DOF question regarding f2.8 vs. f4.

[canonshooter]

I am just thinking of blurred background with surperior subject IQ and I now shoot a 5DMkIII.  I realize two other factors beyond aperture affect DOF so lets assume:

1. Focal length of 70mm
2. Distance to main subject 8-10 feet.

I am asking because I am considering returning my 24-70mm f2.8  MkII for the new f/4 version with IS.  I have had the f/2.8 for about 10 days and I can't control camera shake consistently with anything below 1/100 of a second.  I rented the Tamron version with VC to compare with the Canon.  The VC worked fair, but to consistently get sharp images I needed 1/60 sec plus.  Just not equal to the Canon H-IS.

I am spoiled with my 70-200mm f/2.8 MkII with IS and I can handhold this lens at 1/30 sec with great results.  Before anyone starts suggesting that I practice or change my technique, consider I am 65 and not that steady of hand.

Thanks for your help, I look forward to your replies.

[cicopo]

I'm not sure just what you're trying to do & here's why. The shallow DOF is directly related to the wide aperture & not the slow shutter speed. You can use a very high shutter speed at f2.8 & it won't effect the DOF anymore than a slow shutter speed. You seem to be confusing the background blur that panning at a slow shutter speed induces. I ran into someone a few years ago trying for the background blur by shooting his 70-200 f2.8 L wide open in bright sunlight. He knew it wasn't working out so asked how I do it & I explained that by using f2.8 in really good light it forced the camera to use a very high shutter speed so even if he was panning nicely he couldn't get the desired effect.

So in the simplest of terms the DOF is relative to the aperture, (wide = shallow, small = deep) and whether it's a full frame body (more shallow) or a crop body (deeper DOF for the same aperture).

Here's a good example of panning blur to prove my point, it's at 1/60 sec & f18 (EIGHTEEN) shot by someone who is a smidgeon older than you who also relies on IS whenever possible.

[canonshooter]

Dear cicopo:

 I only referenced shutter speed to provide a reference to my camera shake problem.  I do understand that shutter speed has nothing to do with DOF, only aperture, focal length and distance to subject affect DOF.  I also understand panning to achieve a blurred background and represent movement of the main subject while maintaining focus of that main subject.

What I was asking (given the parameters I listed) is what is the difference in the DOF at f/2.8 compared to f/4 and can similar DOF be achieved with both apertures?

[cicopo]

OK, in that case you can get the answers here. Enter your settings on the left & the answer is on the right. It looks like they are relatively close at that distance.

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

[canonshooter]

Thanks, I could not recall the name of the DOF website.  Now, I have it bookmarked.

[ScottyP]

Every time I look at one of those DOF calculators, I am amazed I get any in-focus shots at all when using f/2.8 (or wider).  So very shallow and unforgiving.

If you have a lot of background distance to work with, and/or if you can get closer to the subject using the highest telephoto possible for the framing, you can get blurry backgrounds at f/4 and at f.5.6 even. 

Especially on a FF camera, which gives you 1 1/3 stops shallower DOF compared to the same aperture on a crop body.  F/4 on a FF body gives you 1/3 shallower DOF than f/2.8 can on a crop body, which is nice.

Good luck!

[canonshooter]

ScottyP, thanks for your reply.  I am really struggling with keeping the 24-70 vII without IS or returning it and preordering the f/4 version with IS.  I feel like I really need the IS and just a little concerned about giving up the blurred background qualities.  I can always boost the ISO on the 5DMkIII and gain the f-stop I would loose.

I agree FF gives much better blurred backgrounds compared to my 7D and that is why I went with the 5DMkIII.  And the new 24-70 vII is a wonderful pair, BUT the lack of IS has me paranoid.  I shoot mainly events/receptions (low light), on-site portraits usually without a tripod.  As you know, most shots look great on the LCD only to get back a see OOF shots that make me want to pull my hair out.

