How Big Can You Print ?

This is a question I'm often asked and I'm normally thinking that a 40"-45" print would be acceptably sharp and detailed.  So when I was asked if I had a photograph that could be printed 50ft wide (yup 50 feet, not 50 inches) by 9ft tall it got me thinking;

Problem 1: How could I produce a photo that would be over 12 times larger than anything I have printed before ?
Solution 1: A panoramic photo - several photos taken in sequence then stitched together to produce one long image.  Applying a bit of mathematics, I found that I would need about 12 images in portrait mode (long side vertical) all stitched together to get enough pixels to produce a 50ft print.  This is ideal since it would be long and narrow and would fit the 50/9ft aspect ratio perfectly.

Problem 2: Using a wide angle lens would mean that I'd have to spin the camera over 360 degrees to take the 12 frames required.  This is just not possible.
Solution 2: I'd have to use a longer lens, say 70mm minimum.  That would mean that I'd only have to spin the camera about 90 degrees and I'd have a better chance of getting a good composition.

Problem 3: The photo has to be taken in a forest.  That means I'd have to keep the foreground (and closer trees) in focus in addition to those in the background.  Longer lenses have a shallower depth of field (i.e. the range of subject that is in focus is short) so if I focus on a close tree, the one in the background will be out of focus.  It is possible to counteract this by reducing the aperture which in turn increases the focus range but that has two drawbacks;
a) The forest is already dark for shooting and reducing the aperture will mean that I need a long exposure time.  long exposure time with branches moving = blur.  
b)  Lenses loose sharpness when reducing the aperture = soft images.
Solution 3: Focus stacking.  This involves focusing on a near subject, take a photo.  without moving the camera, focus on a subject a little further away, take another photo.  Focus further away again... and so on.   These images can then be blended together by taking the sharpest part from each frame and merging them together.  Helicon software has a great application called Helicon Focus that will do this automatically.  So I reckon I'd need a stack of about 8 photos to get the image sharp front to back.

Problem 4: To image stack, the camera must be perfectly stable - any movement or vibration would throw the stack out of alignment resulting in a blurred image.
Solution 4:  There is 2 parts to this solution.  a) use a very stable tripod (I'd do this as a matter of course since I still have a long 0.5 second exposure).  The tripod I use also allows me to release the horizontal pan while locking vertical movement which is great for panoramic shots as the camera stays perfectly level.  b) don't touch the camera to refocus.  For this I would shoot tethered - meaning that the camera is wired to my laptop and the laptop controls the focusing and all other controls.  Again Helicon has an application that allows you to control the camera from a laptop.

Problem 5: I have a laptop and it's pouring with rain in the middle of a damp forest.
Solution 5:  An umbrella (although I think next time I'll rig up a tarp or bring a small tent shelter to give me a bit more space.

Here's the final set up:

Nikon D800 tethered to a Macbook Pro.

Nikon D800 tethered to a Macbook Pro.

So for the one photo, I ended up taking 103 frames all stacked and stitched together and it took about 1 hour to physically take the shot.

It took a further 3 hours to process all the frames and create the single panoramic image.  Another 2 hours to edit in Photoshop.   The file is so large that it takes a long time to work with, so a tip here is to work with a smaller version of the original to get the recipe correct then apply it to the original full size file.  

I left it processing the full size version overnight and in the morning was presented with an image over 70,000 pixels wide (most cameras produce about 4,000 pixels).  It is pin sharp with every frond of lichen visible in the foreground to the pattern of tree bark and individual needles visible in the furthest trees. 

Copy of final image (at about 1.8% of the size)

Copy of final image (at about 1.8% of the size)

So next time someone asks me what size can I print a photo - the answer is; how big would you like it ?