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07-21-2008, 10:49 AM   #1
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Practical tests for focus checking?

Hey all,

I have a Tamron 28-75 that has me worrying about it. I've had it for over a year and a half now, and shooting focus charts (the D70 test chart and Yvon's new test charts), it looks like I have varying BF / FF depending on the focal length, under multiple test conditions (fluorescent lighting, daylight, incandescent).

However, in real life shooting, I cannot ascertain any real issue, but I am worried that this perspective may be ascribed to the fact that I simply do not understand "how sharp is sharp". Perhaps someone could please post a 100% crop of what would be considered "sharp". Most of the time, I attribute misfocused shots simply to user error, but in retrospect (especially with how careful I am with my shots), I am beginning to question whether or not my concerns of BF / FF are legitimate.

Therefore, I am wondering if there are any "practical" tests to check if a lens is back or front focusing. I am somewhat leery of using a test chart (even though it has been "proven" to work properly for many people) because of the seeming discrepancy between what I see when I am shooting in real life conditions, versus what I am being told (that my lens is exhibiting moderate BF/FF depending on the focal length) by the test charts.

Furthermore, I am curious as to whether anyone here on the forums has ever sent their Tamron 28-75 back to Tamron for calibration? I live in Canada, so I would have to send the lens to Amplis (which I assume would pass it on to Tamron Japan). Anyone have anecdotes as to whether or not the calibration is worth the time and effort (I am within warranty, but would hate for my lens to somehow come back in worse "shape" than it currently is in, especially if it is working within spec right now).

Thanks in advance for any reads and replies!

Cheers,

T

07-21-2008, 10:55 AM   #2
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I use a variation of this to check out my range finder cameras:

Use a measuring tape to get a point 10 or 12 feet from the film plane. It doesn't have to be exact (in fact won't be, so don't sweat it). Place camera on a tripod, and aim it at that spot. A textured wall or (like I use) a metal garden table provides the scale, you put some object as the target at exactly the distance, and either other bits (or arrange the tape measure) on either side. Then take your pic wide open.

This gives some indication in a real world situation just how far off you may be. Remember that the in-focus area is larger further than nearer.
07-22-2008, 11:46 AM   #3
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Thanks for the reply! I just would like some clarification as to what you mean by using the "metal garden table" as the scale? Furthermore, is there any particular reason for using 10-12 feet as the "benchmark" point?

Further replies are appreciated!
07-22-2008, 01:07 PM   #4
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I happen to have a table that's made of metal mesh type of material, so I can use it to judge focus if I focus around the middle from an angle above. I suppose you can use bricks etc as well. The point is to be at an angle vs. some flat surface with enough detail to see where the focus is and isn't.

I pick 10-12 feet as that is a typical working distance, to stand in for real life. I suppose say 6 feet would work as well - I'm a bit leery of just how applicable the macro range is to this sort of thing. Actually something like 4-6 feet might be good, especially if you lay a tape measure down the surface, as that will help you judge just how far off (if it is) your focus is. Just make sure that you focus on one known spot!

07-22-2008, 01:11 PM   #5
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Well, this kind of avoids the answer you're looking for, but if your "real world" shots are fine with you, why are you looking to make your self less than happy with your gear? Unless you suspect that your shots could be that much better (and you've shored up your technique, if needed), why go checking your gear?

Just wondering, that's all.
07-23-2008, 12:04 PM   #6
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Every now and then I get misfocused shots that I previously attributed to user error, but even with myself being cognaizent of my focusing and shooting techniques, I still get some misfocused shots which makes me curious as to whether the error lies with the equipment instead of the monkey bethind it
07-23-2008, 05:48 PM   #7
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Download the Pentax ff/bf chart and test using that, thats what it is designed for.
07-23-2008, 08:19 PM   #8
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I have already used that chart, and the results exhibit moderate BF/FF depending on the focal length. However, my problem is that in most cases, real life shots are not consistent with this, but there are the occasional ones that seem to exhibit misfocusing.

07-25-2008, 02:01 AM   #9
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Don't forget the AF system is not 100% accurate 100% of the time, more like 80-90% accurate which means 80-90% of shots will look fine the other 10-20% will not. Thats life and it happens with all systems including Nikon, Canon, Olympus etc.
07-25-2008, 05:44 AM   #10
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QuoteOriginally posted by Fireball Quote
Hey all,

However, in real life shooting, I cannot ascertain any real issue, but I am worried that this perspective may be ascribed to the fact that I simply do not understand "how sharp is sharp". Perhaps someone could please post a 100% crop of what would be considered "sharp". Most of the time, I attribute misfocused shots simply to user error, but in retrospect (especially with how careful I am with my shots), I am beginning to question whether or not my concerns of BF / FF are legitimate.
if you are happy with your shots, that is the final test. I too have a 28-75 and wonder from time to time about sharpness, and then look at what I am criticizing. usually, I am complaining that a single strand of hair, that has a pixle width of 2 is not sharply defined. Sometimes we just look too closely at photos
QuoteQuote:
Therefore, I am wondering if there are any "practical" tests to check if a lens is back or front focusing. I am somewhat leery of using a test chart (even though it has been "proven" to work properly for many people) because of the seeming discrepancy between what I see when I am shooting in real life conditions, versus what I am being told (that my lens is exhibiting moderate BF/FF depending on the focal length) by the test charts.
funny but I have never seen an award winning photo of a lens chart. I think the real problem today is we are all looking for flaws because we can.
QuoteQuote:
Furthermore, I am curious as to whether anyone here on the forums has ever sent their Tamron 28-75 back to Tamron for calibration? I live in Canada, so I would have to send the lens to Amplis (which I assume would pass it on to Tamron Japan). Anyone have anecdotes as to whether or not the calibration is worth the time and effort (I am within warranty, but would hate for my lens to somehow come back in worse "shape" than it currently is in, especially if it is working within spec right now).

Thanks in advance for any reads and replies!

Cheers,

T
I think you have answered your own question, if it is in spec now, then keep it away from service people at all cost.

not to be unfair to service people, but if they make an adjustment it might just make it worse. and still be in spec
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