And do you think there is any chance the f/4 IS version will match the f/2.8 MkII version IQ, because in my opinion it is very, very good?

[ScottyP]

If you thought your 7D gave you good blurred backgrounds with an f/2.8 lens, then you will be even happier with f.4 on the full frame 5D3, as it will be better than that.  If the IS is important, then do it.  Take the $1400.00 difference you are saving with the cheaper 24-105 and buy a lt of other stuff you need or want.

As for the new 24-70 f/4 IS, vs. the f/2.8 mk2, who knows.  Maybe. 

Probably will be sharper than 24-105 because it is newer design, and because it covers a shorter focal range with (presumably) fewer optical compromises.  But probably the kind of difference you'd only be able to see in a photo of a test pattern.  Plus, that will be a long time to wait.  You could buy a used 24-105 on Craigslist or Ebay, and use it for a few years and then sell it for the same thing you paid for it, so it is like a free lens rental!

[Samsen]

 "I am really struggling with keeping the 24-70 vII without IS or returning it and preordering the f/4 version with IS.  I feel like I really need the IS and just a little concerned about giving up the blurred background qualities.  I can always boost the ISO on the 5DMkIII and gain the f-stop I would loose."

Let me make it easy for you.
Just Don't.

Why?

1- IS is invented to compensate for a higher (Wider) F number. At best IS when turned on, is hopped to add one to one and a half F stop. If you can supliment the F with REAL and OPTICAL value (Your current lens with F2.8 as opposed to your proposed lens with F4) why should you ever think of that if you know the concept of IS?

IS don't do magic. If you are feeling better because you are having technology assisting you, then be fair to the lens and compare IS vs Non-IS in the same exact F. A good example would be 70-200mm F2.8 without or with IS or its sister lenses, F4 without or with IS. 

I would think you will not see any benefit of IS with F4 and what you get right now with non IS F2.8 would be almost repeated with shooting F4 with IS, that is at best.

2- In my experience F2.8 24-70mm is one of the best if not simply The Best, walk around lens Canon makes. Please don't believe me and ask it for yourself from several people.

3- You will badly miss the Bokeh F 2.8 is giving you right now. Wait and see if I am not right.

4- If you like to see "Why there is so much hat for the New 24-70 F4" just read this thread: L I N K 

5- The benefit of IS becomes really apparent when using a Tele lens. At 24mm you will hardly see any benefit unless the word IS has some sort of tranquil effect on you. Effect at 70mm is marginal to say the most.

"most shots look great on the LCD only to get back a see OOF shots that make me want to pull my hair out."

Next time make sure to use 10x mag on LCD and you will see all should.

 "usually without a tripod. "

Why don't you consider using it or at least a Monopod. You will gain at least  2 F right there.

[ScottyP]

@Samsen:

A few points regarding your last post:

1.)  Canon claims 4 stops of benefit on its newer IS lenses.  The older systems claimed 3 stops but usually provided about 2 stops of benefit.  That is more than the 1 to 1.5 stops you cite.

2.)  I am not sure what you mean when you say "IS is invented to compensate for a higher (wider) F number".  First of all, a wider aperture is a LOWER F-number, not a higher one.  Secondly, I am not sure I understand the point.

IS does one thing, which is to compensate for camera shake at slower shutter speeds when hand-holding.  One CAN get the shutter speed up by going with a wider aperture, true, but one does not always WANT a super-narrow DOF.  If you are trying to shoot a cluster of 3 or 4 people and they are only 10 - 15 feet away, and they are not lined up shoulder-to-shoulder for you, then you may not be able to get all their faces in sharp focus inside that narrow plane of focus at f/2.8. 

IS is a useful tool for a walk-around lens as much as for telephoto.  True, a telephoto multiplies the camera shake, but your walk around lens will tend to be shot indoors under dim light a lot more than a telephoto is.  If you lower shutter speed to compensate for the dim light, that magnifies camera shake too.