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dicklaxt

Joined: 08 Oct 2011
Posts: 53

 Posted: Thu Oct 27, 2011 1:25 pm    Post subject: My Magnification???? How do I calculate the magnification of my S3is with a Raynox 250? I tried to follow some examples but keep stubbing my toe somewhere the vertical in focus dimension in the viewer of a mm scale is 8.2mm.The size of the sensor is 5.8mm(w) x 4.3mm(h),now with the scale in place and a subject setting adjacent to it the largest vertical in focus dimension of the subject is 6.5 mm .These figures are factual readings but from here on out I am lost or get lost I should say,from the above numbers,,,,,, can the magnification be computed? If so how and what dick
ChrisR

Joined: 14 Mar 2009
Posts: 3060
Location: Near London, UK

 Posted: Thu Oct 27, 2011 1:42 pm    Post subject: Computing is tricky for a combo. SO your in-focus ruler is telling you it's 4.3 /8.2 which is 0.524, = about half life size. It's not that "adjacent" is "in front" ..?
rjlittlefield

Joined: 01 Aug 2006
Posts: 12582
Location: Richland, Washington State, USA

Posted: Thu Oct 27, 2011 1:52 pm    Post subject:

A slightly different spin, but basically just expanding on what ChrisR posted a few minutes ago.

 dicklaxt wrote: can the magnification be computed? If so how and what

It can, and quite easily:

magnification_on_sensor = sensor_size / frame_size_at_subject

Of course you have to work with in-focus sizes.

In your case, I'm not sure what you mean when you say that "the vertical in focus dimension in the viewer of a mm scale is 8.2mm" but then add that "the largest vertical in focus dimension of the subject is 6.5 mm". Does this just mean that the subject doesn't fill the frame?

As I understand your measurements, the calculation would be:

magnification_on_sensor = 0.53 = 4.33 / 8.2

You'd get the same number if you used the subject dimension and the sensor dimension covered by the subject. But that's the hard way to work the problem because you can't directly measure how much of the sensor is covered by the subject.

It's much easier to just image a ruler that completely spans the frame, and use that distance along with the published size of the camera's sensor. I think that's what you've done to get the "8.2mm" and "4.3mm" numbers.

By the way, it's pretty unusual to talk about on-sensor magnification when you're working with a non-removable lens camera. Again, it's much easier and communicates better to tell either the size of the subject, or the total width of the frame at the subject's position.

--Rik
ChrisR

Joined: 14 Mar 2009
Posts: 3060
Location: Near London, UK

 Posted: Thu Oct 27, 2011 2:00 pm    Post subject: What I felt Dick asking, in my water, was If my camera's lens focal length is A and the Raynox FL is B, then I can work out the FL of the combo, to be C then if I focus the camera at a distance which would have been D then I should be able to work out the new focus distance E from which "can the magnification be computed? If so how and what " And the answer is I believe, kinda sorta yes, but it doesn't quite come out right, so use a ruler
rjlittlefield

Joined: 01 Aug 2006
Posts: 12582
Location: Richland, Washington State, USA

 Posted: Thu Oct 27, 2011 2:28 pm    Post subject: Yeah, with combos the calculation based on lens specs hardly ever comes out right. The only simple formula that actually works well is magnification = rear_lens_focal_length / front_lens_focal_length and that one's only correct when the rear lens is focused at infinity. All other situations require much more complicated formulas, typically combined with information you don't know anyway! That way lies madness... --Rik
dicklaxt

Joined: 08 Oct 2011
Posts: 53

 Posted: Thu Oct 27, 2011 3:09 pm    Post subject: I'm not following any of this .I'll just keep clicking them off as I see them and forget the rest. thanks anyway dick
rjlittlefield

Joined: 01 Aug 2006
Posts: 12582
Location: Richland, Washington State, USA

Posted: Thu Oct 27, 2011 3:39 pm    Post subject:

 dicklaxt wrote: I'll just keep clicking them off as I see them and forget the rest.

That works well.

If anybody asks about "magnification", just tell them roughly how big the subject is, and let them figure out anything else they care about.

--Rik
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