Updating Classic Photo Tricks For Cropped Sensors

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Unless you are using film or a professional digital camera body with a full-framed sensor, your camera has a cropped sensor.  Without getting too technical, a cropped sensor is simply a smaller sensor.  And so the camera and its lenses are designed to accommodate.  This causes some differences, such as your lenses will behave like lenses with longer focal lengths, much like a digital zoom.  These differences really affect how your camera captures a shot, which should impact the way you take the shot.  Unfortunately, there are a few classic rules of thumbs – tricks, if you will – that should be updated to accommodate cropped sensors.  And so we shall take this opportunity to update them.

A Brief Background on Crop Factors

Before we go on to update some of the tricks, I think it’s important to discuss crop factors.  Basically, there are two reasons why cropped sensors exist in the first place:  First, a sensor takes up more space than traditional film does (at least for now, I’m sure technology will eventually catch up).  So in order to keep the camera body small and manageable for the typical consumer, the sensor ends up being a bit smaller in and of itself.  Second, full-framed sensors are quite expensive.  There are more sensor beds, and the technology used in full-framed sensors tend to be more complicated and more difficult to build.  And so, for at least the time being, the consumer and mid-level cameras tend to utilize cropped sensors with lens systems to match.

This creates a few complications.  For one, if a lens designed for a full-framed sensor were to throw an image onto a small sensor, there would be a lot of waste of the focal ring.  As you can see in the image at right, a cropped sensor capturing the same scene shows a significantly smaller area.  And so, again without getting into technicalities, the resulting image is almost a false zoom.  Like a digital zoom – which re-crops an existing image – the resulting image appears to be much closer.  In other words, if you were to take the APS-C crop shown here and scale it to match the full-frame shot, the APS-C would look more zoomed in.  This is a bit of a misnomer as the angle of view doesn’t actually change, just the crop itself.  But this will create the appearance of a longer focal length.  And this is why we need to update some of the tricks.

To estimate the apparent focal length when using a cropped sensor, simply multiply the focal length by the crop factor.  For example, a 50mm lens will behave like a 75mm lens on a Nikon APS-C sensor (50mm x 1.5 = 75mm).  Here are some of the more common crop factors, check your manual to be certain:

  • Full-Framed Sensors (Pro bodies):  35mm (the basis for all crop factors)
  • Canon APSH-H (pro-level DSLRs):  1.3
  • Leica M8 and M8.2:  1.33
  • Nikon, Pentax, Konmica Milolta, Sony Alpha and Fuji APS-C:  1.5
  • Canon consumer APS-C:  1.6
  • Foveon X3 (Sigma SD):  1.7
  • Four Thirds (Olympus):  2.0

The Updated Tricks for Cropped Sensors

And now we should explore how these crop factors affect our classic tricks:

The Nifty Fifty – Now the 35mm

Don’t get me wrong, I still love the 50mm lens.  But the classic 50mm prime can now be considered a portrait lens.  Since it behaves like a 75mm and has an angle of view that still behaves like a typical 50mm.  It gets you closer to the model, but since it still has a mid-range angle of view, it won’t introduce noticeable distortion.

Now if you really wanted to be at that coveted 50mm focal length, you should actually consider a 35mm lens.  Again, the angle of view isn’t significantly impacted, but the crop factor will give the appearance of a 50mm lens.  On a Nikon with a crop factor of 1.5, the 35mm acts like a 52.5mm lens and Canon (1.6mm) acts like a 56mm.  Not perfect in either case, but close enough (Photography isn’t an exact science).  Don’t get me wrong, I still love the 50mm lens and I use it very often.  But it does feel a little “zoomy” and tight sometimes.  The 35mm is a much more appropriate middle length for a cropped sensor.

Hand Holding Rule

The classic rule of thumb is that you shouldn’t try to hand hold with a shutter speed equal or less than one over the focal length.  For example, you shouldn’t try to hand-hold a 70mm lens when the ideal exposure requires a shutter speed of 1/70.  The rule worked great for film and I think that’s why the historical rule has carried through.  But when you’re shooting with a cropped sensor, you’ll notice that rule isn’t so very forgiving.  The problem is that you’re considering your lenses actual focal length as opposed to it’s crop factor adjusted focal length.  And so if you’re using a Canon with a 135mm lens, you should probably not hand-hold with a shutter-speed below 1/216 (Adjusted focal length would be 216mm).  Obviously, the camera doesn’t have a shutter speed of 1/216, but it’s still a good rule of thumb.

Edge Sharpness

One rule that doesn’t change is that the center of the photo is going to be the most sharp and have the best image quality (assuming it’s in focus).  However, the edges on a cropped sensor will be relatively more sharp than the edges on a full-framed sensor.  This is of course assuming good lenses, good depth of field, and so on.  The reason is simple:  The true edge of the view – the part closest to the image circle in the example above – is discarded by a cropped sensor (well, point of fact, it never hits the sensor, but I digress).  And so the less-than perfect edge never makes it into your shot.  This is a very subjective concern, but it is a viable one for landscape photographers shooting f/22.  Now that’s not all advantageous.  Suppose you really wanted a nice wide-angle shot?  You’d be losing a good percentage of your shot to the sensor.  That’s one of the reasons why many close-up, macro and some landscape photographers still prefer full-framed sensors.

“In Focus” by D. Travis North

Depth of Field

You won’t have to look far on the web to find the debate about the sensor’s size and it’s impact on the depth of field.  It’s true that a smaller sensor size can result in a smaller depth of field.  But let’s be honest, you’d be splitting hairs.  I ran the calculations with several scenarios, and the difference between a full-framed sensor and a cropped sensor was minimal.  If you’re a calculator type of person and you’re getting hung up on such hair-splitting details, photography may not be for you.  I will concede that if you’re a macro and close-up photographer, tiny increases in the depth of field could make all the difference in a shot.  Full-frame is ideal for macro for this reason.  But for the rest of us, we don’t need such an exact depth of field.  So unless you’re a close-up and macro photographer, this is one issue that shouldn’t vary between full-frame and cropped sensors.

Final Thoughts

As you can see, the size of your sensor can impact your photography more than you might have initially thought.  Fortunately, the classic tricks can still be used with some minimal tweaks.  This is only scratching the surface and I’m sure you have some thoughts and suggestions of your own to update other tricks.  So if you have any suggestions, please feel free to share.

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About Author

D. Travis North is a professional Landscape Architect, a Freelance Photographer and founder of Shutter Photo. Ever since he picked up his first SLR, his father's Nikon N2000, he's been hooked on photography. Travis likes to photograph urban environments, architectural details and has a new-found interest in close-up photography. His work can be found at D. Travis North Photography. Follow Travis on twitter: @dtnorth.

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