My personal macro and tele lens comparison

CrisPhoto wrote:

Hello AndersW,

thanks for your interesting input. I think there are many reasons for back focus, and your example was well chosen. I even thought that my wristwatch sample fools the 300mm lens the way you describe. Because the metallic lines shows some contrasty bokeh. But on the other hand the 60mm lens has similar bokeh and never fails to focus correctly. So bokeh and CA is not always fooling the AF logic.

What you see with your 300 is certainly the same thing as I see with mine. It's not really a matter of the bokeh somehow fooling the AF system though. When I first encountered the problem, I was shooting targets that were not parallel to the sensor, just as your wristwatch. Consequently, something was in focus (as my brain would have it) but behind the point I wanted to focus on. So it looked as though the AF system had somehow missed the target.

That's not what happens though. Even if you shoot a target that is parallel to the sensor, as I did for the samples I posted, the AF system will set focus behind the target, based on what our brains consider perfect focus. However, based on the criterion the AF system uses (maximum contrast) it has set the focus exactly where it should be. There's really nothing wrong with that criterion or with the way the AF system uses it. It's the optics that misbehave when we force the lens to go outside its normal range of operation, causing massive amounts of spherical aberration and therefore a mismatch between maximum contrast on the one hand and what our brain considers maximally sharp on the other.

Besides your explanation - which fits very well to your sample - I have another explanation:

For this, I have to look back at the slow but sharp FT 35/f3.5 lens. The 35mm lens focused veryyyyyy slow (1-3 seconds typically, often more), it was easy to observe the camera acting. It first made a sweep back and forward to find the sharpest distance. Then the focus drive made a big jump and locked at the exact position were the picture supposed to be sharp. I was wondering how the "jump" can be precise enough, but the 35mm lens always had perfect focus.

What if the 300mm lenses are controlled similar, but silent and fast. Maybe they make this jump but they miss the correct point because the macro lens changes the focus behavior quite a lot???

No, I don't think the problems with our 300s when used with close-up lenses has anything to do with the AF mechanics. In both cases, the AF system can move pretty quickly to a certain location. The difference is that FT lenses designed for PDAF only (I didn't check whether the 35/3.5 belongs to the few that are designed to work with both PDAF and CDAF) can only move quickly, in big or small "jumps", whereas native MFT lenses can move as slowly as the body requires to find peak focus (peak contrast) with sufficient precision without having to stop the AF movement from time to time.

BTW: The problem with spherical aberration on the 100-300 when used with a close-up lens is not the same when used with extension tubes. While I still have some further testing to do, my long lens appears to work quite OK with the tubes. Of course, you don't get as much magnification with the tubes (only about 1:3 at most) but you do get a bit more than with the naked lens at 300 (1:5) and you get it at any focal length, combined with great flexibility in terms of working distance (up to about one meter at 300 mm and 1:3).

Have you tested your 75-300 with the tubes? If so, what are your results?

Christof

(P.S.:I am still thankful for your explanation of shutter shock, there was a lot of speculation going round and round. Your explanation as well as the pistol-grip trick made a lot sense for me. I screw my folded tripod under the camera to mimic the pistol grip mass and viola: the shutter shock with the 300mm lens has gone. Thanks!)

You are welcome. Glad to hear the grip helps. The appreciation is mutual. I have followed your earlier test reports with interest and your macro report prompted me to do what I should already have done quite a while ago: Test more systematically how my prospective macro solutions actually work (and not ). As I said, I still have some testing to do, but so far my results are entirely in line with yours. In brief, my preliminary findings are as follows:

75 and close-up lens: No good.

75 and extension tubes: No good.

40-150 and close-up lens: Pretty good

40-150 and extension tubes: Pretty good

100-300 and close-up lens: No good

100-300 and extension tubes: Pretty good