Leica T w/23mm Changing Aperture Automatically

[Jared]

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Hey guys, new to the forum and I had a question about a strange occurrence with my T:

 

Every time that I have the Leica T at f/2 with the 23mm and I go to the closest focusing point with a subject, the camera automatically switches to a different aperture. So for instance I'll be at 2.0, try to get a nice blurry background and the camera will either switch to 2.5 or 2.8 depending on distance. I'm perplexed by why this is happening. No other camera I've ever used has done this. It's almost like the T doesn't want me to shoot with those parameters.

 

And I'm shooting everything in manual mode with RAW selected.

 

Any idea why it's doing this?

 

Thanks.

 

Actually, ALL lenses, regardless of manufacturer, do this if they focus close enough. The camera may or may not indicate that it is happening, but this is neither a feature implemented by Leica or a flaw. As your focus gets closer, the focal length of the lens increases slightly. It's just because you are moving the lens away from the camera body. Your 23mm at infinity focus has become, say, a 28mm at close focus. The physical size of the aperture stays the same. If the focal length is increasing, and the aperture diameter remains fixed, that means the focal ratio has increased. The 23mm just happens to focus close enough for this to be significant.

 

If you want to look it up, it's called "bellows effect".

 

-Jared

[Jared]

Are we certain that aperture cannot be locked to a desired setting?

 

The aperture is locked--determined physically by the lens--but the focal length, and hence the focal RATIO is increasing with close focus. All lenses do this. All.

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[mjh]

The aperture is locked--determined physically by the lens--but the focal length, and hence the focal RATIO is increasing with close focus. All lenses do this. All.

True enough, only that is not what is happening with this lens: the lens actually stops down at close range.

[pico]

True enough, only that is not what is happening with this lens: the lens actually stops down at close range.

 

 

Is that a bad thing?

.

[wildlightphoto]

The aperture is locked--determined physically by the lens--but the focal length, and hence the focal RATIO is increasing with close focus. All lenses do this. All.

 

That's what all lenses used to do before internal focus. Internal-focus lenses are different. The optical configuration changes such that the net effect is the lens has a shorter focal length with the optical center of the lens too far from the focal plane to focus to infinity.

[mjh]

Is that a bad thing?

While it is something that makes you go “Huh?” when you first stumble upon this behaviour I don’t see how this would be an issue in practice. It’s not like a little more depth of field at close range would go amiss.

[Jared]

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True enough, only that is not what is happening with this lens: the lens actually stops down at close range.

 

Really? The iris is actually closing down as you focus closer? You can see it? That something I've never seen before.

 

- Jared

[Jared]

That's what all lenses used to do before internal focus. Internal-focus lenses are different. The optical configuration changes such that the net effect is the lens has a shorter focal length with the optical center of the lens too far from the focal plane to focus to infinity.

 

Even with internal focus, the focal length of the lens generally changes with focus. It's just that the physical length of the lens doesn't change. With most IF lenses, the optical center still moves outward. I agree, though, that I overstated when I said "all" lenses since there are some IF designs where the focal length decreases as you focus exactly enough to keep the framing, and hence completely balance bellows effect. Most IF lenses do not do this since the main purpose is generally to allow AF focus motors to deal with less mass, and to avoid a rotating front element.

 

But it sounds like, in the case of this lens, it is actually stopping down physically--closing the iris. That's a first in my book. Mi would speculate that it's to keep image quality from degrading too badly at close focus, thus allowing a closer focus point than Leica would have been satisfied with at full aperture. Just speculation, though.

 

- Jared

Edited July 16, 2014 by Jared

[wildlightphoto]

Even with internal focus, the focal length of the lens generally changes with focus.

 

Yes, that's what I wrote.

[jonoslack]

Worth mentioning that other lenses do this - the Nikon 100mm macro VR is a notable example.

[henning]

Worth mentioning that other lenses do this - the Nikon 100mm macro VR is a notable example.

 

And Canon 100mm macros, all 180mm and 200mm macro (micro) and most current short (50-60mm FF equiv) macro lenses from all manufacturers. They all tend to be IF, and they all decrease in focal length to achieve their shot focus distances and high reproduction ratios, and also, they use the floating lens elements to correct aberrations throughout the focussing range. Macro lenses have never been better for IQ. Unfortunately, shortening the focal length also decreases subject to lens distance, which aspect I've often found annoying when I've paid a lot of money for long focal length macros, like long ago, when the 200/4 micro-Nikkor came out.

 

A benefit of full IF lenses is that the effective aperture doesn't decrease; i.e., the 200/4 micro-Nikkor mentioned above is still at f/4 effectively at 1:2, even though the focal length is about 130mm now.

 

I don't know about the 23mm Summicron, but I'm assuming that it either has no floating elements or that they change the focal length very little as the lens is close focussed. Either way, the focal length will stay the same but the lens (and aperture) will be further away from the sensor as the lens is close focussed. This will reduce the effective aperture slightly, as mentioned above, but this would not show up in the EXIF or really be otherwise noticeable. The aperture closing down is a different phenomenon and is something I would be extremely annoyed about, but then I'm hardly the target market.

 

Henning

[jonoslack]

I don't know about the 23mm Summicron, but I'm assuming that it either has no floating elements or that they change the focal length very little as the lens is close focussed. Either way, the focal length will stay the same but the lens (and aperture) will be further away from the sensor as the lens is close focussed. This will reduce the effective aperture slightly, as mentioned above, but this would not show up in the EXIF or really be otherwise noticeable. The aperture closing down is a different phenomenon and is something I would be extremely annoyed about, but then I'm hardly the target market.

 

Henning

 

HI Henning

I don't know about the technicalities (not been told). But I understand that Leica restricted the aperture at closer distances (and it does focus quite close) to keep up the image quality, certainly it shows in the exif. . . . So . . . I suspect you would be extremely annoyed, but it seems like a good decision to me.

 

All the best

[henning]

Since I'm not buying the camera I'm not nearly as annoyed :-).

 

Henning

[burkey]

 

I agree. 11 in is amazing anyway.

 

'Just a heads-up, the 23mm f2 does the same thing on the new CL.  It was troubling to me at first but not really a big problem overall.

. . . David

[LexS]

"Exactly...when you do have a reason, it's vitally important. I'd rather Leica (or any other manufacturer) not do me such favors."

Seee it this way: The 23mm T lens is a 2.7 lens with the possibility to go to 2.0 when used at more than very small distances... Typical Leica. I love this lens.