Trying to figure out true focal length

[nmoushon]

At the moment I only have a tripod for my SLR and thus can't track. So I'm trying to figure out what my true focal lengths are for my lenses so I can minimize star trails. I thought that the description of the focal length on the lens was, well what it said it was. But through reading different topics/forums/websites I've figured out that's not always the case. So how do I figure that out?

I have Canon EF-S 18-55mm lens and Canon Zoom Lens EF 100-300mm.

[jgs001]

The focal length of a lens is just that. What you see on the barrel is it. However, and I think this is the bit that's confusing, different sized sensors capture a different field of view. This is because the lens delivers a circle onto the sensor plane. The circle size doesn't change. A full frame camera captures more of the circle than an aps-c sized sensor, leading to an apparent increase in focal length. For fov purposes the canon crop bodies give a fov equivalent to using a lens 1.6x longer on a full frame body. For some numbers a 100mm lens is just that, but the crop sensor body delivers an image that would need a 160mm focal length lens to get the same fov on a full frame camera. Full frame is the same size as the old 35mm film size. Hth and isn't overly complicated.

[nmoushon]

Ok so regards for star trailing when it says 100mm its 100mm focal length. The difference is the FOV it captures? Instead being a true 100mm FOV its really 160mm FOV? Do I need to compensate, length of exposure, for the difference or is it more or less just cropping the final picture?

Heres one of the sites that I read and what Im trying to understand: http://samsastro.com/Astrophotography/Basic.htm

Slide number 12 and the sub text right under it.

[jgs001]

The lens focal length remains the same, you're cropping from the middle. I've read both schools of thought, although I've not tested it specifically with astro, although using the ballpark calculation of 600/focal length to avoid startrails the crop factor doesn't seem to be relevant (but from memory of the last time I did static tripod with a 50mm it works out). I also use 1/50s on my 50mm lens on my crop sensor body as the minimum speed for hand holding and it's working out so far.

Best I can really suggest, is to test it out and see how you get on.

[nmoushon]

Thanks jgs001 for clearing that up a bit. I hope clouds clear for at least a couple hrs tonight so I can do some testing.

[Mr TamiyaCowboy]

the EF-S 18-55mm is what it says. but the EF lens you need to multiply the focal lengths by 1.6.

so your EF 100-300mm is really a 160mm - 480mm .

A lens marked EF-S on a 1.6 crop body will be what it says on lens ie: 18-55mm. the S stands of shortback if i am correct.

a lens marked just EF and used on a 1.6 crop body requires you to multiply the MM shown by 1.6, so a 90-300mm becomes a 114mm-480mm on a 1.6 crop body

[Cornelius Varley]

the EF-S 18-55mm is what it says. but the EF lens you need to multiply the focal lengths by 1.6.

so your EF 100-300mm is really a 160mm - 480mm .

A lens marked EF-S on a 1.6 crop body will be what it says on lens ie: 18-55mm. the S stands of shortback if i am correct.

a lens marked just EF and used on a 1.6 crop body requires you to multiply the MM shown by 1.6, so a 90-300mm becomes a 114mm-480mm on a 1.6 crop body

A 100-300mm zoom lens is always a 100-300mm zoom lens. It doesn't matter what camera it is attached to, the focal length will always stay the same. What changes is the FOV, so a 100-300mm zoom on a DSLR will give the same FOV as a 160-480mm zoom on a 35mm SLR. Put the same zoom lens on a medium format camera and the FOV will become greater.

Peter

[dph1nm]

You need to work out the pixel scale of your camera in arcseconds to determine how much trailing you will see. This depends on the focal length (which is the number on the barrel) and the camera pixel size in microns (which depends on what camera you are using).

NigelM

[nmoushon]

I have a Canon XSi, which has 12.2 megapixels. Don't know how to find the microns.

Thank you Cornelius, that's what I thought jgs001 was saying.

[Mr TamiyaCowboy]

I have a Canon XSi, which has 12.2 megapixels. Don't know how to find the microns.

Thank you Cornelius, that's what I thought jgs001 was saying.

5.2 x 5.2 microns for the XSI

[Auntystatic]

Cornelius is correct about the focal length, I'm afraid Mr Tamiyacowboy is wrong in what he says, sorry. Many people have been caught on the my 100mm is really a 160mm as I have a cropped sensor bandwagon.

See this explanation on the Canon professional website for details, its the best explanation I have seen, so good in fact I carry a couple of copies around in my camera bag.

http://cpn.canon-europe.com/content/education/infobank/lenses/ef-s_and_field_of_view.do