Calculating Needed Lens Backfocus

[Atmos]

For a while I have been considering doing some super macro work. True macro lens' are 1:1 at their closest focus point but I was wondering if there was as way of calculating the back focus required to come into focus at near infinity?

Ideally I'd love to be able to do 5-10:1 macro work :D

[DJScotty]

Are you suggesting you want to stick an extender onto the lens so that it comes to focus at infinity when you have the lens at its closest point to your object?

[Atmos]

:thumbsup:

[DJScotty]

Well i happen to have 2 adapters that allow me to try just that.
It certainly makes a big difference to the zoom but the closest focus point is at the other end of the focus ie not at infinity.
My adapters added about 53mm

[Atmos]

So in short I can add quite a lot! Thanks for that Scott.

[DJScotty]

Colin. Tomorrow I will do some experiments. Give you some very rough numbers in regards to distance from the object with and without the extender at both ends of the focus points. Also some pictures of the zoom difference.
Hopefully that will help you a bit.

Scott

[Atmos]

That would be fantastic Scott!

[JA]

Hello Colin,
Why not try extension tubes and/or bellows? If you want to experiment I could lend you some old Nikon F bellows (if I can find them :) ). They go from 30-40mm to 200+ mm I think. Just add a 50mm lens. You can even use reversing rings for more magn.

Best
JA

[Atmos]

I saw some bellows on Gumtree recently, think they may have been in Adelaide.

My ultimate plan is to connect my QHY163, CFW and FLI Atlas to a Zeiss Macro 100mm. It is only a 1:2 so not a true macro lens but with some added magnification it could be real nice :)

[DJScotty]

Aaaaah. True intentions revealed they are! :D
From my experiments with camera lenses for astrophotography it's all dependant on the back focus distance of the lens. If you have the lens too far from the sensor it won't come to focus.
I might be wrong. All you can do is suck it and see!

[Atmos]

My understanding is that for the most part it stops you from achieving infinity focus (stars) but you have to add a lot more to not be able to focus close. You added ~50mm to yours and could still focus, I just don't think you'd be able to focus at infinity anymore.

[SteveInNZ]

The designation on the lens (ie. 100mm) is the distance between the sensor and the equivalent simple lens when the lens is focused at infinity.
There's a formula for the focus distance of the same lens when it's moved further from the sensor towards the object.
1/f = 1/Obj+1/Sens
Where f is the focal length of the lens, Obj is the distance to the object and Sens is the distance to the sensor.
So if you have a 100mm lens and add 200mm extension, you have
1/100 = 1/x + 1/(100+200)
so 1/x = 1/100 - 1/300
x = 150mm

That means that with your 100mm lens focused at infinity, an object 150mm in front of the lens would be in focus.

This is just the very basics and it gets more complicated, the further you get into it. It's a lot like astrophotography in that respect.

Steve.

[Atmos]

After spending a lot of the morning researching it appears like extension tubes aren't really going to achieve what I am after. The problem is that you need SERIOUS extension before you start getting into the extreme macro level. They're good if you want a cheaper way to convert a non-macro lens into performing at macro scales. Beyond that however, it seems like it is better to just get a dedicated super macro lens.

Currently looking at the Canon 65mm 1-5x macro lens! Get one of those DSLR micro focuser thingys and have some serious patience. Won't be doing it for a little while yet but it is at least a direction to point.

One of the first targets on my list is one of those tiny black sugar ants :P

[axle01]

This is the best place to get extreme macro answers.

http://www.photomacrography.net/forum/index.php?sid=1fa800fa373b119d6f0e1 343ce315bc3

This is one of my set ups.


http://i64.tinypic.com/nmz5as.jpg

Alan

[DJScotty]

Colin,

I have added about 80mm to my macro length and took a photo of a fly.
It does increase the "magnification" but the DOF is incredibly narrow.

Attached is the result

Cheers

EDIT: Just had a look at one of the earlier photos I took. The narrow DOF is probably due to taking it at F2.8.
The other one I took was at F8 and looks a lot better
Just uploaded the other one