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correct focus

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Old Sunday 2nd March 2008, 13:24   #1
andy70437
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correct focus

Hi,
Took this with my Sigma 50-500mm and could someone tell me if it is correctly focussed. Or as good as I can expect.
Aim was centre battery, auto focus on, f5.6, iso 400,at 300mm focal length. Batteries were one inch apart.
Any advice gratefully acknowledged.
Andy
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Old Sunday 2nd March 2008, 14:44   #2
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That's about as good as you'll get without closing your aperture. Try it at something like f16 and you'll have more in focus. As you move towards 500mm less will be in focus as well.
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Old Sunday 2nd March 2008, 21:23   #3
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I am not a fan of the battery test. If the spacing is not spot on and the positive (+) signs are not exactly positioned then you run the risk of getting a false impression. I do not think there are sufficient visual clues to make the test conclusive.

I do not agree that each battery is evenly spaced as you suggest or if they are then they are not in a perfect line IMHO
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Old Monday 3rd March 2008, 16:51   #4
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Don't see why any of that would be a problem, Adrian.

All this test is mean to do is provide an indication that that what was focused on - the battery in the middle - is where the actual point of focus is, rather than some point in front or behind, both of which situations would be indicated by batteries one side or other of the middle one being sharper.

There's no need for precision positioning for that.

Paul,

based on this test, you're looking fine - the middle battery is clearly the sharpest.
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Old Tuesday 4th March 2008, 19:57   #5
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Thanks Gents, as I was wondering. But I`ll take it that`s OK.
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Old Wednesday 5th March 2008, 03:49   #6
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Industry standard is , 1/3rd in focus in front of focal point and 2/3rds behind. Judging by your test the battery in front is more in focus than the one behind. This suggests to me that the focus is slightly out. You may therefore have depthof field problems. It may make little difference in the real world because the degree you are out is minor. You may have a very slight front focus issue but as I said , since the test you have performed is, in my view, unreliable, it is not possible for anyone to advise you with certainty.

For anyone else who has not had a front or back focus problem (I have) it will be difficult for them to grasp what an effect this may have on real world shooting. If your focusing is out it is worth sorting out even if the degree is slight since apart from the usual problems it will mean you may have problems with critical manual focus and your dioptre may be set wrong too! IMHO.

Upshot, carry out a better test and a number of them if you find your real world shots are not up to scratch. I'm sorry if that is not what you wanted to hear.
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Old Wednesday 5th March 2008, 07:09   #7
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does that mean that the camera is out or the lens
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Old Wednesday 5th March 2008, 08:03   #8
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Originally Posted by mark richards View Post
does that mean that the camera is out or the lens
either or both a possibility IMHO.
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Old Thursday 6th March 2008, 07:48   #9
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Originally Posted by a.dancy View Post
Upshot, carry out a better test and a number of them if you find your real world shots are not up to scratch. I'm sorry if that is not what you wanted to hear.
perhaps you could offer some advice as to a better test to do, this sort is what I've used in the past and I'd be interested to learn of more reliable ways of checking this.
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Old Friday 7th March 2008, 00:08   #10
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Peter

I used several methods. The best one for me was a white ruler. At the centre point I placed a black dot using a marker pen to ensure that I was hitting the same focus point each time I did a test...which I did a number of times. I did the same using magazine pages where the contrast with the letters and the page was good. Somehow, or for some reason, the fact that you can read the in and slightly out of focus print enables you to assess the degree of focus (or span of 'field of view' and where it lay) because the content is meaningful. The eye/brain cannot judge things which are more abstract I suppose. I also used a chart designed for the purpose which I got from someone in a camera club. It consisted of lines on a sheet of paper and is the same as the one on photo. net. If you google ' photo.net focus Bob Atkins' you should find such a chart which has the beauty of the lines being cosine adjusted to take account of perspective.

Finally I have a toy bird (how sad) which I set up on branches etc. This enabled me to carry out a host of tests not only for checking focus but helped me learn camera/field skills for things such as DOF and exposure. When I started photography (using film) I could not afford the learning curve cost, so I practised on the toy to help me understand exposure and DOF. It was the best investment I ever made in terms of gear and I didn't get it from a camera shop! I still think it is as good as anything for testing focus. I tried the battery test but (for me) I found the results too variable and I think there are a number of reasons for it, so would not now consider it as an option. It is however only my view and if it works for others then that's great.

Kind regards

Adrian
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Old Friday 7th March 2008, 21:25   #11
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Exif suggest a shutter speed of 1/8th on a 500mm zoom. Although the image looks sharp enough I'm not convinced about the wisdom of assessing correct focus at this speed. Perhaps if the test can be repeated in a better lit location...........
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Old Saturday 8th March 2008, 00:05   #12
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Exif suggest a shutter speed of 1/8th on a 500mm zoom. Although the image looks sharp enough I'm not convinced about the wisdom of assessing correct focus at this speed. Perhaps if the test can be repeated in a better lit location...........

Agreed
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Old Sunday 9th March 2008, 02:42   #13
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Quote:
Originally Posted by ikw101 View Post
Exif suggest a shutter speed of 1/8th on a 500mm zoom. Although the image looks sharp enough I'm not convinced about the wisdom of assessing correct focus at this speed. Perhaps if the test can be repeated in a better lit location...........
How would a faster shutter speed change the (clearly) indicated point of focus, though?

Assuming a stable platform, the shutter speed is pretty much irrelevant. The results of this test show that there is no back or front focus worth worrying about at the selected focal length, aperture and distance from the subject, and that's all it's meant to do.
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Old Sunday 9th March 2008, 11:42   #14
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Quote:
Originally Posted by Keith Reeder View Post
How would a faster shutter speed change the (clearly) indicated point of focus, though?

Assuming a stable platform, the shutter speed is pretty much irrelevant. The results of this test show that there is no back or front focus worth worrying about at the selected focal length, aperture and distance from the subject, and that's all it's meant to do.
Keith....I don't kow why shutter speed makes a difference but I do believe it does. I suppose it is better to have your lens 'snap on' rather than struggle on unreliably. As for the present case taking into account the lens used the focus may be adequate for most real world situations. With my f2.8 300mm lens my guess is that it would not be accurate enough for shallow DOF shots where accurate focus is critical.

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Old Sunday 18th May 2008, 18:07   #15
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Thanks guys, very interesting discussion and I hope I have learnt something useful. I am now off to buy a toy bird . . . . (Thanks Adrian for that!)
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Old Thursday 22nd May 2008, 11:01   #16
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Originally Posted by andy70437 View Post
Hi,
Took this with my Sigma 50-500mm and could someone tell me if it is correctly focussed. Or as good as I can expect.
Aim was centre battery, auto focus on, f5.6, iso 400,at 300mm focal length. Batteries were one inch apart.
Any advice gratefully acknowledged.
Andy
Hi andy thought you might be interested in a Depth of Focus chart for the 50-500 http://www.sigmaphoto.com/images/LensesDepth/APO50-500mmF4-6.3EX_RF(E).pdf
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Old Wednesday 28th May 2008, 19:42   #17
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BIG RON'S FOURTH COROLLARY: Understand the term "depth of field" prior to purchasing a camera.
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