kruiseri
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Depth Of Field Or Something Else

Thu Jul 07, 2011 5:34 pm

Hi,

The other day I was shooting at the local AirPort, this time around I decided to use a faster shutter speed (1/1000 sec) which of course meant a wide open or only slightly dimmed Aperture.

To my surprise, I got several shots where the nose of the AC would be pin sharp, but the tail appeared a bit soft. Surely this is not motion blur as the shutter speed was so fast, so did I encounter a depth of field issue ? This mostly occured on shots that were on the plane coming towards me where there would actually be some difference is distance.
 
JakTrax
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RE: Depth Of Field Or Something Else

Thu Jul 07, 2011 5:50 pm

Almost certainly a DOF issue. The only other thing it could be is an element slightly out but that's unlikely.

Karl
 
dendrobatid
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RE: Depth Of Field Or Something Else

Thu Jul 07, 2011 6:02 pm

No lens works at its best wide open so it is likely to be a combination of that degraded performance and shallow depth of field.

Mick Bajcar
 
kruiseri
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RE: Depth Of Field Or Something Else

Thu Jul 07, 2011 7:05 pm

I was a bit inaccurate about the aperture, this was a Ef 70-200/2.8 lens and it was stopped down to about f 4-5.

The reason I suspect DOF, is that in a sequence of shots this occured mostly when the plane was travelling towards me, once it was perpendicular (i was next to the RWY) everything was sharp.
 
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ptrjong
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RE: Depth Of Field Or Something Else

Thu Jul 07, 2011 11:12 pm

The same often happens to me with my Nikon equivalent of the same lens at f/4 or f/5, while at f/5.6 or f'/6.3 everything is sharp.

I'd think the lens should be good enough, and the DOF still deep enough at f/4 or f/5, to get a whole moving aircraft sharp in most situations, but apparently not. My 300 mm f/4 prime is usually pin sharp wide open.

So is the DOF of the two lenses at f/4 actually rather different, or is the celebrated f.2/8 70-200 actually that much worse than the f/4 prime?

Peter 
The only difference between me and a madman is that I am not mad (Salvador Dali)
 
ckw
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RE: Depth Of Field Or Something Else

Thu Jul 07, 2011 11:32 pm

Quoting ptrjong (Reply 4):
So is the DOF of the two lenses at f/4 actually rather different, or is the celebrated f.2/8 70-200 actually that much worse than the f/4 prime?

The depth of field covering a subject is dependent on 3 factors: focal length, aperture and subject distance. The extent of the DOF increases with distance. A frame filling subject at 200mm will be a lot closer than one at 300mm, so although the DOF of the 300 is less than a 200 in absolute terms, in practice the 50% increase in the shooting distance could result in the whole subject being sharp.

Cheers,

Colin
Colin K. Work, Pixstel
 
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ptrjong
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RE: Depth Of Field Or Something Else

Fri Jul 08, 2011 12:07 am

Quoting ckw (Reply 5):

Thanks Colin

That makes a lot of sense, but I'm experiencing this too when shooting larger aircraft with the 70-210, and smaller aircraft with the 300 at much the same distance (typically, fence to runway or crowd line to display line). Of course, a larger subject will itself require more DOF, but even allowing for that I'd say the prime performs much, much better.
The only difference between me and a madman is that I am not mad (Salvador Dali)
 
JakTrax
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RE: Depth Of Field Or Something Else

Fri Jul 08, 2011 11:46 am

I tend not to go lower than f/5.6; and even then I'd rather the subject (aircraft) be at least 50m away. I've never actually used my 70-200 f/4 at f/4 (which is why it puzzles me when people tell me I 'need to' own a 2.8?).

Karl
 
BHurt
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RE: Depth Of Field Or Something Else

Fri Jul 08, 2011 12:29 pm

JakTrax
I think those people telling you that you need the 2.8 are just repeating the line of all the portrait photographers. Those guys love the wide apertures on the long lenses because the longer focal length increases the DOF effects of a wide aperture. It just means they can take a picture of a person and throw the background farther out of focus than they can otherwise which is something they enjoy.

For aircraft spotters though that's of course not as attractive since you are trying to get an object the size of a wide body jet all in focus.

To the original poster I agree that it's almost certainly a combination of depth of field and reduced performance as dendrobatid mentioned. Wide apertures as a rule are just not as sharp as when a lens is stopped down.

That's not to say the area in focus won't be acceptably sharp, it might even look really good, it's just that chances are that same shot would have been even sharper still if the aperture had been tightened down a little further.
 
ckw
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RE: Depth Of Field Or Something Else

Fri Jul 08, 2011 1:00 pm

Quoting ptrjong (Reply 6):
even allowing for that I'd say the prime performs much, much better.

Well if we're talking strictly about depth of field, that's pure physics - you can calculate what the DOF of any lens will be for a given focal length and aperture - make or model, zoom or prime makes no odds. I forgot to say earlier there is 4th factor which also needs to be included - sensor size (smaller sensors increase DOF). But anyway, DOF is not variable between, say, different makes of lens.

