Leica SL2, IBIS and High Pixel Count: Putting a new telephoto in your bag

Many photographers are quite satisfied with their current camera at 24mp (or even lower).  No doubt, you can get stunning images from well-exposed and focused shots at that level of definition. With the introduction of the Leica SL2 however, you have access to two features that might tip the balance toward this camera body.

Strengths of the Leica SL2

As existing users of the Leica SL2 know, it is an outstanding camera. But it possesses two particular features that are key to extending the effective focal length of your telephoto lenses.

• IBIS, or In-body Image Stabilization. This provides a big boost in sharpness at lower shutter speeds. Photographers can thus use a lower setting ISO for reduced noise and more highlight and shadow detail.
• The high pixel count sensor. This produces images that are 8000+ x 5000+ pixels or higher, on a full format sensor. The result is a 100% increase (or more) in pixel count compared to smaller sensors.

This means a big increase in the potential for cropping (and thus extending your effective lens focal length) without a significant loss in quality. The following discussion assumes you are using a very high-quality lens, typically produced by Leica.  It will be more challenging to obtain a satisfactory result with lower-quality lenses.

Let’s assume you already have a 135mm lens in your bag.  How can you make this lens effectively longer and maintain quality?  Furthermore, are there tricks available to help with quality when you crop the image? I’ll attempt to answer these questions below. Starting with a 135mm field of view, we can find out how the image will be affected by reducing that field of view (by cropping) to match longer lenses.  The table below refers to the SL2 (full-format) sensor with 8368×5584 pixels (47mp).

The impact of cropping on pixel count

First, look at some horizontal fields of view, the cropped horizontal and vertical pixel count, and the equivalent MP of the cropped image, assuming you maintain the 3:2 image aspect ratio after cropping:

Most post-processing software will show you horizontal and vertical pixel dimensions.  Note 1 below shows the field-of-view calculation method for any lens.

In Lightroom or Photoshop (and most other editing applications), you can crop to these dimensions, while maintaining the 3:2 aspect ratio, or you may choose to use a different aspect ratio for your final image (that is, 1:1, 4:3, and so on)

Another example:  With the same sensor as the example above, an image shot with a 250mm lens, cropped to a 400mm equivalent field of view will have a very usable 5258×3522 pixel count.

To address quality, you must first determine how the image will be displayed. For projection, or display, you should know that the typical display width for most computer monitors or projectors is approximately 1920 pixels, with some ultra-high definition displays having up to 3840 pixels in width. The good news in this case is your cropped image, even at a 300mm-equivalent field of view, has more than adequate pixels to be displayed electronically at high quality without further enhancement.

What about prints?

For prints, you need to consider the print size.  Good quality prints can be made at 300 dpi (dots per inch), but may also be quite appealing at lower dpi values depending upon the printing process, print size and viewing distance.  Assuming you wish to use 300 dpi, an 8.5×11” print therefore needs an image of approximately 2550×3300 pixels, and a 12×18” print will need approximately 3600×5400 pixels. 

For all who are more familiar with it, here come metric figures. The “300mm” crop in the above example is more than enough for an A4 print in a fine 300dpi resolution. 6000×4000 pixels will safely give you a 50×34cm print if again 300dpi is the threshold. In fact, even 60×40cm can be achieved safely with a 24mp image. The uncropped 47mp file is good enough for a stunning 75×50cm print with just under 300dpi.

The Leica SL2 brings added flexibility

With the crops in the table above, you can get a very high quality 8.5×11” print without any further enhancement.  When you wish to print to 12×18”, using the crop that equals a 300mm lens angle of view, for example, you can try print settings that specify 300 dpi for a 12×18” print dimension.  Lightroom and Photoshop will automatically add the needed dpi in a process called “up-sampling”. First set the image size in inches and the dpi to 300. 

Lightroom does this in the Print settings, and in Photoshop, you simply need to re-size the image and save it as a new file before printing.  This may, in fact, provide excellent print quality.  I have produced a stunning 24×36” print from a 24MP image with this method, and I have a 12×18” print from an M8 (10.3mp) image (3936×2624) that is amazingly also good.

Software to the rescue

If the resulting print quality does not meet your needs, it may be worth trying one of the available applications that are designed to increase image size. This is especially for very severe crops where you are using only a small area of the original image. Topaz Gigapixel AI is one such application, using artificial intelligence to ‘fine tune’ the final image before printing.  Selective sharpening may also be advised.

Another more extreme example is shown below. The original uncropped image of the robin is 8368 pixels wide (taken at 379mm zoom) and the cropped version is approximately ¼ the width in pixels.  The resultant cropped image is equivalent to approximately 1532mm focal length.  A sharp lens and the IBIS capability of the Leica SL2 both help.

I used Topaz Gigapixel AI to enhance the extreme crop (sharpness, apparent grain, etc.). You can probably get similar results in Topaz Photo AI or Photoshop Super Zoom (Neural Filter).  These applications can add pixels back into the image, using AI as a way to provide a realistic result. 

The above process will work to extend any focal length lens mounted on the Leica SL2, in case you don’t have a longer lens in your bag. Here also is an opportunity to take full advantage of the older lenses you may have. The bottom line: there may not be a compelling need to burden yourself with a bulky and heavy long lens for the occasional wildlife shot. Travel light!

Notes for the mathematically inclined

1. To calculate the horizontal field of view for any lens:  

Horiz.  FOV (deg) = 2arctan(h/2f) where, 

h=horizontal image size, mm (36 for full format)

f=lens focal length, mm

Example, 18mm lens, FOV=2arctan(36/36)=90 degrees

2. Horizontal and vertical pixel counts vary linearly with the inverse of the angle of view (see table above)

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