Can AF Fine tuning be non consistent?

Started Feb 27, 2012 | Discussions thread
SteveL54 Senior Member • Posts: 2,701
Re: Can AF Fine tuning be non consistent?

jkjond wrote:

Your samples use different apertures so I can't see what conclusions you could draw regarding focus. You also used a target square on to your camera.

If testing for front/back focus you'd get more useful information shooting at an oblique angle - a common approach is to lay a tape measure on a table and focus on an exact place on the measure using max aperture for shortest dof.

Targeting the oblique angled scale is very controversial. Since the AF point is not really a point it can be influenced by a significant range of markings along the scale. So one can't be certain where the true AF null point should be on the scale. Having it out to the side of a parallel target is good as long it can't influence the AF sensor. If a single scale is out to one side then lateral alignment to the target (and scale) is important.

The test you set up is more useful for comparing lens performance over a range of apertures (edge to edge performance), or different lenses.

I didn't read what measures you took to eliminate camera shake, but this is also imperative - though in the real world, if you shoot hand held and have poor technique you will suffer from soft shots unless using very fast shutter speeds or flash.

Camera shake is one of the most problematic variables in achieving sharp images.


Mirror slap is a common cause,

I would't say it's common because I've only encountered it under specific circumstances. If one has a cheap tripod then I suppose it will be more common. And as sensor densities go up it will be noticed more when those photos are viewed at 100%.

I only encountered mirror slap in any significant way while using my two larger telephotos. For my Nikon system that would be the 70-200mm VR1. For my Canon system it would be the 100-400L.

I was once surprised to compare some lab shots that I had taken using my 70-200 against similar shots taken with my smaller 28-200mm. The shots taken with the smaller zoom where noticably better despite otherwise similar settings. The reason was mirror slap. The bigger zooms have their own mount point. This provides a significant moment arm for the mirror motion to impart momentum to the camera.

That and a couple of other sessions with the bigger zooms is what prompted me to get a really good tripod (Gitzo 1325) and ball head (ARCA Swiss Mono Ball B1)

and if you camera hasn't got mirror lock up, then there will always be some camera movement unless you lock the camera on a bench and rest a bag of cement on top of it.

I think that's overstating the case a bit. Unless one has a cheap tripod, using a small prime with the camera body directly coupled to the tripod head isn't that much of a problem.

I just ran a test using my lighter tripod Gitzo GT1531 and Markins Q3 ball head with my D300 RRS L-plate and 35mm f/1.8. I found no detectable softness in the image due to mirror slap. I tested at the most common mirror slap shutter speeds 1/10 to 1/3 second.

Some cameras have far worse mirror slap than others. I watch my hotshoe mounted bubble level when using a tripod, its quite surprising how much it wavers when I do the first shutter click to raise the mirror - small wavers

I only saw waivering while I was making adjustments to the camera. On timer release there was no detectable motion of the bubbles during mirror movement.

but put a long lens on a camera any deflection is magnified.


All that being said I should add some more words of caution about mirror slap since that is the greatest potential gotcha with the test procedure I describe. (There are a lot of cheap tripods out there.)

Thanks for the feedback.

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