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08-28-2013, 11:42 AM   #1
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75mm VS 90mm angle of view

All right, I have difficulties understanding anything beyong basic mathematics, and I will present the problem in as simplistic a manner as possible.

The angle of view on 75 mm focal length on 35mm film is: 32 degrees.

The angle of view on 90mm mm focal length on 35mm film is: 26.9 degrees.

How many steps would I have to take back with the 90mm lens in order to attain the angle of view of 32 degrees?

All the online angle of view calculators only take into account the focal length; the factor of the photographer himself moving is not included in the equation. Are there any online tools that will solve the puzzle for me or does anyone here know of a simple way of calculating this?

08-28-2013, 12:10 PM - 1 Like   #2
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QuoteOriginally posted by DominicVII Quote
All right, I have difficulties understanding anything beyong basic mathematics, and I will present the problem in as simplistic a manner as possible.

The angle of view on 75 mm focal length on 35mm film is: 32 degrees.

The angle of view on 90mm mm focal length on 35mm film is: 26.9 degrees.

How many steps would I have to take back with the 90mm lens in order to attain the angle of view of 32 degrees?

All the online angle of view calculators only take into account the focal length; the factor of the photographer himself moving is not included in the equation. Are there any online tools that will solve the puzzle for me or does anyone here know of a simple way of calculating this?
You need an infinite number of steps back, because the angle of view is a function of focal length and sensor size. I suspect, however, what you are really discussing is how many steps back to get the same magnification of a specific subject at a specific distance with a 90mm as opposed to your 75mm lens.

The answer is really simple math.

Image size = subject size x Focal length / subject distance

Since the image size on the sensor wants to remain the same, and the subject size remains the same, you will move back by the ratio of focal lengths, I.e. 90/75 times the subject distance at 75mm
08-28-2013, 12:25 PM   #3
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This is where the expression "zoom with your feet" falls apart, because zooming (changing the angle of view) and actually moving the camera achieve two very different results. A 90mm lens will always take a picture with a more compressed perspective than a 75mm, unless you crop a 75mm picture to the same angle of view as the 90mm.
08-28-2013, 12:30 PM   #4
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well, you are using the DIAGONAL Angle of View, which is more difficult to interpretate than horizontal AoV (or vertical, depending on application)
90mm Hor AoV on 135 format = 26°
75mm Hor AoV on 135 format = 22°
(source: Field Of View Calculator)

So, the difference is very small, so common sense tells me youŽll have to move very little to achieve the same subject size in the frame. But for an strict result, keep in mind that the distance to the subject will also has to be taken into account.


Last edited by carrrlangas; 08-28-2013 at 12:36 PM.
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