often carry several) that has low distortion at a focal length that will work.

Distortion Introduced by Tilting the Lens and Easel?

Its obvious that the tilted easel will introduce distortion in the direction of the axis of the tilt. That stretching is what reduces the distortion that you’re trying to eliminate. I don’t know if it introduces stretching or compression in the direction of the tilt but, from what I’ve read, a longer focal length lens reduces the required tilt to achieve the same effect. For this project, I used my 63mm EL Nikkor that I usually use for printing 35mm negatives on 8″x10″ paper. The tilt of the lens seems quite small. The picture below shows the lens exactly as adjusted in this case.

Focusing on a Tilted Easel

My Peak enlarger focusing aid was not very useful with the easel propped up as it was here, so I used a magnifying glass. I had to adjust the lens tilt only twice to get the image in reasonable focus in all four corners of the 8″x8″ image area. I made two prints at f5.6 and I could see the grain structure in all four corners of the print, but decided to do a print at f8.0 to see if there was any improvement. The f8.0 print was indeed a little crisper on one corner, but I needed to look through the small higher magnification inset of the magnifying glass to notice it.

Is Doing This Worth All the Trouble?

If you’re trying to salvage a negative that you really like except for the distortion, I’d conclude that it is definitely worth the trouble. What would make it better is a more stable tilting mechanism for the easel probably using a smaller easel if you’re making smaller prints. In this case I was using a 16×20 Saunders easel propped up on one side with a 3.5″ piece of wood. I used some foam non-slip drawer lining material to keep the easel from sliding around on the baseboard.

One thing I didn’t think of when I built the tilting lens stage is that it would be handy if the lens stage could be rotated so the easel could always be tilted toward me as I have it set up in the pictures above. The negative and easel must have a fixed relationship, but the tilt of the lens doesn’t have to be long one axis of the frame or the other. For the negative I used, most of tilt was along the image vertical access (for converging horizontal lines), but it could have used some tilt along the horizontal access for a small amount of vertical distortion (very slightly converging of vertical lines). In other words, the best lens tilt for some images may require a compound tilt that includes both a horizontal and vertical component. If the lens stage could be rotated, it would be easier to parse how the tilt is divided between each of the two axes.

Now that I’ve finally broken the ice, I will be more willing to go the extra mile to correct a small, but annoying, case of “crooked” verticals running up along the edges of a print.