aperture calcs

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Fresherjohn

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I've looked into the differences f-stops make in the past, but earlier tried to compare a 30mm f1.4 lens to a 17mm f2.8
after reading some info on http://www.uscoles.com/fstop.htm
i put the calculations into my own spreadsheet and found a 30mm f1.4 lens has an aperture area of 361 sq.mm, but a 17mm 2.8 only 29sq.mm.
For low light photography I would assume this would make the 30mm f1.4 better, but out of interest I entered 150mm f2.8 and the aperture area becomes a massive 9016sq.mm

I'm sure this doesnt mean its better in a low light situation, but it doesnt make sense to me?

if anyone can unpickle my brain id be most grateful.

thanks
 
Your starting down the "light gathering power" route, this is normally reserved for those doing astronomy where they need maximum light gathering power and their aperture is the size of the lens or mirror, so for them the bigger the better.

You only need to know in order to use a lower ISO and hence less noise or a faster shutter speed, the F1.4 has the edge there as it has a 1 stop advantage over the 150mm you quoted, thus you will be able to drop the ISO from 3200 - 1600 or increase the shutter speed from say 1/60th to 1/125th.

However if you where taking a long exposure of a star field then the 150mm F2.8 would gather more light than the F1.4 and produce a better image.

F1.2 and even F0.95 lenses are available, but then your talking from over £1000 to several thousand.
 
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The 150mm f2.8 does indeed gather more light but It's spread over a much larger area in the focal plane than the light from the 50mm f1.4. The aperture of the lens is a direct indication of the illumination at the sensor and, of course you need to select a focal length for the job in hand rather than consider them all.
 
I'm not quite sure I understand the original post, but the aperture size of any given lens is the focal length divided by the f/ number. The f/ number denotes how much light reaches the sensor/film. No matter what the focal length, an f/1.8 lens should always be the same brightness in the viewfinder, within reason. The t-stop of a lens is the actual light transmission, which can vary ever so slightly depending on how much glass is in it.

An 85mm f/1.8 lens has an aperture size of 47.2mm
(85 / 1.8 = 47.2)

A 50mm f/1.8 lens has an aperture size of 27.8mm
(50 / 1.8 = 27.8)

This is why lenses with a longer focal length, even although they may have the same aperture, appear to have a larger barrel. You can see it really obviously on the 50 and 85 I quoted - they are the same aperture but the "hole" in the 85 is huge whereas the 50 is just a modest size, smaller than a 10pence piece.

I don't know if any of that helped at all but it's some useless info anyway :p
 
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thanks all.
i think this bit answers my question.

trencheel303 said:
No matter what the focal length, an f/1.8 lens should always be the same brightness in the viewfinder, within reason.
Click to expand...

In my mind I'd considered the focal length to be relational to the amount of light which could be captured. In reality its just the wider lens's which have the bigger apertures due to the glass.

so a photo shot at 17mm 2.8 would look as bright as one shot at 30mm 2.8, it'd just be wider.
:thumbs:
 
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