# Photographing Neowise with 600mm lens

Started 3 months ago | Questions thread
Re: Photographing Neowise with 600mm lens
2

Let us look at the MATH. Light gathering is about the physical area of the aperture opening and how long the shutter can remain open. A physical shutter can only let in so much light. Turning up the ISO doesn’t increase light, only using a bigger aperture can increase the light that falls on the digital sensor for any given amount of time. Increasing the amount of time that light is allowed into the camera will increase the total amount of light as well. So let us look at the statistics for your 500mm f/5.6 lens and my 85mm f/1.4 lens and compare the results.

500mm lens at f/5.6 has a physical aperture area of 6261 mm^2

85mm lens at f/1.4 has a physical aperture area of 2895 mm^2

500 Rule for 500mm lens = 1 sec

500 Rule for 85mm lens = 6 sec

For 500mm f/5.6 lens: 1 sec x 6261 = 6261 factor of light

For 85mm f/1.4 lens: 6 sec x 2895 = 17371 factor of light

85mm f/1.4 lens gathers 2.77x more light than does 500mm f/5.6 lens when both are using the 500 Rule (or any rule you choose).

For each second, the 500mm f/5.6 lens lets in more total light, a little more than twice as much, but since the 85mm f/1.4 lens can allow up to 6 sec before the stsr trails get noticeable, it has 2.77 x more over all light gathering capability. This is for a static mount of course. Put that 500mm f/5.6 on a good tracking mount and that changes everything. However, to track with such a long FL lens you will need a very good tracking mount, and those are rather pricey.

Bottom line, if you want more detail using a static mount, go with a smaller FL And faster lens. You will get better detail, less noise, and over all better image quality.

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