Best focal length.

[Steve Clay]

I'm looking at getting an old OM lense for my OMD EM 10 M/43 camera with the intention of mounting it on my azgti  to start learning some astrophotography. There is a 80_200 zoom on ebay that would give 160_400 zoom on the m43. Would this be suitable? Or should I look for a prime, if so what FL.

Cheers

Steve

[vlaiv]

Best way to determine FL of the lens is to use field of view tools available online like this one:

https://astronomy.tools/calculators/field_of_view/

Select imaging mode, and input camera sensor specs, then select different focal lengths (use custom scope) and examine a few targets that you are interested in to see what sort of field of view you'll get on them.

In general - if you want wide field / milky way sort of photography - go with less than 50mm. For regular wide field - like constellations and such, go for about 50-100mm, 150-200mm are very close to wide field scope focal lengths, so you will be able to aim for larger nebulae and even some galaxies with such focal lengths (although images will still be wide field and not close ups).

You should also keep in mind working resolution (depends on focal length and sensor pixel size) and mount tracking capabilities, but at these focal lengths you don't really need to worry, they are quite forgiving. However, you might consider getting a wedge for AzGTI and putting it in EQ mode for this.

[GlenM]

I used to have a m43 Olympus OMD EM 10 II. A great little camera. I used Samyang lenses on this. I was lucky by receiving good ones to begin with, but the QC can be a little iffy for sure. Buy from a reputable retailer.

 

Glen.

 

[rickwayne]

Stars are very demanding objects for a camera lens, best to use a prime instead of a zoom.

Another good way to play around with focal lengths for a given size sensor is the planetarium program Stellarium. You can enter a variety of sensors and focal lengths, and go hunting about the sky framing objects, rotating the frame, etc.

[Knight of Clear Skies]

135mm lenses tend to work well, possibly because the old ones have a simple optical design. 50mm lenses are usually good at about f4-f4.5.

[Steve Clay]

9 hours ago, Knight of Clear Skies said:

135mm lenses tend to work well, possibly because the old ones have a simple optical design. 50mm lenses are usually good at about f4-f4.5.

Is that 135 @ 35mm so a 70mm ish on a micro 4/3 with 2x crop factor?

[alacant]

20 hours ago, Steve Clay said:

135 @ 35mm so a 70mm ish

Hi. I think it's best to keep the lens specification the same. A 135mm lens is just that.

20 hours ago, Steve Clay said:

135mm lenses tend to work well

+1.

IfI understand correctly, I think that means a 135mm lens. Not a 70mm lens.

Perhaps best as @vlaiv suggests,  use the field of view calculator rather than think in terms of crop factor.

Cheers and clear skies 

**EDIT: wondering whether a Takumar 55mm would be close enough...

 

Edited May 19, 2019 by alacant

[JamesF]

54 minutes ago, alacant said:

Hi. I think it's best to keep the lens specification the same. A 135mm lens is just that.

Exactly so.  A 135mm lens is always a 135mm lens.  Change the sensor size and the field of view changes, sure, but it's still a 135mm lens.

James