Focal lengths and interchangeable lenses

The standard focal length gives a view that is, approximately, what you can see with your unaided eye.

A standard 35mm camera has an area exposed 36 x 24 mm. A 3:2 aspect ratio.

The Nikon D5100, has an exposed area of 23.6 x 15.6 mm, also a 3:2 aspect ratio. This means that the angle of view of a 35mm camera is approximately 1.5 x that of the Nikon D5100. Thus, the approximate focal length for D5100 lenses in 35mm format can be calculated by multiplying the focal length of the lens by 1.5 (the crop factor). I have two lenses for this camera, a wide-angle 18-55mm and a telephoto lens, 70-300mm. The standard 35mm  equivalent on the D5100 camera would be 50mm ÷ 1.5 = 33.3mm,  approximately 35mm.

I also, on occassion use my Panasonic Lumix FZ45. The sensor on this camera is 6.08 x 4.56 mm. It has an optical 24x and when this is set to a 4:3 aspect ratio the focal range of the lens is 4.5 mm to 108mm. Each zoom point is the equivalent of 4.5mm.  To calculate the 35mm equivalent I need to multiply this by 5.5. Which means this camera has a 35mm equivalent focal range of 25mm to 600mm. The standard focal length for the Lumix FZ45 would be 2x zoom, 9mm which is approximately 49.5mm on a 35mm equivalent.

The following exercise demonstrates the effect of changing lenses from one focal length to another. Simply, the amount of view that can be taken in.

This task required an open view, with some details in the middle distance. Hence I decided to visit a local golf club and selected  a view across the driving range, towards the New Dubai skyline. I set my D5100 camera with 18-55mm lens on the tripod. There were a few false starts here as the very bright, and hot, afternoon sun made the view was too bright for the camera to operate. Adjusting the camera settings did not help. Eventually I found an angle which seemed to satisfy both me and the camera and I focused on the buildings just above the palm trees.

1. 18mm or 27mm on 35mm equivalent, F5/6, ISO 100, 1/800s

A wide-angle view of the driving range.

2. 24mm or 36mm on 35mm equivalent, f/5.6, ISO 100, 1/800

The angle of view has closed in slightly. The water hut to the right is no longer visible. Sections of both, the golf stands in front and golf cart to the left are also gone.

3. 34mm or 51mm on 35mm equivalent, F?5.6, ISO 100, 1/1000s

This would be the standard focal length for the D5100, approximately 35mm.

4. 45mm or 67.5mm on 35mm equivalent, F/5.6, ISO 100, 1/800s

The towers in the distance now appear closer and share prominence in the frame with the driving range.

5. 55mm or 82.5mm on 35mm equivalent, F/5.6, ISO/100, 1/1000s

As the focal length increases the angle of view closes in. The towers, that were initially in the distance, now could arguably be the subject of the photograph.

6. 70mm or 105mm on 35mm equivalent, F/13, ISO 100, 1/125s

The longer focal length is magnifying the trees and buildings, making them appear closer.

7. 100mm or 150mm on 35mm equivalent, F/13, ISO 100, 1/125s

As the focal length on the telephoto lens continues to increase the angle of view closes in further around the camera focal point.

8. 135mm or 202.5mm on 35mm equivalent, F/20, ISO 100, 1/60s

9. 210mm or 315mm on 35mm equivalent, F/20, ISO 100, 1/50s

10. 300mm or 450mm equivalent, F/20, ISO 100, 1/50s

With a focal point of 300mm the angle of view is so narrow that only the treetops and a section of the towers are visible. It has made them appear closer. If you compare this image to image 1, taken with a 18mm focal point, you will notice that the centre of image 1 is identical to image 10, only smaller.

This was a useful, practical exercise as I am fairly new to the idea of interchangeable lenses, having previously relied on the zoom function on my camera to take me from wide-angle to telephoto angles.

Freeman notes that prior to digital photography the standard format of camera was the 35mm camera, with a 3:2 aspect ratio. Now, however, physical width of film is no longer a constraint. As the two cameras that I use have different aspect ratios I found calculating their focal lengths against a ’35mm standard’ useful. It was surprising to note that the Lumix’s telephoto zoom has the 35mm equivalent of a 600mm focal length.


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