Under exposing Sigma 150mm macro

[Mitchell]

Back in May I started a thread about Effective aperture, a concept I was unaware of until I experienced it on my Sigma 150mm f2.8 OS macro lens. I was experiencing a 2 stop underexposure with this lens when used for close up/macro with a Canon 7D2. This was irritating because the amount of underexposure and therefore compensation varied with distance and the lighting conditions.
I thought it worth reporting that, following discussions with Sigma, I discovered they had a firmware upgrade they claimed would resolve the problem so I recently sent it off. Today it was returned to me ( within a week!) and although conditions here are not particularly conducive for photography it seems that this rechipping has indeed resolved the problem as far as I can tell from a few test shots.
Sigma produce some great lenses but sometimes get criticised for poor service, I have to say I cannot fault them particularly if the improvement I have so far seen in the lens holds up to further testing

[odds]

...underexposure with this lens when used for close up/macro with a Canon 7D2.

I believe Canon cameras do not compensate the exposure ("effective aperture") for close up/macro. My Nikons do.


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Odd S.

[pschlute]

Am I right in thinking that the change in effective aperture at very close focussing is due to a change in the lens's focal length ?

ie. does the "optical centre" of the lens move forward at minimum focus, and thus the same "observed" entrance pupil size renders the lens's actual aperture slower than indicated ?

Or does the "observed" entrance pupil get smaller, thus creating the slower aperture. Or perhaps a combination of both ?

Presumably TTL open aperture metering takes this into account ?

[DanK]

If you are using through the lens metering, the camera’s metering system should automatically take care of this. The camera is responding to the effective f stop, not the nominal one. If you are using TTL metering and are having exposure problems, the problem has to lie with the firmware of the lens. I have done a great deal of macro photography with 4 different Canon bodies and 3 different Canon macro lenses, and I have never had a problem with metering at close distances, up to about 2:1 magnification. Canon cameras do report the nominal rather than the effective f stop, but this has no effect on metering.

Here is a brief but good explanation of effective f stop that makes this point on the macro page of the tutorial section of this site.


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[Manfred M]

Am I right in thinking that the change in effective aperture at very close focussing is due to a change in the lens's focal length ?

ie. does the "optical centre" of the lens move forward at minimum focus, and thus the same "observed" entrance pupil size renders the lens's actual aperture slower than indicated ?

Or does the "observed" entrance pupil get smaller, thus creating the slower aperture. Or perhaps a combination of both ?

Presumably TTL open aperture metering takes this into account ?

Peter - I believe you are 100% right here. Both the focal length and maximum aperture change when focusing a fixed focal length lens. I suspect that the "effective aperture" rule comes from the days of manual photography and has fallen by the wayside with current cameras with TTL metering (especially mirrorless cameras).

For rechipping to fix this suggests a firmware design error on Sigma's part.

[Geoff F]

I have the Sigma 180 macro lens and that is about 2 stops darker than the earlier 180 lens. So for cloudy conditions I go back to my previous lens and just use the new one for bright sunshine, often with a bit of fill flash as well.

[DanK]

Both the focal length and maximum aperture change when focusing a fixed focal length lens. I suspect that the "effective aperture" rule comes from the days of manual photography and has fallen by the wayside with current cameras with TTL metering

I have to say that I am somewhat puzzled because there are two things going on, and I don't understand the connection.

From what I have read: One factor is that focal length increases as you get very close, and since f-stop is defined as the ratio of focal length to aperture, the f-stop increases as you get very close. This is the issue described in the tutorial.

The second issue is that magnification decreases the light reaching any given sensor area. This is a phenomenon that is very familiar to anyone who works with extension tubes.

These must be different manifestations of the same phenomenon, but I haven't sorted the connection for myself.

But I think the bottom line is that it shouldn't matter for proper exposure. The fact that I can't sort this out even though I have done manhy hundreds of macro images with varying equipment indicates that in practice, you don't need to worry about it for purposes of proper exposure if you are using TTL metering and your equipment is working properly.

There are two practical implications of this nonetheless. One is that focusing accurately becomes increasingly difficult as magnification increases. I find this to be an issue when I do studio macro at levels substantially above 1:1. The second is that as the tutorial says, effective f-stop really is the functional f-stop, so the diffraction limit drops in terms of nominal f-stop as you get very close.