I wonder in your case if what your seeing is actually a problem with the lens itself - a mis-aligned element, or possibly front focusing (ie. the DOF is greater than you realise because the area in focus is starting well ahead of the subject).

Quoting JakTrax (Reply 7):
which is why it puzzles me when people tell me I 'need to' own a 2.8

Well there's the option of a greater shutter speed range, low light capability and of course better selective focus. In many cases the wide aperture may also mean faster and more accurate focus (as the camera determines focus with the lens wide open). Traditionally, manufacturers also tended to keep their best glass in fast lenses, so there is a general quality issue, though I think this not so much the case now. Against all this you have size/weight and cost.

Personally I love fast lenses and the look they give, but for practical purposes I do most of my work with Canon f4 lenses which seem to me a good compromise on quality/size and price.

Cheers,

Colin
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ptrjong
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RE: Depth Of Field Or Something Else

Fri Jul 08, 2011 4:07 pm

Quoting ckw (Reply 9):
I wonder in your case if what your seeing is actually a problem with the lens itself - a mis-aligned element, or possibly front focusing (ie. the DOF is greater than you realise because the area in focus is starting well ahead of the subject).

Possibly, but maybe blaming my copy is a bit premature.

If any other users of an f/2.8 70-200s or 80-200 read this, I''d like to ask: what f number do you regard as a minimum when shooting distant aircraft?

Peter 
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kruiseri
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RE: Depth Of Field Or Something Else

Fri Jul 08, 2011 9:24 pm

Quoting JakTrax (Reply 7):
(which is why it puzzles me when people tell me I 'need to' own a 2.8?).

Karl


Well some of us shoot also other things than planes   f4 70-200 is quite useless in a hockey rink...
 
ua935
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RE: Depth Of Field Or Something Else

Fri Jul 08, 2011 11:17 pm

Quoting dendrobatid (Reply 2):
No lens works at its best wide open

Sorry, but what a load of ****.

My 500 F4 is the same across the range, 300 2.8 just the same, if they didn't then they would not justify the cost.

Aircraft spotting, that is something you do with binoculars or a telescope, wide open super tele's, maybe not for many on here but they are fantastic wide open and that is why we own them.

If you don't own them don't comment but to say that no lens works at its best wide open is a far reaching and ill founded comment.

Perhaps you don't own a lens that you can shoot wide open and stand by the results, if you did you would not make such a statement. Anet screeners are not the be all and end all of photography, try having a look at some of the wide open results on proper photography sites such as Fredmiranda etc.

[Edited 2011-07-08 16:19:58]
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cpd
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RE: Depth Of Field Or Something Else

Sat Jul 09, 2011 4:18 am

Quoting dendrobatid (Reply 2):
No lens works at its best wide open so it is likely to be a combination of that degraded performance

What is described here is a classic case of a narrow depth of field, and not "degraded performance".

The EF 70-200mm F/2.8 stopped to F/4 or F/5 won't have any of these degraded performance issues. It's a premium grade lens. We have numerous examples of that lens at work that can be used at whatever aperture you wish with no degraded performance (on EOS-1D and EOS 5D Mk.II).

The simple fix for this issue should be to change the aperture to maybe F/6.3 or F/7.1, even F/8.0 perhaps and increase the ISO setting to maintain the desired shutter speed.

[Edited 2011-07-08 21:22:42]
 
aviopic
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RE: Depth Of Field Or Something Else

Sat Jul 09, 2011 8:12 am

Quoting kruiseri (Reply 3):
I was a bit inaccurate about the aperture, this was a Ef 70-200/2.8

Foca Length - Distance - Aperture = DOF
200mm - 50m - 4 = 10m
200mm - 50m - 5.6 = 15m
200mm - 50m - 8 = 21m

200mm - 100m - 4 = 40m
200mm - 100m - 5.6 = 50m
200mm - 100m - 8 = 96m
All DOF's are +/-
DOF calculator
http://www.dofmaster.com/dofjs.html

Apps are available for Android and probably iphone too.

Quoting ptrjong (Reply 4):
So is the DOF of the two lenses at f/4 actually rather different, or is the celebrated f.2/8 70-200 actually that much worse than the f/4 prime?

The theory behind DOF.
http://en.wikipedia.org/wiki/Depth_of_field

Quoting JakTrax (Reply 7):
I've never actually used my 70-200 f/4 at f/4 (which is why it puzzles me when people tell me I 'need to' own a 2.8?).

AF works through a wide open lens.
Thus a "faster" lens can shoot faster, has faster AF performance and better low light AF even stopped down.

Quoting ua935 (Reply 12):
Sorry, but what a load of ****.

It is rare for me to visit this forum these days but when I do it doesn't take long to know why I spend little time here.

Quoting ua935 (Reply 12):
My 500 F4 is the same across the range

No, it isn't.

