Aperture has a big role to play in how your macrophotography images come out.
With the smaller sensor on the Olympus/Panasonic Four Thirds system, there is the issue of decreasing resolution and thus image detail with small apertures as a result of the physical limitations imposed by diffraction.
At the same time we want to increase depth of field and this requires small apertures, thus creating opposing image quality effects on the aperture we choose.
So what aperture is best?
To answer this I ran some simple tests which will ONLY precisely apply to using the Olympus ZD 50mm f/2.0 macro lens combined with the excellent Olympus EC-20 2x teleconverter, however, the general principles can be applied to ANY camera system.
For simplicity, aperture f stops mentioned below are those shown in the camera – ie. f/8 is actually f/4 on a lens and multiplied by 2x to account for the 2x teleconverter. The camera automatically does this for you and records the calculated aperture in the image EXIF data.
Check out the tests of aperture versus depth of field at 1:1 macro (this should apply to ANY camera system at 1:1 macro as depth of field is a function of subject magnification and aperture and is independent of sensor size or lens focal length – at least that is my understanding of the theory).
Check out the tests of aperture vs resolution. Although these are not at 1:1 macro, the relationship of resolution vs aperture should be applicable for all focus points with this combination. Perhaps surprisingly, diffraction limitations really only started to become evident at 100% pixel peeping at f/16 which was still not much different to the best resolution at f/5.6. This is surprising because it seems to be contrary to what would be expected according to table 3 on this article which suggests on this sensor, diffraction should be limiting resolution to 2mp at f/16 when clearly the results seem to be MUCH better than that.
In general for optimum DOF vs resolution with the 50mm + EC-20, use:
- f/5.6-f/8 for flat surfaces, and,
- f/11-16 for non-flat surfaces (eg. bugs) – f/22 if DOF more important than resolution.
- f/11 (f/5.6×2) if in doubt will give excellent compromise.
- this suggests that the 2x effect of the teleconverter in light loss terms could possibly be ignored when factoring in diffraction issues
To avoid the shutter lag and pre-flash using the Ring Flash, you can set the Ring Flash to manual exposure and adjust as needed according to your macro combination, distance of flash to subject, aperture and ISO you select.
For simplicity, using this combination at 1:1 macro, ie. manual focus to closest focus with Ring Flash attached, you can try this setting:
NB. my Ring Flash seemed to be overexposing at wide apertures so if you use these, double check the exposure with the histogram.
Finally, does the EC-20 degrade image quality?
To test this, I imaged a maple leaf at 1:2 macro magnification – at the closest focus of the ZD 50mm at f/8 alone, and at ~double this distance for the 50mm macro + EC-20 teleconverter (at f/11 – f/5.6×2) so that the image magnification was pretty much identical. I used AF for both.
These are pixel peeping 100% crops near the centre:
First, the macro lens alone:
and, the macro lens + EC-20 teleconverter:
In terms of image detail, I must admit, I can’t see much difference although for some reason the EC20 image has more contrast which appears to give it more detail which is counter to what we would otherwise expect.
This is how good this teleconverter is – no wonder Olympus claims it is one of the sharpest ever made!
Since writing this post, dpreview.com have posted their review of the ZD 50mm f/2.0 macro which confirms how good this lens is optically – although it really needs a SWD version with focus range limiter switch – hopefully Olympus will be updating this soon.
