# Telephoto Reach With Various Digital Cameras

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It’s common for photo writers to state, “A 100mm lens on a Micro Four Thirds camera has the equivalent reach of 200mm on full frame”. While the field of view is equivalent to 200mm FF, the reach is not necessarily equivalent.

To understand reach, the important metric is pixel density or pixels per mm. You can calculate this for any sensor by dividing the width in pixels by the width in millimeters. Here are some:

Point and Shoot or Superzoom Cameras

20mp 1/2.3 sensor = 844 pixels per mm
16mp 1/2.3 sensor = 755 pixels per mm
20mp “One inch” sensor = 416 pixels per mm

Interchangeable Lens Cameras

20mp micro 4/3 = 300 pixels per mm
24mp APS-C = 256 pixels per mm (I own this, see below)
Sony 61mp FF = 266 pixels per mm (I also own this)
Canon 45mp R5 FF = 228 pixels per mm
Sony 42mp FF = 220 pixels per mm
Sony 24mp FF = 168 pixels per mm
Canon 20mp R6 FF = 152 pixels per mm
Sony 12mp FF = 119 pixels per mm

Consider a given telephoto lens photographing a bird at a distance which results in an image 1mm tall on the sensor. Holding these conditions constant, that image will be 1mm tall on all sensors. But the number of pixels will depend on the pixel density above.

Comparing the 20mp 4/3 camera with 300 pixels per mm to the 24mp FF camera with 168 pixels per mm, the ratio is near 2x. It’s actually 1.79x. So a 100mm lens on this 4/3 camera has the reach of a 179mm lens on this FF camera. In this case, the common belief just happens to be close to correct.

But the 4/3 reach is only 1.13x the 61mp FF camera because 300/266=1.13. Thus a 100mm lens on a 20mp 4/3 camera only has the reach of a 113mm lens on a 61mp FF camera.

Pixels per mm is the key metric of reach, but only if the lens can resolve this density. The resolution of the 4/3 lens must be as good or better than 300 lines per mm of the sensor. Fortunately, most ILC lenses do resolve the pixel density of the camera they were made for.

But unfortunately, point and shoot superzoom lenses fall short of resolving the extremely high pixel density of their sensors. You can be pretty sure that the resolution of the wide-range zoom lens of the 20mp 1/2.3 sensor in the Canon SX70 does not reach the sensor’s 844 lines per mm. If it did, wildlife photographers would abandon their \$10,000 FF telephoto rigs and venture forth into the wild with Canon SX70s.

It’s important to consider pixel density when selecting a camera for shooting wildlife. For example, a Cameralabs video demonstrated fabulous tracking of seagulls in flight with the new Sony a7S III. But with its 12mp sensor, your telephoto lens would have to be 1.4 times as long as you’d need with a 24mp FF sensor to achieve the same bird image size in pixels. And it would have to be 2.15 times as long as you’d need on a 24mp APS-C camera. (For a given sensor size, pixel density is proportional to the square root of the total pixels. So doubling a 12mp FF sensor to 24mp increases pixel density by the square root of 2, or 1.4x).

How many pixels tall will that bird be on your sensor? You can calculate that easily.

Image ht. on sensor = Target ht. x Focal Length / Distance to target

Example: A 300mm tall bird, shot with a 0.4-meter lens (400mm) at a distance of 10 meters would be 12mm tall on the sensor. Multiply 12mm times your sensor’s pixels per mm to get the height in pixels.

I like to feature the bird’s head, as in these bird “portraits”. So pixel density and thus telephoto reach is very important to me.

I own the two cameras shown above in bold type in the table above. Though one is APS-C and the other is full-frame, their pixel densities are nearly equal and both cameras have about the same reach with a given lens. The main difference is the full frame camera has a wider field of view. But I tend to crop that away and feature the center only. So with the full-frame camera, I’d just crop away more of the original shot. Thus I grab the lighter weight APS-C when hiking out to the birding spots.

About the author: Alan Adler lives in Los Altos, California. He has been an avid photographer for 60 years. He is also a well-known inventor with about 40 patents. His best-known inventions are the Aerobie flying ring and the AeroPress coffee maker.