Aperture diameter

[penn10]

Please help me verify my understand of aperture opening diameter and amount of

lights coming in.

 

Use two lens as example, 50mm f/1.4 and 85mm f/1.4. For 50mm f/1.4, the

maximum aperture diameter will be about 35.7. For 85mm f/1.4, it is 60.7.

 

Does this mean, althought both lens are set to f/1.4, 85mm is able to capture

much more light than the 50mm? Therefore allowing a much faster shutter speed

under the same lighting condition?

 

Thanks!

[tommyinca]

No. They are both are f1.4 lens so they will yield the same shutter speed. Longer focal length and larger image circle needs larger lens diameter. That the way things work in this side of the universe.

[penn10]

That's what I thought, thanks Tommy!

[eric merrill]

The 85mm captures more light at the front of the lens.

 

By the time that light gets to the back, it falls off more than the 50mm.

 

That's why both have the same aperture. By the time the light is focused at the back, it has the same intensity even though there was more light initially entering the 85mm lens.

 

That's vastly oversimplified, but it gets the point across. I think. :)

 

 

Eric

[penn10]

Thanks Eric, just curious, why would the light lose more in 85mm? Is it because the longer distance it needs to travel? Or is it because lens elements?

[Hector Javkin]

Thanks Eric, just curious, why would the light lose more in 85mm?

 

Because it's magnified more. The longer lens brings objects closer. One way to think of it (not really accurate) is that more of the light goes out to the walls of the lens.

[tom_luongo1]

Frank, for the record, when you talk about "aperture opening diameter" it means the diameter of an idealized simple lens and not the actual diameter of the aperture blades wherever they happen to be inside the lens.

 

But otherwise, your calculations are correct. The front of an 85 f/1.4 lens must be at least 60.7mm wide. The front of an 85 f/1.8 lens must be at least 47.2mm wide. And the front of the 50/1.4 must be 35.7mm. I just pulled these lens out of my camera bags and confirmed the measurement.

 

It might help to think of the light gathering abilities of the lenses in terms of Field-Of-View. Photons bounce off a wall and come to your camera lens and are focused onto the image sensor or film frame. A longer lens will see a smaller fraction of the wall so fewer photons will reach the sensor. (This is all a consequence of the fact that the longer lens "magnifies more".)

 

For example, a 50mm lens sees an entire wall. A 100mm will see 1/4 of the same wall because it has 1/2 the FOV in both the vertical and horizontal dimensions. So the 100mm will have to collect four times the light.

 

Either lens at f/2 will deliver the same amount of light to the image sensor. At f/2, the effective diameter of the 50mm lens is 25mm and the effective diameter of the 100mm lens is 50mm. Apply the formula for area of a circle (pi/2)*(d^2) and you'll see the effective area (i.e. light gathering ability) of the 100mm lens at f/2 is indeed four times the area of the 50mm lens at f/2. (And pretty much like Hector says, most of the lost light goes into the lens walls and/or aperture blades.)

 

One purpose of the f-stop terminology is to eliminate all these calculations for the photographer. All you really need to know is that for the same lighting conditions and shutter speed, the same f-stop will give you the same exposure.

[penn10]

Great explanation, Tom, learned a lot! Thanks.