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How do you read the focus distance window? (1 image)
The main thing I'd like to do is get perfect spot on manual focusing. In order to do this with my old 18-55mm lens I could zoom full in, focus, then zoom out and I knew I'd be spot on at full wide..

With the 28-135mm lens there is different focusing at different zoom levels, this is frustrating, but I think the camera gives me a clue with this focusing window. But how do I read it?.

Consider this example:.

Image control:Zoom outZoom 100%Zoom inExpand AllOpen in new window.

If I'm fully zoomed in to 135mm and perfectly focused at the pictured focus setting, what do I need to set the focus to to be perfectly focused at 28mm?.

I thought that since it appears that 135mm is focused at 13 ft according to the focus window, then when I zoom back to 28mm then I just need to set the focus to 13ft under the 28mm mark and it should be exactly. But all my trials at this, with a tripod and even using the exact same apeture, it's focused at 135mm but blurry at 28mm. Why doesn't this work!? The distance has NOT changed between camera and subject. Is it because I have a Rebel XT with a crop sensor, and my lens is full frame? If that's the case is there math I can do like 13ft x 1.6 or something? I just want to get in focus ..

Comments (5)

Yumology wrote:.

The main thing I'd like to do is get perfect spot on manual focusing.In order to do this with my old 18-55mm lens I could zoom full in,focus, then zoom out and I knew I'd be spot on at full wide..

A lens who remains focused when zoomed is called parfocal. Some are, some are not..

With the 28-135mm lens there is different focusing at different zoomlevels, this is frustrating, but I think the camera gives me a cluewith this focusing window. But how do I read it?.

Well, it does not. Let's see. Lowest, white text: focus distance (meteres). Above, in green, focus distance in feet. The red markers indicate the corrections when shooting infrared film with a filter, because infrared light refracts differently..

If I'm fully zoomed in to 135mm and perfectly focused at the picturedfocus setting, what do I need to set the focus to to be perfectlyfocused at 28mm?.

Well. It seems the 28-135 is not parfocal.http://www.rogercavanagh.com/helpinfo/30_parfocal.stmhttp://photo.net/bboard/q-and-a-fetch-msg?msg_id=003rJd.

So, refocus at 28. The distance scale is just an indicator..

I thought that since it appears that 135mm is focused at 13 ftaccording to the focus window, then when I zoom back to 28mm then Ijust need to set the focus to 13ft under the 28mm mark and it shouldbe exactly..

Definitely not. Those are the infrared correction marks..

But all my trials at this, with a tripod and even usingthe exact same apeture, it's focused at 135mm but blurry at 28mm. Whydoesn't this work!? The distance has NOT changed between camera andsubject. Is it because I have a Rebel XT with a crop sensor, and mylens is full frame? If that's the case is there math I can do like13ft x 1.6 or something? I just want to get in focus .

No, it's because the lens is not parfocal. Probably the kit zoom ain't, either, but it's wider and shorter, therefore harder to spot the problem. To get in focus... focus .

/d/n..

Comment #1

I don't think you can get an accurate focus by dialling in the distance - it should be used as a guide - useful if you need to set a manual speed light output perhaps. I have a Nikon 18-200mm which physically lengthens when zooming - the minimum focus distance for this lens remains the same at wide or telephoto and I believe this distance is from the subject to the image sensor NOT the subject to the front lens - this also requires a refocussing when changing zoom using a tripod although subject to sensor has not changed. It maybe that different lens constructions have other characteristics - perhaps the lens manufacturer would be place to start - most have a web site where you can e'mail a tecnical query. Happy snapping,Pete..

Comment #2

Yumology wrote:.

The main thing I'd like to do is get perfect spot on manual focusing.In order to do this with my old 18-55mm lens I could zoom full in,focus, then zoom out and I knew I'd be spot on at full wide..

Probably not. Today most zoom lenses change focus as you zoom. If the lens has variable aperture, then it changes focus as you zoom. If the lens has a constantant aperture over the entire zoom range, then it probably holds focus as you zoom..

As stated by other posters, the focus scale is a guide only, I would not put too much stock in it's accuracy. If you want to manual focus, look carefully at the image in your viewfinder..

The greatest of mankind's criminals are those who delude themselves into thinking they have done 'the right thing.'- Rayna Butler..

Comment #3

Devnull wrote:.

I thought that since it appears that 135mm is focused at 13 ftaccording to the focus window, then when I zoom back to 28mm then Ijust need to set the focus to 13ft under the 28mm mark and it shouldbe exactly..

Definitely not. Those are the infrared correction marks..

That those are not IR focusing marks since they go from 28-135 just like the lens..

So if the shot is perfectly focused at 10m when zoomed to 135 (the 10 will be under the red 135) then to focus at 10m when zoome back to 28mm will require the 10m to be under the red 28 mark. You are correct about it not being parfocal, thus the range of focus marks. IR would be a constant offset from each focus mark..

As an aside, almost no current lenses (I know of none in the last 5 years) are marked for IR anymore and that looks like a current lens..

Feel free to prove me wrong on the above assertions. I've been wrong before and might be wrong now......but I don't think so..

A member of the rabble in good standing..

Comment #4

LM2 wrote:.

Devnull wrote:.

I thought that since it appears that 135mm is focused at 13 ftaccording to the focus window, then when I zoom back to 28mm then Ijust need to set the focus to 13ft under the 28mm mark and it shouldbe exactly..

Definitely not. Those are the infrared correction marks..

That those are not IR focusing marks since they go from 28-135 justlike the lens..

I am not an optic engineer, but I am pretty sure there is a different light path at different zoom ratios, hence the difference..

So if the shot is perfectly focused at 10m when zoomed to 135 (the 10will be under the red 135) then to focus at 10m when zoome back to28mm will require the 10m to be under the red 28 mark. You arecorrect about it not being parfocal, thus the range of focus marks.IR would be a constant offset from each focus mark..

Ok, don't take my word. Try it .

As an aside, almost no current lenses (I know of none in the last 5years) are marked for IR anymore and that looks like a current lens..

Well, I can't contradict you regarding new lenses and all. For sure, the EF-S lenses are NOT marked for infrared, since there is an IR filter on digital sensors..

However, 28-135 is quite an old lens..

Feel free to prove me wrong on the above assertions. I've been wrongbefore and might be wrong now......but I don't think so..

I think you are this time  There is such a thing as a lens manual .

Also, the 17-40 has the IR scale, and it's parfocal. Strangely, I don't see the red dot on the 50mm 1.2L, so you might be right about new designed lenses..

/d/n..

Comment #5

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This question was taken from a support group/message board and re-posted here so others can learn from it.

 

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