Confusion

vintageslrs

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I have read elsewhere on the internet and, I believe here at this forum too, that when using legacy lenses on let's say a G1--the f stop changes as viewed by the camera. Example a Hexanon 40mm F1.8 becomes (35mm terms) an 80mm F3.6.
But I just read on page 183 on David Busch's book on the GF1, that although the focal length doubles--the f stop remains the same. So in the example used it is (again in 35mm film thought process) an 80mm f1.8.
Which is the truth?
From the few photos I have taken with my G1---I always thought it appeared not the change the aperture size effect....but up till today everything I read says it does.
Confusion.

thanks
 

grebeman

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Hi Bob,
The aperture of an adapted lens on m4/3 will be the same as when the lens is used on it's intended format, that is a physical property of the lens, however the depth of field that the lens produces will change, so in judging the depth of field the f/4 mark on the lens barrel will be actually f/8 when used on m4/3, the f/5.6 mark on the lens barrel will be actually f/11 on m4/3 and of course the focal length will give the equivalent of a lens of twice the focal length when judged in 35mm format terms.

Barrie
 

Grant

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Well, I hope David Busch didn't say it exactly like that.

The F value and the focal length of the lens remains the same regardless of what camera you have it on.

What changes is the crop factor. A 40 mm F/1.8 on a M43s camera behaves like an 80 F1.8 camera on a 35 mm system as far as magnification goes. This is regardless of where it is a legacy lens or not.
 

GaryAyala

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Well said Grant ... but, (the big but), technically speaking ... isn't it the crop factor which changes the FOV?

Gary

PS- I enjoyed scrolling down your photo site.
G
 

Grant

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Well said Grant ... but, (the big but), technically speaking ... isn't it the crop factor which changes the FOV?

Gary

PS- I enjoyed scrolling down your photo site.
G
You are absolutely right! I only chose the term crop factor over FOV because I figured, for new people, it would be easier to visualize. Of course I could be wrong :rolleyes:
 

GaryAyala

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You are absolutely right! I only chose the term crop factor over FOV because I figured, for new people, it would be easier to visualize. Of course I could be wrong :rolleyes:
Gotcha ... things start getting sorta weird when talking about crop sensors.

G
 

vintageslrs

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Crop factor, field of view and now crop circles--LOL.
You folks are great...I got a big smile from that!!! :biggrin:

so, if I understand correctly the aperture does not change (for exposure purposes).
but the effect of the aperture's depth of field does--that is where the f1.8 acts like an f3.6---for depth of field purposes only.

Do I have have it right now?
Thank you all for trying to end my confusion! :thumbup:
 

GaryAyala

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Circle of Confusion anyone?

(and technically it's not your Chem Study Group - aka drinking buds - from college ... but in retrospect the shoe seems to fit ...)
 

Streetshooter

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Bob,
You have it right enuff to make photos.

Gary, Circle of confusion. Hmmm
Depends on the Aspect Ratio. My GF1 does square and rectangles.
Where did ya find the circle thing?
Don
 

carpandean

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I like to think about it this way:

Say, I have a 40mm f/1.8 and I put it on a full-frame camera and take a picture. Then, in software, I crop out the center of the picture (half the height, half the width), and blow it up x2, the resulting image is exactly what I would get from that lens on a :43: camera. Technically, the pixel density (MP/sensor size = pixels/in^2) of the two sensors should be the same and the same aspect ratio should be selected (technically, :43: only specifies the diagonal) for the equivalence to hold.

As pointed out above, this would not be exactly the same as the image that you would get from an 80mm f/1.8 (or 80mm f/3.6) on the FF camera. It has similarities, but is not 100%. FOV-wise, it would be similar to any 80mm lens on the FF and light-wise, it would be similar to any f/1.8. These are relatively simple geometric properties.

However, DOF-wise, it is not so easy to tell without doing a bit of geometry and calculations. It would actually have a smaller DOF than the 40mm f/1.8 on the FF, but that's not a fair comparison, because the FOV is different, too. What f-stop 80mm on the FF would it have the same DOF as the ? I don't know exactly. There are two offsetting effects: shorter focal length (40mm on :43: as opposed to 80mm on FF) leads to increased DOF, but smaller sensor size leads to decreased DOF. I've read that the focal length effect would be larger, meaning that the DOF would be higher on the :43: image. In other words, the 40mm f/1.8 on :43: would have an equivalent DOF of an 80mm with a larger f-number (smaller aperture) on a full-frame camera. I don't believe (but haven't verified) that it works out to 2x.
 

vintageslrs

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OK....Great!
now for the sake of accuracy--this is what David Busch said "Yet, your f/stop remains the same (that is, a 300mm f/4 becomes a very fast 600mm f/4 lens)."

I did not want to mis-quote David.

thanks
 

carpandean

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It's funny, because "that is, a 300mm f/4 becomes a very fast 600mm f/4 lens" seems to imply that you are getting something that you couldn't get as easily as before. Something like "wow, 600mm f/4, can you imaging how much that would have cost for a full-frame system?!" When, in reality, all you had to do was blow up the center of the image from your 300mm f/4 on the full-frame camera to get roughly the same thing (again, assuming roughly the same pixel density on the sensor.) You're not getting something for nothing. In fact, you're leaving out a lot of the image that you were getting from the same lens.
 

Amin Sabet

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When, in reality, all you had to do was blow up the center of the image from your 300mm f/4 on the full-frame camera to get roughly the same thing (again, assuming roughly the same pixel density on the sensor.) You're not getting something for nothing.
Current 4/3 sensors have higher pixel density than any of the current 35mm format cameras. So if the lens is sharp enough to outresolve both sensors and the subject distance is fixed, then the 4/3 camera with the 300mm lens is going to capture more detail than the blown up center crop from the 35mm format camera with the same 300mm lens.
 
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