Quoting ua935 (Reply 12):
300 2.8 just the same

No, it's not.

Quoting ua935 (Reply 12):
if they didn't then they would not justify the cost.

A lens never does, unless you need one.
The truth lives in one’s mind, it doesn’t really exist
 
XT6Wagon
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RE: Depth Of Field Or Something Else

Sat Jul 09, 2011 9:04 am

Quoting ua935 (Reply 12):
Quoting dendrobatid (Reply 2):No lens works at its best wide open
Sorry, but what a load of ****.

Certainly "no cheap lens" might be a valid point, but a 70-200 F2.8 Isn't a cheap lens.... If it wasn't sharp at 2.8 no one would buy it since the F4 version is a good bit lighter and cheaper.

Anyway, like others have said, reserve the 2.8 for things that don't need a huge DOF like airliners.
 
chris78cpr
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RE: Depth Of Field Or Something Else

Sat Jul 09, 2011 1:53 pm

Quoting ua935 (Reply 12):
Quoting dendrobatid (Reply 2):
No lens works at its best wide open

Sorry, but what a load of ****.

My 500 F4 is the same across the range, 300 2.8 just the same, if they didn't then they would not justify the cost.

Aircraft spotting, that is something you do with binoculars or a telescope, wide open super tele's, maybe not for many on here but they are fantastic wide open and that is why we own them.

If you don't own them don't comment but to say that no lens works at its best wide open is a far reaching and ill founded comment.

Perhaps you don't own a lens that you can shoot wide open and stand by the results, if you did you would not make such a statement. Anet screeners are not the be all and end all of photography, try having a look at some of the wide open results on proper photography sites such as Fredmiranda etc.

I was about to go into a small rant and rave but it seems Simon said everything i was going to say. So that +1.

Chris
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timz
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RE: Depth Of Field Or Something Else

Sat Jul 09, 2011 11:19 pm

Quoting XT6Wagon (Reply 15):
Certainly "no cheap lens" might be a valid point, but a 70-200 F2.8 Isn't a cheap lens.... If it wasn't sharp at 2.8 no one would buy it since the F4 version is a good bit lighter and cheaper.

Depends what you mean by "sharp". As you'd expect, the graphs on dpreview.com show the 70-200L sharper at f/5.6.

"Sharpness on APS-C is certainly very good, if not quite outstanding. As usual, the lens is weakest at the extremes of the zoom range, and best in the middle. Naturally it also tends to be softest wide open, and gives optimum results at apertures of F5.6-F11..."

Like he said, you don't have to wonder whether depth of field is your problem-- you can calculate DOF any time you want.

[Edited 2011-07-09 16:21:27]
 
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clickhappy
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RE: Depth Of Field Or Something Else

Sun Jul 10, 2011 12:58 am

The lens is sharper in the center at 5.6, but falls off more as you move to the edges. F/8, or even f/11, give you a much more uniform sharpness across the entire image, and thus are easier to post-process.
 
ckw
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RE: Depth Of Field Or Something Else

Mon Jul 11, 2011 12:52 am

Quoting ua935 (Reply 12):
My 500 F4 is the same across the range, 300 2.8 just the same, if they didn't then they would not justify the cost.

You just have to look at Canon's own MTF charts to see this is not in fact the case. Certainly they chart better than many other lenses, but Canon (or anyone else for that matter) has yet to produce a perfect variable aperture lens.

Of course whether the fall off in performance is significant is a different matter. I have no hesitation using my 500 f4 wide open, and it is better at the edge wide open than lesser lenses in the centre stopped down. I would agree that for all practical purposes, the difference between f4 and f8 on the 500mm is irrelevant.

Cheers,

Colin
Colin K. Work, Pixstel
 
dendrobatid
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RE: Depth Of Field Or Something Else

Mon Jul 11, 2011 8:38 am

Quoting ua935 (Reply 12):
Quoting dendrobatid (Reply 2):
No lens works at its best wide open

Sorry, but what a load of ****.

I am equally sorry if I have offended your pride but I stand by what I said. I have no doubt that your lens performs very well wide open but it will still perform even better stopped down a bit - no lens performs at its best wide open, though it is evident that primes perform better than zooms in this respect - their design is far simpler.

http://reithian.com/mtf.htm

If you and chris78cpr do not understand the mtf charts there is an explanation in the top right corner. The fast and expensive lenses of which you are evidently so proud perform very well wide open but they like every lens performs better stopped down.

Thank you Willem, at least you got the point !

Mick Bajcar
 
XT6Wagon
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RE: Depth Of Field Or Something Else

Mon Jul 11, 2011 9:54 am

Quoting timz (Reply 17):
Depends what you mean by "sharp". As you'd expect, the graphs on dpreview.com show the 70-200L sharper at f/5.6.

The difference is that most people will have to pixel peep to tell the difference in sharpness with a good copy of a 70-200mm L at 2.8 Vs 5.6. if that even shows it.

A cheap

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