Hyperfocal question

Derek.Laurence

Derek.Laurence

Suspended / Banned
Messages
231
Name
Derek
Edit My Images
Yes
Hi everyone


Simple question.....

Why is the hyperfocal distance for large print pictures twice that of regular print?

I just downloaded a chart and almost got caught out with the wrong one.

Surely if you're looking to keep as much in relative focus as possible then print size wouldn't matter?
Or have I missed the point?

I'd be grateful if someone could enlighten me. :-)
 
Not quite true. The lens only EVER focuses on one plane at a fixed distance from the lens; in front and behind that distance any object will be 'slightly' oof.

Depth of Focus is the distance within the subject range of 'acceptable' focus, or where the fuzzyness is so small as to be 'imperceptable'....

But of course, more you enlarge your image..... more 'perceptable' that fuzziness will become, wont it?
 
It's simple, the bigger you project an image, the smaller the depth of field appears to be, this includes large prints. You can see this yourself when looking at the camera LCD and then looking at the same image on your monitor, things that looked sharp on the LCD suddenly look out of focus. Now magnify this to a large print 2-3x the size of your monitor and your going to find even more is OOF!

Hyperfocal focussing relies on the furthest away bits of a scene only just barely being in acceptable focus - apply the theory above and you find they no longer are focussed, so you'd need to focus elsewhere in order to keep them sharp, at the expense of close objects of course.

Someone recently posted a very nice test of hyperfocal focussing vs infinity focussing, the aim being to prove that focussing at infinity was actually better, which I would agree with adding a caveat that it only holds true if items in the distance are the important bit of a scene. I apply a 'bokeh test': would the image still be good if I was to blur the background? If so, then Hyperfocal, if not then infinity. (Although in reality I don't use HFD, I focus on the subject or at infinity as necessary)

Hmm, longer reply than I had planned.
 
Thank you for all your replies folks

I had wriiten a follow up question but a combination of the last two posts has given me some clarity.

I think it was the "at the expense of close objects" that made the penny drop!

Thank you again for the replies ( if your reply hadnt been as long allan, I wouldnt have seen the difference for the two distances)

Derek
 
All depth of field calcs are based on the assumption that if you view a 10in print from a distance equal to the diagonal (ie 12in) then anything up 0.2mm wide will be seen as equally sharp. Everything in the formula is worked back from that internationally agreed standard. Good DoF calculator here http://www.dofmaster.com/dofjs.html

DoF calcs also assume that with larger size prints, the viewing distance will be adjusted accordingly and viewed from a further distance, as it normally is (maintaining the length of the diagional thing) and in that case the calcs hold true. If however, you view at 100% on screen from close distance, as we often do these days, then the calcs go out of the window.
 
ah....even more clarity hoppy. Thanks muchly

Im currently searching for that infinity vs hyperfocal focusing thread but with no success as of yet :(
 
Derek.Laurence said:
Hi everyone


Simple question.....

Why is the hyperfocal distance for large print pictures twice that of regular print?

I just downloaded a chart and almost got caught out with the wrong one.

Surely if you're looking to keep as much in relative focus as possible then print size wouldn't matter?
Or have I missed the point?

I'd be grateful if someone could enlighten me. :-)
Click to expand...

Well, it does...and it doesn't.
The print size and viewing distance determines the "circle of confusion" (COC) which is essentially "perceptual sharpness." (the zooming in factor)

Most DOF calculators assume an image 2x as large will be viewed 2x as far away(so that you can see the whole thing w/out scanning). If this is the case the COC doesn't change and the hyperfocal distance doesn't change.

But if an image is 2x as large and viewed from the same distance then the COC does change and therefore the hyperfocal distance also changes.
 
Thanks for all the replies people.

I find that you get so much information even for the simplest of questions!

I'm so much clearer on it all now but will give everything a more in depth look once I don't have my 2yo bouncing all over me!
 
You must log in or register to reply here.
Back
Top