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Large Digital Mosaics as a Substitute for Large Format Film

by Roger N. Clark

Introduction

If you like to make large "knock your socks off" prints, the traditional method of achieving the required image quality is to work with large format film (4x5, 5x7, 8x10 inch film and larger). For example, a 4x5 sheet of film, drum scanned on a high-end scanner can produce 12,000 x 16,000 pixel images and larger, far higher than current digital cameras. An alternative method to make large pixel count images is to mosaic many smaller images together (either film or digital). But large format view cameras have interesting advantages that ordinary digital cameras and lenses do not have. The main advantages of a view camera are the tilts and shifts of the film plane and the lens plane (Scheimpflug rule). Tilts and shifts are used to change perspective and change the plane of best focus to help obtain a greater depth of field. Can mosaics of digital camera images do the same thing, and can they achieve the same resolution needed to make large sharp prints? This articles discusses some of these issues.


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Figure 1. A 59-frame mosaic using 8.2-megapixel digital camera. The full resolution image is 15,250 x 12,571 pixels. The morning view is from Shrine Pass, Colorado and the tallest mountain to the right of center is the Mount of the Holy Cross.

The image in Figure 1 shows an example of a digital mosaic that included 59 frames from a Canon 1D Mark II (8.2 megapixel) digital camera. The lens used was a Canon 28-135 mm IS f/3.5-5.6 zoom lens working at f/11 and 65mm focal length. The exposure time on every frame (camera on manual) was 1/80th second at ISO 100 with a circular polarizer. Only jpeg images were recorded. The images were recorded in 4 rows using a custom panoramic tripod head with the camera mounted so that the long axis of the frame was vertical (portrait mode). The camera was used in autofocus mode on the center sensor so the focal point of each image was different and optimum for that frame. The idea with this strategy is that a faster f/ratio could be used than that typically used with large format cameras requiring a larger depth of field (from the nearest point to the horizon). The horizontal field of view of the final image is approximately 90 degrees, similar to about a 63 mm lens on a 4x5 camera.

The images were assembled using PTGui. PTGui allowed for control and compensation of lens distortion, and final image projection. Note the horizon is flat, even though in this very wide angle image, the horizon is not centered (wide angle images often suffer from curved horizons unless the horizon is centered). This image suffers from some tilting of the trees at the left and right edges. PTGui allows for correction of this with a slightly different projection (I am still experimenting and learning the program). The output of PTGui was selected as a photoshop file with both the final image and all the layers from each image. Some post PTGui photoshop work was needed to change the areas merged for best focus between the many frames. The photoshop psb file was about 5.0 Gigabytes in size. The final 16-bit per pixel photoshop file after the layers were flattened is 1.2 Gigabytes. The jpeg acquisition format was a test to see image quality from such a simple and lowest signal-to-noise ratio image set from the DSLR (the jpeg quality was the highest the camera could produce). The exposure time was set to not saturate the clouds and the land was underexposed. The land portions of the image were selected and brightened. The curves tool was used to give a characteristic curve similar to that of slide film.

To illustrate the detail in the final image, 100% crops, from the areas indicated in Figure 2 are displayed in Figure 3. The crops indicate that the image is very sharp in all portions of the frame, both near and far.

How sharp is the final image compared to film? While a 4x5 film image was not obtained at the same time as the mosaic, I have obtained many 4x5 images on slide film. Figures 4 and 5 illustrate high-end drum scans of Fujichrome Velvia 50 4x5 images. The Figure 4 scan produced an image size of 16,200 x 12,800 pixels, similar to the size of the digital mosaic shown here. The 4x5 image in Figure 4 was taken with a 90 mm lens at f/64 with front tilts of the lens plane to help the depth of field. The Figure 5 scan gave a 16,200 x 12,800 pixel image, and was obtained with a 185 mm lens at f/45. The film grain is clearly apparent, and edges are soft in comparison to the digital images in Figure 3.


Figure 2. Areas shown in the red boxes have 100% crops shown in Figure 3.


