Value Micro adjustment tool - received it - love it -

tdodd said:
By maintaining the 50X ratio between target distance and focal length the target will remain the same size in the frame no matter which focal length you use.
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Yes, that's very true.

tdodd said:
Unless your DOF really is as little as 1cm or less then I personally do not think that minor errors in alignment (say 1-2mm) will be significant.
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Another thing with using 50x FL is that, for any aperture, DoF will also be constant. At f2.8 that's going to be 25cm! Even at f1 the DoF will be 10cm.
 
hollis_f said:
Another thing with using 50x FL is that, for any aperture, DoF will also be constant.
Click to expand...

I do believe that's true.

hollis_f said:
At f2.8 that's going to be 25cm! Even at f1 the DoF will be 10cm.
Click to expand...

That's the figure you get back from DOFMaster, but that figure only holds true for a "normal sized print" viewed from "normal viewing distance", when using a 1.6 crop body. That's roughly a 10x8 (or 9x6) print viewed from about 12" away.

If you are going to analyse your AF accuracy by studying images at 100%, especially if from a 7D, then you are enlarging the image significantly beyond 10x8 and the apparent DOF will shrink. In other words, fitted to screen you may see a DOF of around 25cm when judging against the scale of your ruler, but viewed at 100% it will be far less. On my 132ppi monitor a 7D file would be 5184/132 = 39" in the long dimension. In other words it would be enlarged around 4X more than your typical print. That is going to make the softer areas outside the plane of focus far more obvious.

I can't find the link at the moment but there is an online DOF calculator which allows you to input display size as well as viewing distance in order to complete the DOF calculation for "non standard" enlargements. I'll add a link if/when I can find it.

EDIT : Doh! How hard was that? Here it is - http://www.cambridgeincolour.com/tutorials/DOF-calculator.htm. It even makes provision for differing visual acuity of the individual concerned.

According to that calculator the DOF for a 200mm f/2.8 lens shot from 10m away on a 7D would be just 7cm when viewed at 100% on my monitor....

20100922_181620_.JPG


Now that will help you pinpoint your focusing fairly accurately, but I still don't think an alignment error of a couple of mils in your focus target alignment will matter.
 
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tdodd said:
I do believe that's true.



That's the figure you get back from DOFMaster, but that figure only holds true for a "normal sized print" viewed from "normal viewing distance", when using a 1.6 crop body. That's roughly a 10x8 (or 9x6) print viewed from about 12" away.

If you are going to analyse your AF accuracy by studying images at 100%, especially if from a 7D, then you are enlarging the image significantly beyond 10x8 and the apparent DOF will shrink. In other words, fitted to screen you may see a DOF of around 25cm when judging against the scale of your ruler, but viewed at 100% it will be far less. On my 132ppi monitor a 7D file would be 5184/132 = 39" in the long dimension. In other words it would be enlarged around 4X more than your typical print. That is going to make the softer areas outside the plane of focus far more obvious.

I can't find the link at the moment but there is an online DOF calculator which allows you to input display size as well as viewing distance in order to complete the DOF calculation for "non standard" enlargements. I'll add a link if/when I can find it.

EDIT : Doh! How hard was that? Here it is - http://www.cambridgeincolour.com/tutorials/DOF-calculator.htm. It even makes provision for differing visual acuity of the individual concerned.

According to that calculator the DOF for a 200mm f/2.8 lens shot from 10m away on a 7D would be just 7cm when viewed at 100% on my monitor....

20100922_181620_.JPG


Now that will help you pinpoint your focusing fairly accurately, but I still don't think an alignment error of a couple of mils in your focus target alignment will matter.
Click to expand...

I think the recommendation for 50x focal length as a distance for AF checking is mainly to prevent people from testing at artificially close range, and then throwing up untypical errors. Namely those A4 print-out sheets you download from the web which often get tested at well under 2ft, which is just asking for trouble and seems to be very prone to user error on top.

50x is not bad for the generally shorter focal lengths that we now use on crop format cameras, but it can be too long for full frame and obviously more so with longer lenses. I think the manufacturers of some of these commercial test targets suggest 25x.

Another way of looking at it is to test at a typical focusing distance for normal use, or some kind of sensible distance within that range. For me, what that boils down to is about 5-10ft as a distance for testing, though if that resulted in an AF adjustment that didn't deliver extremely good accuracy from under three feet to infinity, the lens would be going straight back. Any lens that isn't plumb within its optimum operating range at that kind of shooting distance has no place in my bag.

But so far I have had no reason for complaint and in fact rather the opposite - the AF testing I have done has left me very impressed with my 5D2 and L lenses with a very high level of accuracy and consistency, both of which are to a much higher standard than I ever need in practise.

Or to put that another way, if I ever get a missed focus, it's going to be my fault rather than the camera's. An example of a shallow depth of field close portrait at say f/2, DoF around a couple of cms and if you want to be ultra critical you maybe want to be within about half that. Now the camera will be accurate to within a few mms or even less in that situation, but unless the camera is on a tripod and the subject is actually stone dead, then there is no way you can possibly work to that standard as live human beings.
 
Hi guys

I received my Spyder LensCal yesterday. Must say I'm very happy with it. Yes you can make your own set ups for a lot less than the cost of this, but I like the way it folds up, is quick to set up and has a spirit bubble to ensure you have it level.

Anyway I micro adjusted my 135L which i had a feeling wasn't focussing right.

Here's the before shot


as you can see it is front focussed which confirms my initial suspicions.

Anyway after a few shots later here it is with a +8 Micro adjustment


As you can see this has made quite a difference. :)
 
I did get a chance to use mine last night.

Phil, did you use the recommended 50x focal length as the distance between lenscal and camera?
 
This looks like a handy tool, I've been meaning to check the calibration with lenses for a while.
Are there any places selling this in the UK yet as a quick google shows zilch.
 
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