# reason # 43 for the 12-40mm/2.8 - Astral Photography

Started 5 months ago | Photos thread
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 Re: reason # 43 for the 12-40mm/2.8 - Astral Photography In reply to drpoop, 5 months ago

drpoop wrote:

CrisPhoto wrote:

drpoop wrote:

alternatively, I used my 7.5 mm f 3.5 FE to grab this shot. It's not the best composition, but is an example of how f 3.5 (even with 30 sec exposure vs 25 sec at 12mm) doesn't grab as much detail as f 2.8

Very good shot of the milky way, in my area there is to much light pollution for this.

While I share your enthusiasm with astro and the 12-40 as well, one little remark:

The difference you observe between 12/f2.8 and 7.5/f3.5 is not the aperture alone. It is the difference in light gathering area.

The difference is like this:

• 12mm, f2.8: =>entrance diameter 12mm/2.8=4,2mm => entrance area = 18mm²
• 7.5mm, f3.5:=>entrance diameter 7.5mm/3.5=2,1mm => entrance area =4.5mm²

As you already observed, the 12/2.8 gathers 4 times (400%) more light from the distant star while the aperture value is only 25% "bigger".

Christof

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OM-D + Sam7.5, PL25, O60, O75
P12-35, O75-300

Hey, thanks for the detailed explanation. Its handy to know the math behind it.

Thanks too.

Oh, and as you mention maths, ups, I calculated square instead of circle. The area is d*d*pi/4. But anyhow, the stars remain the same and the brightness difference of 400% too ...

Christof

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OM-D + Sam7.5, PL25, O60, O75
P12-35, O75-300

CrisPhoto's gear list:CrisPhoto's gear list
Olympus E-M1 Panasonic Lumix G Vario 7-14mm F4 ASPH Olympus Zuiko Digital ED 50-200mm 1:2.8-3.5 SWD Olympus Zuiko Digital 1.4x Teleconverter EC-14 Olympus M.Zuiko Digital ED 40-150mm 1:4-5.6 R +6 more
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