BF & FF compensation

[cupic]

i should have included some examples but in the meantime i used a K20D with the sigma 100-300 f/4 lens to shot some dragonfly's and noticed that the concrete behind the DF was more in focus than the subject.is this the case of BF and if so would some +VE compensation be the order of the day.Or is it just the facts on DOF






cheers

[I @ M]

Cupic, the first thing that comes to mind for me is --- how close were you to the subject? --- if you are inside the minimum focus distance by just a little bit then the focus will be "stuck" at the minimum focus distance and the image will appear to be in focus behind the subject.

[cupic]

Thanks for that Andrew it may seem that I creeped into the 180cm minimum focus distance so that may be the culprit


cheers

[Ozzi Paul]

Try a +2 close up filter. I was using an old Sig 100-300 f4.5-5.6 for a little while and found that a +2 filter would allow me to put the lens very close to the subject.
I tried it on a small flower at 200mm and could have the lens within a few inches and could fill the frame with the centre part of the flower that was only as big as a thumbnail.

[arthurking83]

I may be wrong here as I'm looking at a lower resolution, and hence lower detailed version of the image that you have access too Cupic, but my eyes are seeing more detail and sharpness in the concrete in front of the DF.

I drew a line to show you where I reckon the plane of focus appears to be:
4421547310_3870734e2d_o.jpg

I remember Andrew had focus distance issues with his copy and sent it in for a service/calibration and it came back better.
Great lens BTW!

[cupic]

Yes I concur that it appears to be FF issues.Is this easily rectified?


chers

[OzzieTraveller]

G'day Cupic

It's a nice pic regardless of your concerns :-)

You state that the minimum focus distance is 180cm ~ that's a hell of a long way out
For many years now I have used Close-Up Lenses on long zooms and they are magic for what they can do for you

They come in several "strengths" and diameters, and screw into the accessory / filter ring on the front of the lens

a "+1 dioptre" lens focuses at 1 metre
a "+2 dioptre" lens focuses at 1/2 metre
a "+3 dioptre" lens focuses at 1/3 metre

They come in two types - a simple, single-element lens from Hoya or suchlike, costing about $60 for a 58mm dia, and a multi-element compound lens from 2-3 others but costing $200-odd

If you were to experiment with a +1 lens, you would immediately find
a) the focus would be down to 100cm ... probably 75cm using the main-lens focus ring, and
b) at the point of focus, you still have 100% of your zoom available - it's not lost as if you were using extn tubes etc

Hope this helps a bit
Regards, Phil

[cupic]

The original question was in relation to the correction that can be made in the menu of the K20D to correct lenses that display FF and BF .Anyone used this technique?


cheers

[jibbonpoint]

The wings on the insect seem to be in focus & that's what cupic has focused on. The lens is not vertical to the plane that we are all looking at; that is the brick that the insect has alighted on. Therefore the centre of the depth of field won't be vertical either; it will be at the same angle as the lens.

I reckon the lens is OK.

Coises; I can't get my scanner to work to post a sketch of what I mean. I'll keep trying

[jibbonpoint]

See if this makes sense:-

[cupic]

The DOF is about right ,if the EXIF isnt there I have included this.The stats are

f/9 ,1/4000s, -1EV, 640ISO @ 300mm



cheers

[James T]

Based on one shot in uncontrolled circumstances, my money would be on user error before lens problems. (No offence). Did you just miss focus slightly on this one?

AF would be just as likely to grab the concrete as the insect. How did you shoot it? Tripod, handheld? Very easy to move, just a small amount before/as you press the shutter, thus missing the focus.

[jibbonpoint]

Call me stubborn, but I don't believe that there's anything wrong with the focus. I've added the wings to the above chart as a demo.



Next, you could try the Depth of Field calculator:-

http://www.dofmaster.com/dofjs.html

And just to make sure, download & print the Focus Chart contained in this page & set the lens up accordingly which will reveal any BF or FF issues:-

http://focustestchart.com/chart.html

You will notice the chart is specific to a 45 degree angle, therefore it has been constructed to allow for any parralex error in viewing a shot that cupit has taken.

Let's know how you go.

[cupic]

I have checked and the DOF is quite narrow band literally mm's on both sides as for your diagram(are you sure that you didn't use goggle maps very accurate ).


cheers

[jibbonpoint]

Well, there you go. That's why the tip of the wing on the insects RH side was out of focus. In effect my Rembrandt should have shown the wing on the LH side slanted towards the lens & the other, further away.

So you won't need to flog the lens off on e-bay, now.