Back Focus distance for DSLR lenses?

[SteveL]

Hi all

I`m toying with the idea of using a DSLR lens attached to one of my CCDs, and was wondering if someone could explain to me where I can find the amount of back focus required for various lenses.

For example, if the lens is for a Canon 350D, where would I look for back focus information?

And do different camera makes have different back focus distances?

The whole "super wide" imaging appeals to me, especially if I can hang the system off the EQ6 while I am imaging something else at the same time, but I just need to know about this whole back focus issue.

Many thanks for your help!

[brendant]

Steve, you may find this chart useful in working out the distance to the sensor from the rear flange of the lens

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

Brendan

[SteveL]

Fantastic, just waht I was looking for!

Systems with short flange-to-film distances include Canon's FD (42.00 mm) and EF mounts (44.00 mm), Konica's AR (40.70 mm), and Olympus's Four Thirds System (38.67 mm). These can readily take many other brands of lens.

Systems with larger flange-to-film distances include Nikon's F-mount (46.50 mm), Olympus's OM mount (46.00 mm), and the Pentax K mount/M42 (45.46 mm) universal thread mount. These lenses can be adapted to many camera bodies.

[jgs001]

Steve, the Canon EF-S lenses actually use an even shorter back focus distance than EF lenses. It means they can be made smaller and lighter, but limits them to crop sensor bodies (as the mirrors in full frame bodies will clout the rear of the lens). Just something to bear in mind when you're thinking about lenses.

[brendant]

If the distance is measured from the mounting flange both EF and EFS lenses are identical (44mm) as John rightly says the rear element of EFS lenses can protrude into the camera body further, this is because of the smaller mirror box on APS-C cameras, on a full frame camera 5D etc.. the mirror would touch this rear element, obviously something to bear in mind if using EFS lenses. EFS lenses cannot be fitted to full frame cameras.

Brendan

[SteveL]

So ideally a larger back focus lens would be better to allow for inline filters, and adapters.... is that correct?

[jgs001]

I think so. I know that with the EF-S lenses on Canon bodies, you can't use things like the clip filters or teleconverters as the rear element connects with them. Probably best to stick with the EF lenses. Also, it might be worth talk to Steve (Steppenwolf) as I think he's has done something like this already.

[Merlin66]

Not sure what you mean about your "in-line filters and adaptors" ????

I use my old Zuiko Oly lenses on the 350D; just needs a thin Oly>Canon adaptor ( Ex-Hong Kong) and seems to work very well...

Any filters I use are up the front end ( usually use 2" astro filters for "specials" ie UHC-S, Ha etc) Limits the aperture to 48mm but like many things in life.... a compromise.

[jgs001]

If you want to use something like the Astronomic CLS EOS clip filter, you cannot use EF-S lenses due to the intrusion into the camera body of the rear element of the lens. For anything that's an EF mount that's not an issue, like an Oly lens on an EF adapter. I believe the S in EF-S is used to indicate short back focus.

[SteveL]

Not sure what you mean about your "in-line filters and adaptors" ????

Something like this:

[brendant]

Correct Steve, if you want to get your filter wheel between the lens and the CCD go for as long a back focus distance as you can, obviously depends on the thicknesses of the various adaptors you will need to introduce to connect everything together, going for screw fit lenses will be the easiest to adapt. Also need to allow for the depth of the CCD inside the camera body, tricky to determine and they do not always publish.

Brendan

[Psychobilly]

You only need to be spot on with the mount to sensor distance if you want the focus marks on the lens to be correct....for astro work you just need to make sure you can focus at "infinity" somewhere within the focus range of the lens...

as an example the 500mm lens that I use as a guide scope with my DSI II focuses stars at the 70m mark on the focus scale not at the infinity mark becasue the mount to sensor distance isnt correct...

Billy...

[Merlin66]

Steve,

Got you now!

I use a "modified" Mogg adaptor between the Zuiko lenses and the DSIproII, MX7c, webcam etc.

Generally the distance from the rear lens flange (on the film cameras) to the focal plane was 55mm......

[SteveL]

OK, another dumb question here: Do the "auto focus" / "auto everything" lenses work fine without a camera to drive them? If I manually focus and set aperture, will they work correctly? Or do I need to looking at a "manual only" lens of some sort?

[Psychobilly]

Autofocus lenses (except fot the very earliest ones which were self contained addons ) can still be manually focused...

aperture - they will tend to be fully open by default...although you can get lenses which can me manually stopped down..

