100-300 with Kenko extension rings - focus range

Started Jan 11, 2013 | Discussions thread
Tony Rogers
Contributing MemberPosts: 555Gear list
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Re: Image Quality
In reply to _sem_, Jan 16, 2013

_sem_ wrote:

Tony Rogers wrote:

I have just run a series of tests at 100, 200 and 300mm with 26mm extension tubes to get an idea of the relative image quality at different apertures.

100mm - Centre sharpness is very good from F/5.6 onwards. The corners are horribly soft. Slight improvement in corners at higher F stops.

200mm - Centre sharpness is very good from F/5.6 onwards. The corners are softer with the extreme corners being very soft. Slight improvement in corners at higher F stops.

300mm - Centre sharpness is very good from F/7.1. No real improvement with higher F stops. Extreme corners a soft.

With my test target (pre-printed pattern on paper) I can't see a lot of difference in centre image quality between 100, 200, and 300mm at these F stops. Corners get much better at longer focal lengths. Magnification is approximately the same for all focal lengths!

So the quality is better with the 500D or not?

Is the corner softness actual softness or field curvature (so that you can improve corners by refocussing)?

I'm not sure yet since I have not been able to try any real subjects. My butterflies won't be back for at least two months yet! The 500D doesn't have soft corners. However, the extension tubes seem good at about 300mm FL/1200mm distance which the 500D can't do at all!

The corner softness (at 100mm FL) does not seem to be improved by focusing. There is also lots of CA in the corners. Crop those out!

F/7.1 is best at 300mm which is the same result that I have found with the lens on its own and is a lot better than F/5.6 (wide open). No great surprise I suppose. Given the corner softness at shorter focal lengths, I would advise setting F/7.1 and leaving it there for all focal lengths. I would probably only use the shorter focal lengths when I can get in close AND there is a need for a slower shutter speed (if indeed it that makes sense? If the magnification is the same at different focal lengths, can you use a slower shutter speed for shorter focal lengths and still avoid shake? Or do they all require the same shutter speed because the field of view is the same? Hmm...)

Macro with long effective FL (and working distance) is challenging regarding technique. With typically limited ambient light you need relatively long exposures to get useful DoF, which is already halfway into diffraction blur. A tripod with a focusing rail is recommended. Stabilization usually doesn't help enough. Shake/motion blur may be avoided freezing motion with flash at x-sync speed. Flash power should be relatively low to keep the pulse short; but a power-eating diffuser or bounce-card of a suitable size is recommended to avoid harsh shadows and tame specular highlights; so flash should preferably be off-camera, close to the subject.

At some point, I will get to try out the extension tubes with other lenses which will be in true macro territory. Thanks for the advice.

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