DSLR - how many mm is sensor "inside" ?

[kirkster501]

Hi all, when working out the distance the CCD sensor has to be behind a FR how many mm is the sensor inside a DSLR please? Inparticualr a Canon 450d?

Thanks, Steve

[Digz]

Most DSLRs have a focal plane mark:

http://www.photokonnexion.com/?page_id=6469

i guess you could use this to work out the measurement.

[billhinge]

Its called the flange focal distance or lens register, each manufacturer has their own standard

see here for a list of common ones, eg Canon 44mm, so the distance from the flange to the mark at the back is 44mm

http://en.wikipedia.org/wiki/Flange_focal_distance

[kirkster501]

Many thanks guys. So when a focal reducer says it needs x amount between sensor and FR, is this 44mm (or whatever) part of the number that needs to be added with spacers to get the distance required? Lets say they say, for example, 76mm. Does it have to be EXACTLY that distance? Of course since this is after the focuser this cannot be focused away so must be exact right? Spacers are not normally made in such exact lengths though?

Thanks, Steve

[Digz]

Exact is good, but I think you might get away with being a millimetre or two out either side, depending on the camera spec. Of course I have no experience of this so I may be wrong......

[steppenwolf]

The usual spacing specified is around 55mm - this is made up of, nominally, 45mm for the camera mounting face to sensor distance + the nominal 10mm depth of a suitable 'T' adapter. Some focal reducers/field flatteners have a more generous allowance into the 70mm region.

[Merlin66]

You can get various variable length T thread adaptors ie Baader -these can then be locked to maintain the distance.

http://www.telescope-service.com/baader/adaptors/adaptors.html#Large

[earth titan]

I found when mounting a coma corrector on to a DSLR, that the quoted focal distance was a 'guide' and it still required some fine tuning.

Typed by me on my fone, using fumms... Excuse eny speling errurs.

[kirkster501]

The usual spacing specified is around 55mm - this is made up of, nominally, 45mm for the camera mounting face to sensor distance + the nominal 10mm depth of a suitable 'T' adapter. Some focal reducers/field flatteners have a more generous allowance into the 70mm region.

So is the distance between FR and sensor and "allowance" then? If they say 70mm must it be that *exactly*? Or is it "up to" that?

I want to buy one of these. for my RC8. I love this scope but its a little slow.

It says:

-- Optimal back focus: 85 mm - if you change the distance, the kompressor factor will change. You can change the

distance between M48 connection thread to the sensor from 70mm to 90mm with good results.

I am using a DSLR at the moment but going to get a CCD in a few months. So with my 44mm camera distance what distance do i need to make up do you think? Get it to between 70 and 90mm.

Rgds, Steve

[earth titan]

I had a come corrector which quoted 70mm required and I used this on a DSLR. I found I needed around 67mm to get right. Just a quick bit if maths to work out the distance required, less the focal plane distance of the camera = spacer required.

I bought an adjustable spacer ring to get ti right.

When I get a field flattener for the current scope, I will need to redo the spacers.

Typed by me on my fone, using fumms... Excuse eny speling errurs.