Bronica SQ and close up filters

Kev M

Kev M

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Some of you will probably know that the 150mm lens for a Bronica SQA has a pretty woeful close focussing distance of 1.5 metres making it useless for a tight head & shoulders portrait. I'd read that this can be improved with the addition of a +1 close up filter and was lucky enought o wind a genuine Bronica filter on evil bay last week.

:eek:Shocked isn't really the word for it, it's like a bloody macro lens now and is really hard to focus. I've used close up filters on my DSLR and you'e got to stack a few of them to get this kind of effect.

Anyone else found the sme thing or is the Bronica filter a bit more than the +1 indicated.

On a more technical note is there anyway of knowing how much I need to compensate for the loss of DoF?

Cheers,
Kev
 
Unless I'm missing the question here. Very possible. Your dof shouldn't change with a close up filter. Unlike extention tubes and bellows the filter isn't extending the focal length, it's just magnifying the image and essentially giving you a crop of the normal field of view.
 
dazzajl said:
Unlike extention tubes and bellows the filter isn't extending the focal length, it's just magnifying the image and essentially giving you a crop of the normal field of view.
Click to expand...

Intereshting Mishter Bond, very intershting. I assumed that as I was now focusing at a closer distance my DoF would be reduced. Might have to break open a pack of polaroid and waste it on some experimentation when I have time.
 
dazzajl said:
Unless I'm missing the question here. Very possible. Your dof shouldn't change with a close up filter. Unlike extention tubes and bellows the filter isn't extending the focal length, it's just magnifying the image and essentially giving you a crop of the normal field of view.
Click to expand...

Actually, a 1 dioptre lens added to the main lens is changing the focal length.

Here is some maths:

The original lens with a focal length of 150mm (or 0.15m) is 6.67D

Add 1 dioptre to that and you get 7.67D

That is a 130mm lens*.

As it is still in the position required for a 150mm lens, it is a bit too far forward of the film plane to focus on infinity but has no problem with closer subjects.

(* 1 divided by the focal length in metres gives the lens power in dioptres).

And extension tubes and bellows don't change the focal length, they just move the lens forward allowing closer objects to come into focus.


Steve.
 
Kev M said:
I was now focusing at a closer distance my DoF would be reduced.
Click to expand...

Your lens is now shorter in focal length so in theory you should have more DOF. However, your aperture settings are now not quite right so that may offset it or compound it (I'm not doing the maths to work out which way it goes!).


EDIT: OK then, here is the maths I wasn't going to do:

F Number = focal length / diameter of aperture so your 150mm lens at f8 has an opening of 18.75mm dia.

With the +1D lens on the front, focal length is now 130mm and the opening is still 18.75 but the actual F No. is now 130/18.75 = 6.93

So although the lens is set for F8, it is really F6.93

This will reduce the DOF working against the increase you get by having a shorter focal length.




Steve.
 
Cheers for the explanation Steve,

Looks like stopping down .5-1 stops should bring me back to what's expected.

Am I right in thinking that when using tubes or bellows it affects the exposure somehow which has to be compensated for but with the close up lens it doesn't need it?
 
Kev M said:
Am I right in thinking that when using tubes or bellows it affects the exposure somehow which has to be compensated for but with the close up lens it doesn't need it?
Click to expand...


This: http://www.salzgeber.at/disc/index.html explains how it works with large format cameras with bellows but the principle is the same as using an extension tube.

formel.JPG


Google 'bellows factor' for more info.


Steve.
 
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