24-70 L Choice for Shallow DOF

[SonomaBear]

This may have been answered elsewhere and I should know the answer... All things being equal, an i mage taken by both lenses (24-70 f2.8 and 24-70 f4) of same subject, same body, same settings, both wide open, other than shutter speed one stop different, will the images appear similar (identical)????
IMHO, the IS of the f4 should balance the one stop shutter difference. Have I overlooked anything?
Is there a mathematic difference to depth of field based upon max aperture?

[ebiggs1]

"That said, the IS will be very helpful."

Maybe better said, IS can be very helpful.  Tim gave another good explaination of how IS can help.  But to take it further, IS is varible as stated.  It works best starting with one stop and gradually gets less effective the more stops you try.  It does not automatically turn off at 4 stops.

Also you should consider, just as you don't want to yse IS on with a tripod, you don't want to use it from a moving platform.  A car for instance.  This can confuse the IS.

Which brings up the last point, IS will be more or less effective for different people.  The better you are at holding and shooting correctly the better the IS will perform for you.  You may be that individual that get consistant 4 stop shots.  Maybe not.

IS is not a feature I "must have".  But if two lenses are offered one with and one without, I will pick the IS version every time.

Remember in the end the only person that needs to be happy with the results, is you.

[Skirball]

---

@SonomaBear wrote:

I'm using a 70D (crop sensor) effectively turning the 28-70 into a 45~112 which is perfect for candids (I rarely do formal portraits)... giving me 12 inch DOF at 15 feet at f4 (said the calculator) -- just perfect for candids with ISO on auto.

---

That doesn't sound right to me.  What calculator did you use?  Maybe it is, just doesn't sound right.  I'd expect a much wider DoF at 70mm and 15 feet at f/4 on a crop.  Both in terms of my true DoF (what is technically in focus), and the effect on my background (I wouldn't expect much bokeh unless the background was really far back.  Just based off experience, not calculators.

[Carnac]

Skirtball - you are correct. DOF on a 70D, 70mm, 15', and f4 is 2.11 feet (near to far).

Even though with a crop sensor camera, you set the actual FL (70mm) on the calculator. By setting the camera body, the crop factor is calculated corectly.

I guess you could say about a foot if you are thinking of a foot in front and a foot behind). From "www.DOFMASTER.com". Jim

ps - this site does not have the option of a 70D, but any of the Canon 1.6 crop sensor cameras will give the correct DOF (I used the 60D).