Samyang 85mm Camera Lens

[AstroComet3]

Hi everyone. So I've got a primary imaging rig but I need something portable to grab and go for dark-sky locations. I have already chosen a star tracker but what do you all think of the Samyang 85mm 1.4 camera lens? I looked at the 135mm but it is way out of my budget and from what I've seen, the 85mm can produce some stunning shots. For those in America, you may know Samyang as Rokinon. Clear skies! 

[willcastle]

I have an 85mm lens (sigma art) and I pointed it at Orion the other night. The stars and focus were pin sharp, even at f1.4. A single, 3 second exposure showed the nebula clearly. My only reservation was that it still gave an extremely large field of view. It would definitely require tracking to fully make the most of it however. Are you familiar with the “rule of 500”?

[AstroComet3]

Oh cool! No I am not...tell me more! 

[willcastle]

The rule of 500 tells you an approximation for how long you can take an exposure before you get star trails in your image. You take your focal length and divide 500 by that number. So for example, if you had an 18mm lens, you would do 500 divided by 18 and that gives you 27.7 seconds of exposure. 

That is for a full frame Dslr however. If you have an APS-C sensor (like a canon 80D), you have to multiply your focal length by 1.6. So that same 18mm Lens would be 500 divided by (18 x1.6), so 500 divided by 28.8, which would only allow you to make a 17 second exposure. 

Now, for an 85 mm lens, you would get about a 5-6 second exposure for a full frame camera and around a 3-4 second exposure on an APS-C camera. 

It means you are massively restricted on exposure time. But then I balanced it out by setting the aperture to f1.4 which means you collect more light. Even so, a 3 second exposure did show some slight star trailing. Ideally you would want to buy a mount that can track the stars. It would solve the issue 

[AstroComet3]

Cool, thank you so much! I am thinking of purchasing the SkyWatcher Star Adventurer for my Canon 1100D (which without it would only be able to do 5.9 seconds). 

[willcastle]

You are welcome! The canon 1100D has an APS-C sensor (same as my camera), so it would barely do more than 3 seconds. 

The star adventurer looks like a great piece of kit (and I seriously love Sky-Watcher products) so it would drastically improve your results but bare in mind you would need a pretty stable tripod to attach it to