Figure 3. 100% crops from Figure 2. If these crops were printed at 300 ppi, the full image would be 50.8 x 41.9 inches.


Figure 4. Example 100% crops from a large format film image scanned on a drum scanner. The film was Fujichrome Velvia, scanned on a Linotype-Hell, Heidelberg Tango scanner resulting in a 16,200 x 12,800 pixel image. If these crops were printed at 300 ppi, the full image would be 54 x 42.7 inches. The full image can be seen by clicking on the title: Wildflowers in the Maroon Bells - Snowmass Wilderness of Colorado.


Figure 5. Another example of 100% crops from a large format film image scanned on a drum scanner. The film was Fujichrome Velvia, scanned on a Linotype-Hell, Heidelberg Tango scanner resulting in a 16,200 x 12,800 pixel image. The original full resolution image is 15,986 x 12,628 pixels. The image was obtained with a 185 mm lens at f/45. If these crops were printed at 300 ppi, the full image would be 53.3 x 40.1 inches. The full image can be seen by clicking on the title: Mount Sneffels Reflection.

Comparisons of the 100% crops from the film scans (Figures 4 and 5) with those from the digital mosaic (Figure 3) show a dramatic difference in sharpness. The differences include the fact that a faster f/ratio of the digital resulted in less diffraction effects, and the film has a lower Modulation Transfer Function (MTF) for recording the fine image detail in these images.

Another factor in large format landscape photography is exposure time. The use of a polarizer and slow f/ratios (typically f/22 to f/64) requires long exposure times, typically 0.5 to 2 seconds in full sunlight. The use of faster f/ratios allows faster exposure times. In fact, at the time the digital mosaic in Figure 1 was obtained, wind was blowing the flowers and a large format image at the required f/45 for this image would have resulted in many blurry flowers. If I was carrying my 4x5 equipment, I would not have attempted an image due to the wind.

Pros to Digital Mosaics

Cons to Digital Mosaics

Other Lessons

When doing landscape photos with a large depth of field, the depth of field is usually from the foreground to the background, so that the best focal point is changing vertically. Most pano-heads hold the camera in portrait mode with the long dimension of the camera's image plane vertically. This places the maximum dimension of the image plane covering the most depth of field. A better way for such photos is to place the camera in a horizontal, or landscape mode. This minimizes the depth of field requirement for each frame in the image. The lesson is to have a pano head that you can do either orientation and use the orientation that minimizes depth of field requirements for any particular image.

When learning, I recommend to have about 50% overlap from frame to frame. This helps to ensure that one does not have poor focus in some areas with extreme depth of field.

Larger pixel cameras would enable fewer images to achieve the same final resolution. For example, if an 8-megapixel camera requires 50 to 60 frames to surpass 4x5 film, then only 25 frames would be needed with a 16-magapixel camera.

Conclusions

I believe I have demonstrated I can make superior images with digital mosaics than I can get with 4x5 Velvia film drum scanned. And I can do this by carrying less weight in the field, and taking less time per image in the field. There is a trade for field time and post processing time, with digital helping reduce time in the field, so more images can be acquired. But there is a cost in post processing time though that time is greatly reduced circa 2012 and beyond with faster computers and better software.

I will not get rid of my 4x5, as I feel I need it for imaging large moving events (e.g. like frame filling fireworks). But I will use my 4x5 much less, as I can get the quality I want with digital mosaics most of the time, and including conditions where I could not get a 4x5 image. 2015: I have not needed a 4x5 view camera for several years.

See other examples in the MOSAICS GALLERY.

Large Format Gallery


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References and Further Reading

http://www.largeformatphotography.info .

http://www.largeformatphotography.info/how-to-focus.html .

http://en.wikipedia.org/wiki/View_camera shows operation of a view camera, including tilts. .

Introduction to Large Format, Nature Photographers Online Magazine. .

http://www.ptgui.com .


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First Published August 18, 2006.
Last updated December 31, 2015.