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04-20-2010, 11:50 AM   #1
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Focusing closer than infinity reduces light entering lens!

While using my M 135mm f/3.5 I thought about how much it extends when focusing from infinity to close, about half an inch or a bit more. Then it occurred to me that this would cause less light to enter when close-focused than when focused to infinity, as the length increases. I placed the camera on a flat surface facing out a window at a static scene, focused to infinity & used the green button to meter: 1/50s. With the focus ring turned halfway: 1/40s. With focus ring to closest focus: 1/30s.

I tried this several times & got the same results, the ISO was on 200 the whole time. Granted that the exact focal length varies with focus distance (focus breathing) so what's in the frame is not exactly the same, but I hadn't expected so much of a difference.

So my f/3.5 lens is only f/3.5 at infinity, when close-focused it loses almost a full stop & is closer to an f/4.5!

Which other lenses does this apply to? Are internal-focus lenses exempt from this effect?

04-20-2010, 12:05 PM   #2
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Actually, a lot of lenses will have different exposure readings at different focal lengths. It is not just a matter of the physical extension, but what the glass is doing inside, one element being moved away from another and so on. I do not think that internal-focus lenses are necessarily exempt, either.

This can be a difficult test to do unless you are sure the illumination of your subject is exactly the same. I suggest an internal shot where you can control the light.
04-20-2010, 12:12 PM   #3
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QuoteOriginally posted by Jonathan Mac Quote
While using my M 135mm f/3.5 I thought about how much it extends when focusing from infinity to close, about half an inch or a bit more. Then it occurred to me that this would cause less light to enter when close-focused than when focused to infinity, as the length increases. I placed the camera on a flat surface facing out a window at a static scene, focused to infinity & used the green button to meter: 1/50s. With the focus ring turned halfway: 1/40s. With focus ring to closest focus: 1/30s.

I tried this several times & got the same results, the ISO was on 200 the whole time. Granted that the exact focal length varies with focus distance (focus breathing) so what's in the frame is not exactly the same, but I hadn't expected so much of a difference.

So my f/3.5 lens is only f/3.5 at infinity, when close-focused it loses almost a full stop & is closer to an f/4.5!

Which other lenses does this apply to? Are internal-focus lenses exempt from this effect?

You already saw, that the lens extends, when you focus at near distances. That is the reason. The distance between thew lens and sensor increases and thus the illumination at the sensor plain drops. The relationship between sensor-to-lens distance and illumination level is given by the according to the inverse square law.

This will be even more pronounced if you add an extension tube or bellows.

With IF lenses in theory this effect should vanish. Instead most IF lenses exhibit a much more pronounced reduction in focal length at near distances.

Ben
04-20-2010, 12:17 PM   #4
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this applies to all lenses,

the loss between minimum and maximum focus may change a s a function of the minimum focus limit. Clearly a 135mm lens that focuses to 2 meters will have a different light fall off than a lens that focuses to 1 meter.

Effectively each lens should have the same level of fall off between infinity and (for example ) 1:10 magnification.

Note that this may not exactly apply for internal focusing lenses. I have not studied the real impact of intenal focusing, since some internal focusing methods actually change the focal length, since focal length is only measured at infinity

04-20-2010, 03:17 PM   #5
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QuoteOriginally posted by Lowell Goudge Quote
this applies to all lenses,

the loss between minimum and maximum focus may change a s a function of the minimum focus limit. Clearly a 135mm lens that focuses to 2 meters will have a different light fall off than a lens that focuses to 1 meter.

Effectively each lens should have the same level of fall off between infinity and (for example ) 1:10 magnification.

Note that this may not exactly apply for internal focusing lenses. I have not studied the real impact of intenal focusing, since some internal focusing methods actually change the focal length, since focal length is only measured at infinity
IF is, I think a"problematic" technology, as each manufacturer or each lens will behave indivually.
The "loss" of light is simple geometry in conventional lenses, but not for IF lenses.

Ben
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