I tend to preffer manual focus lenses as autofocus ones tend to be a bit "loose" when manually focussed...so loose in fact that they can very easily change focus - I use bluetack or tape to lock them once focussed...

I would Look for some qaulity Canon or Nikon manual era Glass...

here are soem quick and diry 1.25" adaptors I made for M42 thread and Minolta AF mount lenses...

http://picasaweb.google.co.uk/PsychoBillyUK/CCDImagersWithSLRLenses#5162853204427922882

With reference to back focus distances.. T mounts are 55mm flange to Focal Plane but other lenses vary canoncs haev the shortest back focus whiuch is one of the reasons why its faitly eay to use other manufacturers lenses with canons as there some meat to play with for adaptors...... Nikon have ket the Same distance since there earliest film cameras through to the lastes DSLRs...

Billy...

[SteveL]

Just picked up a couple of Nikon F lenses from my local camera shop. A Sigma 28-105mm (f/3.8-22) and a slightly older Soligor 35-200mm (f/3.8-22). There was also a great 18-35mm lens, but I could hear something rattling around inside it, and the whole focus assembly was very loose and easy to turn (not something I`d like to use!). They are sending that one back to be looked at, and will give me call if it comes back in a usable condition.

I hear of people having to step down one click when using them for astro work. Is that a good rule of thumb, or only for faster lenses that go down to f/1.8-2.8?

[themos]

It applies to all of them, I should think. Definitely on my 135mm which is f/3.5. Ultimately, you'll have to experiment. And it's not just one stop, look at this:

http://astrosurf.com/buil/50mm/test_us.htm

[SteveL]

Looks like I have some fun and games ahead of me then

[Psychobilly]

Generally you'd expect primes to perform better than zooms... and they are often a stop or more faster at a given focal length.

I used to get reasonable results using smaller sensors as you only using the "best" part of the "image" ...

Here's the one that you want....

http://www.sigmaphoto.com/lenses/lenses_all_details.asp?id=3349&navigator=3

Or if your on a budget (credit crunch and all that...)

Its smaller brother....

http://www.sigmaphoto.com/lenses/lenses_all_details.asp?id=3308&navigator=3

Billy...

[SteveL]

Last question for today, honest!

Just so I know I havent lost the plot here, if I want to plug these lenses into my SNPP or The Sky FOV indicators:

Aperture is the size of the front lens (shown on outside ring of the lens - 62mm and 67mm for my two lenses I have here)

Focal Ratio is aperture / focal length

Where does the 1.6x digital multiplier come into it that I see mentioned in a lot of places with regard to DSLR lenses? Does this mean that actually the focal length needs to be multipled by 1.6 before I plug it into the above calculations? (28mm focal length becomes 44.8mm?)

[MartinB]

Just the thread I needed!! Which filter drawer is that Steve and does the camera mount come with it? What is that arrangement at the back of the drawer? Is it a push fit for the QHY8 nose piece.

I have some Zeiss Jena M42 lenses I would be interested in using. I'm guessing they would need a similar spacing to the M42 Practikas, would that be right?

[SteveL]

i`ll be getting this one for the SXV:

http://www.gerdneumann.net/v2/english/eng_filterdrawer.html

(Filter drawer main body T2/M48, 2" drawer, M48 to Nikon adapter)

I`ll also be trying it with the QHY8 to see if there is enough back focus.

[Psychobilly]

The 62 and 67 are more likely to refer to the filter thread...

The multiplier.. is an interestign one as its to do with the Equiavlent FL need to match the FOV on a 35mm (or full frame sensor)

A quick snap and submit it to astromometry ?

Billy....

[SteveL]

Just as soon as I get all the bits delivered, assembled, and point it all at a clear dark sky, i`ll know

[themos]

for M42 lenses look in this list:

http://m42.artlimited.net/site_lenses.php

Aperture is the size of the front lens (shown on outside ring of the lens - 62mm and 67mm for my two lenses I have here)

I don't think aperture is quoted anywhere. I agree with psychobilly that those numbers tell you the diameter of the filter that the lens can take (on the outer face).

Where does the 1.6x digital multiplier come into it

It comes from the fact that the digital sensors are of different size to 35mm film - they are smaller. They are trying to tell you that you will capture less fov with the lens if you have a digital sensor. That's an effective 1.6X magnification effect or whatever. Ignore it.