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?

[ScottyP]

Yes absolutely. Wider aperture makes shallower DOF.

Look up a depth of field calculator and fiddle with the aperture variable and watch the DOF change.

[SonomaBear]

Thank you Scotty... an old man can learn a new trick.
I always thought that DOF was nil at max aperture rather than a slight difference based upon the max. So a 1.4 prime will had a bit less DOF than a 1.8 all else being equal.
Since I shoot so many portraits at 85mm, perhaps I'll get the EF85 f1.8 instead of using a variable aperture zoom.
I went to http://www.dofmaster.com/dofjs.html for info.

[Skirball · Accepted solution]

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@SonomaBear wrote:
Thank you Scotty... an old man can learn a new trick.
I always thought that DOF was nil at max aperture rather than a slight difference based upon the max. So a 1.4 prime will had a bit less DOF than a 1.8 all else being equal.
Since I shoot so many portraits at 85mm, perhaps I'll get the EF85 f1.8 instead of using a variable aperture zoom.
I went to http://www.dofmaster.com/dofjs.html for info.

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Yes.  And if you're looking for truly thin DoF, then get a fast prime over the zoom.  1.8 is a whole lot thinner than 2.8.  More so than the difference between 2.8 and 4.  Once you go below 2.8-ish you really have to pay attention to exactly where you're focusing.

The 85mm 1.8 is a great choice for portraiture.

[ScottyP]

My 85mm f/1.8 does a very good job. Very sharp and focusing is fast.

If it is dim light indoors and you need a quick shutter speed to freeze moving subjects it is very noticeably better (2x better) at this than even an f/2.8 lens. Way way better (4 x better) than an f/4 lens.

It gives nice shallow DOF wide open and the background bokeh is nice.

The only weak point is it's tendency to show some purple fringing, but you only see it in a high contrast shots (like tree branches against a bright cloud) and this is easily corrected in post processing.

[Carnac]

Google DOF calculator - there are lots of free ones.

At 70mm on a full frame body (Canon 5d in this example) at a distance of 10 feet the DOF is:

at f4    = 1.47 feet

at f2.8 = 1.03 feet

The f2.8 lens has about 30% less DOF wide open at 10 feet.

Things that effect DOF are; sensor size, focal length of lens, distance to subject, and aperture.

Jim

[ebiggs1]

If you are deciding on a portrait lens, you might want to consider the 70-200mm f2.8 offerings.  More and more of the pros are going to this lens catagory for portrait lenses.

I doubt you are going to shoot many portraits at f1.4 or f1.8.

[SonomaBear]

Thank you ebiggs1 --

I use my 70-200 f2.8 occasionally for portraits but it is best only for known shooting rather than candids -- that big ole white lens can scare children and elderly!  And my 70-200 is very old (from my 650 days) so there is no IS for those cloudy days.  

But now that I am using a 70d instead of a 20d things have changed a bit... higher noiseless ISO images allow higher shutter speeds.  The 70-200 f2.8 with 1/200th do not need IS een for this old man! 

The REAL surprise is the "kit" lens -- EF-S 18-135 IS STM -- is nothing short of phenominal even though the background diffusion is less than a 2.8 or 1.8 -- but the bokeh is gorgeous.

[TCampbell]

The "angle of view" (how much fits in the frame) will be the same between the f/2.8 and f/4 versions assuming the same camera body and same focal length.

The background blur will NOT be the same if they are using different f-stops (e.g. one at f/4 and one at f/2.8). 

The IS of the f/4 version DOES NOT balance out with the 1 stop gain in the f/2.8 version!  Here's why:

"IS" is designed to help reduce or eliminate blur due to "camera movement" but it can do NOTHING to deal with "subject movement".  

If the reason you want a faster shutter speed is because you are photographing a moving subject... then the f/2.8 lens has the advantage because it can gather enough light to let you double the shutter speed and that might be enough to avoid motion blur due to subject movement.  

If you are photographing a stationary subject but you are "hand holding" the camera, and you want to avoid blur caused by camera movement at low speeds, then the f/4 IS lens would have more of an advantage... and actually MORE of an advatnage than the f/2.8 lens becuse f/2.8 is just one stop.  Canon's "IS" (actually "Hybrid IS") is a 4-stop IS system.  This means it may be able to allow you to shoot 4 stops "slower" than would otherwise be possible with a hand-held shot.  That's a lot more than the 1 stop gain you get from an f/2.8 lens with no IS.  But 4 stops isn't a guarantee... it's just highly probable.  It's the most probable at 1 stop... and the least probable at 4 stops.  The more you push it... the less likely it is to succeed.  And of course at 5 stops it's now "improbable" that it will be able to stablize the shot... but you could still get lucky.

[SonomaBear]

Thank you, Tim

In the standard lens range, I am not (rarely) shooting moving subjects and almost always hand held (macro & primes for tripods).  That said, the IS will be very helpful.

I was fixated on the background blur of my L lenses at f2.8 but I think now, due to the education that y'all provided, that the 28-70 f4 will do very well for MY shooting style.

Moving subjects are fodder for my 70-200 f2.8L oldie goldie handheld, so bless the Canon 2.8 glass wide open or nearly so.

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.

Low light, my trusty old 50mm f1.8 is amazing and its perspective is 80MM on the 70D -- poifect!

Thanks again Everyone